21st Century Astronomy 4th Edition by Laura Kay, Stacy Palen, Brad Smith & Blumenthal TEST BANK

TEST BANK

21st Century Astronomy FOURTH EDITION

LAURA KAY ◆ STACY PALEN ◆ BRAD SMITH ◆ GEORGE BLUMENTHAL

MICHAEL HOOD CAROL HOOD MICHAEL LOPRESTO

DONALD TERNDRUP

Contents

Assessment Guidelines Based on the Science of Learning vii Chapter 1

Why Learn Astronomy? 1

Chapter 2

Patterns in the Sky—Motions of Earth 15

Chapter 3

Motion of Astronomical Bodies 29

Chapter 4

Gravity and Orbits 44

Chapter 5

Light 59

Chapter 6

The Tools of the Astronomer 74

Chapter 7

The Birth and Evolution of Planetary Systems 88

Chapter 8

The Terrestrial Planets and Earth’s Moon 103

Chapter 9

Atmospheres of the Terrestrial Planets 120

Chapter 10 Worlds of Gas and Liquid—The Giant Planets 135 Chapter 11 Planetary Adornments—Moons and Rings 150 Chapter 12 Dwarf Planets and Small Solar System Bodies 164 Chapter 13 Taking the Measure of Stars 178 Chapter 14 Our Star—The Sun 191 Chapter 15 Star Formation and the Interstellar Medium 204 Chapter 16 Evolution of Low-Mass Stars 217

Chapter 17 Evolution of High-Mass Stars 231 Chapter 18 Relativity and Black Holes 245 Chapter 19 The Expanding Universe 261 Chapter 20 Galaxies 275 Chapter 21 The Milky Way—A Normal Spiral Galaxy 289 Chapter 22 Modern Cosmology 305 Chapter 23 Large-Scale Structure in the Universe 319 Chapter 24 Life 331

CHAPTER 1

Why Learn Astronomy?

CONCEPT MAP Sec 1.1 1. Astronomy I. Definition i. Astronomy loosely translated means “patterns among the stars” (MC: 1) II. Your Place in the Universe i. Your address: street, city, town, country, Earth, Sun, Milky Way, Local Group, Virgo Supercluster, universe (MC: 3, SA: 1) ii. Solar System: classical versus dwarf planets (TF: 1, 2, MC: 1–4, 7, SA: 2) iii. Milky Way: contains 200 to 400 billion stars (MC: 3, 5–7) iv. Local Group (MC: 3, 8) v. Virgo Supercluster (TF: 3, MC: 3) vi. Universe: contains hundreds of billions of galaxies, roughly as many stars as in the Milky Way (TF: 4) vii. Much of the universe is made of dark matter, and all of space is permeated by dark energy (TF: 5, MC: 9, 10) III. Scale of the Universe i. Speed of light, c  3  108 m/s (MC: 11) ii. d  v  t (TF: 2, MC: 12–17) iii. Light year is a measure of distance (TF: 6, MC: 12, SA: 3) iv. Distance versus time comparison: circumference of the Earth versus snapping your fingers (MC: 19, 20, SA: 4–6) IV. Origin and Evolution of Universe

i. Age of universe: 13.7 billion years (MC: 21) ii. Big Bang created the initial chemical elements: H, He, Li, Be, B (TF: 7, MC: 22, 23) iii. Stars manufactured the other chemical elements from nuclear burning and explosions (TF: 7, 8, MC: 24, 25, SA: 7) iv. Solar System formed v. Life evolved on Earth Sec 1.2 2. Science Involves Exploration and Discovery I. Evolution of Astronomy from New Technology i. Satellites, e.g., Sputnik, lunar exploration, Solar System exploration (TF: 10, MC: 26, SA: 8, 9) ii. Space-based astronomy used for high spatial resolution and access to wavelengths blocked by the atmosphere (TF: 10, MC: 27, SA: 9, 10) iii. Cross disciplines: astronomy, physics, chemistry, geology, planetary science iv. Computers: a new important tool for astronomers (SA: 9) Sec 1.3 3. Science Is a Way of Viewing the World I. Scientific Method i. Scientific method (MC: 28, 29, SA: 11) ii. Rational inquiry iii. Facts iv. Hypothesis (MC: 28, 30, SA: 11) v. Theory (MC: 28, 30, SA: 12) vi. Testable predictions/falsifiable (TF: 10, 11, MC: 28, SA: 11) vii. Physical laws viii. Scientific principle ix. Occam’s razor (MC: 31, 32) x. Cosmological principle (TF: 12, MC: 33, 34, SA: 13, 14) II. Scientific Knowledge Changes and Evolves i. Scientific knowledge continually evolves, usually slowly and gradually, because of new information (MC: 28) ii. Even when a theory is accepted as true, it may need revision later when new data comes along (TF:

14, MC: 28, 35, SA; 12) iii. Scientific revolutions: e.g. Newton/gravity, Einstein/special and general relativity, and quantum mechanics (MC: 36, SA: 15) iv. “Modern Physics,” post-quantum mechanics III. Challenges to Science i. Cultural/philosophical/religious influences ii. Scientific facts and theories need to be judged based on their predictions and observations alone iii. Scientists never ignore data just because it doesn’t fit their theory Sec 1.4 4. Patterns Make Our Lives and Science Possible I. Patterns Point Out Underlying Scientific Principles i. Patterns point out underlying scientific principles (TF: 15, MC: 37, SA: 16, 17) ii. Examples of patterns: rise/setting of Sun, seasons (MC: 38, SA: 16, 17) II. Mathematical Tools i. Mathematics is the “language of science,” a tool to quantify and compare patterns (MC: 39, SA: 18) ii. Scientific notation (MC: 40–43) iii. Ratios and proportionalities (MC: 44–49) iv. Units (MC: 50, 51) v. Reading a graph (MC: 52) vi. Slope of a line (MC: 53) Sec 1.5 5. Thinking Like an Astronomer I. What Is a Planet? i. Pluto was reclassified as a dwarf planet in 2006 (TF: 2, MC: 54–56, SA: 19) Sec 1.6 6. Origins: An Introduction I. Astrobiology i. Astrobiology: study of whether there is life elsewhere in the Solar System and the universe (MC: 57, SA: 20)

TRUE/FALSE 1. Our Sun is one of the most massive and luminous stars in the Milky Way. ANS: F DIF: Easy REF: Section 1.1 MSC: Factual TOP: 1IIii 2. Pluto is the only dwarf planet in our Solar System.

ANS: F DIF: Easy REF: Section 1.1 MSC: Factual TOP: 1IIii | 5Ii 3. The Local Group is a member of the Virgo Supercluster.

ANS: T DIF: Medium REF: Section 1.1 MSC: Factual TOP: 1IIiv | 1IIv 4. There are nearly 1,000 times more galaxies in the observable universe as there are stars in the Milky Way. ANS: F DIF: Medium REF: Section 1.1 MSC: Factual TOP: 1IIvi 5. A great majority of the matter in our universe is not visible.

ANS: T DIF: Easy REF: Section 1.1 MSC: Factual TOP: 1IIvii 6. A light-year is a unit commonly used in astronomy as a measure of time.

ANS: F DIF: Easy REF: Section 1.1 MSC: Factual TOP: 1IIIiii 7. Human beings are composed almost entirely of elements that were created in the Big Bang.

ANS: F DIF: Medium REF: Section 1.1 MSC: Applied TOP: 1IVii | 1IViii 8. The heavy elements that make up most of Earth were formed via nuclear fusion in the center of the Sun. ANS: F DIF: Easy REF: Section 1.1 MSC: Factual TOP: 1IViii 9. The invention of satellites advanced astronomy because telescopes on satellites can observe wavelengths of electromagnetic radiation, such as X-rays, that cannot penetrate the Earth’s atmosphere.

ANS: T DIF: Easy REF: Section 1.2 MSC: Factual TOP: 2Iii | 2Iiii 10. If a scientific theory cannot be tested, it is assumed to be true.

ANS: F DIF: Easy REF: Section 1.3 MSC: Conceptual TOP: 3Ivi 11. A crucial component of a scientific theory is that it is able to be tested by observations and thus proven true or false.

ANS: T DIF: Easy REF: Section 1.3 MSC: Conceptual TOP: 3Ivi

12. The Copernican principle states that there is nothing special about our local region of the universe.

ANS: T DIF: Easy REF: Section 1.3 MSC: Conceptual TOP: 3Ix 13. One consequence of the principle of universality is that gravity works the same here on Earth as it does on the planet Jupiter.

ANS: T DIF: Medium REF: Section 1.3 MSC: Conceptual TOP: 3Ix 14. Once a scientific theory is declared to be true, it is believed from that time onward.

ANS: F DIF: Medium REF: Section 1.3 MSC: Conceptual TOP: 3IIii 15. Science proceeds by presuming that observed patterns in nature can be attributed to an underlying physical explanation.

ANS: T DIF: Easy REF: Section 1.4 MSC: Factual TOP: 4Ii

MULTIPLE CHOICE 1. The word astronomy means: a. “patterns among the stars” b. “to study the stars” c. “discovering the universe” d. “the movement of the stars” e. “personality traits set by the stars” ANS: A DIF: Easy REF: Section 1.1 MSC: Factual TOP: 1Ii

2. According to the figure below, if you were to specify your address in the universe, listing your membership from the smallest to largest physical structures, it would be:

a. Earth, Local Group, Solar System, Andromeda, the universe b. Earth, Solar System, Local Group, Milky Way, the universe c. Earth, Solar System, Milky Way, Local Group, Virgo Supercluster, the universe d. Earth, Solar System, Milky Way, Fornax Supercluster, the universe e. Earth, Fornax Supercluster, Milky Way, Solar System, the universe ANS: C DIF: Difficult REF: Section 1.1 MSC: Factual TOP: 1IIi | 1IIv

3. Which of the following is FALSE? a. The Local Group is a member of the Virgo Supercluster, which contains thousands of galaxies. b. The Local Group contains two large spiral galaxies and a few dozen dwarf galaxies. c. Our Solar System has eight classical planets. d. The Milky Way galaxy contains approximately 100 million stars. e. The Virgo Supercluster is one of many superclusters in the universe ANS: D DIF: Difficult REF: Section 1.1 MSC: Factual TOP: 1IIii | 1IIiii | 1IIiv | 1IIv 4. The number of classical planets in our Solar System is: a. eight b. nine c. twelve d. six e. four ANS: A DIF: Easy REF: Section 1.1 MSC: Factual TOP: 1IIii 5. According to the figure below, the Earth is located approximately:

a. at the center of the Milky Way b. near the center of the Milky Way c. about halfway out from the center of the Milky Way d. at the farthest outskirts of the Milky Way e. outside the Milky Way, which is why we can see it as a band across the night sky ANS: C DIF: Easy REF: Section 1.1 MSC: Applied TOP: 1IIiii

6. What is the approximate number of stars in the Milky Way? a. 10 million b. 300 million c. 10 billion d. 300 billion e. 1 trillion ANS: D DIF: Medium REF: Section 1.1 MSC: Factual TOP: 1IIiii 7. If the diameter of the Milky Way is approximately 100,000 light-years, then our galaxy is _________ times larger than our Solar System. For reference, Pluto’s orbit has an approximate diameter of 80 AU. a. 100 b. 1,000 c. 10,000 d. 106 e. 108 ANS: E DIF: Difficult REF: Section 1.1 MSC: Applied TOP: 1IIiii 8. The Local Group is the environment around: a. the Earth-Moon system b. the Sun that contains about a dozen stars c. the Sun that contains over a million stars d. the Milky Way that contains a few dozen galaxies e. the Milky Way that contains a few thousand galaxies ANS: D DIF: Medium REF: Section 1.1 MSC: Factual TOP: 1IIiv 9. The majority of the mass in our universe is made up of: a. planets b. stars c. galaxies d. dust e. dark matter ANS: E DIF: Medium REF: Section 1.1 MSC: Factual TOP: 1IIvii

10. The majority of the energy in our universe is: a. radiated by stars from the nuclear fusion going on in their cores b. the kinetic energy found in the collisions of galaxies c. the gravitational potential energy of superclusters d. emitted in radioactive decays of unstable elements e. made up of dark energy that permeates space ANS: E DIF: Difficult REF: Section 1.1 MSC: Factual TOP: 1IIvii 11. The speed of light is approximately: a. 3,000 km/s b. 30,000 km/s c. 300,000 km/s d. 3 million km/s e. 3 billion km/s ANS: C DIF: Medium REF: Section 1.1 MSC: Factual TOP: 1IIIi 12. The average distance between the Earth and Sun is 1.5  1011 m, and light from the Sun takes approximately _________ to reach Earth. a. 8 seconds b. 8 minutes c. 8 hours d. 8 days e. 8 years ANS: B DIF: Easy REF: Section 1.1 MSC: Applied TOP: 1IIIii 13. If an event were to take place on the Sun, how long would it take for the light it generates to reach us? a. 8 minutes b. 11 hours c. 1 second d. 1 day e. It would reach us instantaneously.

ANS: A DIF: Medium REF: Section 1.1 MSC: Applied TOP: 1IIIii

14. After the Sun, the next nearest star to us is approximately _________ away. a. 8 light-seconds b. 80 light-minutes c. 40 light-hours d. 4 light-years e. 200 light-years ANS: D DIF: Difficult REF: Section 1.1 MSC: Factual TOP: 1IIIii 15. One of the nearest stars is Alpha Centauri, whose distance is 4.2  1016 m. How long does it take light to travel from Alpha Centauri to us? a. 1.25 seconds b. 8.3 minutes c. 4.4 years d. 560 years e. 6,200 years ANS: C DIF: Medium REF: Section: 1.1 MSC: Applied TOP: 1IIIii 16. The distance to the nearest large spiral galaxy, the Andromeda galaxy, is 2.4  1022 m. How long does it take light to travel from Andromeda to us? a. 4.4 years b. 360 years c. 1.2 thousand years d. 2.5 million years e. 4.5 billion years ANS: D DIF: Medium REF: Section 1.1 MSC: Applied TOP: 1IIIii

17. The distance to the center of the Virgo cluster of galaxies is 5  1023 m. How long does it take light to travel from these galaxies to us? a. 7,000 years b. 54,000 years c. 120,000 years d. 12 million years e. 54 million years ANS: E DIF: Medium REF: Section 1.1 MSC: Applied TOP: 1IIIii 18. A light-year is a unit commonly used in astronomy as a measure of: a. time b. speed c. mass d. distance e. acceleration ANS: D DIF: Medium REF: Section 1.1 MSC: Factual TOP: 1IIIiii 19. The figure below measures distances in the amount of time it takes light to travel. If the circumference of Earth is a snap of your fingers (1/7 sec), the diameter of the Solar System is approximately equal to:

a. the length of a quick lunch b. the time to turn a page in a book c. the length of the work day d. the time you spent in high school e. a human lifetime ANS: C DIF: Difficult REF: Section 1.1 MSC: Applied TOP: 1IIIiv

20. If you compared the diameter of the Earth, which is 13,000 km, to 1 second, then what unit of time would be equivalent to the size of the Milky Way, whose diameter is 1021 m, and what significant milestone would this time correspond to in our evolution? a. 2 million years, the length of time humans have existed on Earth b. 30,000 years, the length of time humans have lived in North America c. 400 years, the length of time humans have been exploring the skies with telescopes d. 4 billion years, the age of the Solar System e. 14 billion years, the age of the universe ANS: A DIF: Difficult REF: Section 1.1 MSC: Applied TOP: 1IIIiv 21. Our universe is approximately 13.7 _________ years old. a. million b. billion c. trillion d. thousand e. hundred ANS: B DIF: Easy REF: Section 1.1 MSC: Factual TOP: 1IVi 22. The early universe was composed mainly of which two elements? a. hydrogen and helium b. carbon and oxygen c. hydrogen and oxygen d. carbon and iron e. nitrogen and oxygen ANS: A DIF: Easy REF: Section 1.1 MSC: Factual TOP: 1IVii

23. Which presently observed element or isotope was NOT produced in appreciable amounts in the very early universe shortly after the Big Bang? a. hydrogen b. helium-4 c. deuterium d. carbon e. helium-3 ANS: D DIF: Medium REF: Section 1.1 MSC: Factual TOP: 1IVii 24. Which is an important element in the composition of your body that was produced by nuclear fusion inside a star or an explosion of a star? a. iron b. calcium c. oxygen d. carbon e. all of the above ANS: E DIF: Easy REF: Section 1.1 MSC: Factual TOP: 1IViii 25. The most massive elements such as those that make up terrestrial planets like Earth were formed: a. in the early universe b. inside stars and supernovae c. through meteor collisions d. in the core of Earth e. during the formation of the Solar System ANS: B DIF: Medium REF: Section 1.1 MSC: Factual TOP: 1IViii 26. An unmanned robotic spacecraft has NOT landed on: a. an asteroid b. Mars c. Venus d. Pluto e. Titan, Saturn’s largest moon ANS: D DIF: Medium REF: Section 1.2 MSC: Factual TOP: 2Ii

27. Which is NOT an advantage of placing telescopes in space? From space, a telescope: a. has better access to high energy radiation such as ultraviolet light and X-rays b. has better access to low energy radiation such as infrared and microwave radiation c. avoids the blurring of images caused by the Earth’s atmosphere d. avoids light pollution from big cities e. is closer to the objects being observed ANS: E DIF: Easy REF: Section 1.2 MSC: Applied TOP: 2Iii 28. Which of the following is FALSE? a. A scientific theory is an undisputed fact. b. If continual testing of a hypothesis shows it to be valid, it may become an accepted theory. c. A hypothesis must always have one or more testable predictions. d. A scientific theory may eventually be proven wrong when scientists acquire new data. e. Scientific observations are used to test a hypothesis.

ANS: A DIF: Medium REF: Section 1.3 MSC: Conceptual TOP: 3Ii | 3Iiv | 3Iv | 3Ivi | 3IIi | 3IIii 29. The scientific method is a process by which scientists: a. prove theories to be known facts b. gain confidence in theories by failing to prove them wrong c. show all theories to be wrong d. test the ideas of Aristotle e. survey what the majority of people think about a theory ANS: B DIF: Medium REF: Section 1.3 MSC: Applied TOP: 3Ii 30. A _________ becomes a _________ when repeated testing of its predictions does not disprove it. a. hypothesis; scientific method b. theory; scientific revolution c. phenomenon; theory d. hypothesis; theory e. law; theory ANS: D DIF: Medium REF: Section 1.3 MSC: Applied TOP: 3Iiv | 3Iv

31. _________ is the idea that the simplest explanation for a phenomenon is usually the correct one. a. Newton’s hypothesis b. Occam’s razor c. Aristotle’s test d. Einstein’s excuse e. The Copernican principle ANS: B DIF: Difficult REF: Section 1.3 MSC: Conceptual TOP: 3Iix 32. If you have a stuffy nose, a fever, chills, and body aches and a doctor treats you for the flu rather than four separate diseases that account for each of your symptoms, this is an application of: a. Newton’s hypothesis b. Occam’s razor c. Aristotle’s test d. Einstein’s relativity e. Copernican principle ANS: B DIF: Difficult REF: Section 1.3 MSC: Applied TOP: 3Iix 33. The cosmological principle states that: a. the universe is expanding in all directions at the same rate b. a unique center of the universe exists c. the universe looks the same everywhere and in all directions as long as you look on large enough spatial scales d. physical laws change from place to place in the universe e. the universe is in a “steady state” ANS: C DIF: Medium REF: Section 1.3 MSC: Conceptual TOP: 3Ix 34. Because of _____________, we can conclude that gravity works the same way on Earth as it does on Mars. a. Newton’s theory of relativity b. Einstein’s special theory of relativity c. Sagan’s planetary principle d. the principle of universality e. the hypothetical statute ANS: D DIF: Medium REF: Section 1.3 MSC: Conceptual TOP: 3Ix

35. A scientific theory can be shown to be wrong if: a. cultural beliefs evolve to contradict it b. scientists gather new data that contradicts its predictions c. it cannot explain all phenomena d. it was first proposed as a conjecture e. a majority of people do not accept it ANS: B DIF: Easy REF: Section 1.3 MSC: Conceptual TOP: 3IIii 36. Albert Einstein is best known for his revolutionary theory of: a. relativity b. quantum mechanics c. astronomy d. electricity e. mathematics ANS: A DIF: Easy REF: Section 1.3 MSC: Factual TOP: 3IIiii 37. When you see a pattern in nature, it is usually evidence of: a. a theory being displayed b. quantum mechanics in action c. a breakdown of random clustering d. an underlying physical law e. A decrease in entropy ANS: D DIF: Easy REF: Section 1.4 MSC: Factual TOP: 4Ii

38. The figures above show the night sky as it appears for an observer in the United States at the same time of the night but at four different seasons of the year. Which conclusion below is NOT reasonable based on these observations? a. Constellations do not change their location relative to one another, but which constellations appear in the night sky does change from season to season. b. There are some constellations such as Ursa Minor, Ursa Major, Cassiopeia, and Cephus that are always seen in the night sky. c. Some constellations such as Capricornus and Sagittarius are only visible during summer and fall. d. A good time to harvest crops would be when the constellation Pegasus is directly overhead. e. A good time to plant crops would be when the constellation Sagittarius is directly overhead.

ANS: E DIF: Medium REF: Section 1.4 MSC: Applied TOP: 4Iii 39. The language of science is: a. Greek b. mathematics c. calculus d. Java e. Latin ANS: B DIF: Easy REF: Section 1.4 MSC: Factual TOP: 4IIi 40. Scientific notation is used in astronomy primarily because it allows us to: a. write very large and very small numbers in a convenient way b. talk about science in an easy way c. change easy calculations into hard calculations d. change hard calculations into easy calculations e. explain science to engineers ANS: A DIF: Easy REF: Section 1.4 MSC: Factual TOP: 4IIii

41. The number 123,000 written in scientific notation is: a. 1.23  106 b. 1.23  105 c. 1.23  10 3 d. 1.23  10 6 e. 1.23  103 ANS: B DIF: Easy REF: Section 1.4 MSC: Applied TOP: 4IIii 42. (6  105)  (3  10 2) a. 1.8  103 b. 1.8  104 c. 1.8  106 d. 1.8  103 e. 1.8  10 3 ANS: B DIF: Medium REF: Section 1.4 MSC: Applied TOP: 4IIii 43. (1.2  109 )  (4  10 3)  a. 3  106 b. 3  105 c. 3  1010 d. 3  1011 e. 3  1012 ANS: D DIF: Medium REF: Section 1.4 MSC: Applied TOP: 4IIii 44. If the radius of circle B is twice the radius of circle A, and the area of a circle is proportional to the radius squared (A

r2), then the ratio of the area of circle B to that of circle A is:

a. 4 b. 0.5 c. 0.25 d. 2 e. 1.414 ANS: A DIF: Easy REF: Section 1.4 MSC: Applied TOP: 4IIiii

45. If the radius of circle B is 5 times the radius of circle A, then the ratio of the area of circle B to that of circle

A is: a. 25 b. 5 c. 0.2 d. 0.04 e. 0.025 ANS: A DIF: Medium REF: Section 1.4 MSC: Applied TOP: 4IIiii 46. If the radius of sphere B is 5 times the radius of sphere A, then the ratio of the volume of sphere B to the volume of sphere A is: a. 0.008 b. 0.2 c. 5 d. 25 e. 125 ANS: E DIF: Medium REF: Section 1.4 MSC: Applied TOP: 4IIiii 47. The volume of a sphere is related to its radius by the formula . Using algebra to invert this formula, we find that: a. b. c. d. e.

ANS: B DIF: Difficult REF: Section 1.4 MSC: Applied TOP: 4IIiii

48. The area of a circle is related to its diameter by the formula . Using algebra to invert this formula, we find that: a. b. c. d. e.

ANS: D DIF: Medium REF: Section 1.4 MSC: Applied TOP: 4IIiii 49. The type of mathematics that deals with infinitesimal changes is called: a. algebra b. calculus c. arithmetic d. geometry e. topology ANS: B DIF: Easy REF: Section 1.4 MSC: Factual TOP: 4IIiii 50. If the speed of light is 3  105 km/s and 1 km  0.62 miles, what is the speed of light in miles/hr? a. 670 million miles/hr b. 670 thousand miles/hr c. 186 miles/hr d. 186 thousand miles/hr e. 3.2 billion miles/hr ANS: A DIF: Difficult REF: Section 1.4 MSC: Applied TOP: 4IIiv 51. The orbital period of Mercury is 1.9 years. What is its orbital period in units of seconds? a. 60,000 seconds b. 6 million seconds c. 6 billion seconds d. 600 billion seconds e. 60 million seconds ANS: C DIF: Difficult REF: Section 1.4 MSC: Applied TOP: 4IIiv

52. Which of the graphs shown below illustrates a linear relationship between the variables x and y?

a. (a) b. (b) c. Both (a) and (b) show linear relationships. d. Neither (a) nor (b) show linear relationships.

ANS: A DIF: Medium REF: Section 1.4 MSC: Applied TOP: 4IIv

53. What is the slope of the line in the figure shown below?

a. 0.1 km/hr b. 1 km/hr c. 6 km/hr d. 10 km/hr e. 60 km/hr ANS: D DIF: Difficult REF: Section 1.4 MSC: Applied TOP: 4IIvi 54. Pluto is classified as a: a. planet b. dwarf planet c. asteroid d. comet e. meteroid ANS: B DIF: Easy REF: Section 1.5 MSC: Factual TOP: 5Ii 55. Pluto’s status was changed from a classical planet to a dwarf planet primarily because of its: a. lack of an atmosphere b. small mass c. large distance d. ice-covered surface e. many moons.

ANS: B DIF: Medium REF: Section 1.5 MSC: Factual TOP: 5Ii

56. The decision to classify Pluto as a dwarf planet was made by: a. its discoverer Clyde Tombaugh in 1930 b. the president of the U.S. Naval Observatory in 2001 c. the International Astronomical Union in 2006 d. the Secretary General of the United Nations in 2001 e. a vote of the majority of Americans in 2006 ANS: C DIF: Difficult REF: Section 1.5 MSC: Factual TOP: 5Ii 57. The study of whether or not life exists elsewhere in the Solar System and beyond is called: a. origins b. biochemistry c. cosmology d. astrobiology e. exoplanetology ANS: D DIF: Medium REF: Section 1.6 MSC: Factual TOP: 6Ii

SHORT ANSWER 1. Suppose you were writing to a pen pal in another universe. What address would you put on the envelope that included all the major structures in which we reside? (Hint: Your cosmic address should begin with “Earth” and end with “the universe.”)

ANS: The address would be: Earth, the Solar System, the Milky Way, the Local Group, Virgo Supercluster, the universe.

DIF: Medium REF: Section 1.1 MSC: Factual TOP: 1IIi 2. What is the only thing that makes the Sun an exceptional star?

ANS: The fact that it is our star!

DIF: Easy REF: Section 1.1 MSC: Conceptual TOP: 1IIii 3. What would you say to someone who said, “It would take light-years to get to the Andromeda galaxy”?

ANS: You would have to tell them that light-years is a unit of distance not time.

DIF: Medium REF: Section 1.1 MSC: Applied TOP: 1IIIiii

4. If you compare the diameter of the Earth to 1 minute of time, then what interval of time would represent the diameter of the Solar System? Assume the diameter of the Solar System is approximately 80 AU. ANS: The diameter of the Earth is 2  6378 km  1.3  107 m, and 80 AU  80  1.5  1011 m  1.2  1013 m. Thus, the diameter of the Solar System would be represented by: 1.2  1013 m  (1 minute) / (1.3 107 m)  9.4  105 minutes  1.8 years.

DIF: Medium REF: Section 1.1 MSC: Conceptual TOP: 1IIIiv 5. Using the method of comparing times to get a handle on the large distances in astronomy, compare the size of Earth to the size of the visible universe. Start by making the size of the Earth comparable to a snap of your fingers, which lasts about 1/7 seconds. Show your computation.

ANS: If the size of Earth is like a snap of your fingers (1/7 seconds), the size of the visible universe would be 13.7 billion years  3  4.5 billion years  3 times the age of the Solar System.

DIF: Medium REF: Section 1.1 MSC: Applied TOP: 1IIIiv 6. Using the method of comparing distances to time intervals to get a handle on the large distances in astronomy, compare the diameter of our Solar System, which is 6  1012 to the diameter of the Galaxy, which is 1.2  1021 by calculating the time it would take for light to travel these diameters. For reference, the speed of light is 3  108 m/s.

ANS: The time it takes light to travel across the diameter of the Solar System is t  d / v  6  1012 m/(3  108 m/s)  20,000 s  (1 hr/3600 s)  5.5 hr. The time it takes light to travel across the diameter of the Galaxy is t  1.2  1021 m/(3  108 m/s)  4  1012 s  (1 hr/3600 s)  (1 day/24 hr)  (1 yr/365 day)  130,000 yr.

DIF: Difficult REF: Section 1.1 MSC: Applied TOP: 1IIIiv 7. Describe briefly why the phrase “we are stardust” is literally true.

ANS: Massive stars make heavy elements during their lifetime. When they eventually explode in a supernova, some of these heavy elements, as well as additional ones that are created in the explosion itself, are ejected into space, where they eventually cool and form new solar systems and everything in them, including us.

DIF: Medium REF: Section 1.1 MSC: Conceptual TOP: 1IViii

8. On which objects in our solar system, other than Earth, have humans actually landed? On which objects have unmanned robot spacecraft landed?

ANS: Humans have landed only on the Moon. Unmanned spacecraft have landed on Mars, Venus, asteroids, and Saturn’s moon Titan.

DIF: Medium REF: Section 1.2 MSC: Factual TOP: 2Ii 9. Describe two important technological developments in the last 100 years that have greatly increased our ability to study the universe and describe how each did so.

ANS: The invention of spacecraft allowed us to launch telescopes above Earth’s atmosphere, giving us a much clearer view of the universe and access to wavelengths of radiation that do not penetrate the Earth’s atmosphere. Computers can rapidly collect and analyze large amounts of data which allows us to get results more quickly and efficiently.

DIF: Medium REF: Section 1.2 MSC: Applied TOP: 2Ii | 2Iii | 2Iiv 10. Describe two important reasons why the ability to put telescopes in space dramatically affected the science of astronomy.

ANS: Images taken from telescopes in space do not suffer the blurring that is due to light’s passage through the Earth’s atmosphere, and thus the images are much sharper. Also, some wavelengths of radiation do not penetrate through the Earth’s atmosphere, such as ultraviolet light or X-rays, and thus they can only be observed from space.

DIF: Easy REF: Section 1.2 MSC: Applied TOP: 2Iii 11. Describe the main steps involved in the scientific method.

ANS: First you make a hypothesis and then you make a prediction based on your hypothesis. Finally, you test your prediction through experimentation to prove or disprove your original hypothesis. You revise your hypothesis, if necessary, when the experiments disagree with your hypothesis.

DIF: Medium REF: Section 1.3 MSC: Conceptual TOP: 3Ii | 3Iiv | 3Ivi 12. How would you respond to someone who stated that “Evolution is not proven; it is just a theory”?

ANS: You would need to explain that in science, a theory is not something that is proven, rather it our best explanation based on available data. Thus calling something a theory does not diminish its importance.

DIF: Difficult REF: Section 1.3 MSC: Applied TOP: 3Iv| 3Iii

13. What two pre-Renaissance beliefs are contradicted by the cosmological principle?

ANS: (1) Earth is at the center of our universe, and (2) celestial objects are made of a different substance than Earth and obey different rules.

DIF: Medium REF: Section 1.3 MSC: Factual TOP: 3Ix 14. Describe the two main aspects of the cosmological principle.

ANS: (1) What we see around us is representative of what the universe is like in general, and (2) the physical laws valid on Earth are valid everywhere.

DIF: Easy REF: Section 1.3 MSC: Conceptual TOP: 3Ix 15. Describe two ways in which Einstein’s new theories changed commonly accepted scientific views of his time.

ANS: Mass and energy are manifestations of the same phenomenon. Thus you can convert one into the other. Time and space are not separable, but are intimately related to one another. Thus Newton’s Law of gravity is only a special case of a more general law Einstein called general relativity.

DIF: Medium REF: Section 1.3 MSC: Factual TOP: 3IIiii 16. There are many different areas of science, but a common factor in each is the evaluation and analysis of patterns. What patterns does astronomy deal with? (Describe it in general and give at least one concrete example.)

ANS: Astronomy deals with patterns related to celestial objects. One example is that patterns in the sky mark the changing of seasons, the coming of rains, the movement of herds, and the planting and harvesting of crops. An additional example is that the Sun rises and sets at a specific time because the Earth goes around the Sun. DIF: Easy REF: Section 1.4 MSC: Conceptual TOP: 4Ii | 4Iii 17. An observed pattern in nature is usually a sign of some underlying physical reason. Give an example of this in astronomy, citing the pattern and the reason behind it.

ANS: The Sun rises and sets each day. This pattern is due to the Earth’s daily revolution on its axis. The stars visible in the sky at a given time of day change throughout the year, but the pattern repeats every year. This is due to the Earth’s motion around the Sun in one year.

DIF: Easy REF: Section 1.4 MSC: Conceptual TOP: 4Ii | 4Iii

18. It is often said that “mathematics is the language of science.” Explain why this is true.

ANS: Math is a formal system used when describing and analyzing patterns, and explaining the reasons for patterns is the heart of science. Thus math is the language of science.

DIF: Easy REF: Section 1.4 MSC: Conceptual TOP: 4IIi 19. Describe the scientific reasons why astronomers reclassified Pluto as a dwarf planet in 2006.

ANS: The reason is similar to why the first “minor planets” found in the 1800s are now known as asteroids. Since its discovery in 1930, Pluto has been found NOT to be unique, but rather one of many similar objects that lie in the Kuiper Belt beyond the orbit of Neptune. This includes the dwarf planet Eris.

DIF: Medium REF: Section 1.5 MSC: Applied TOP: 5Ii 20. What is the primary concern of the subfield of astronomy known as astrobiology?

ANS: Astrobiology is the investigation of the possibility that life exists elsewhere in our solar system and beyond.

DIF: Easy REF: Section 1.6 MSC: Factual TOP: 6Ii

CHAPTER 2

Patterns in the Sky—Motions of Earth

CONCEPT MAP Sec 2.1 1. A View from Long Ago I. Night Sky (MC: 1) i. Constellations (TF: 1) Sec 2.2 2. The Earth Spins on Its Axis I. Important Concepts i. Celestial sphere (SA: 1, 3–5) ii. Equator, latitude, longitude on Earth (MC: 2–8, SA: 2, 6) iii. Horizon (MC: 4, 9, 15, SA: 1–3, 6, 10, 20) iv. Meridian (TF: 2, MC: 9, 10–13, SA: 3) v. Zenith/nadir (MC: 5, 9, 14, SA: 3) vi. North/South celestial poles (MC: 4–6, 9, 15, SA: 1) vii. Ecliptic (MC: 16, SA: 5) viii. Location of Polaris in the night sky (MC: 3, SA: 6) II. Relative Motions and Frame of Reference (SA: 7) III. Motion Due to Earth’s Daily Revolution on Its Axis i. Rotation rate  360 degrees/24 hours (TF: 3, MC: 10–13, 17–19, SA: 8) IV. Observed Effect on Stars’ Motions in the Night Sky i. Rising and setting of celestial objects on the eastern and western horizons, respectively (TF: 4, 5 MC: 10–13, 17–23, SA: 9, 10)

ii. Circumpolar stars (TF: 5) iii. How the observed star motions change with an observer’s longitude and latitude (TF: 3, SA: 9, 10) Sec 2.3 3. Revolution about the Sun Leads to Changes during the Year I. Earth Orbits the Sun at an Average Distance of 1 AU i. 1 AU  1.5  108 km ii. Earth rotates on its axis in the same direction it orbits the Sun (counterclockwise as viewed looking down on the North Pole) (TF: 6, MC: 10–13) II. Orbital Period i. 365.24 days  3651⁄4 days (MC: 25–26) ii. Consequence: At a given time of day, a star/constellation’s position in the sky will vary throughout the year (MC: 25–26) iii. Sun’s apparent path over the year defines the ecliptic plane and constellations of the zodiac (MC: 10–13, 27) III. The Tilt of Earth’s Equator with Respect to Earth’s Orbital Plane i. Tilt  23.5° (MC: 7, 8, 28–31) ii. Seasons result because the angle of Sun’s rays more perpendicular to Earth’s surface in summer, and the surface density of the energy received is larger (TF: 7, MC: 30–33, SA: 11–12) iii. Seasons are NOT due to Earth being physically closer to the Sun in summer (a common misconception) (TF: 7) iv. Sun’s azimuth at rising/setting or altitude at noon changes during the year (TF: 8–9, MC: 34) v. Vernal and autumnal equinoxes (TF: 10, SA: 5, 13) vi. Winter and summer solstices (TF: 11, MC: 8, 35–37, SA: 2, 13) IV. Location of the Earth’s N/S Pole on the Celestial Sphere Moves with Time i. Precession, period  26,000 years (MC: 38) ii. The “North Star” will not always be Polaris (MC: 39) Sec 2.4 4. The Motions and Phases of the Moon I. Moon Phases i. Important terms: full, new, crescent, gibbous, waxing, waning (TF: 12, MC: 17–19, 40–44, 38, SA: 14)

ii. Predicting the Moon’s phase given the position of the Sun, Earth, and Moon and vice versa (MC: 17–19, 42, 43–48, SA: 14–16) II. Periods i. Sidereal period (27.32 days)—orbital period measured by an outside observer (with respect to background stars) (MC: 45) ii. Synodic period (29.53 days)—the time required for the Moon to go through a complete cycle of phases (MC: 17–19, 41, 43–45) iii. Moon’s sidereal period is NOT equal to the Moon’s synodic period because of the Earth and Moon’s motion around the Sun (MC: 45, 49) iv. Predicting the Moon’s position in the sky given its phase and time of day; predicting the rising and setting times of the Moon (TF: 12, MC: 17–19, SA: 14, 17) III. Moon’s Rotation (MC: 50) i. Rotation rate  1 sidereal period ii. Consequence: synchronous rotation—Moon always keeps the same side toward Earth (TF: 13, SA: 18) Sec 2.5 5. Cultures and Calendars I. Leap Year i. Occurs because the Earth’s orbital period is 3651⁄4 days (MC: 51–53, SA: 19) II. Gregorian Calendar (SA: 19) Sec 2.6 6. Eclipses: Passing through a Shadow I. Geometry of Eclipses i. Lunar eclipses (MC: 46, 54, SA: 20) ii. Solar eclipses (TF: 14, 15, MC: 47–48, 55–56, SA: 21) II. Degree of Eclipse (MC: 55–56) i. Total, partial, or annular eclipse (MC: 54) ii. Umbra (MC: 57) iii. Penumbra (MC: 57) III. Eclipses Are Rare i. 5.2° tilt of Moon’s orbital plane relative to Earth’s orbital plane (MC: 58, SA: 22)

ii. Line of nodes (SA: 22) iii. Eclipse frequency: 2 of each type per 11 months (MC: 58–59, SA: 22) iv. Total/partial eclipses can be seen only at specific locations on the Earth (TF: 15) Sec 2.7 7. Origins: The Obliquity of Earth I. Obliquity i. Change in the degree of Earth’s axial tilt (SA: 12)

TRUE/FALSE 1. Constellations are arbitrary groupings of stars in the sky.

ANS: T DIF: Easy REF: Section 2.1 MSC: Factual TOP: 1Ii 2. The meridian is half of an imaginary circle in the sky that passes through an observer’s zenith and both celestial poles.

ANS: T DIF: Easy REF: Section 2.2 MSC: Factual TOP: 2Iiv 3. Locations along the equator are the only place on Earth where you can see the entire celestial sphere (during the day or night) over the course of 24 hours.

ANS: T DIF: Easy REF: Section 2.2 MSC: Factual TOP: 2IIIi | 2IViii 4. If a star rises due east on the horizon, it will set due west on the horizon six hours later.

ANS: F DIF: Easy REF: Section 2.2 MSC: Applied TOP: 2IVi 5. For an observer in the Northern Hemisphere, as he or she looks north, stars travel in a clockwise direction around the north celestial pole over the course of the night.

ANS: F DIF: Easy REF: Section 2.2 MSC: Factual TOP: 2IVi | 2IVii 6. Earth revolves around the Sun in the same direction Earth spins about its axis.

ANS: T DIF: Easy REF: Section 2.3 MSC: Factual TOP: 3Iii 7. The seasons on Earth are caused by the change in distance between the Sun and Earth.

ANS: F DIF: Medium REF: Section 2.3 MSC: Factual TOP: 3IIIii | 3IIIiii

8. The altitude of the Sun as it crosses the meridian changes during the year.

ANS: T DIF: Medium REF: Section 2.3 MSC: Factual TOP: 3IIIiv 9. A person who lives at the equator will see the Sun directly overhead at noon every day of the year.

ANS: F DIF: Medium REF: Section 2.3 MSC: Applied TOP: 3IIIiv 10. On the autumnal equinox, the lengths of both day and night are 12 hours.

ANS: T DIF: Easy REF: Section 2.3 MSC: Factual TOP: 3IIIv 11. The longest day of the year in the Northern Hemisphere occurs on the summer solstice.

ANS: T DIF: Easy REF: Section 2.3 MSC: Factual TOP: 3IIIvi 12. When in the New Moon phase, the moon will be visible in the eastern sky at sunrise.

ANS: T DIF: Medium REF: Section 2.4 MSC: Applied TOP: 4Ii | 4IIiv 13. The fact that we always see the same side of the Moon indicates that the Moon does not rotate about an axis.

ANS: F DIF: Medium REF: Section 2.4 MSC: Conceptual TOP: 4IIIii 14. When a solar eclipse occurs, the Sun lies between the Earth and Moon.

ANS: F DIF: Easy REF: Section 2.6 MSC: Conceptual TOP: 6Iii 15. When a solar eclipse occurs, typically more people will witness it as a partial eclipse than as a total eclipse.

ANS: T DIF: Medium REF: Section 2.6 MSC: Factual TOP: 6Iii | 6IIIiv

MULTIPLE CHOICE 1. There are _________ constellations in the entire sky. a. 12 b. 13 c. 88 d. hundreds of e. thousands of ANS: C DIF: Easy REF: Section 2.1 MSC: Factual TOP: 1I

2. What defines the location of the equator on Earth? a. the axis around which Earth rotates b. where the ground is the warmest c. the tilt of Earth’s rotational axis relative to its orbit around the Sun d. the orbit of Earth around the Sun e. all of the above ANS: A DIF: Easy REF: Section 2.2 MSC: Factual TOP: 2Iii 3. If the star Polaris has an altitude of 35°, then we know that: a. our longitude is 55° b. our latitude is 55° c. our longitude is 35° d. our longitude is 35° e. our latitude is 35° ANS: E DIF: Medium REF: Section 2.2 MSC: Applied TOP: 2Iii | 2Iviii 4. At a latitude of 50°, how far above the horizon is the north celestial pole located? a. 0° b. 40° c. 50° d. 90° e. It is not visible at that latitude.

ANS: C DIF: Medium REF: Section 2.2 MSC: Applied TOP: 2Iii | 2Iiii | 2Ivi 5. At what latitude is the north celestial pole located at your zenith? a. 0° b. 30° c. 60° d. 90° e. This occurs at every latitude.

ANS: D DIF: Medium REF: Section 2.2 MSC: Applied TOP: 2Iii | 2Iv | 2Ivi

6. At what latitude is the north celestial pole at your horizon? a. 0° b. 30° c. 60° d. 90° e. This can never happen.

ANS: A DIF: Medium REF: Section 2.2 MSC: Applied TOP: 2Iii | 2Iiii | 2Ivi 7. For a person who lives at a latitude of 40°, when is the Sun directly overhead at noon? a. only on the summer solstice b. only on the winter solstice c. only on the vernal and autumnal equinoxes d. never e. always ANS: D DIF: Medium REF: Section 2.3 MSC: Applied TOP: 2Iii | 3IIIi 8. For a person living in Vancouver, Canada, at latitude of 49°, the Sun will reach a maximum height above the Southern horizon on winter solstice of a. 41.0° b. 17.5° c. 25.5° d. 37.0° e. 64.5° ANS: B DIF: Difficult REF: Section 2.3 MSC: Applied TOP: 2Iii | 3IIIi | 3IIIvi 9. The meridian is defined as an imaginary circle on the sky on which lie the: a. celestial equator and vernal equinox b. north and south celestial poles c. zenith and the north and south celestial poles d. zenith and east and west directions e. celestial equator and summer solstice ANS: C DIF: Medium REF: Section 2.2 MSC: Factual TOP: 2Iiv | 2Iv | 2Ivi Figure 1

10. Assume you are observing the night sky from a typical city in the United States with a latitude of 40°.

Using Figure 1, which constellation of the zodiac would be nearest to the meridian at midnight in midSeptember? a. Scorpius b. Taurus c. Pisces d. Aquarius e. Leo ANS: D DIF: Medium REF: Section 2.3 MSC: Applied TOP: 2Iiv | 2IIIi | 2IVi | 3Iii | 3IIiii 11. Assume you are observing the night sky from a typical city in the United States with a latitude of 40°.

Using Figure 1, which constellation of the zodiac would be nearest to the meridian at 6 P.M. in midSeptember? a. Scorpius b. Taurus c. Pisces d. Aquarius e. Leo ANS: A DIF: Difficult REF: Section 2.3 MSC: Applied TOP: 2Iiv | 2IIIi | 2IVi | 3Iii | 3IIiii 12. Assume you are observing the night sky from a typical city in the United States with a latitude of 40°.

Using Figure 1, which constellation of the zodiac would be nearest to the meridian at 10 P.M. in mid-May? a. Aries b. Libra c. Capricornus d. Gemini e. Sagittarius ANS: B DIF: Difficult REF: Section 2.3 MSC: Applied TOP: 2Iiv | 2IIIi | 2IVi | 3Iii | 3IIiii

13. Using Figure 1, what time of the day or night will the zodiac constellation Gemini rise in March? a. 2 P.M. b. 8 P.M. c. 2 A.M. d. 8 A.M. e. noon ANS: A DIF: Difficult REF: Section 2.3 MSC: Applied TOP: 2Iiv | 2IIIi | 2IVi | 3Iii | 3IIiii 14. The direction directly overhead of an observer defines his or her: a. meridian b. celestial pole c. nadir d. circumpolar plane e. zenith ANS: E DIF: Easy REF: Section 2.2 MSC: Factual TOP: 2Iv 15. No matter where you are on Earth, stars appear to rotate about a point called the: a. zenith b. celestial pole c. nadir d. meridian e. equinox ANS: B DIF: Easy REF: Section 2.2 MSC: Factual TOP: 2Ivi 16. The apparent path of the Sun across the celestial sphere over the course of a year is called the: a. prime meridian b. ecliptic c. circumpolar plane d. celestial equator e. eclipse ANS: B DIF: Easy REF: Section 2.2 MSC: Factual TOP: 2Ivii

17. At what time does a full Moon rise? a. 12 midnight b. 12 noon c. 6 A.M. d. 6 P.M. e. 3 P.M. ANS: D DIF: Easy REF: Section 2.4 MSC: Applied TOP: 2IIIi | 2IVi | 4Ii | 4Iii | 4IIii | 4IIiv 18. What time does a third-quarter Moon rise? a. 12 midnight b. 12 noon c. 3 P.M. d. 6 A.M. e. 6 P.M. ANS: A DIF: Medium REF: Section 2.4 MSC: Applied TOP: 2IIIi | 2IVi | 4Ii | 4Iii | 4IIii | 4IIiv 19. At which of the possible times below could the waxing gibbous moon be seen rising? a. 3 P.M. b. 9 A.M. c. 11 P.M. d. 5 A.M. e. 8 P.M. ANS: A DIF: Difficult REF: Section 2.4 MSC: Applied TOP: 2IIIi | 2IVi | 4Ii | 4Iii | 4IIii | 4IIiv

20. A friend takes a time-lapse picture of the sky, as shown below. What direction must your friend have been facing when the picture was taken?

a. north b. east c. south d. west e. directly overhead ANS: A DIF: Medium REF: Section 2.2 MSC: Applied TOP: 2IVi 21. A friend takes a time-lapse picture of the sky, as shown below. What direction must your friend have been facing when the picture was taken?

a. north b. east c. south d. west e. directly overhead ANS: B DIF: Difficult REF: Section 2.2 MSC: Applied TOP: 2IVi

22. A friend takes a time-lapse picture of the sky, as shown below. What direction must your friend have been facing when the picture was taken?

a. north b. east c. south d. west e. directly overhead ANS: C DIF: Medium REF: Section 2.2 MSC: Applied TOP: 2IVi 23. A friend takes a time-lapse picture of the sky, as shown below. What direction must your friend have been facing when the picture was taken?

a. north b. east c. south d. west e. directly overhead ANS: D DIF: Difficult REF: Section 2.2 MSC: Applied TOP: 2IVi

24. How far away on average is the Earth from the Sun? a. 1 light-second b. 1 light-minute c. 1 astronomical unit d. 1 light-hour e. 1 light-year ANS: C DIF: Easy REF: Section 2.3 MSC: Factual TOP: 3Ii 25. You and a friend go outside to view the stars at midnight tonight. Six months later, you go outside to find the stars in exactly the same position in the sky as when you and your friend viewed them. What time is it?

Assume you can see the stars at any time, day or night. a. 6 A.M. b. noon c. 6 P.M. d. midnight e. This can never happen.

ANS: B DIF: Difficult REF: Section 2.3 MSC: Applied TOP: 3IIi | 3IIii 26. If you go out at exactly 9 P.M. each evening over the course of one month, the position of a given star will move westward by tens of degrees. What causes this motion? a. the Earth’s rotation on its axis b. the revolution of the Earth around the Sun c. the revolution of the Moon around the Earth d. the revolution of the Sun around the Earth e. the speed of the star through space ANS: B DIF: Easy REF: Section 2.3 MSC: Applied TOP: 3IIi | 3IIii

27. The ecliptic is defined by the motion of _________ in the sky. a. the Moon b. the Sun c. the planets d. Polaris e. the stars ANS: B DIF: Easy REF: Section 2.3 MSC: Factual TOP: 3IIiii 28. When the Northern Hemisphere experiences fall, the Southern Hemisphere experiences: a. spring b. summer c. fall d. winter ANS: A DIF: Easy REF: Section 2.3 MSC: Applied TOP: 3IIIi 29. When the Northern Hemisphere experiences summer, the Southern Hemisphere experiences: a. spring b. summer c. fall d. winter ANS: D DIF: Easy REF: Section 2.3 MSC: Applied TOP: 3IIIi 30. If the Earth’s axis were tilted by 5°, instead of its actual tilt, how would the seasons be different than they are currently? a. The seasons would remain the same. b. Summers would be warmer. c. Winters would last longer. d. Winters would be warmer. e. Summers would last longer.

ANS: D DIF: Medium REF: Section 2.3 MSC: Factual TOP: 3IIIi | 3IIIii

31. If the Earth’s axis were tilted by 35°, instead of its actual tilt, how would the seasons be different than they are currently? a. The seasons would remain the same. b. Summers would be colder. c. Winters would be shorter. d. Winters would be colder. e. Summers would be shorter.

ANS: D DIF: Medium REF: Section 2.3 MSC: Factual TOP: 3IIIi | 3IIIii 32. We experience seasons because: a. the Earth’s equator is tilted relative to the plane of the solar system b. the Earth is closer to the Sun in summer and farther from the Sun in the winter c. the length of the day is longer in the summer and shorter in the winter d. the Earth moves with a slower speed in its orbit during summer and faster during winter e. one hemisphere of Earth is closer to the Sun than the other hemisphere during the summer ANS: A DIF: Medium REF: Section 2.3 MSC: Applied TOP: 3IIIii 33. Earth is closest to the Sun when the Northern Hemisphere experiences: a. spring b. summer c. fall d. winter ANS: D DIF: Difficult REF: Section 2.3 MSC: Factual TOP: 3IIIii 34. During which season (in the Northern Hemisphere) could you see the Sun rising from the furthest north? a. winter b. spring c. summer d. fall e. The Sun always rises directly in the east.

ANS: C DIF: Medium REF: Section 2.3 MSC: Applied TOP: 3IIIiv

35. The day with the smallest number of daylight hours over the course of the year for a person living in the

Northern Hemisphere is the: a. summer solstice (June 1) b. vernal equinox (March 21) c. winter solstice (Dec. 22) d. autumnal equinox (Sept. 23) e. The number of daylight hours is always the same.

ANS: C DIF: Easy REF: Section 2.3 MSC: Applied TOP: 3IIIvi 36. The day with the smallest number of daylight hours over the course of the year for a person living in the

Southern Hemisphere is the: a. summer solstice (June 1) b. vernal equinox (March 21) c. winter solstice (Dec. 22) d. autumnal equinox (Sept. 23) e. The number of daylight hours is always the same.

ANS: A DIF: Medium REF: Section 2.3 MSC: Applied TOP: 3IIIvi 37. On which day of the year does the Sun reach its northernmost point in the sky? a. vernal equinox b. summer solstice c. autumnal equinox d. winter solstice e. The sun always reaches the same altitude.

ANS: B DIF: Easy REF: Section 2.3 MSC: Factual TOP: 3IIIvi 38. The Earth’s rotational axis precesses in space and completes one revolution every: a. 200 years b. 1,800 years c. 7,300 years d. 26,000 years e. 51,000 years ANS: D DIF: Easy REF: Section 2.3 MSC: Factual TOP: 3IVi

39. Which of the following stars will be the North Star in 12,000 years? a. Polaris b. Deneb c. Vega d. Thuban e. Sirius ANS: C DIF: Medium REF: Section 2.3 MSC: Factual TOP: 3IVii 40. In regard to the phase of the Moon, the term waxing means: a. less than half-illuminated b. more than half–illuminated c. becoming smaller d. increasing in brightness e. decreasing in brightness ANS: D DIF: Easy REF: Section 2.4 MSC: Factual TOP: 4Ii 41. If you see a full moon tonight, how long would you have to wait to see the next full moon? a. 1 week b. 2 weeks c. 3 weeks d. 4 weeks e. 5 weeks ANS: D DIF: Easy REF: Section 2.4 MSC: Factual TOP: 4Ii | 4IIii 42. If a person on Earth currently views the Moon in a waxing crescent phase, in what phase would the Earth appear to a person on the Moon? a. waxing crescent b. waxing gibbous c. waning gibbous d. waning crescent e. new ANS: C DIF: Difficult REF: Section 2.4 MSC: Applied TOP: 4Ii | 4Iii

43. If tonight the Moon is in the waxing gibbous phase, in three days the Moon will most likely be in the: a. new phase b. full phase c. third-quarter phase d. first-quarter phase e. waxing crescent phase ANS: B DIF: Easy REF: Section 2.4 MSC: Applied TOP: 4Ii | 4Iii | 4IIii 44. If there is a full moon out tonight, approximately how long from now will it be in the third-quarter phase? a. three to four days b. one week c. two weeks d. three weeks e. one month ANS: B DIF: Easy REF: Section 2.4 MSC: Applied TOP: 4Ii | 4Iii | 4IIii 45. Which of the following is FALSE? a. Everyone on Earth observes the same phase of the Moon on a given night. b. The phases of the Moon cycle with a period that is longer than its sidereal period. c. In some phases, the Moon can be observed during the day. d. The observed phase of the Moon changes over the course of one night. e. A full Moon can be seen on the eastern horizon at sunset.

ANS: D DIF: Easy REF: Section 2.4 MSC: Applied TOP: 4Iii | 4IIi | 4IIii | 4IIiii Figure 2 46. In Figure 2, at which position must the moon be located in order for a lunar eclipse to occur? a. 1 b. 2 c. 3 d. 4 ANS: D DIF: Easy REF: Section 2.6 MSC: Conceptual TOP: 4Iii | 6Ii

47. In Figure 2, at which position must the moon be located in order for a solar eclipse to occur? a. 1 b. 2 c. 3 d. 4 ANS: B DIF: Easy REF: Section 2.6 MSC: Conceptual TOP: 4Iii | 6Iii 48. During which lunar phase do solar eclipses occur? a. new b. first quarter c. full d. third quarter ANS: A DIF: Easy REF: Section 2.6 MSC: Conceptual TOP: 4Iii | 6Iii 49. The Moon’s sidereal period is 2.2 days shorter than the period during which the Moon’s phases change because: a. the Moon always keeps the same side turned toward the Earth b. the Earth must rotate so an observer can see the Moon c. the Moon’s orbit is tilted with respect to the Earth’s rotational axis d. the Earth moves significantly in its orbit around the Sun during that time e. the Moon’s orbital speed varies ANS: D DIF: Medium REF: Section 2.4 MSC: Conceptual TOP: 4IIiii 50. The Moon undergoes synchronous rotation, and as a consequence the: a. rotational period of the Moon equals the orbital period of the Moon around the Earth b. rotational period of the Moon equals the rotational period of the Earth c. rotational period of the Moon equals the orbital period of the Earth around the Sun d. orbital period of the Moon around the Earth equals the rotational period of the Earth e. Moon does not rotate as it orbits the Earth ANS: A DIF: Easy REF: Section 2.4 MSC: Conceptual TOP: 4III

51. Leap years occur because: a. the Earth’s orbital period around the Sun is decreasing b. the Earth’s orbital period is 365.24 days c. the Gregorian calendar contains only 11 months d. the Earth speeds up in its orbit when it comes closest to the Sun e. a calendar month is not the same as a lunar month ANS: B DIF: Easy REF: Section 2.5 MSC: Conceptual TOP: 5Ii 52. How often do leap years occur? a. almost every 3 years b. almost every 4 years c. almost every 5 years d. almost every 8 years e. almost every 10 years ANS: B DIF: Easy REF: Section 2.5 MSC: Factual TOP: 5Ii 53. How often would we have leap years if Earth’s orbital period were 365.1 days? a. every year b. every 2 years c. every 4 years d. every 10 years e. We would not need to have leap years.

ANS: D DIF: Medium REF: Section 2.5 MSC: Applied TOP: 5Ii 54. A partial lunar eclipse occurs when: a. the Sun appears to go behind the Moon b. the Moon passes through part of the Earth’s shadow c. the Moon shadows part of the Sun d. the Earth passes through part of the Moon’s shadow e. the Moon passes through part of the Sun’s shadow ANS: B DIF: Easy REF: Section 2.6 MSC: Conceptual TOP: 6Ii | 6IIi

55. If you are lucky enough to see a total solar eclipse, you must be standing in the: a. Moon’s umbra b. Moon’s penumbra c. Earth’s umbra d. Earth’s penumbra e. Sun’s umbra ANS: A DIF: Medium REF: Section 2.6 MSC: Applied TOP: 6Iii | 6II 56. If you are observing a partial solar eclipse, you must be standing in the: a. Moon’s umbra b. Moon’s penumbra c. Earth’s umbra d. Earth’s penumbra e. Sun’s umbra ANS: B DIF: Medium REF: Section 2.6 MSC: Applied TOP: 6Iii | 6II 57. A solar-powered spacecraft is traveling through the Moon’s shadow. Which part(s), if any, of the Moon’s shadow will cause the spacecraft to completely lose power? a. umbra b. penumbra c. annulus d. both umbra and penumbra e. The spacecraft will never lose power.

ANS: A DIF: Medium REF: Section 2.6 MSC: Applied TOP: 6IIii | 6IIiii 58. Solar and lunar eclipses are rare because: a. the Moon’s orbital plane is tipped by 5.2° relative to the plane defined by the Earth’s equator b. the Moon’s orbital plane is tipped by 5.2° relative to the Earth’s orbital plane c. the Moon’s orbital plane is tipped by 23.5° relative to the plane defined by the Earth’s equator d. the Moon’s orbital plane is tipped by 23.5° relative to the Earth’s orbital plane e. the Moon’s orbital plane is tipped by 5.2° relative to the galactic plane ANS: B DIF: Medium REF: Section 2.6 MSC: Conceptual TOP: 6IIIi | 6IIIiii

59. Approximately how often do lunar eclipses occur? a. twice every year b. three times every year c. once per month d. twice every 11 months e. once every 11 years ANS: D DIF: Difficult REF: Section 2.6 MSC: Factual TOP: 6IIIiii

SHORT ANSWER 1. On what place(s) on Earth can you stand and have the celestial equator be at the same altitude for all 360° of its circumference?

ANS: You can stand at either the North Pole or the South Pole.

DIF: Medium REF: Section 2.2 MSC: Applied TOP: 2Ii | 2Iiii | 2Ivi 2. For an observer in Seattle, Washington, which is located at latitude  47°, what is the lowest possible altitude one might see the Sun on the meridian over the course of the year? Approximately what time of the day and year will this occur?

ANS: For an observer in Seattle, Washington, the celestial equator will be at an altitude of 90°

47° 

43° above the southern horizon. The Sun will be located at its southern most position on the celestial sphere on the winter solstice, which is 23.5° south from the celestial equator. Therefore, the Sun will be on the meridian at noon on the winter solstice with an altitude of 43°

23.5°  19.5° above the southern

horizon.

DIF: Difficult REF: Section 2.2 MSC: Applied TOP: 2Iii | 2Iiii | 3IIIvi 3. Draw a dome representing the visible sky. Label the horizon, meridian, zenith, and each of the four cardinal directions (north, east, south, and west).

ANS: The drawing should look like a dome, with the ground portion labeled as the horizon, the topmost part of the dome labeled as the zenith, and the cardinal directions labeled on the horizon with north, east, south, and west at 90 degrees from each other, clockwise. Finally, the meridian should be a line drawn from the north, through the zenith, to the south.

DIF: Medium REF: Section 2.2 MSC: Factual TOP: 2Ii | 2Iiii | 2Iiv |2Iv

4. The center of the Milky Way lies approximately 30° south of the celestial equator. From what latitudes on the Earth is it impossible to view the center of our galaxy?

ANS: At latitudes  90°

30°  60°, it would be impossible to see the center of our galaxy because it

would lie below the horizon.

DIF: Medium REF: Section 2.1 MSC: Applied TOP: 2Ii 5. The position of the autumnal equinox lies at the intersection of which two great celestial circles on the celestial sphere?

ANS: The autumnal equinox lies at the intersection of the celestial equator and the ecliptic.

DIF: Difficult REF: Section 2.3 MSC: Factual TOP: 2Ii | 2Ivii | 3IIIv 6. How is the observed height of Polaris above the horizon related to an observer’s latitude? (Hint: Consider three cases of observers located at the equator, the North Pole, and latitude  45°.)

ANS: The observed height of Polaris above the horizon is equal to an observer’s latitude. For an observer at the equator (latitude  0°), Polaris is on the horizon. For an observer at the North Pole (latitude  90°), Polaris is at the zenith or 90° above the horizon. For an observer at latitude  45°, Polaris is 45° above the horizon.

DIF: Medium REF: Section 2.2 MSC: Applied TOP: 2Iii | 2Iiii | 2Iviii 7. If you are standing on the equator and shoot a cannonball directly north, where would you expect it to land?

ANS: The cannonball would land to the northeast of your position. Since you are standing on the equator, you have the fastest ground speed of any location on Earth. Once the cannonball is fired, it is given a velocity in the northern direction. However, the cannonball retains the ground speed of the equator also. Since the ground speed of the northern latitudes is lower than that of the equator, the cannonball will appear to travel northeast instead of straight north!

DIF: Difficult REF: Section 2.2 MSC: Conceptual TOP: 2II 8. Earth has an average radius of approximately 6.4  103 km. What is the average speed, in units of km/s, of the ground at the Earth’s equator due to the daily rotation of Earth if there are 8.64  104 seconds per day?

ANS: Here the students need to convert the radius of Earth to its circumference: C  2 r  2  3.14159  6.4  103  4.02  104 km. Divide this distance by 8.64  104 s, and we get a speed of 0.465 km/s  1,676 km/hr.

DIF: Difficult REF: Section 2.2 MSC: Applied TOP: 2IIIi 9. Consider an observer located on the equator. If the observer sees a star directly overhead at 10 P.M., where will that star be located in the night sky at 3 A.M.?

ANS: The star will be visible low on the western horizon.

DIF: Easy REF: Section 2.2 MSC: Applied TOP: 2IVi | 2IViii 10. Consider an observer located on the equator. If the observer sees a star directly overhead at 8 P.M., where will that star be located in the night sky at midnight? How far above the horizon will it be or will it have set?

ANS: The star will move westward by an amount that is equal to (12 hr the star will be 90°

8 hr)  360°/24 hr  60°, and

60°  30° above the western horizon.

DIF: Medium REF: Section 2.2 MSC: Applied TOP: 2IVi | 2IViii 11. Earth experiences seasons due to the tilt of its axis. What are two consequences of this tilt that contribute to the seasons?

ANS: (1) Variation in the length of daylight (2) Variation in the directness of the Sun’s rays DIF: Medium REF: Section 2.3 MSC: Applied TOP: 3IIIii 12. What would be the effect on the seasons if the tilt of the Earth’s axis were 10° rather than 23.5°?

ANS: If the tilt of the Earth’s axis were smaller, there would be a less dramatic temperature shift between the seasons because the angle of the Sun’s rays would vary less and the length of day/night would be more equal throughout the year.

DIF: Easy REF: Section 2.3 MSC: Applied TOP: 3IIIii | 7Ii 13. What makes the equinoxes and solstices special?

ANS: The equinoxes occur when the Sun is directly above the equator; the entire world experiences a 12-hour day and a 12-hour night. The solstices occur when the Sun is farthest from the equator (north or south). On these days, one hemisphere experiences its longest day and shortest night, while the other hemisphere experiences its shortest day and longest night.

DIF: Easy/Medium REF: Section 2.3 MSC: Factual TOP: 3IIIv | 3IIIvi 14. If the Moon was full three days ago, what phase will it be in tonight, and when will it rise and set?

ANS: The Moon’s phase cycles on a 29.5-day period. Therefore, the Moon tonight will be approximately

halfway between the full and third-quarter phases, and thus it will be in the waning gibbous phase. It will be on an observer’s eastern horizon and rising halfway between 6 P.M. and midnight, which is 9 P.M. It will set 12 hours later at 9 A.M. DIF: Medium REF: Section 2.4 MSC: Applied TOP: 4Ii | 4Iii | 4IIiv 15. Based on the location of the moon in the diagram below, draw a picture of how the moon would appear to an observer located on Earth.

ANS: The drawing should show a third-quarter moon, where the left half of the moon’s face will be lit up and the right half will be in darkness.

DIF: Medium REF: Section 2.4 MSC: Conceptual TOP: 4Iii 16. Based on the location of the moon in the diagram below, draw a picture of how the moon would appear to an observer located on Earth.

ANS: The drawing should show a waxing gibbous moon, where more than half of the moon’s right face will be lit up and less than half of the left face will be in darkness.

DIF: Difficult REF: Section 2.4 MSC: Conceptual TOP: 4Iii

17. As the month passes, the Moon appears to rise later in the day or night when compared to the previous day. Explain why this happens.

ANS: In general, objects appear to rise and set due to Earth’s rotation. While Earth rotates once every 24 hours, the Moon also orbits around Earth roughly once a month in the same direction as Earth’s rotation. Therefore, over 24 hours, the Moon has moved slightly from its original position, and Earth has to rotate a little more before the Moon appears to rise again the next day.

DIF: Medium REF: Section 2.4 MSC: Applied TOP: 4IIiv 18. Explain why we always see the same side of the Moon from Earth.

ANS: The amount of time it takes for the Moon to rotate once about its axis is exactly equal to the amount of time it takes to orbit once around Earth.

DIF: Easy REF: Section 2.4 MSC: Conceptual TOP: 4IIIii 19. How does today’s Gregorian calendar differ from the calendars of more ancient civilizations, such as the Chinese, the Egyptians, and the Babylonians?

ANS: The Gregorian calendar is based on the tropical year, based on the motion of the Earth around the Sun. The others are lunar calendars based on the motion of the Moon around the Earth. The Gregorian calendar also includes leap years to avoid the shifting of the seasons due to the fact that the Earth orbits the Sun in 365.24 days.

DIF: Medium REF: Section 2.5 MSC: Factual TOP: 5Ii | 5II 20. Draw a picture below showing the Moon’s location relative to the Earth and the Sun during a lunar eclipse.

ANS: The Moon, Earth, and Sun should all be drawn in a straight line with the Earth in between the Moon and the Sun. DIF: Medium REF: Section 2.6 MSC: Applied TOP: 6Ii 21. Draw a picture below showing the Moon’s location relative to the Earth and the Sun during a solar eclipse.

ANS: The Moon, Earth, and Sun should all be drawn in a straight line with the Moon in between the Earth and the Sun. DIF: Medium REF: Section 2.6 MSC: Applied TOP: 6Iii 22. Explain why the eclipse seasons occur roughly twice every 11 months, rather than twice per year.

ANS: This happens because the plane of the Moon’s orbit slowly wobbles, completing one full “wobble”

every 18.6 years. Because the wobble is in the opposite direction from the Moon’s orbit, the eclipse seasons occur less than six months apart.

DIF: Difficult REF: Section 2.6 MSC: Applied TOP: 6IIIi | 6IIIii | 6IIIiii

CHAPTER 3

Motion of Astronomical Bodies

CONCEPT MAP Sec 3.1 1. Motions of the Planets in the Sky I. Observed Motions of the Planets i. Geocentric model (MC: 1) ii. Retrograde motion (MC: 2) iii. Ptolemy added epicycles to correct inadequacies of the geocentric model (MC: 3–5, SA: 1) Sec 3.2 2. Earth Moves I. Development of the Heliocentric Model i. Nicolaus Copernicus (TF: 1, MC: 6, 7) ii. Heliocentric Model (MC: 7, SA: 2) II. Determining the Scale of the Solar System i. Inferior and superior planets (MC: 8–14, SA: 3) ii. Conjunction, opposition and greatest elongation (MC: 12–15, SA: 4–6) iii. Sidereal period (MC: 16, SA: 7) iv. Synodic period (MC: 17, SA: 7) v. Inferior planets: 1/P  1/E  1/S; superior planets: 1/P  1/E

1/S (MC: 18–21, SA: 7–9)

vi. Retrograde motion (TF: 2, MC: 22, 23, SA: 10–12) vii. Heliocentric model triumphs over geocentric model (TF: 2, MC: 24, SA: 11) Sec 3.3 3. An Empirical Beginning: Kepler’s Laws

I. Empirical Data Meets Theory i. Empirical: based on observational data (MC: 25, 26) ii. Tycho Brahe and Kepler observe planet positions versus time (TF: 3) iii. Ptolemaic model (even with addition of epicycles) does not fit the data on retrograde motion, but Copernicus shows the heliocentric model fits better (TF: 3, MC: 27, 28, SA: 9) II. Kepler’s Laws i. Orbital Period (P) (MC: 29, 30) ii. Kepler’s first law: planets move on elliptical orbits with the Sun at one focus (TF: 4, MC: 31 ) iii. Ellipse properties: foci, semimajor axis (A) and semiminor axis (B), eccentricity (e) (MC: 32–34, SA: 13) iv. Kepler’s second law: planets sweep out equal areas in equal time (TF: 5, MC: 35) v. Planet’s speed varies as it orbits in an ellipse; fastest when near the Sun and slowest when farther from the Sun (TF: 6, 7, MC: 36–39, SA: 14) vi. Kepler’s third law: P2

A3 (TF: 8, 9, MC: 40–49, SA: 15)

Sec 3.4 4. Galileo: The First Modern Scientist I. Galileo Galilei i. Galileo observed moons orbiting Jupiter ii. Galileo observed phases of Venus, which could only be explained with a heliocentric model of the Solar System (MC: 50, SA: 16) iii. Prior to Newton, Galileo discovered that all objects accelerate down inclined planes at the same rate regardless of their mass; derived the value of g, which becomes Newton’s gravitational constant iv. Galileo finds that objects in motion stay in motion at constant speed unless acted on by an unbalanced force v. Inertia (MC: 51) vi. Galileo writes “Dialogue Concerning the Two Chief World Systems” in support of heliocentric model, conflicting with Catholic Church and leading to house arrest Sec 3.5 5. Newton’s Laws of Motion I. Newton Discovers Laws of Motion

i. Newton’s first law: objects at rest stay at rest; objects in motion stay in motion (MC: 52) ii. Inertial frame of reference (SA: 17) iii. Newton’s second law: motion is changed by unbalanced forces iv. Velocity (v): speed and direction (MC: 53–55) v. Acceleration (a): the rate of change in velocity (MC: 56–58, SA: 18) vi. Mass (m): the property of matter that resists changes in motion vii. Proportionality viii. Newton’s second law: F  ma (TF: 10, MC: 59–61, SA: 19, 20) ix. Newton’s third law: for every force, there is an equal and opposite force x. Action and reaction (MC: 61, SA: 20) Sec. 3.6 6. Origins: Planets and Orbits

TRUE/FALSE 1. Nicolaus Copernicus was the first to propose that Earth revolved around the Sun. ANS: F DIF: Medium REF: Section 3.1 MSC: Factual TOP: 2Ii 2. In the heliocentric system, retrograde motion is not an actual motion of the planet but appears to occur because of the Earth’s movement.

ANS: T DIF: Medium REF: Section 3.2 MSC: Conceptual TOP: 2IIvii 3. Johannes Kepler obtained accurate data on the positions of the planets in the sky over time, which Galileo used to prove that planets revolved around the Sun. ANS: F DIF: Easy REF: Section 3.3 MSC: Factual TOP: 3Iii | 3Iiii 4. Planets orbit the Sun on circular orbits.

ANS: F DIF: Easy REF: Section 3.3 MSC: Factual TOP: 3IIii 5. Planets with circular orbits travel at the same speed at all points in their orbits, whereas planets with elliptical orbits change their speeds at different points in their orbits.

ANS: T DIF: Medium REF: Section 3.3 MSC: Factual TOP: 3IIiv

6. A planet travels fastest in its orbit when it is closest to the Sun. ANS: T DIF: Easy REF: Section 3.3 MSC: Factual TOP: 3IIv 7. As we move farther from the Sun, the circumferences of planetary orbits are larger, and the speeds at which planets travel increase.

ANS: F DIF: Easy REF: Section 3.3 MSC: Factual TOP: 3IIv 8. Even though they move at faster average speeds, the outer planets in the Solar System have longer periods than the inner planets.

ANS: F DIF: Difficult REF: Section 3.3 MSC: Applied TOP: 3IIvi 9. Kepler’s third law written as P2  A3 holds true mathematically for our Solar System only if the period is expressed in years and the semimajor axis is expressed in AU. ANS: T DIF: Easy REF: Section 3.3 MSC: Applied TOP: 3IIvi 10. Newton’s second law says that more massive objects are accelerated more than less massive objects when the same force is applied.

ANS: F DIF: Medium REF: Section 3.3 MSC: Conceptual TOP: 5Iviii

MULTIPLE CHOICE 1. At the center of the geocentric model of the Solar System is the: a. Sun b. Moon c. Earth d. Venus e. Jupiter ANS: C DIF: Easy REF: Section 3.1 MSC: Factual TOP: 1Ii

2. Retrograde motion is seen when ____________ due to the Earth’s motion. a. stars change their position in the sky with respect to background stars b. stars rise in the west and set in the east c. planets rise in the west and set in the east d. planets change the direction in which they wander across the night sky e. planets orbit the Sun in the opposite direction ANS: D DIF: Medium REF: Section 3.1 MSC: Factual TOP: 1Iii 3. How did Ptolemy “fix” the geocentric system? a. He introduced retrograde motion. b. He introduced prograde motion. c. He moved the Sun to the center. d. He introduced epicycles. e. He introduced Earth’s motion.

ANS: D DIF: Medium REF: Section 3.1 MSC: Factual TOP: 1Iiii 4. When the geocentric model of the solar system did not match the observed positions of the planets: a. Tycho Brahe made measurements of higher accuracy and showed the geocentric model was correct b. Ptolemy added epicycles to the geocentric model to match the observed data c. Galileo argued that the Sun revolved around the Earth d. Kepler was inspired to create the theory of gravity e. Copernicus proposed the heliocentric mode ANS: B DIF: Difficult REF: Section 3.1 MSC: Factual TOP: 1Iiii 5. Who of the following was NOT a proponent of the heliocentric model of the solar system? a. Galileo b. Copernicus c. Newton d. Ptolemy e. Aristarchus ANS: D DIF: Easy REF: Section 3.1 MSC: Factual TOP: 1Iiii

6. _________ was the first person to introduce a mathematical heliocentric model of the Solar System from which accurate predictions could be made of planets’ positions. a. Nicolaus Copernicus b. Tycho Brahe c. Johannes Kepler d. Galileo Galilei e. Isaac Newton ANS: A DIF: Easy REF: Section 3.2 MSC: Factual TOP: 2Ii 7. Based on his observations of the planets, Copernicus calculated the relative distance of the planets from the Sun using the heliocentric model, and these distances were: a. 10 times too large b. exactly correct c. close to the correct values, with errors less than 0.5 AU d. accurate, but not as accurate as Ptolemy’s values e. two times too small ANS: C DIF: Medium REF: Section 3.2 MSC: Applied TOP: 2Ii | 2Iii 8. Which of the following are the inferior planets? a. Mercury b. Mercury and Mars c. Mercury and Venus d. Mars e. Mercury, Mars, and Pluto ANS: C DIF: Medium REF: Section 3.2 MSC: Factual TOP: 2IIi 9. Which of the following are superior planets? a. Mars b. Earth and Venus c. Venus, Mars, Jupiter, and Saturn d. Earth, Jupiter, and Saturn e. Mars, Jupiter, and Saturn ANS: E DIF: Medium REF: Section 3.2 MSC: Factual TOP: 2IIi

10. An inferior planet is one that is: a. smaller than Earth b. larger than Earth c. closer to the Sun than Earth d. farther from the Sun than Earth e. made of lighter materials than Earth ANS: C DIF: Easy REF: Section 3.2 MSC: Factual TOP: 2IIi 11. A superior planet is one that is: a. smaller than Earth b. larger than Earth c. closer to the Sun than Earth d. farther from the Sun than Earth e. made of heavier materials than Earth ANS: D DIF: Easy REF: Section 3.2 MSC: Factual TOP: 2IIi Figure 3 12. Based on Figure 1, a superior planet would be seen high overhead at midnight: a. when at opposition b. when at eastern quadrature c. when at conjunction d. when at western quadrature e. throughout its orbit ANS: A DIF: Difficult REF: Section 3.2 MSC: Applied TOP: 2IIi | 2IIii 13. Based on Figure 1, a superior planet at opposition: a. would rise at noon and set at midnight b. would rise at midnight and set at noon c. would rise at sunset and set at sunrise d. would rise at sunrise and set at sunrise e. would rise at 8 and set at 8 ANS: C DIF: Difficult REF: Section 3.2 MSC: Applied TOP: 2IIi | 2IIii

14. Based on the figure below, an inferior planet would be farthest from the Sun and therefore most easily visible at:

a. inferior conjunction b. superior conjunction c. greatest eastern elongation d. greatest western elongation e. at either greatest eastern or western elongation ANS: E DIF: Difficult REF: Section 3.2 MSC: Applied TOP: 2IIi | 2IIii 15. When the Sun, the Earth, and a planet all lie along a straight line, the planet is at: a. quadrature b. opposition c. greatest elongation d. conjunction e. either opposition or conjunction ANS: E DIF: Difficult REF: Section 3.2 MSC: Conceptual TOP: 2IIii

16. The amount of time a planet takes to orbit the Sun is called its _________ period. a. synodic b. sidereal c. prograde d. retrograde e. geocentric ANS: B DIF: Easy REF: Section 3.2 MSC: Factual TOP: 2IIiii 17. The time it takes for a planet to come back to the same position relative to the Sun is called its _________ period. a. synodic b. sidereal c. heliocentric d. geocentric e. prograde ANS: A DIF: Medium REF: Section 3.2 MSC: Factual TOP: 2IIiv 18. If a superior planet is observed from Earth to have a synodic period of 1.2 years, what is its sidereal period? a. 0.54 years b. 1.8 years c. 2.3 years d. 4.0 years e. 6.0 years ANS: E DIF: Medium REF: Section 3.2 MSC: Applied TOP: 2IIv 19. If the synodic period of Mars is observed from Earth to be 2.1 years, what is Mars’s sidereal period? a. 5.3 years b. 0.47 years c. 1.9 years d. 3.4 years e. 0.69 years ANS: C DIF: Difficult REF: Section 3.2 MSC: Applied TOP: 2IIv

20. If the sidereal period of Jupiter is 11.9 years, what is Jupiter’s synodic period as observed from Earth? a. 2.3 years b. 0.84 years c. 0.92 years d. 1.09 years e. 1.5 years ANS: D DIF: Difficult REF: Section 3.2 MSC: Applied TOP: 2IIv 21. If the synodic period of Venus is observed from Earth to be 1.6 years, Venus’ sidereal period is _____ years. a. 1.9 years b. 0.45 years c. 0.28 years d. 1.6 years e. 0.62 years ANS: E DIF: Medium REF: Section 3.2 MSC: Applied TOP: 2IIv 22. In the ________ model of the Solar System, ________ motion is only an apparent, not a real, motion. a. geocentric; retrograde b. heliocentric; retrograde c. geocentric; prograde d. heliocentric; prograde e Galilean; prograde ANS: B DIF: Difficult REF: Section 3.2 MSC: Conceptual TOP: 2IIvi 23. When the Earth catches up to a slower moving outer planet and passes it like a faster runner overtaking a slower runner in an outside lane, the planet: a. exhibits retrograde motion b. slows down because it feels the Earth’s gravitational pull c. decreases in brightness as it passes through the Earth’s shadow d. moves into a more elliptical orbit e. exhibits prograde motion ANS: A DIF: Easy REF: Section 3.2 MSC: Factual TOP: 2IIvi

24. Astronomers argued that the heliocentric model of the Solar System was simpler than the geocentric model, based on: a. the observation that the planets do not move relative to the background stars b. the fact that the Moon orbits the Earth c. the fact that the Sun is more massive than the Earth d. the observed retrograde motions of the planets e the observed timing of lunar and solar eclipses ANS: D DIF: Medium REF: Section 3.2 MSC: Conceptual TOP: 2IIvii 25. An empirical science is one that is based on: a. assumptions b. calculus c. computer models d. observed data e. hypotheses ANS: D DIF: Medium REF: Section 3.3 MSC: Factual TOP: 3Ii 26. Which laws are based entirely on observational data without having any theoretical framework behind them? a. physical laws b. Galileo’s laws of planetary motion c. Newton’s laws of motion d. deductive laws e. empirical laws ANS: E DIF: Easy REF: Section 3.3 MSC: Factual TOP: 3Ii

27. Observations of what astronomical events allowed astronomers to definitively determine that the heliocentric model of the solar system was correct? a. total eclipses of the Sun b. the precise motions of planets across the celestial sphere c. motion of bright stars on the celestial sphere d. the timing of the equinoxes e. the timing of the solstices ANS: B DIF: Easy REF: Section 3.3 MSC: Factual TOP: 3Iiii 28. The fact that Kepler’s heliocentric model of the Solar System predicted _________ more easily and accurately than the geocentric model is an illustration of how scientific theories evolve by the scientific method. a. solar eclipses b. lunar eclipses c. retrograde motion of planets d. prograde motion of planets e. the duration of the seasons ANS: C DIF: Medium REF: Section 3.3 MSC: Factual TOP: 3Iiii 29. The time it takes a planet to complete one full orbital revolution is commonly known as its: a. period b. frequency c. orbital domain d. velocity e. eccentricity ANS: A DIF: Easy REF: Section 3.3 MSC: Factual TOP: 3IIi

30. You find a moon orbiting a planet and you measure the distance between the moon and the planet once a night over the course of a few weeks. Its behavior is shown in the figure below where the time shown is the time since your first observation. What is this moon’s orbital period?

a. 22 days b. 20 days c. 18 days d. 11 days e. 6 days ANS: A DIF: Difficult REF: Section 3.3 MSC: Applied TOP: 3IIi 31. If the Sun is located at one focus of Earth’s elliptical orbit, what is at the other focus? a. Earth b. the Moon c. another planet d. nothing e. Jupiter ANS: D DIF: Easy REF: Section 3.3 MSC: Factual TOP: 3IIii

32. The average distance between a planet and the Sun is given by the _________ of its elliptical orbit. a. radius b. semiminor axis c. eccentricity d. semimajor axis e. distance between the foci ANS: D DIF: Easy REF: Section 3.3 MSC: Factual TOP: 3IIiii 33. A circle has an eccentricity of _________ and a line has an eccentricity of _________. a. 1; 0 b. 1; 1 c. 0; infinity d. 0; 1 e. infinity; 0 ANS: D DIF: Difficult REF: Section 3.3 MSC: Factual TOP: 3IIiii 34. The eccentricity of the majority of the planetary orbits in our Solar System is approximately: a. 0 b. 1 c. 0.5 d. 0.2 e. infinity ANS: A DIF: Medium REF: Section 3.3 MSC: Factual TOP: 3IIiii

Figure 2 35. In Figure 2, a planet orbits the Sun. The line connecting the planet and Sun sweeps out three areas labeled A, B, and C, during three different time intervals. If the duration of the time intervals are the same (meaning t2

t1  t4

t3  t 6

t5), how are the sizes of these areas related?

a. A  B  C b. C  B  A c. A  C  B d. B  A  C e. A, B, and C have the same size.

ANS: E DIF: Easy REF: Section 3.3 MSC: Applied TOP: 3IIiv 36. In Figure 2, a planet orbits the Sun. During which of the three sections (A, B, or C) will the planet have the

lowest average velocity? a. A b. B c. C d. The average velocity is the same for sections A, B, and C. e. The information given is insufficient to answer this question.

ANS: C DIF: Easy REF: Section 3.3 MSC: Applied TOP: 3IIv 37. Kepler’s second law says that if a planet is in an elliptical orbit around a star, then the planet moves fastest when the planet is: a. farthest from the star b. closest to the star c. exceeding the escape velocity d. experiencing zero acceleration e. located at one of the foci ANS: B DIF: Easy REF: Section 3.3 MSC: Applied TOP: 3IIv

38. Which of the following is TRUE about a comet that is on an elliptical orbit around the Sun? a. The comet’s speed is greatest when it is farthest from the Sun. b. The comet’s speed is greatest when it is nearest the Sun. c. This comet’s speed is zero. d. The comet’s speed is constant because its mass and the Sun’s mass stay approximately the same. e. The eccentricity is very low.

ANS: B DIF: Easy REF: Section 3.3 MSC: Applied TOP: 3IIv 39. During a certain comet’s orbit around the Sun, its closest distance to the Sun is 0.6 AU, and its farthest distance from the Sun is 35 AU. At what distance will the comet’s orbital velocity be the largest? a. 35 AU b. 17.8 AU c. 1.2 AU d. 0.6 AU e. The comet’s velocity is constant no matter what its distance is.

ANS: D DIF: Easy REF: Section 3.3 MSC: Applied TOP: 3IIv 40. Kepler’s third law for our Solar System can be expressed mathematically as: a. P  A b. P2  A2 c. P2  A3 d. P3  A2 e. P  A2 ANS: C DIF: Easy REF: Section 3.3 MSC: Factual TOP: 3IIvi 41. Kepler’s third law is a relationship between an orbiting object’s: a. gravitational force and mass b. acceleration and mass c. velocity and period d. period and semimajor axis e. semimajor axis and velocity ANS: D DIF: Easy REF: Section 3.3 MSC: Factual TOP: 3IIvi

42. Suppose an asteroid had an orbit with a semimajor axis of 4 AU. How long would it take for it to orbit once around the Sun? a. 76 years b. 45 years c. 8 years d. 16 years e. 2 years ANS: C DIF: Medium REF: Section 3.3 MSC: Applied TOP: 3IIvi 43. If Jupiter has an orbital period of 12 years, what value is closest to its average distance from the Sun? a. 2 AU b. 25 AU c. 10 AU d. 5 AU e. 144 AU ANS: D DIF: Medium REF: Section 3.3 MSC: Applied TOP: 3IIvi 44. If Mercury has an orbital period of about 88 days, what is its average distance from the Sun? a. 0.2 AU b. 0.01 AU c. 0.05 AU d. 0.4 AU e. 0.7 AU ANS: D DIF: Difficult REF: Section 3.3 MSC: Applied TOP: 3IIvi 45. The dwarf planet named Eris orbits the Sun with a semimajor axis of 68 AU. Using Kepler’s third law, Eris’s orbital period is: a. 26 years b. 130 years c. 72 years d. 240 years e. 560 years ANS: E DIF: Medium REF: Section 3.3 MSC: Applied TOP: 3IIvi

46. A comet orbits the Sun with a semimajor axis of 90 AU. Using Kepler’s third law, the comet’s orbital period is approximately a. 850 years b. 630 years c. 410 years d. 180 years e. 90 years ANS: A DIF: Medium REF: Section 3.3 MSC: Applied TOP: 3IIvi 47. If Neptune has a semimajor axis of 19 AU, its orbital period is: a. 45 years b. 83 years c. 130 years d. 220 years e. 380 years ANS: B DIF: Difficult REF: Section 3.3 MSC: Applied TOP: 3IIvi 48. Kepler’s third law says that a comet with a period of 160 years will have a semimajor axis of: a. 30 AU b. 50 AU c. 90 AU d. 140 AU e. 210 AU ANS: A DIF: Medium REF: Section 3.3 MSC: Applied TOP: 3IIvi

49. Which equation represents the relationship of the planet’s period to its semimajor axis in data shown in the figure below?

a. P  A b. P2  A2 c. P3  A2 d. P2  A3 e. P  A3 ANS: D DIF: Easy REF: Section 3.3 MSC: Applied TOP: 3IIvi 50. Galileo’s telescopic observations of _________ led him to conclude that the heliocentric model of the Solar System was correct. a. motion of Jupiter and Saturn b. motion of Venus c. moons of Jupiter and phases of Venus d. phases of the Moon e. epicycles of Mars ANS: C DIF: Medium REF: Section 3.4 MSC: Factual TOP: 4Iii

51. The natural tendency of an object to resist changes in motion is called: a. inertia b. weight c. acceleration d. mass e. velocity ANS: A DIF: Medium REF: Section 3.4 MSC: Factual TOP: 4Iv 52. Newton’s first law states that objects in motion: a. eventually come to rest b. experience an unbalanced force c. experience a nonzero acceleration d. stay in motion e. must be subject to zero friction ANS: D DIF: Easy REF: Section 3.5 MSC: Factual TOP: 5Ii 53. If you travel 20 miles from home to school in 30 minutes, what is your average velocity? a. 20 mph b. 40 mph c. 0.7 mph d. 5 mph e. 600 mph ANS: B DIF: Medium REF: Section 3.5 MSC: Applied TOP: 5Iiv 54. If you travel at a velocity of 30 mph during a 15-mile trip from home to school, how long does the trip take? a. 2 hours b. 0.2 hours c. 0.5 hours d. 5 hours e 750 hours ANS: C DIF: Easy REF: Section 3.5 MSC: Applied TOP: 5Iiv

55. If you travel at a velocity of 60 mph for a 5-hour trip, how far did you travel? a. 300 miles b. 120 miles c. 12 miles d. 0.4 miles e. 240 mph ANS: A DIF: Medium REF: Section 3.5 MSC: Applied TOP: 5Iiv 56. What is your acceleration if you go from 0 to 60 mph in 4 seconds? a. 60 mph/s b. 30 mph/s c. 15 mph/s d. 8.5 mph/s e. 240 mph/s ANS: C DIF: Medium REF: Section 3.5 MSC: Applied TOP: 5Iv 57. If you start from rest and accelerate at 15 mph/s for 5 seconds, how fast will you be traveling at the end? a. 75 mph b. 45 mph c. 3 mph d. 12 mph e. 20 mph ANS: A DIF: Difficult REF: Section 3.5 MSC: Applied TOP: 5Iv 58. If you start from rest and accelerate at 10 mph/s and end up traveling at 60 mph, how long did it take? a. 1 seconds b. 6 seconds c. 600 seconds d. 0.6 seconds e. 200 seconds ANS: B DIF: Difficult REF: Section 3.5 MSC: Applied TOP: 5Iv

59. If you apply a force of 10 N to a grocery cart and get an acceleration of 0.5 m/s2, then the mass of the grocery cart is: a. 5 kg b. 0.05 kg c. 20 kg d. 50 kg e. 0.20 kg ANS: C DIF: Medium REF: Section 3.5 MSC: Applied TOP: 5Iviii 60. You apply a force of 10 N to a grocery cart in order to get an acceleration of 0.5 m/s2. If you apply a force of 20 N to the same grocery cart, its acceleration will be: a. 10 m/s2 b. 1 m/s2 c. 0.5 m/s2 d. 0.25m/s2 e. 20 m/s2 ANS: B DIF: Difficult REF: Section 3.5 MSC: Applied TOP: 5Iviii 61. If a 100-kg astronaut pushes on a 5,000-kg satellite and the satellite experiences an acceleration of 0.1 m/s2, what is the acceleration experienced by the astronaut in the opposite direction? a. 5 m/s2 b. 10 m/s2 c. 50 m/s2 d. 0.1 m/s2 e. 1000 m/s2 ANS: A DIF: Medium REF: Section 3.5 MSC: Applied TOP: 5Iviii | 5Ix

SHORT ANSWER 1. Why were epicycles used in the geocentric system? Who first introduced epicycles?

ANS: Ptolemy introduced epicycles to explain accurate measurements made of retrograde motion of the planets.

DIF: Medium REF: Section 3.1 MSC: Factual TOP: 1Iiii

2. Who was the first notable historical figure to argue that the Earth orbits the Sun? Name two other people that were instrumental in arguing for the heliocentric model.

ANS: Aristarchus was a Greek who was the first notable historical figure. Others include Copernicus and Galileo.

DIF: Medium REF: Section 3.2 MSC: Factual TOP: 2Iii 3. Based on the figure shown below, explain why an inferior planet is most likely to be seen when it is at one of its greatest elongations.

ANS: An inferior planet is most easily seen when it appears to be the farthest away from the Sun in an observer’s night sky, as opposed to day sky. That means the planet is most easily visible when it is at greatest eastern elongation.

DIF: Medium REF: Section 3.2 MSC: Applied TOP: 2IIi

Figure 3

4. Based on Figure 3, explain why, when a superior planet is in opposition, it will be visible from Earth all night long.

ANS: Opposition means that it is opposite the Sun in the sky. Based on the figure, we can see that the planet will rise in the observer’s sky at sunset and set at sunrise. Thus the planet is visible all night long.

DIF: Medium REF: Section 3.2 MSC: Applied TOP: 2IIii

5. Based on Figure 3, explain why a superior planet, when it is at conjunction, will not be seen at all from Earth during the night.

ANS: Conjunction means that the planet is aligned with the Sun in the sky, so it will rise and set with the Sun; thus it will not visible during the night.

DIF: Medium REF: Section 3.2 MSC: Applied TOP: 2IIii 6. Based on the figure shown below, explain why an inferior planet would not be able to be seen at all from Earth when it is in conjunction.

ANS: Conjunction means that the planet is aligned with the Sun in the sky, so it will rise and set with the Sun and not be visible due to the brightness of the Sun. DIF: Medium REF: Section 3.2 MSC: Applied TOP: 2IIii 7. Explain how the synodic and sidereal periods of a planet are defined. Why are they not the same? Explain how they are related to one another.

ANS: The synodic is the time it takes for a planet to come to the same place in an observer’s sky relative to the Sun. The sidereal period is the orbital period of the planet as measures with respect to the background stars. They are not the same because the Earth moves in orbit, too. For inferior planets, 1/P  1/E  1/S and for superior planets, 1/P  1/E

1/S where E is the Earth’s orbital period, S is the syn-

odic period and P is the sidereal period.

DIF: Medium REF: Section 3.2 MSC: Applied TOP: 2IIiii | 2IIiv | 2IIv

8. Assume that at sunset today, Jupiter appears to be 20 degrees away from the Sun. If the sidereal period of Jupiter is 12 years, when will it next appear exactly in this same position relative to the Sun? ANS: Jupiter will be the same distance away from the Sun in one synodic period because that’s the definition of the synodic period. If P  12 years, E  1 year, and 1/P  1/E then S  1/(1

1/S for this superior planet,

(1/12))  12/11  1.09 years.

DIF: Difficult REF: Section 3.2 MSC: Applied TOP: 2IIv 9. Explain how the Occam’s razor argument influenced whether people believed in the heliocentric or the geocentric model of the Solar System.

ANS: Occam’s razor is the argument that the simplest explanation of a pattern in nature is usually the correct one. Because Copernicus’ heliocentric model was able to more accurately predict the positions of the planets, especially during retrograde motion, and it was simpler than Ptolemy’s addition of epicycles in the heliocentric model, the heliocentric model became the one people believed.

DIF: Difficult REF: Section 3.2, 3.3 MSC: Applied TOP: 2IIv | 3Iiii 10. In the heliocentric model of the Solar System, does retrograde motion occur for superior or inferior planets?

(It might help you to draw some illustrations to answer this question.)

ANS: Retrograde motion occurs for both superior and inferior planets. It naturally occurs for a planet when the Earth passes a slower moving, superior planet in its orbit or a faster moving, inferior planet passes the Earth in its orbit.

DIF: Medium REF: Section 3.2 MSC: Conceptual TOP: 2IIvi 11. How was retrograde motion explained in the geocentric system?

ANS: Ptolemy postulated that instead of moving in a circular orbit, a planet moved along a circle called an epicycle, and the center of the epicycle then moved around in a circular orbit around the Earth.

DIF: Medium REF: Section 3.2 MSC: Conceptual TOP: 2IIvi | 2IIvii 12. Explain what is meant by retrograde motion only being an “observational artifact” in the heliocentric system.

ANS: Retrograde motion is not an actual motion; it only appears to happen because of the Earth’s motion relative to the other planet.

DIF: Easy REF: Section 3.2 MSC: Conceptual TOP: 2IIvi

13. What do we customarily call the semimajor axis of a circular orbit? What is the value of the eccentricity of a circle? What might the value of the eccentricity be for a comet on a very elliptical orbit around the Sun? ANS: The semimajor axis of a circular orbit is called the radius of the circle. The eccentricity of a circle is 0. The eccentricity of a comet on a very elliptical orbit around the Sun would be close to 1.

DIF: Easy REF: Section 3.3 MSC: Factual TOP: 3IIiii 14. In a period of three months, a planet travels 30,000 km with an average speed of 10.5 km/s. Some time later, the same planet travels 65,000 km in three months. How fast is the planet traveling at this later time?

During which period is the planet closer to the Sun? ANS: At the later time, because the planet is traveling a greater distance in the same amount of time, it must be moving faster. If the time intervals are the same, then the speed is proportional to the distance traveled. Using ratios: 10.5 km/s/30,000 km  X km/s/65,000 km. Solving for X, X  65,000 km 10.5 km/s/30,000 km  22.75 km/s. Because the planet is moving faster in the second instance, it also must be closer to the Sun at this time.

DIF: Difficult REF: Section 3.2 MSC: Applied TOP: 3IIv 15. Saturn has a semimajor axis of 9.6 AU. How long does it take Saturn to orbit once around the Sun? ANS: Using Kepler’s third law, P2

A3 and comparing it to the Earth’s orbital period of one year and

semimajor axis of 1 AU, Saturn’s period P is equal to (P/1 yr)2  (9.6 AU/1 AU)3 P  1 yr  (9.6)3/2  9.61.5 yr  30 yr DIF: Difficult REF: Section 3.2 MSC: Applied TOP: 3IIvi 16. According to Aristotle, what is the natural state of all objects? In practical terms, what does this mean for moving objects? How did Galileo disagree with Aristotle’s theory?

ANS: According to Aristotle, the natural state of all objects is a state of rest, and all moving objects eventually stop. Galileo disagreed, stating that unbalanced forces on an object make a moving object stop.

DIF: Medium REF: Section 3.4 MSC: Conceptual TOP: 4Iii 17. In terms of frames of reference, explain why an object moving in a straight line at constant speed remains in motion.

ANS: There is no difference between rest and constant motion, so all the features of an object being in constant motion are precisely the same as an object at rest. A frame of reference that is in constant motion will stay in constant motion just as a frame of reference that is at rest will stay at rest.

DIF: Medium REF: Section 3.5 MSC: Conceptual TOP: 5Iii 18. Name the two ways in which an object’s motion (meaning its velocity) can experience a nonzero acceleration.

ANS: The object can have a nonzero acceleration if the speed of its velocity changes or if the direction of the velocity changes.

DIF: Easy REF: Section 3.5 MSC: Factual TOP: 5Iv 19. What acceleration would result from a 5-N force acting on a 3-kg object? (Recall that 1 N  1 kg m/s2.)

ANS: a  F/m  5N/3 kg  1.67 m/s2 DIF: Medium REF: Section 3.5 MSC: Applied TOP: 5Iviii 20. If a 100-kg asteroid collides with Earth, causing the asteroid to decelerate in one second from 1,000 m/s to 0 m/s, what acceleration will Earth experience according to Newton’s third law? (For reference, Earth has a mass of approximately 6  1024 kg.)

ANS: According to Newton’s third law, the force that Earth imparts on the asteroid to slow it down is the same force the asteroid imparts on Earth. According to Newton’s second law, the size of the force equals mass times acceleration (or deceleration). Therefore, 100 kg  1,000 m/s2  6  1024 kg  X (where X is the deceleration experienced by Earth) X  100 kg  1,000 m/s2/(6  1024 kg) X  1.67  10 20 m/s2 (a negligible amount).

DIF: Difficult REF: Section 3.5 MSC: Applied TOP: 5Iviii | 5Ix

CHAPTER 4

Gravity and Orbits

CONCEPT MAP Sec 4.1 1. Gravity Is a Force between Any Two Objects Due to Their Masses I. Gravity i. Gravity: a concept that explains motions of the planets, any orbiting bodies, and the motions of all objects in the universe ii. Galileo measures the acceleration on the Earth due to gravity and finds it’s the same no matter the object’s mass (neglecting air resistance) (TF: 1, 2, MC: 1, 2) iii. Acceleration due to gravity: g  GM/R2 (MC: 3, SA: 1–4) iv. Weight: Fweight  mg (TF: 3, MC: 4–8) v. Newton’s Law of Gravitation: Fgrav  Gm1m2/r2 (MC: 9–16, SA: 5–8) vi. Inverse square law (TF: 4) vii. The force of gravity from a spherically symmetric object outside that body is the same as if all its mass were concentrated at a point at the object’s center Sec 4.2 2. Orbits Are One Body “Falling around” Another I. Orbits Arise When One Body Freely Falls around Another Body i. Newton’s cannonball thought experiment ii. Circular velocity: (MC: 17–20, SA: 9, 10) iii. Free fall (TF: 5, MC: 21, SA: 11) iv. “Weightless” is a misnomer (TF: 5, SA: 11) v. Satellites

vi. Uniform circular motion: circular orbits with constant speed have nonzero acceleration and require a force (TF: 5, 6) vii. Centripetal force viii. Newton’s law of gravitation (P2  42A3/GM) explains Kepler’s third law (P2

A3)

II. Orbits i. Bound orbits are circles or ellipses ii. Unbound orbits are hyperbolas or parabolas (TF: 7) iii. Escape velocity: (TF: 8, MC: 22–25 , SA: 10, 12) iv. Center of mass v. Newton’s law of gravitation can be applied to any orbiting body to derive the mass of the body that it orbits: M 42A3/GP2 (MC: 26–31, SA: 13, 14) Sec 4.3 3. Tidal Forces on the Earth I. The Causes of Tides i. Ocean tides on Earth rise and fall twice per day ii. Tides on Earth are the result of the differences in gravitational force depending on the relative distance from the Sun and Moon (TF: 9, MC: 32) iii. At a given latitude, high tides occur at the longitude closest to the Moon and longitude farthest from the Moon; low tide occurs midway in longitude between those experiencing high tide (MC: 33–35, SA: 15) iv. Tidal force on Earth from a body of mass M: Ftidal(M)  2GM MEarth REarth/d3 (SA: 16, 17) v. Tidal force on Earth due to the Moon is about two times that of the Sun (TF: 10, MC: 36, SA: 16, 17) vi. Spring tides: especially strong tides when Moon’s and Sun’s gravitational pulls combine; occur around new and full Moon phases (MC: 37, 38, 41, 42) vii. Neap tides: especially low tides when the Moon’s and Sun’s gravitational pulls are at right angles and work against one another; occur around first and third quarter Moon phases (MC: 41–43, SA: 18) Sec 4.4 4. Tidal Effects on Solid Bodies I. Tidal Friction i. Tidal forces heat the interior of an orbiting body

ii. Tidal locking: orbital period of an orbiting object equals its rotational period (MC: 44) iii. Due to tidal locking, the Moon always keeps the same side facing Earth (MC: 45, 46, SA: 19) iv. Because of the tidal force of the Moon on the Earth’s oceans, Earth’s rotation rate is slowing, the Moon is getting farther from the Earth, and its orbital period is getting larger (TF: 11, MC: 47–50) v. Spin-orbit resonance (MC: 51) II. Roche Limit i. Roche limit for a planet: 2.5 R (MC: 52, 53, SA: 20) ii. Moons that wander inside the Roche limit can be torn apart and create planetary rings (TF: 12, MC: 52, 53, SA: 20) iii. Small objects are held together by forces stronger than their self-gravity and that’s why satellites in orbit inside the Roche limit can survive III. Gravity in a Three-Body System i. 5 Lagrangian equilibrium points (MC: 54) ii. Tidal forces play a role in making planetary rings and in galaxy–galaxy collisions/mergers (MC: 55) Sec 4.5 5. Origins I. Tidal Forces and Life i. Tides may have played a role in facilitating the development of life on Earth by shaping the margins between the oceans and land where the chemical reactions necessary for life may have begun (MC: 56)

TRUE/FALSE 1. All objects on Earth, regardless of the size of their mass, fall with the same acceleration.

ANS: T DIF: Easy REF: Section 4.1 MSC: Factual TOP: 1Iii 2. More massive objects fall faster in the Earth’s gravitational field than less massive objects.

ANS: F DIF: Medium REF: Section 4.1 MSC: Factual TOP: 1Iii 3. Mass and weight are different names for the same physical property of matter.

ANS: F DIF: Medium REF: 4.1 MSC: Conceptual TOP: 1Iiv

4. Once two objects are far enough away from each other, they no longer exert any gravitational attraction on each other.

ANS: F DIF: Medium REF: Section 4.1 MSC: Applied TOP: 1Ivi 5. An astronaut orbiting the Earth in a circular orbit at a constant speed does not feel the force of gravity because his or her acceleration is zero.

ANS: F DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2Iiii | 2Iiv | 2Ivi 6. The only acceleration you experience on a merry-go-round is when it starts or stops.

ANS: F DIF: Medium REF: Section 4.2 MSC: Applied TOP: 2Ivi 7. An unbound orbit results when a satellite has a velocity greater than the escape velocity.

ANS: T DIF: Easy REF: Section 4.2 MSC: Factual TOP: 2IIii 8. The escape velocity on the Moon is less than that on Earth.

ANS: T DIF: Easy REF: Section 4.2 MSC: Applied TOP: 2IIiii 9. Ocean tides on Earth are caused primarily by the gravitational pull of the Sun. ANS: F DIF: Medium REF: Section 4.3 MSC: Factual TOP: 3Iii 10. The tidal force on the Earth due to the Moon is about two times stronger than that due to the Sun. ANS: T DIF: Medium REF: Section 4.3 MSC: Factual TOP: 3Iv 11. Because of the Moon’s tidal effect on the oceans, the Moon’s orbital distance is shrinking and eventually the Moon will collide with the Earth.

ANS: F DIF: Difficult REF: Section 4.4 MSC: Factual TOP: 4Iiv 12. Moons that wander inside of a planet’s Roche limit can be torn apart due to tidal forces.

ANS: T DIF: Easy REF: Section 4.4 MSC: Factual TOP: 4IIii

MULTIPLE CHOICE 1. In the absence of air friction, a 0.001-kg piece of paper and a 0.1-kg notebook are dropped from the same height and allowed to fall to the ground. How do their accelerations compare?

a. The accelerations are the same. b. The notebook’s acceleration is 100 times faster than the paper’s acceleration. c. The notebook’s acceleration is 1,000 times faster than the paper’s acceleration. d. The paper’s acceleration is 100 times faster than the notebook’s acceleration. e. The paper’s acceleration is 1,000 times faster than the notebook’s acceleration.

ANS: A DIF: Medium REF: Section 4.1 MSC: Applied TOP: 1Iii 2. Two rocks (call them S and T) are released at the same time from the same height and start from rest.

Rock S has 20 times the mass of rock T. Which rock will fall faster if the only forces involved are each rock’s mutual gravitational attraction with Earth? a. Rock S b. Rock T c. Both rocks will fall at the same rate. d. not enough information is available to answer ANS: C DIF: Easy REF: Section 4.1 MSC: Applied TOP: 1Iii

3. According to the scales in the figure, about how many times stronger is gravity on Earth than on the Moon?

a. 20 b. 3 c. 2 d. 6 e. They are the same.

ANS: D DIF: Medium REF: Section 4.1 MSC: Applied TOP: 1Iiii 4. Which of the following properties of an astronaut changes when he or she is standing on the Moon, relative to when the astronaut is standing on Earth? a. weight b. mass c. inertia d. all of the above e. Nothing changes.

ANS: A DIF: Easy REF: Section 4.1 MSC: Applied TOP: 1Iiv

5. Suppose you are suddenly transported to a planet with 1/4 the mass of Earth but the same radius as the Earth. Your weight would _________ by a factor of _________. a. increase; 4 b. increase; 16 c. decrease; 4 d. decrease; 16 e. increase; 2 ANS: C DIF: Medium REF: Section 4.1 MSC: Applied TOP: 1Iiv 6. Suppose you are suddenly transported to a planet that had 1/4 the radius of Earth but the same mass as the Earth. Your weight would _________ by a factor of _________. a. increase; 4 b. increase; 16 c. decrease; 4 d. decrease; 16 e. decrease; 8 ANS: B DIF: Medium REF: Section 4.1 MSC: Applied TOP: 1Iiv 7. If you weighed 150 lbs on Earth, what would you weigh on Mars? For reference, Mars has a mass that is 0.1 times the Earth’s mass and Mars has a radius that is 0.5 times the Earth’s radius. a. 30 lbs b. 110 lbs c. 75 lbs d. 60 lbs e. 15 lbs ANS: D DIF: Difficult REF: Section 4.1 MSC: Applied TOP: 1Iiv

8. If you weighed 100 lbs on Earth, what would you weigh at the upper atmosphere of Jupiter? For reference, Jupiter has a mass that is about 300 times the Earth’s mass and a radius that is 10 times the Earth’s radius. a. 10,000 lbs b. 3,000 lbs c. 1,000 lbs d. 300 lbs e. 30 lbs ANS: D DIF: Difficult REF: Section 4.1 MSC: Applied TOP: 1Iiv 9. _________ hypothesized that planetary motions could be explained by a force arising from the attraction between the mass of the planet and Sun that decreased with the square of the distance between them. a. Johannes Kepler b. Isaac Newton c. Tycho Brahe d. Nicolaus Copernicus e. Galileo Galilei ANS: B DIF: Easy REF: Section 4.1 MSC: Factual TOP: 1Iv

10. The force of gravity that an object has is directly proportional to its: a. inertia b. size c. mass d. density e. distance ANS: C DIF: Easy REF: Section 4.1 MSC: Factual TOP: 1Iv

11. Two rocks (call them S and T) are a distance of 50 km from one another. Rock S has 20 times the mass of rock T. Considering only their mutual gravitational force, which rock will accelerate faster in response to gravity? a. rock S b. rock T c. Both rocks will have the same acceleration. d. not enough information available to answer ANS: B DIF: Medium REF: Section 4.1 MSC: Applied TOP: 1Iv 12. If the distance between the Earth and Sun were cut in half, the gravitational force between these two objects would: a. decrease by 4 b. decrease by 2 c. increase by 2 d. increase by 4 e. decrease by 8 ANS: D DIF: Easy REF: Section 4.1 MSC: Applied TOP: 1Iv 13. According to the progression shown in the figure above, if the distance between two objects is increased to four times its original value, the gravitational force between the two objects would be _________ times its original value. a. 1/2 b. 1/32 c. 1/4 d. 1/16 e. 4 ANS: D DIF: Easy REF: Section 4.1 MSC: Applied TOP: 1Iv

14. The force of gravity between the Earth and the Sun is _________ the force of gravity between the Earth and the Moon. For reference, the average distance between the Earth and the Moon is 0.003 AU, the mass of the Moon is 7  1022 kg, and the mass of the Sun is 2  1030 kg. a. 86,000 times larger than b. 260 times larger than c. 140 times smaller than d. 6,400 times smaller than e. the same as ANS: B DIF: Difficult REF: Section 4.1 MSC: Applied TOP: 1Iv 15. The force of gravity between Saturn and the Sun is _________ the force of gravity between the Earth and the Sun. For reference, Saturn is approximately 100 times more massive than Earth, and the semimajor axis of Saturn’s orbit is 10 AU. a. 10 times smaller than b. 1,000 times larger than c. 1,000 times smaller than d. 100 times larger than e. approximately equal to ANS: E DIF: Difficult REF: Section 4.1 MSC: Applied TOP: 1Iv 16. Mercury orbits the Sun with an average distance of 0.4 AU, and its mass is 0.06 times that of the Earth.

The gravitational force that the Sun exerts on Mercury is _______________ times the force of gravity that the Sun exerts on the Earth. a. 20 b. 6 c. 4 d. 0.4 e. 0.1 ANS: D DIF: Difficult REF: Section 4.1 MSC: Applied TOP: 1Iv

17. If an object is moving in a circular orbit at a constant speed, which of the following is FALSE? a. Its acceleration is not zero. b. Its acceleration is zero. c. Its velocity is not zero. d. There is an unbalanced force acting on it. e. All the above statements are true.

ANS: B DIF: Medium REF: Section 4.2 MSC: Conceptual TOP: 2Iii 18. If we wanted to increase the Hubble Space Telescope’s altitude above the Earth and keep it in a stable orbit, we also would need to: a. increase its orbital speed b. increase its weight c. decrease its weight d. decrease its orbital speed e. increase its mass ANS: D DIF: Medium REF: Section 4.2 MSC: Applied TOP: 2Iii 19. The Hubble Space Telescope orbits at an altitude of 600 km above the Earth’s surface. Assuming it is in a stable circular orbit, what is its velocity? For reference, the Earth’s radius is 6,400 km and Earth’s mass is 6  1024 kg. a. 240,000 m/s b. 7,500 m/s c. 51,000 m/s d. 64,000 m/s e. You also must know the mass of the Hubble Space Telescope to determine its speed.

ANS: B DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2Iii

20. How fast is the Moon moving as it orbits the Earth? For reference, the Moon’s orbit is approximately circular with a radius equal to 400,000 km, and the Moon’s orbital period is 27 days. a. 1 km/s b. 10 km/s c. 50 km/ d. 100 km/s e. 500 km/s ANS: A DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2Iii 21. Astronauts orbiting Earth in the space shuttle feel weightless in space because: a. they are farther away from the Earth b. they eat less food while in orbit c. the gravitational pull of the Moon counteracts the Earth’s gravitational pull d. they are in constant free fall around the Earth e. they are in space where there is no gravity ANS: D DIF: Medium REF: Section 4.2 MSC: Applied TOP: 2Iiii | 2Iiv 22. What is the escape velocity from Mars if its mass is 6  1023 kg and its radius is 3,400 km? a. 2,400 m/s b. 4,900 m/s c. 8,600 km/s d. 12,000 m/s e. 25,000 km/s ANS: B DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2IIiii 23. What is the escape velocity from a large asteroid if its mass is 6  1021 kg and its radius is 2,400 km? a. 98 km/s b. 210 km/s c. 340 m/s d. 580 m/s e. 12,400 m/s ANS: D DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2IIiii

24. If you have two moons that have the same radius, but Moon A is denser and has 2 times the mass of Moon B, how do their escape velocities compare? a. Moon A has an escape velocity that is 1.4 times larger than Moon B. b. Moon A has an escape velocity that is 1.4 times smaller than Moon B. c. Moon A has an escape velocity that is 2 times smaller than Moon B. d. Moon A has an escape velocity that is 2 times larger than Moon B. e. Because gravity affects all masses the same, the escape velocities are the same.

ANS: A DIF: Easy REF: Section 4.2 MSC: Applied TOP: 2IIiii 25. If a satellite sent to Mars is designed to return a rock sample to Earth, how fast must the satellite be launched from its surface in order to escape Mars’s gravity? For reference, Mars has a mass of 6  1023 kg and a radius of 3,400 km. a. 100 m/s b. 5,000 m/s c. 20 m/s d. 20,000 m/s e. You must know the mass of the satellite to determine the answer.

ANS: B DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2IIiii 26. If you measured the orbital period of the Moon and the distance between the Earth and the Moon then you could calculate: a. the mass of the Moon b. the sum of the masses of the Earth and the Moon c. the average distance between the Earth and Sun d. the radius of the Earth e. the radius of the Moon ANS: B DIF: Medium REF: Section 4.2 MSC: Conceptual TOP: 2IIv

27. If you found an exoplanet whose mass was the same as Jupiter’s, but the planet orbited its star with a period of 2 years and a semimajor axis of 1 AU, what would be the mass of its star? For reference, Jupiter has a semimajor axis of 5.4 AU and an orbital period of 12 years. a. 0.25 M b. 0.5 M c. 2.0 M d. 1.5 M e. not enough information available to answer ANS: A DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2IIv 28. Titan, the largest moon of Saturn, has an orbital period of 16 days and a semimajor axis of 1.2  109 m.

Based on this information, what is Saturn’s mass? For reference, the Earth’s mass is 6  1024 kg. a. 290 MEarth b. 130 MEarth c. 90 MEarth d. 40 MEarth e. 4 MEarth ANS: C DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2IIv 29. You find a moon orbiting a planet. The moon has a period of 10 days, and the average distance between the moon and planet is 106 km. What is the planet’s mass? Note that the mass of Jupiter is 1.9  1027 kg. a. 0.1 MJupiter b. 0.4 MJupiter c. 1 MJupiter d. 4 MJupiter e. 10 MJupiter ANS: B DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2IIv

30. If you discovered a planet orbiting another star, and the planet had an orbital period of 2 years and a semimajor axis of 1 AU, what would be the mass of its parent star? You can assume the planet’s mass is much less than the star’s mass. a. 0.25 M b. 0.5 M c. 1.0 M d. 1.25 M e. 2.0 M ANS: A DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2IIv 31. Assume that a planet just like Earth orbits the bright star named Sirius. If this Earth-like planet orbits with a semimajor axis of 1 AU and an orbital period of 7 months, what is the mass of Sirius? a. 3 M b. 12 M c. 8 M d. 5 M e. 17 M ANS: A DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2IIv 32. Tidal forces are caused by: a. the weight of the water in the oceans on the ocean floor b. the strength of the gravitation pull of the Moon on Earth c. the difference between the weight of the water on the ocean floor at high and low tide d. the difference between the strength of the gravitiational pull of the Moon and Sun on either side of Earth e. the strength of the gravitation pull of the Moon and the Sun on Earth ANS: D DIF: Easy REF: Section 4.3 MSC: Factual TOP: 3Iii

Figure 1

33. According to Figure 1, a high tide at a given location will occur ____ time(s) a day. a. one b. three c. two d. four e. eight ANS: C DIF: Medium REF: Section 4.3 MSC: Applied TOP: 3Iiii 34. According to Figure 1, the approximate amount of time between two high tides at a given location is: a. 3 hours b. 8 hours c. 6 hours d. 12 hours e. 24 hours ANS: D DIF: Medium REF: Section 4.3 MSC: Applied TOP: 3Iiii 35. According to Figure 1, the approximate amount of time between a high tide and a low tide at a given location is: a. 3 hours b. 8 hours c. 6 hours d. 12 hours e. 24 hours ANS: C DIF: Medium REF: Section 4.3 MSC: Applied TOP: 3Iiii

36. Lunar tides are approximately _________ solar tides. a. 2 times weaker than b. 2 times stronger than c. 200 times weaker than d. 200 times stronger than e. the same strength as ANS: B DIF: Easy REF: Section 4.3 MSC: Factual TOP: 3Iv Figure 2

37. According to Figure 2, spring tides occur when the lunar and solar tides _________ resulting in _________ tides. a. add; above average b. partially cancel out; above average c. add; below average d. partially cancel out; below average e. completely cancel out; no ANS: A DIF: Easy REF: Section 4.3 MSC: Applied TOP: 3Ivi 38. According to Figure 2, spring tides occur at which phases of the Moon? a. third quarter b. new and full c. first and third quarter d. full e. new ANS: B DIF: Medium REF: Section 4.3 MSC: Applied TOP: 3Ivi

39. According to Figure 2, when the Moon is in between Earth and the Sun, _________ tides occur. a. spring b. no c. neap d. high e. low ANS: A DIF: Easy REF: Section 4.3 MSC: Applied TOP: 3Ivi 40. According to Figure 2, when Earth is in between the Moon and the Sun, _________ tides occur. a. spring b. no c. neap d. high e. low ANS: A DIF: Easy REF: Section 4.3 MSC: Applied TOP: 3Ivi 41. According to Figure 2, neap tides occur when the lunar and solar tides _________, resulting in _________ tides. a. add; above average b. partially cancel out; above average c. add; below average d. partially cancel out; below average e. completely cancel out; no ANS: D DIF: Easy REF: Section 4.3 MSC: Applied TOP: 3Ivii 42. According to Figure 2, neap tides occur at which phases of the Moon? a. new and full b. third quarter c. first quarter and third quarter d. full e. new ANS: C DIF: Medium REF: Section 4.3 MSC: Applied TOP: 3Ivii

43. When the Sun and Moon are separated by 90 degrees in the sky, _________ tides occur on Earth when the strength of the tides are _________ than normal. a. spring; smaller b. spring; larger c. lunar; smaller d. neap; smaller e. neap; larger ANS: D DIF: Medium REF: Section 4.3 MSC: Factual TOP: 3Ivii 44. The moon keeps the same hemisphere facing the Earth because the _________ is equal to the _________. a. rotational period of the Earth; orbital period of the Moon around the Earth b. orbital period of the Earth; orbital period of the Moon around the Earth c. orbital period of the Moon around the Earth; rotational period of the Earth d. rotational period of the Moon; orbital period of the Moon around the Earth e. rotational period of the Earth; orbital period of the Earth ANS: D DIF: Medium REF: Section 4.4 MSC: Factual TOP: 4Iii

45. In the figure shown below, the person on the Moon is standing at the same location on the Moon as the Moon rotates around the Earth. Based on this, we see that the Moon’s rotational period is equal to:

a. the Earth’s rotational period b. half the Earth’s rotational period c. the Moon’s orbital period d. half the Moon’s orbital period e. the Earth’s orbital period around the Sun ANS: C DIF: Easy REF: Section 4.4 MSC: Applied TOP: 4Iiii 46. The Moon always keeps the same face toward Earth because of: a. tidal locking b. tidal forces from the Sun c. tidal forces from Earth d. tidal forces from the Earth and Sun e. all sides of the moon face Earth at one time or another ANS: A DIF: Easy REF: Section 4.4 MSC: Factual TOP: 4Iiii

47. Because of the tidal force between the Earth and Moon: a. the Earth’s rotation rate is decreasing b. the Moon’s distance from Earth is increasing c. the Moon’s orbital period is increasing d. the Moon’s rotational period is increasing e. All of the above are true.

ANS: E DIF: Difficult REF: Section 4.4 MSC: Factual TOP: 4Iiv 48. The distance between Earth and the Moon: a. will never change b. is slowly decreasing c. is slowly increasing d. will increase or decrease depending on future changes in the tides on the Moon due to the Earth e. will increase or decrease depending on future changes in the tides on the Earth due to the Moon ANS: C DIF: Easy REF: Section 4.4 MSC: Factual TOP: 4Iiv 49. The Earth’s rotation rate is slowing because of: a. radioactive decays in its core b. relativistic effects of gravity c. tidal forces from the Moon d. the gravitational force of the Sun e. gravitational drag from dark matter ANS: C DIF: Easy REF: Section 4.4 MSC: Applied TOP: 4Iiv 50. The distance between Earth and the Moon is increasing because of: a. the expansion of the Universe b. the expansion of the Solar System c. tidal forces from the Moon d. tidal forces from the Moon and Sun e. dark energy ANS: C DIF: Easy REF: Section 4.4 MSC: Applied TOP: 4Iiv

51. Because of tidal forces, for every _________ time(s) it rotates on its axis, Mercury revolves around the Sun _________ time(s). a. 1; 1 b. 2; 3 c. 3; 2 d. 10; 1 e. 20; 1 ANS: C DIF: Difficult REF: Section 4.4 MSC: Factual TOP: 4Iv 52. Which one of the statements below about a planet’s Roche limit is FALSE? a. The Roche limit is about 2.5 times the radius of gaseous planets. b. Objects orbiting closer to a planet than the Roche limit are likely to be ripped apart by tidal forces. c. The Roche limit is where tidal forces from an orbiting object are equal to its internal self-gravity. d. Orbiting objects beyond the Roche limit from the planet do not get ripped apart by tidal forces. e. The ring systems around giant planets are located beyond the Roche limit.

ANS: E DIF: Easy REF: Section 4.4 MSC: Factual TOP: 4IIi | 4IIii 53. Which of the statements below are TRUE about the Roche limit of a giant planet? a. It is about equal to the radius of the planet. b. It is the closest to the planet that moons normally are found. c. It is the closest to the planet that rings will be found. d. It is the farthest from the planet that moons normally are found. e. Because they have no solid surfaces, giant planets do not have a Roche limit.

ANS: B DIF: Medium REF: Section 4.4 MSC: Factual TOP: 4IIi | 4IIii 54. In a system with three gravitating bodies, there are _________ stable locations called _________ points for the third object to orbit in lockstep with the two more massive objects. a. 3; tidal b. 2; tidal c. 4; Roche d. 2; Lagrange e. 5; Lagrange ANS: D DIF: Medium REF: Section 4.4 MSC: Factual TOP: 4IIIi

55. Tidal forces can affect: a. moons b. galaxies c. planets d. satellites e. all of the above ANS: E DIF: Easy REF: Section 4.4 MSC: Factual TOP: 4IIIii 56. _________ may have been instrumental in shaping the interface between Earth’s land and oceans where the chemistry needed to develop life may have occurred. a. Meteor showers b. Collisions with comets c. Earth’s Moon d. Techtonic activity e. A collision with a Mars-sized object ANS: C DIF: Medium REF: Section 4.5 MSC: Factual TOP: 5Ii

SHORT ANSWER 1. How would the acceleration due to gravity on a planet that is 16 times as massive as Earth and 4 times its radius compared to the acceleration of gravity on the Earth?

ANS: For that planet, the acceleration due to gravity is gplanet  GM/R2  G(16M) / (4R)2  (16GM) / (16R2)  GM/R2  g. Therefore, the acceleration due to gravity would be the same for that planet as for Earth.

DIF: Difficult REF: Section 4.1 MSC: Applied TOP: 1Iiii 2. Saturn has 95 times the mass of Earth, and its atmosphere extends outward 9.5 times the Earth’s radius.

How does the acceleration due to gravity at the edge of Saturn’s atmosphere compare to that on the Earth?

ANS: g  GM/R2; thus gS/gE  (GMS/RS2) / (GME/RE2)  (MS/ME)  (RE/RS)2  95  (1/9.5)2  1.05.

DIF: Difficult REF: Section 4.1 MSC: Applied TOP: 1Iiii

3. Mars has about one tenth the mass of Earth and half the Earth’s radius. How does the acceleration of gravity on Mars compare to that on Earth?

ANS: g  GM/R2; thus gM/gE  (GMMM/RM2) / (GMME/RE2)  (MM/ME)  (RE/RM)2  (1/10)  22  0.4.

DIF: Difficult REF: Section 4.1 MSC: Applied TOP: 1Iiii 4. Is there a difference in your weight between when you are on top of a mountain at 1,000 meters above sea level compared to when you are sitting in a classroom at 10 meters above sea level?

ANS: Yes, you weigh less on the mountaintop because the distance between you and the center of the Earth (R) is larger. However, the difference in weight is so small that it isn’t noticeable, because the Earth is so large (R  6,370 km) compared to the 1 km difference in these altitudes.

DIF: Easy REF: Section 4.1 MSC: Applied TOP: 1Iiii 5. Show that the gravitational pull on Earth from the Sun is about 200 times the gravitational pull on Earth from the Moon. For reference, Moon’s mass is 7.3  10 22 kg, the Sun’s mass is 2  1030 kg, the EarthMoon distance is 3.8  105 km, and the Earth-Sun distance is 1.5  108 km.

ANS: Fgrav  GM1M2/r2; thus FSun/FMoon  (GMSME/dES2)/(GMMME/dEM2)  (MSun/MMoon)  (dEM/dES)2  (2  1030 kg /7.3  1022 kg )  (3.8  105 km /1.5  108 km)2  176, which is approximately 200.

DIF: Medium REF: Section 4.1 MSC: Applied TOP: 1Iv 6. How much stronger is the gravitational force of the Sun on the Earth compared to the gravitational force of the Sun on Pluto? Note that Pluto’s semimajor axis is 40, AU, and Pluto’s mass is 0.002 times the mass of the Earth.

ANS: For a planet, the force of gravity between it and the Sun is F  GMSunM/d2; thus FP/FE  (GMSunMP/dP2)/(GMSunME/dE2)  (MP/ME)  (dE/dP)2  0.002  (1/40)2  0.002 / 402  1.3  106. Therefore the force of gravity between the Sun and Pluto is about a million times less than that between the Sun and the Earth.

DIF: Medium REF: Section 4.1 MSC: Applied TOP: 1Iv 7. How does the force of gravity between the Sun and Mercury compare to the gravitational force between the Sun and the Earth? Note that the semimajor axis of Mercury’s orbit is 0.4 AU, and Mercury’s mass is 0.06 times the mass of the Earth.

ANS: For a planet, the force of gravity between it and the Sun is F  GMSunM/d2; thus FM/FE  (GMSunMM/dM2)/(GMSunME/dE2)  (MM/ME)  (dE/dM)2  0.06  (1/0.4)2  0.06 / 0.42  0.38. Therefore

the force of gravity between the Sun and Mercury is only 38% that between the Sun and the Earth.

DIF: Medium REF: Section 4.1 MSC: Applied TOP: 1Iv 8. How does the force of gravity between the Sun and Jupiter compare to the gravitational force between the Sun and the Earth? Note that the semimajor axis of Jupiter’s orbit is 5.2 AU, and Jupiter’s mass is 320 times the mass of the Earth.

ANS: For a planet, the force of gravity between it and the Sun is F  GMSunM/d2; thus FJ/FE  (GMSunMJ/dJ2) / (GMSunME/dE2)  (MJ/ME)  (dE/dJ)2  320  (1/5.2)2  320 / 5.22  12. Therefore the force of gravity between the Sun and Jupiter is 12 times larger than that between the Sun and the Earth.

DIF: Medium REF: Section 4.1 MSC: Applied TOP: 1Iv 9. The International Space Station orbits at an altitude of 400 km above the Earth’s surface. Assuming it is in a stable circular orbit, what is its velocity? For reference, the Earth’s radius is 6,400 km, and Earth’s mass is 6  1024 kg and G  6.7  10 11 N m2/kg2.

ANS: Recall that 1N  1 kg m/s2. Thus vcirc  (GM/r)1⁄2  (6.7  10 11 m3/(kg s2)  6  1024 kg / ((6400  400)  103 m))1⁄2  7,700 m/s  7.7 km/s.

DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2Iii 10. Explain what the terms circular velocity and escape velocity mean. Give the formula for each and explain what each mathematical symbol represents.

ANS: The circular velocity is the velocity that an object needs in order to maintain a stable orbit around an object of mass M at a distance r from it, and the circular velocity is equal to vcirc  (GM/r)1⁄2. The escape velocity is the minimum velocity an object would have to be given if it were to escape the gravity of the object it orbits, whose mass is M. If the object is a distance R from the object it orbits, then the escape velocity is equal to vesc  (2GM/R)1⁄2.

DIF: Easy REF: Section 4.2 MSC: Applied TOP: 2Iii | 2IIiii 11. Explain the difference between being weightless and being in free fall.

ANS: The only way to be truly weightless is to have no mass; feeling weightless comes from being on a moon or planet that weighs much less than Earth. Free fall occurs when you are in orbit around something. As long as you don’t land on the object but simply continue to “fall” around it, the constant magnitude of the acceleration experienced is physically equivalent to zero acceleration. If you are free-falling, you interpret this as not experiencing the force of gravity between yourself and the object you are orbit-

ing. You are functionally weightless except for the “microgravity” you feel between yourself and the other objects that are orbiting with you.

DIF: Medium REF: Section 4.2 MSC: Conceptual TOP: 2Iiii | 2Iiv 12. What is the velocity one would need to give a satellite in order for it to escape from the Solar System (meaning escape the Sun’s gravity) if it was launched from the Earth at a distance of 1 AU from the Sun? Give your answer in units of m/s.

ANS: The escape velocity from the Sun would be where R  1AU  1.5  1011m and the mass of the sun is M  2.0  1030 kg.

DIF: Medium REF: Section 4.2 MSC: Applied TOP: 2IIiii 13. What two pieces of information would you need to obtain about one of the moons of the planet Jupiter in order to measure the mass of Jupiter? What formulae would you use to determine the mass?

ANS: You would need to know the semimajor axis of the moon’s orbit and the moon’s orbital period. You would use Newton’s version of Kepler’s third law to calculate the mass: M  42A3/GP2.

DIF: Medium REF: Section 4.2 MSC: Applied TOP: 2IIv 14. You discover a moon orbiting a planet. The moon has an orbital period of three weeks, and the average distance between the moon and planet is 1.2  106 km. What is the planet’s mass? Compare its mass to that of Jupiter, which is 1.9  1027 kg.

ANS: Use the equation M  42A3/GP2, where G  6.67  10 11 Nm2/kg2. First convert the units of G,

P, and A so they contain combinations of only the units of kg, m, and s.

G  6.67  10 11 m3/kg s2 P  3 weeks  (7 day/1 week)  (24 hr/1 day)  (3,600 sec/1 hr)  1.81  106 s A  1.2  109 m M  42(1.2  109 m)3/(6.67  10 11 m3/kg s2  (1.81  106s)2)  3.1  1026 kg M  3.1  1026 kg  (MJupiter/1.9  1027 kg)  0.16 MJupiter DIF: Difficult REF: Section 4.2 MSC: Applied TOP: 2IIv

15. Consider the figure below that illustrates the tidal bulge on the Earth’s oceans due to the Moon and four people at different longitudes on the Earth from the point of view of an observer looking down on the North Pole of the Earth. If you were to arrive at the beach and find that the Moon was visible in the western half of the sky, then is the tide most likely to be coming in and the water level rising; or is the tide going out and the water level going down? Explain the rationale for your answer.

ANS: If the Moon is in the western half of the sky, then you would be located somewhere between the person shown on top, for whom the Moon is just west of the meridian, and the person shown on the right, for whom the Moon is about to set. Therefore the tide would be going out and the water level going would be going down.

DIF: Medium REF: Section 4.3 MSC: Applied TOP: 3Iiii 16. Which is larger, the tidal force on the Earth due to the Moon or the tidal force on the Earth due to the Sun, and by approximately how much? Explain conceptually why this is possible given that the gravitational force of the Sun on the Earth is 200 times larger than the gravitational force of the Moon on the Earth.

ANS: Tides are caused by the difference between the gravity affecting one side of an object and the other. The tidal force on the Earth due to another body of mass M is F  2GMMERE/d3, where d is the distance between the body and the Earth. When you do the calculation, the tidal force of the Moon on the Earth is about two times that of the Sun. Even though the mass of the moon is smaller than the Sun, its much closer distance makes the tidal force larger.

DIF: Medium REF: Section 4.3 MSC: Conceptual TOP: 3Iiv | 3Iv 17. Show that the tidal force on Earth from the Moon is approximately two times the tidal force on Earth from the Sun. For reference, Moon’s mass is 7.3 1022 kg, the Sun’s mass is 2  1030 kg, the Earth-Moon distance is 3.8  105 km, and the Earth-Sun distance is 1.5  108 km.

ANS: The tidal force on the Earth due to a body of mass M at a distance d is given by F  2GM ME RE/d3. Thus the ratio of the tidal force on the Earth due to the Moon compared to the Sun is FM/FS 

(2GMM ME RE/dEM3)/(2GMS ME RE/dES3)  (MM/MS)  (dES/dEM)3  (7.3  10 22 kg / 2  1030 kg)  ( 1.5  108 km / 3.8  105 km)3  2.2.

DIF: Difficult REF: Section 4.3 MSC: Applied TOP: 3Iiv | 3Iv 18. Consider the Figure below that illustrates the tidal bulge on the Earth’s oceans due to the Moon and Sun from an observer looking down on the North Pole of the Earth. At what phase of the Moon will the lowest tides of the year occur? Explain the rationale for your answer either in words or with a sketch.

ANS: The lowest tides of the year are called neap tides, and they occur when the Sun’s gravitational pull is at right angles to the Moon’s gravitational pull. The neap tides are shown in part (b) of the figure, and they occur for the first-quarter moon (top of the panel) and the third-quarter moon (bottom of the panel).

DIF: Medium REF: Section 4.3 MSC: Applied TOP: 3Ivii 19. Explain why the Moon rotates in the same amount of time as it takes to orbit once around the Earth.

ANS: The Moon’s tidal locking is caused by tidal friction that results in the side of the Moon that is heavier always facing the Earth.

DIF: Easy REF: Section 4.4 MSC: Conceptual TOP: 4Iiii 20. Explain what the Roche limit is and how it is related to rings around giant planets.

ANS: This Roche limit is the distance from a large planet at which the tidal forces on an object, such as a moon, are equal to the self-gravity that holds the object together. It is equal to approximately 2.5 times the radius of a large planet. Outside this distance moons can exist, but inside this distance they are likely to be torn apart by tidal forces and, in doing so, create planetary rings.

DIF: Easy REF: Section 4.4 MSC: Conceptual TOP: 4IIi | 4IIii

CHAPTER 5

Light

CONCEPT MAP Sec 5.1 1. The Speed of Light I. Light Carries Information i. We learn about the universe by analyzing the light that comes to us from great distances II. Speed i. Ole Rømer (1670s)—measured speed of light using the observed time delay of the moons of Jupiter (MC: 1) ii. Speed of light is constant: c  300,000 km/s (TF: 1, MC: 2–4, SA: 1) iii. Light travels slower when it travels through a medium other than vacuum (MC: 5) iv. Distances: light year and parsec (MC: 6) Sec 5.2 2. Light Is an Electromagnetic Wave I. Wave Nature of Light i. Electric and magnetic fields and forces ii. Changing electric or magnetic fields generate electromagnetic waves iii. James Clerk Maxwell’s description of light II. Waves Are Characterized by Amplitude, Speed, Frequency, and Wavelength i. Amplitude, frequency (f), period (P), and wavelength () (MC: 7, 8) ii. f  1/P (TF: 2) iii.   c P  c/f (MC: 9–14, SA: 1) III. A Wide Range of Wavelengths Make Up the Electromagnetic Spectrum

i. Electromagnetic spectrum: gamma rays, X-rays, ultraviolet, visible, infrared, microwave, radio (TF: 3, MC: 4, 15–18, SA: 1, 2, 6) ii. Nanometer (1 nm  10 9 m) IV. Light Is a Wave, but It Is Also a Particle i. Einstein and the photoelectric effect (MC: 4, 19, 20) ii. Photons (MC: 21) iii. E  hf  hc/ (MC: 14, 18, 22) Sec 5.3 3. The Quantum Mechanical View of Matter I. Atoms Make Up Most Matter i. Atoms: protons, neutrons, and electrons (MC: 23) ii. Bohr model of the atom (SA: 3) iii. Heisenberg uncertainty principle: x p  h iv. Electron cloud (MC: 24, SA: 3) II. Atoms Have Discrete Energy Levels i. Atoms have discrete energy levels and a ground state III. An Atom’s Energy Levels Determine the Wavelengths It Can Emit and Absorb i. Electronic transitions lead to absorption or emission of photons with   hc /E (MC: 25, SA: 2, 4) ii. Emission spectra: cloud of glowing gas (TF: 4, MC: 26, 27) iii. Absorption spectra: stars, cool gas in front of a light source (TF: 5, MC: 28, 29) IV. Emission and Absorption Lines Are the Spectral Fingerprints of Atoms i. Discrete energy levels in atoms that are unique to each chemical element (TF: 6, 7) ii. Observing spectra allows astronomers to measure chemical composition, temperature, density, and pressure (SA: 5) V. How Are Atoms Excited and Why Do They Decay? i. Spontaneous emission (SA: 6, 7) Sec 5.4 4. The Doppler Effect—Motion toward or Away from Us I. Doppler Effect i. Rest wavelength: rest ii. Objects moving toward you are blueshifted; objects moving away from you are redshifted (TF: 8,

MC: 30–33, SA: 5, 8–10) iii. obs  (1  vr/c) rest (MC: 34–37, SA: 8, 11) iv. Radial velocity: vr Sec 5.5 5. Light and Temperature I. Temperature Balance i. Static, unstable, and dynamic equilibrium ii. Thermal equilibrium (MC: 39) II. Temperature Is a Measure of How Energetically Particles Are Moving i. Temperature scales and absolute zero (MC: 40) ii. Kinetic energy: EK  1⁄2 mv2 iii. Thermal motion: v III. Hotter Means More Luminous and Bluer i. Continuous spectrum is thermal radiation ii. Luminosity (L) iii. Blackbody (or Planck) spectra IV. The Stefan-Boltzmann Says That Hotter Means Much More Luminous i. F   T4 (MC: 41–44, SA: 3, 12–14) V. Wien’s Law Says That Hotter Means Bluer i. max  (2,900,000 nm K)/T (TF: 3, 9, MC: 41, 44–49, SA: 5, 13) Sec 5.6 6. Light and Distance I. Distance Determines Brightness i. Luminosity vs. brightness (TF: 10; SA: 16) ii. Brightness: B  L / (4 r2) (MC: 50–52, SA: 5, 13, 14, 17) Sec 5.7 7. Origins: Temperatures of Planets I. Thermal Equilibrium Allows Us to Use Radiation to Predict Planet Temperatures i. Planets radiate energy to reach equilibrium temperature (SA: 18) ii. Albedo (a): fraction of light reflected by a planet (MC: 53)

iii. T  (279 K)[(1

a)1⁄2 / dAU]1⁄2 (MC: 54–57; SA: 19, 20)

TRUE/FALSE 1. A radio photon travels slower than a gamma ray photon.

ANS: F DIF: Easy REF: Section 5.1 MSC: Factual TOP: 1IIii 2. The period of a wave is inversely proportional to the frequency of that wave.

ANS: T DIF: Easy REF: Section 5.2 MSC: Factual TOP: 2IIii 3. Your average body temperature is approximately 310 K, thus most of the light you radiate is at ultraviolet wavelengths.

ANS: F DIF: Medium REF: Section 5.5 MSC: Applied TOP: 2IIIi | 5Vi 4. An emission line is produced when an atom absorbs a photon of a specific energy.

ANS: F DIF: Easy REF: Section 5.3 MSC: Factual TOP: 3IIIii 5. A thin cloud of cool gas on top of a hotter blackbody will radiate an emission spectrum.

ANS: F DIF: Medium REF: Section 5.3 MSC: Conceptual TOP: 3IIIiii 6. The emission spectrum of a helium atom is identical to that of a carbon atom.

ANS: F DIF: Medium REF: Section 5.3 MSC: Conceptual TOP: 3IVi 7. The emission lines of a given atom occur at the exact same energies as that atom’s absorption lines.

ANS: T DIF: Medium REF: Section 5.3 MSC: Conceptual TOP: 3IVi 8. If an object is moving toward you, the light you see from it will be blueshifted.

ANS: T DIF: Easy REF: Section 5.4 MSC: Factual TOP: 4Iii 9. Cooler objects radiate more of their total light at shorter wavelengths than hotter objects.

ANS: F DIF: Medium REF: Section 5.5 MSC: Factual TOP: 5Vi 10. The luminosity of an object is independent of its distance from you.

ANS: T DIF: Easy REF: Section 5.6 MSC: Factual TOP: 6Ii

MULTIPLE CHOICE

1. The speed of light was first determined by which scientist? a. Galileo b. Newton c. Kepler d. Rømer e. Einstein ANS: D DIF: Easy REF: Section 5.1 MSC: Factual TOP: 1IIi 2. The speed of light in a vacuum is: a. 300,000 m/s b. 300,000 mph c. 300,000 km/s d. 300,000,000 mph e. infinite ANS: C DIF: Easy REF: Section 5.1 MSC: Factual TOP: 1IIii 3. If the Sun instantaneously stopped giving off light, what would happen on Earth? a. It would immediately get dark. b. It would get dark 8 minutes later. c. It would get dark 27 minutes later. d. It would get dark 1 hour later. e. It would get dark 24 hours later.

ANS: B DIF: Medium REF: Section 5.1 MSC: Applied TOP: 1IIii 4. What is the difference between visible light and X-rays? a. Speed; X-rays go faster than visible light. b. Speed; X-rays go slower than visible light. c. Wavelength; X-rays have a shorter wavelength than visible light. d. Wavelength; X-rays have a longer wavelength than visible light. e. X-rays are made up of particles, while visible light is made up of waves.

ANS: C DIF: Easy REF: Section 5.2 MSC: Conceptual TOP: 1IIii | 2IIIi | 2IVi

5. How does the speed of light traveling through a medium (such as air or glass) compare to the speed of light in a vacuum? a. It is the same as the speed of light in a vacuum. b. It is always less than the speed of light in a vacuum. c. It is always greater than the speed of light in a vacuum. d. Sometimes it is greater than the speed of light in a vacuum, and sometimes it is less, depending on the medium. e. Light can’t travel through a medium, it only can go through a vacuum.

ANS: B DIF: Easy REF: Section 5.1 MSC: Factual TOP: 1IIiii 6. A light-year is a unit that is used to measure: a. time b. wavelength c. speed d. energy e. distance ANS: E DIF: Easy REF: Section 5.1 MSC: Factual TOP: 1IIiv 7. Use the figure below to answer the following question:

Which of these statements about the amplitude and wavelength of the two waves shown is correct?

a. Wave A has a larger amplitude and a larger wavelength. b. Wave B has a larger amplitude and a larger wavelength. c. Wave A has a larger amplitude, and Wave B has a larger wavelength. d. Wave B has a larger amplitude, and Wave A has a larger wavelength. e. Wave A and B have the same amplitude and wavelength.

ANS: D DIF: Easy REF: Section 5.2 MSC: Applied TOP: 2IIi 8. If the frequency of a beam of light were to increase, its period would _________ and its wavelength would _________? a. decrease; increase b. increase; decrease c. increase; increase d. decrease; decrease e. stay the same; stay the same ANS: D DIF: Medium REF: Section 5.2 MSC: Applied TOP: 2IIi 9. The fact that the speed of light is constant as it travels through a vacuum means that: a. photons with longer wavelengths have lower frequencies b. radio wave photons have shorter wavelengths than gamma ray photons c. X-rays can be transmitted through the atmosphere around the world d. ultraviolet photons have less energy than visible photons e. all of the above are true ANS: A DIF: Medium REF: Section 5.2 MSC: Conceptual TOP: 2IIiii 10. If the wavelength of a beam of light were to double, how would that affect its frequency? a. The frequency would be four times larger. b. The frequency would be two times larger. c. The frequency would be two times smaller d. The frequency would be four times smaller. e. There is no relationship between wavelength and frequency.

ANS: C DIF: Medium REF: Section 5.2 MSC: Applied TOP: 2IIiii

11. Which formula denotes how the speed of light is related to its wavelength and frequency? a. c  f b. c /f c. c  f/ d. c  1/f e. There is no relationship between wavelength and frequency.

ANS: A DIF: Easy REF: Section 5.2 MSC: Factual TOP: 2IIiii 12. Light with a wavelength of 600 nm has a frequency of: a. 2  105 Hz b. 5  107 Hz c. 2  1010 Hz d. 5  1012 Hz e. 5  1014 Hz ANS: E DIF: Medium REF: Section 5.2 MSC: Applied TOP: 2IIiii 13. Which of the following lists different types of electromagnetic radiation in order from the smallest wavelength to the largest wavelength? a. radio waves, infrared, visible, ultraviolet, X-rays b. gamma rays, ultraviolet, visible, infrared, radio waves c. gamma rays, X-rays, infrared, visible, ultraviolet d. X-rays, infrared, visible, ultraviolet, radio waves e. radio waves, ultraviolet, visible, infrared, gamma rays ANS: B DIF: Medium REF: Section 5.2 MSC: Factual TOP: 2IIiii 14. As wavelength increases, the energy of a photon _________ and its frequency _________. a. increases; decreases b. increases; increases c. decreases; decreases d. decreases; increases ANS: C DIF: Medium REF: Section 5.2 MSC: Factual TOP: 2IIiii | 2IViii

15. The color of visible light is determined by its: a. speed b. wavelength c. mass d. distance from you e. size ANS: B DIF: Easy REF: Section 5.2 MSC: Factual TOP: 2IIIi 16. How do the wavelength and frequency of red light compare to the wavelength and frequency of blue light? a. Red light has a longer wavelength and higher frequency than blue light. b. Red light has a longer wavelength and lower frequency than blue light. c. Red light has a shorter wavelength and higher frequency than blue light. d. Red light has a shorter wavelength and lower frequency than blue light.

ANS: B DIF: Easy REF: Section 5.2 MSC: Factual TOP: 2IIIi 17. What wavelengths of light can the human eye see? a. 3.8 m to 7.5 m b. 3.8 nm to 7.5 nm c. 380 cm to 750 cm d. 380 nm to 750 nm e. 3.8 m to 7.5 m ANS: D DIF: Easy REF: Section 5.2 MSC: Factual TOP: 2IIIi 18. Which of the following photons carry the smallest amount of energy? a. a blue photon of the visible spectrum, whose wavelength is 450 nm b. an infrared photon, whose wavelength is 10 5 m c. a red photon in the visible spectrum, whose wavelength is 700 nm d. a microwave photon, whose wavelength is 10 2 m e. an ultraviolet photon, whose wavelength is 300 nm ANS: D DIF: Medium REF: Section 5.2 MSC: Applied TOP: 2IIIi | 2IViii

19. Einstein showed that the _________ could be explained if photons carried quantized amounts of energy. a. warping of space and time b. Heisenberg uncertainty principle c. photoelectric effect d. theory of special relativity e. Bohr model of the atom ANS: C DIF: Medium REF: Section 5.2 MSC: Factual TOP: 2IVi 20. Light is: a. a wave b. a particle c. both a particle and a wave d. neither a particle nor a wave ANS: C DIF: Medium REF: Section 5.2 MSC: Conceptual TOP: 2IVi 21. Saying that something is quantized means that it: a. is a wave b. is a particle c. travels at the speed of light d. can only have discrete quantities e. is smaller than an atom ANS: D DIF: Medium REF: Section 5.2 MSC: Factual TOP: 2IVii 22. A red photon has a wavelength of 650 nm. An ultraviolet photon has a wavelength of 250 nm. The energy of an ultraviolet photon is _________ the energy of a red photon. a. 2.6 times larger than b. 6.8 times larger than c. 2.6 times smaller than d. 6.8 times smaller than e. the same as ANS: A DIF: Medium REF: Section 5.2 MSC: Applied TOP: 2IViii

23. Why is an iron atom a different element than a carbon atom? a. A carbon atom has fewer neutrons in its nucleus than an iron atom. b. An iron atom has more protons in its nucleus than a carbon atom. c. An iron atom has more electrons than a carbon atom. d. A carbon atom is bigger than an iron atom. e. All of the above are true.

ANS: B DIF: Medium REF: Section 5.3 MSC: Conceptual TOP: 3Ii 24. In the quantum mechanical view of the atom, an electron is best thought of as: a. a cloud that is centered on the nucleus b. a particle orbiting the nucleus c. free to orbit at any distance from the nucleus d. a particle inside the nucleus e. All of the above are true.

ANS: A DIF: Medium REF: Section 5.3 MSC: Conceptual TOP: 3Iiv 25. The n  5 electronic energy level in a hydrogen atom is 1.5  10 19 J higher than the n  3 level. If an electron moves from the n  5 level to the n  3 level, then a photon of wavelength: a. 1.3 nm, which is in the ultraviolet region, is emitted b. 1.3 nm, which is in the ultraviolet region, is absorbed c. 1,300 nm, which is in the infrared region, is absorbed d. 1,300 nm, which is in the infrared region, is emitted e. no light will be absorbed or emitted ANS: D DIF: Difficult REF: Section 5.3 MSC: Applied TOP: 3IIIi 26. When an electron moves from a higher energy level in an atom to a lower energy level: a. the atom is ionized b. a continuous spectrum is emitted c. a photon is emitted d. a photon is absorbed e. the electron loses mass ANS: C DIF: Easy REF: Section 5.3 MSC: Applied TOP: 3IIIii

27. If you observe an isolated hot cloud of gas, you will see: a. an absorption spectrum b. a continuous spectrum c. an emission spectrum d. a rainbow spectrum e. a dark spectrum ANS: C DIF: Easy REF: Section 5.3 MSC: Applied TOP: 3IIIii 28. Which of these objects would emit an absorption spectrum? a. an incandescent light bulb b. a fluorescent light bulb c. an isolated hot gas cloud d. a hot, solid object e. a thin, cool gas cloud that lies in front of a hotter blackbody ANS: E DIF: Easy REF: Section 5.3 MSC: Applied TOP: 3IIIiii 29. If you observe a star, you will see: a. an absorption spectrum b. a continuous spectrum c. an emission spectrum d. a rainbow spectrum e. a dark spectrum ANS: A DIF: Easy REF: Section 5.3 MSC: Applied TOP: 3IIIiii 30. The Doppler shift can be used to determine the _________ of an object. a. energy b. temperature c. radial velocity d. color e. three-dimensional velocity ANS: C DIF: Easy REF: Section 5.4 MSC: Factual TOP: 4Iii

31. A spaceship is traveling toward Earth while giving off a constant radio signal with a wavelength of 1 meter.

What will the signal look like to people on Earth? a. a signal with a wavelength less than 1 m b. a signal with a wavelength more than 1 m c. a signal moving faster than the speed of light d. a signal moving slower than the speed of light e. a signal with a wavelength of 1 m, moving the normal speed of light ANS: A DIF: Easy REF: Section 5.4 MSC: Applied TOP: 4Iii 32. You observe the spectrum of two stars. A section of these spectra are shown below. The spectra are different because star A is:

a. cooler than star B b. farther away from us than star B c. moving toward us faster than star B d. made of different elements than star B e. larger than star B ANS: C DIF: Medium REF: Section 5.4 MSC: Applied TOP: 4Iii

33. Which of these stars would have the biggest redshift? a. a star moving slowly toward you b. a star moving fast toward you c. a star moving slowly away from you d. a star moving fast away from you e. a star that is not moving away from you or toward you ANS: D DIF: Easy REF: Section 5.4 MSC: Factual TOP: 4Iii 34. You are driving on the freeway when a police officer records a shift of 7 nm when he or she hits you with a radar gun that operates at a wavelength of 0.1 m. How fast were you going? a. 43 mph b. 83 mph c. 21 mph d. 65 mph e. 47 mph ANS: E DIF: Medium REF: Section 5.4 MSC: Applied TOP: 4Iiii 35. You record the spectrum of a star and find that a calcium absorption line has an observed wavelength of 394.0 nm. This calcium absorption line has a rest wavelength is 393.3 nm; what is the radial velocity of this star? a. 5,000 km/s b. 500 km/s c. 50 km/s d. 5 km/s e. 0.5 km/s ANS: B DIF: Medium REF: Section 5.4 MSC: Applied TOP: 4Iiii

36. If you find that the H-alpha line in a star’s spectrum occurs at a wavelength of 656.45 nm, what is the star’s radial velocity? Note that the rest wavelength of this line is 656.30 nm. a. 150 km/s away from you b. 150 km/s toward you c. 350 km/s toward you d. 70 km/s away from you e. 70 km/s toward you ANS: D DIF: Difficult REF: Section 5.4 MSC: Applied TOP: 4Iiii 37. The spaceship shown in the figure below is traveling at a speed of 15,000 km/s to the left while it sends out a signal with a wavelength of 4 meters.

If astronomers living on planets A and B measure the radio waves coming from the space ship, what wavelengths will they measure? a. Planet A measures 6 meters, and planet B measures 2 meters. b. Planet A measures 2 meters, and planet B measures 6 meters. c. Planet A measures 4.2 meters, and planet B measures 3.8 meters. d. Planet A measures 3.8 meters, and Planet B measures 4.2 meters. e. Both Planet A and Planet B measure 4 meters.

ANS: D DIF: Difficult REF: Section 5.4 MSC: Applied TOP: 4Iiii 38. What does it mean to say that an object is in thermal equilibrium? a. It isn’t absorbing any energy. b. It isn’t radiating any energy. c. It is radiating more energy than it is absorbing. d. It is absorbing more energy than it is radiating. e. It is absorbing the same amount of energy that it is radiating.

ANS: E DIF: Easy REF: Section 5.5 MSC: Factual TOP: 5Iii

39. At what temperature does water freeze? a. 0 K b. 32 K c. 100 K d. 273 K e. 373 K ANS: D DIF: Medium REF: Section 5.5 MSC: Factual TOP: 5Iii 40. The Kelvin temperature scale is used in astronomy because: a. at 0 K an object has absolutely zero energy b. water freezes at 0 K c. water boils at 100 K d. hydrogen freezes at 0 K e. the highest temperature possible is 1000 K ANS: A DIF: Easy REF: Section 5.5 MSC: Factual TOP: 5IIi 41. As a blackbody becomes hotter, it also becomes _________ and _________. a. more luminous; redder b. more luminous; bluer c. less luminous; redder d. less luminous; bluer e. more luminous; stays the same color ANS: B DIF: Medium REF: Section 5.5 MSC: Factual TOP: 5IVi | 5Vi 42. Compare two blackbody objects, one at 200 K and one at 400 K. How much larger is the flux from the 400 K object, compared to the flux from the 200 K object? a. two times larger b. four times larger c. eight times larger d. sixteen times larger e. They have the same flux.

ANS: D DIF: Medium REF: Section 5.5 MSC: Applied TOP: 5IVi

43. Consider an incandescent light bulb. If you wanted to turn a 10 W light bulb into a 100 W light bulb, how would you change the temperature of the filament inside the bulb? a. Raise its temperature by a factor of 3.2. b. Raise its temperature by a factor of 1.8. c. Raise its temperature by a factor of 10. d. Lower its temperature by a factor of 2.6. e. Lower its temperature by a factor of 5.4.

ANS: B DIF: Difficult REF: Section 5.5 MSC: Applied TOP: 5IVi 44. You observe a red star and a blue star and are able to determine that they are the same size. Which star is hotter, and which star is more luminous? a. The red star is hotter and more luminous. b. The red star is hotter, and the blue star is more luminous. c. The blue star is hotter and more luminous. d. The blue star is hotter, and the red star is more luminous. e. They have the same luminosities and temperatures.

ANS: C DIF: Medium REF: Section 5.5 MSC: Factual TOP: 5IVi | 5Vi 45. At what wavelength does your body radiate the most given that your temperature is approximately that of the Earth, which is 300 K? a. 10 5 m b. 10 3 m c. 10 2 m d. 10 m e. 1,000 m ANS: A DIF: Medium REF: Section 5.5 MSC: Applied TOP: 5Vi

46. If a star has a peak wavelength of 290 nm, what is its surface temperature? a. 1000 K b. 2000 K c. 5000 K d. 10,000 K e. 100,000 K ANS: D DIF: Difficult REF: Section 5.5 MSC: Applied TOP: 5Vi Figure 1 47. If Jupiter has a temperature of 165 K, then at what wavelength does its spectrum peak? Use the electromagnetic spectrum in Figure 1 to answer this question. a. 18 nm—orange visible wavelengths b. 1,800 mm—microwave wavelengths c. 1,800 nm—infrared wavelengths d. 18,000 nm—ultraviolet wavelengths e. 18,000 nm—infrared wavelengths ANS: E DIF: Medium REF: Section 5.5 MSC: Applied TOP: 5Vi 48. If the typical temperature of a red giant is 3000 K, at what wavelength is its radiation the brightest? Use the electromagnetic spectrum in Figure 1 to help you answer this question. a. 1 m—infrared wavelengths b. 1 m—red visible wavelengths c. 20 m—infrared wavelengths d. 20 m—red visible wavelengths e. 700 nm—red visible wavelengths ANS: A DIF: Medium REF: Section 5.6 MSC: Applied TOP: 5Vi

49. Why do some stars in the sky appear blue, while other stars appear red? a. The red stars are hotter on their surfaces than the blue stars. b. The blue stars are hotter on their surfaces than the red stars. c. The blue stars are closer to us than the red stars. d. The red stars are closer to us than the blue stars. e. The blue stars are moving away from us faster than the red stars.

ANS: B DIF: Medium REF: Section 5.6 MSC: Applied TOP: 5Vi 50. Star A and star B appear equally bright in the sky. Star A is twice as far away from Earth as star B. How do the luminosities of stars A and B compare? a. Star A is twice as luminous as star B. b. Star B is twice as luminous as star A. c. Star A is four times as luminous as star B. d. Star B is four times as luminous as star A. e. Stars A and B have the same luminosity.

ANS: C DIF: Medium REF: Section 5.6 MSC: Applied TOP: 6Iii 51. Star C and star D have the same luminosity. Star C is twice as far away from Earth as star D. How do the brightnesses of stars C and D compare? a. Star C appears four times as bright as star D. b. Star C appears twice as bright as star D. c. Star D appears twice as bright as star C. d. Star D appears four times as bright as star C. e. Stars C and D appear equally bright.

ANS: D DIF: Medium REF: Section 5.6 MSC: Applied TOP: 6Iii

52. The average red giant in the night sky is about 1,000 times more luminous than the average mainsequence star. If both kinds of stars have about the same brightness, how much farther away are the red giants compared to the main-sequence stars? a. 32 times farther b. 1,000 times farther c. 65 times farther d. 5.6 times farther e. The red giants and main sequence stars have approximately the same distances.

ANS: A DIF: Difficult REF: Section 5.6 MSC: Applied TOP: 6Iii 53. A black car left in the sunlight becomes hotter than a white car left in the sunlight under the same conditions because: a. the white car absorbs more sunlight than the black car b. the white car reflects more sunlight than the black car c. the black car absorbs only blue photons and reflects red photons, whereas the white car absorbs only red photons and reflects blue photons d. the atoms in the black car are smaller than the atoms in the white car ANS: B DIF: Easy REF: Section 5.7 MSC: Applied TOP: 7Iii 54. Which of these planets would be expected to have the highest average temperature? a. a light-colored planet close to the Sun b. a dark-colored planet close to the Sun c. a light-colored planet far from the Sun d. a dark-colored planet far from the Sun e. There is not enough information to know which would be hotter.

ANS: B DIF: Medium REF: Section 5.7 MSC: Applied TOP: 7Iiii

55. Which of the following factors does NOT directly influence the temperature of a planet? a. the luminosity of the Sun b. the distance of the planet from the Sun c. the albedo of the planet d. the size of the planet e. the atmosphere of the planet ANS: D DIF: Easy REF: Section 5.7 MSC: Factual TOP: 7Iiii 56. An asteroid with an albedo of 0.1 and a comet with an albedo of 0.6 are orbiting at roughly the same distance from the Sun. How do their temperatures compare? a. They both have the same temperature. b. The comet is hotter than the asteroid. c. The asteroid is hotter than the comet. d. You must know their sizes to compare their temperatures. e. You must know their compositions to compare their temperatures.

ANS: C DIF: Medium REF: Section 5.7 MSC: Conceptual TOP: 7Iiii 57. If Saturn has a semimajor axis of 10 AU and an albedo of 0.7, what is its expected temperature? a. 130 K b. 15 K c. 35 K d. 170 K e. 65 K ANS: E DIF: Difficult REF: Section 5.7 MSC: Applied TOP: 7Iiii

SHORT ANSWER 1. Compare and contrast the wavelengths, frequencies, speeds, and energies of red and blue photons.

ANS: Red and blue photons both travel at the speed of light, which is 3  108 m/s. Red photons have longer wavelengths, lower frequencies, and lower energy levels than blue photons.

DIF: Easy REF: Section 5.2 MSC: Factual TOP: 1IIii | 2IIiii | 2IIIi

2. The difference in energy between the n  2 and n  1 electronic energy levels in the hydrogen atom is 1.6  10 18 J. If an electron moves from the n  1 level to the n  2 level, will a photon be emitted or absorbed? What will its energy be, and what type of electromagnetic radiation is it? Use the electromagnetic spectrum shown above to answer this question.

ANS: The n  2 energy level is higher than the n  1 energy level, so a photon with energy equal to 1.6  10 18 J must be absorbed to make this transition. Its wavelength is equal to   hc/E  (6.6  10 34 J s  3  108 m/s)/1.6 10 18 J  1.2  10 7 m  120 nm, which is in the ultraviolet region.

DIF: Difficult REF: Section 5.3 MSC: Applied TOP: 2IIIi | 3IIIi 3. Describe, in your own words, the fundamental problem with the Bohr model of the atom. (That is, why doesn’t an electron orbit a nucleus like a planet does the Sun?) ANS: In this model, the electron is constantly undergoing acceleration, and therefore would constantly be giving off electromagnetic radiation. This would cause the electron to quickly spiral into the nucleus.

DIF: Difficult REF: Section 5.3 MSC: Conceptual TOP: 3Iii | 3Iiv 4. The figure below shows an energy level diagram for a hydrogen atom, with the electron in the n  4 level.

If this atom emitted a photon, what energies could this photon have?

ANS: To emit a photon, the electron needs to drop to a lower level, and the energy of the photon will be equal to the difference between the two energy levels. So, an atom in this state could give of a photon with 0.6 eV, 2.5 eV or 12.7 eV of energy.

DIF: Easy REF: Section 5.3 MSC: Applied TOP: 3IIIi

5. Name four physical properties of an object that we can determine by analyzing the radiation that it emits, and briefly describe how these properties are determined. Cite the names of any laws that apply.

ANS: We can learn the following: (1) measure the spectrum, determine the wavelength where the most photons are emitted, and use Wein’s law to derive the temperature of the object; (2) measure the spectrum and determine the radial velocity of the object using the Doppler shift; (3) measure the spectrum and determine the chemical composition of the object from the absorption or emission lines it emits; and (4) measure the distance of an object by comparing its luminosity and brightness.

DIF: Difficult REF: Section 5.6 MSC: Factual TOP: 3IVii | 4Iii | 5Vi | 6Iii 6. Why do we see black lines in an absorption spectrum if the absorbed photons are (almost) instantaneously reemitted by the atoms in the cloud?

ANS: Originally all the light was traveling in the same direction, but absorbed photons, when reemitted, can be emitted in any direction. The number of photons emitted in the same direction they were originally traveling is just a small fraction of the total number of photons.

DIF: Difficult REF: Section 5.3 MSC: Factual TOP: 3Vi 7. How are atoms excited and why do they decay?

ANS: An atom becomes excited when an electron absorbs just the right amount of energy to allow it to jump up to a higher energy level. Although it is possible to cause an atom to decay by causing the electron to emit just the right amount of energy and jump back down to a lower level, most of the time this simply happens randomly and spontaneously.

DIF: Easy REF: Section 5.3 MSC: Conceptual TOP: 3Vi 8. Suppose you observe a star emitting a certain emission line of helium at 584.8 nm. The rest wavelength of this line is 587.6 nm. How fast is the star moving? Is it moving toward you or away from you?

ANS: The Doppler formula states that the radial velocity of an object is directly proportional to the shift of the spectral line it emits. The exact formula is . Plugging in values gives us νr  (584.8 nm

587.6 nm)

 3  105 km/s/ 587.6 nm  -1,430 km/s. The negative sign indicates that the emission line has been blue shifted, and thus the star is moving toward us.

DIF: Medium REF: Section 5.4 MSC: Applied TOP: 4Iii | 4Iiii

9. For a star that lies in the plane of Earth’s orbit around the Sun, how does the observed wavelength of the hydrogen absorption line at 656.28 nm in its spectrum change in wavelength (if at all) with the time of year?

ANS: The wavelength of this absorption line will get longer, then shorter, as Earth goes around the Sun and first moves toward the star, then sideways relative to it, then away from the star, sideways again, and toward the star again.

DIF: Easy REF: Section 5.4 MSC: Applied TOP: 4Iii 10. Imagine a satellite is orbiting a planet as shown in the image below. This satellite gives off radio waves with a constant wavelength of 1 meter. An observer on Earth then measures the signal from the satellite when it is at the locations labeled A–D. For each of these locations, how does the wavelength received compare to the wavelength that the satellite gave off?

ANS: A: Received signal will be exactly 1 meter, since the satellite is not moving toward or away from the Earth.

B: Received signal will be greater than 1 meter, since the satellite is moving away from the Earth.

C: Received signal will be exactly 1 meter, since the satellite is not moving toward or away from the Earth.

D: Received signal will be less than 1 meter, since the satellite is moving toward the Earth.

DIF: Difficult REF: Section 5.4 MSC: Applied TOP: 4Iii 11. A spaceship approaches Earth at 0.9 times the speed of light and shines a powerful searchlight onto Earth.

How fast will the photons from this searchlight be moving when they hit Earth?

ANS: At the speed of light, 3  105 km/s.

DIF: Easy REF: Section 5.4 MSC: Applied TOP: 4Iiii 12. How much would you have to change the temperature of an object if you wanted to increase its flux by a factor of 100?

ANS: Because flux is proportional to T4, you would have raised the object’s temperature by a factor of 1001⁄4  3.16.

DIF: Medium REF: Section 5.5 MSC: Applied TOP: 5IVi 13. Imagine you observed three different stars: a red star, a blue star, and a yellow star. You are able to determine that each of these stars has the same radius. Answer each question below and explain how you know.

A: Which star is hottest?

B: Which star is the most luminous?

C: Which star is the brightest?

ANS: A: The blue one is the hottest because Wien’s law says hotter objects radiate at shorter or bluer wavelengths.

B: The blue one is also the most luminous. The Stefan-Boltzmann law says that the hotter an object is, the larger the flux will be; thus the hottest star is also the most luminous because they all have the same radius.

C: You cannot tell from the information given. The brightness of a star depends on both luminosity and distance. Since you don’t know the distances to these stars, you can’t know which one is the brightest.

DIF: Difficult REF: Section 5.6 MSC: Applied TOP: 5IVi | 5Vi | 6Iii

14. Astronomers have now found a large number of exoplanets, which are planets that orbit around stars other than the Sun. Imagine astronomers found a planet identical to the Earth orbiting a star that had the same radius as the Sun, but with a temperature that is twice the temperature of the Sun. How far would this new planet need to be away from its star to have the same average temperature as the Earth?

ANS: In order to have the same average temperature as the Earth, the planet must receive the same brightness from its star that the Earth receives from the Sun. A star with a temperature two times that of the Sun would have 24  16 times the flux of the Sun. Since the two stars have the same radius, this new star’s luminosity would also be 16 times that of the Sun. Brightness is proportional to 1/distance2, so if this new planet was times further from its star than the Earth is from the Sun, it would have the same brightness as the Sun does when viewed from the Earth. So, this planet would need to be 4 AU from its star.

DIF: Difficult REF: Section 5.6 MSC: Applied TOP: 5IVi | 6Iii 15. If you want a blackbody’s peak wavelength to be cut in half, by how much do you need to increase its temperature?

ANS: Wien’s law states that peak(2,900,000 nm K)/T. Since we want to cut the peak wavelength in half, we need peak,new / peak,old  1⁄2. peak,new / peak,old  [(2,900,000 nm K)/Tnew]/ [(2,900,000 nm K)/Told]  Told / Tnew  1⁄2. So, the temperature must double to cut the peak wavelength in half.

DIF: Difficult REF: Section 5.5 MSC: Applied TOP: 5Vi 16. If you were driving down a deserted country road and you saw a light in the distance, what would you need to measure or know about it in order to calculate how far away it was?

ANS: You would need to know the light’s luminosity and measure its brightness.

DIF: Easy REF: Section 5.6 MSC: Conceptual TOP: 6Ii 17. Imagine you see a streetlamp that is 100 meters away from you and is 10,000 times more luminous than a firefly. How close would you have to be to the firefly to make it look as bright as the streetlamp?

ANS: For objects with equal brightness, L

d2, and the firefly has to be the square root of 10,000 or 100

times closer than the street lamp to have the same brightness. Thus, the firefly must be 1 meter away.

DIF: Difficult REF: Section 5.6 MSC: Applied TOP: 6Iii

18. How can the average temperature of the Earth stay approximately constant, even though the Earth is always getting energy from the Sun? ANS: The Earth is also giving off energy into space in the form of blackbody radiation. The Earth is in thermal equilibrium, so it gives off as much energy as it takes in, keeping the temperature constant.

DIF: Medium REF: Section 5.7 MSC: Applied TOP: 7Ii 19. What two factors control a planet’s surface temperature if it has no atmosphere?

ANS: A planet’s distance from the Sun and its albedo determine its temperature.

DIF: Easy REF: Section 5.7 MSC: Applied TOP: 7Iiii 20. What would you expect the temperature of a comet to be if its distance was 100 AU from the Sun? Assume that it is very icy and reflective so that its albedo is equal to 0.6. Does it matter what the radius of the comet is?

ANS: No, it does not matter what the radius of the comet is. As derived in Math Tools 5.4, the temperature of any object in the solar system will be equal to , where a is the albedo and dAU is the distance from the Sun measured in units of AU. Thus, the comet’s temperature is

DIF: Difficult REF: Section 5.7 MSC: Applied TOP: 7Iiii

CHAPTER 6

The Tools of the Astronomer

CONCEPT MAP Sec 6.1 1. Optical Telescopes I. Early History i. 13th century—lens (Latin for lentil) used to improve vision ii. 1608—Hans Lippershey (Netherlands) invents the telescope (TF: 1, MC: 1) iii. 1610—Galileo Galilei (Italy) uses the telescope to view the heavens (MC: 2) II. Refractors and Reflectors i. Refracting telescopes (MC: 3, 4, SA: 1–3) ii. Aperture, focal length, and magnification (TF: 2, MC: 5–7) iii. Light-gathering power

Area  D2/4 (TF: 2, MC: 6, 7, 8, SA: 4)

iv. Reflecting telescopes (MC: 4, 9, 10, SA: 5, 6) III. When Light Doesn’t Go Straight i. Refraction: index of refraction (MC: 11–13, SA: 7) ii. Blue wavelengths are refracted more than red wavelengths; leads to chromatic aberration (TF: 3, MC: 3, 12, 15, 16, SA: 7) iii. Reflection: angle of incidence  angle of reflection (MC: 17, 18, 19, SA: 5) iv. Dispersion (MC: 20, SA: 8) IV. Observatory Locations i. Want dry, high, away from light pollution and nearest the equator (SA: 9, 10) V. Optical and Atmospheric Limitations i. Resolution (MC: 21, 22, SA: 11, 12) ii. Diffraction limit: ➔  2.06  105(/D) arcsec (MC: 6, 23–28, 46, 47, SA: 12, 14)

iii. Astronomical seeing (TF: 4, MC: 21, SA: 13–15) iv. Adaptive optics (MC: 30, SA: 16) Sec 6.2 2. Optical Detectors and Instruments I. Human Eye i. Rods, cones, retina, pupil (MC: 31) ii. Integration time (human eye: 100 ms) (TF: 5, MC: 32, SA: 17) iii. Quantum efficiency (human eye QE  0.1) (TF: 5, MC: 3) II. Photographic Plates i. Emulsion, developing, fragile glass plates (MC: 34) ii. QE  1 to 3 percent, long exposure times possible; detects fainter objects (MC: 35, SA: 17) iii. Nonlinearity (MC: 36) III. Charged-Coupled Devices i. Charge-coupled device (CCD): ultrathin wafer of silicon, 2D array of pixels, cooled to reduce electronic noise (MC: 33, 34, SA: 9, 16, 17) ii. Linear response (MC: 37, SA: 9, 17) iii. High quantum efficiency (80 percent or higher) for visible and near-infrared light (MC: 37, SA: 9, 18) iv. Large format (100 million pixels) (MC: 37, SA: 9, 18) v. Digital output (MC: 37, SA: 9, 18) IV. Spectrographs i. Spectrograph/spectrometer (MC: 38) ii. Prism versus diffraction grating (TF: 7, MC: 38) iii. Interference (MC: 39, 40) iv. Spectral dispersion/diffraction (MC: 37, SA: 8) Sec 6.3 3. Radio and Infrared Telescopes (SA: 14) I. Atmospheric Windows i. Visible, radio (MC: 40, SA: 10) ii. EM radiation—ultraviolet, X-rays, infrared—absorbed by Earth’s atmosphere (MC: 41–43, SA: 10) II. Radio Telescopes i. 1930s—Karl Jansky (Bell Labs, USA)—discovers radio emission from the galactic center; birth of

radio astronomy (MC: 44) ii. Late 1930s—Grote Reber (USA) leads development of radio astronomy (MC: 26, SA: 19) iii. Large radio “dishes,” e.g., Arecibo, and arrays, e.g., very large array (TF: 8, MC: 45) iv. Interferometer: D  distance between two distant telescopes (MC: 28, 46, 47, SA: 12) III. Infrared Telescopes i. Water vapor absorbs infrared photons (MC: 48, 49) ii. Examples: Kuiper Airborne Observatory/SOFIA (MC: 49) Sec 6.4 4. Getting above the Earth’s Atmosphere: Orbiting Observatories I. Access to EM Wavelengths Absorbed by Earth’s Atmosphere i. Examples: telescopes on rockets and satellites; great observatories: Hubble Space Telescope (HST), Chandra and Spitzer Observatories (SA: 10, 19) ii. Disadvantages: cost, repair (MC: 50, SA: 20) Sec 6.5 5. Getting Up Close with Planetary Spacecraft (SA: 14) I. Satellites in Earth’s Orbit i. Remote sensing (MC: 51) II. Planetary Exploration i. Flyby missions (TF: 9, MC: 52, SA: 19, 20) ii. Orbiters mapping a planet’s surface yield the highest spatial resolution images (TF: 9, MC: 53, SA: 21) iii. Landers, rovers and atmospheric probes (MC: 54) iv. Sample returns (MC: 55) Sec 6.6 6. Other Astronomical Tools (SA: 14) I. Experiments and Simulations i. Particle accelerators, e.g., LHC (TF: 10, MC: 56, SA: 19) ii. Neutrinos (TF: 11, MC: 57, 58, SA: 19) iii. Gravity waves (MC: 59, SA: 19, 21) iv. High-speed computers—essential to analyzing and interpreting observations (MC: 60, SA: 19)

Sec 6.7 7. Origins: Microwave Telescopes That Detect Radiation from the Big Bang I. History i. Penzias and Wilson discover the CMB (MC: 56, 61) ii. Missions: RELIKT-1, COBE, BOOMERANG, WMAP (MC: 61)

TRUE/FALSE 1. Hans Lippershey invented the telescope in the early 1600s.

ANS: T DIF: Easy REF: Section 6.1 MSC: Factual TOP: 1Iii 2. Point sources look brighter when viewed through a telescope.

ANS: T DIF: Easy REF: Section 6.1 MSC: Conceptual TOP: 1IIii | 1IIiii 3. The path of red light is deflected by a larger angle than blue light when passing through a glass prism.

ANS: F DIF: Medium REF: Section 6.1 MSC: Applied TOP: 1IIIii 4. The image quality of most optical telescopes is limited by atmospheric refraction experienced by light as it passes through the Earth’s atmosphere.

ANS: T DIF: Medium REF: Section 6.1 MSC: Factual TOP: 1Viii 5. How faint an object we can see with our eyes is limited only by the integration time of our eye (how often it processes input).

ANS: F DIF: Medium REF: Section 6.2 MSC: Factual TOP: 2Iii | 2Iiii 6. Astronomers usually study distant galaxies by looking at them directly through the eyepiece of a telescope.

ANS: F DIF: Easy REF: Section 6.2 MSC: Factual TOP: 2IIIi 7. The dispersing element in a spectrograph could be a prism or a diffraction grating.

ANS: T DIF: Easy REF: Section 6.2 MSC: Factual TOP: 2IVii 8. The 305-m Arecibo radio telescope has a resolution that is no better than that of the human eye.

ANS: T DIF: Easy REF: Section 6.3 MSC: Factual TOP: 3IIiii 9. The highest spatial resolution images that we have of many planets’ surfaces were obtained with the Hubble Space Telescope.

ANS: F DIF: Medium REF: Section 6.5 MSC: Factual TOP: 5IIi 10. Understanding the results of high-energy collisions between elementary particles can give astronomers insight into conditions that were present during the earliest moments of the universe.

ANS: T DIF: Easy REF: Section 6.6 MSC: Conceptual TOP: 6Ii

11. Unlike photons, neutrinos are relatively easy to detect.

ANS: F DIF: Medium REF: Section 6.6 MSC: Factual TOP: 6Iii

MULTIPLE CHOICE 1. The telescope was invented by: a. Galileo Galilei, an Italian inventor b. Hans Lippershey, an eyeglass maker in the Netherlands c. Gote Reber, a German cabinetmaker d. Tycho Brahe, a Danish astronomer e. Johannes Kepler, a German astronomer ANS: B DIF: Easy REF: Section 6.1 MSC: Factual TOP: 1Iii 2. Which of the following was NOT discovered by Galileo using a telescope? a. The Moon has a heavily cratered surface. b. Jupiter has four moons that orbit around it. c. Mars has a polar ice cap similar to the Earth. d. The planet Venus goes through phases similar to those of the Moon. e. The Milky Way is a collection of countless numbers of individual stars.

ANS: C DIF: Easy REF: Section 6.1 MSC: Factual TOP: 1Iiii 3. Why can a compound lens combat a refracting telescope’s chromatic aberration? a. Red light is absorbed by a larger amount than blue light. b. Red light is refracted by a larger amount than blue light. c. Blue light is refracted by a larger amount than red light. d. Blue light is absorbed by a larger amount than red light. e. A compound lens cannot combat chromatic aberration.

ANS: C DIF: Medium REF: Connections 6.1 MSC: Conceptual TOP: 1IIi | 1IIIii

4. One reason to prefer a reflecting over a refracting telescope is: a. its lack of chromatic aberration b. its shorter length for the same aperture size c. there is no aperture limit d. its lighter weight for larger apertures e. all of the above are valid reasons ANS: E DIF: Easy REF: Section 6.1 MSC: Factual TOP: 1IIi |1IIiv 5. The magnification of a telescope depends on the focal length of the telescope and: a. the size of the aperture b. the type of telescope (refracting vs. reflecting) c. the wavelengths being observed d. the focal length of the eyepiece e. the angular resolution of the telescope ANS: D DIF: Easy REF: Section 6.1 MSC: Factual TOP: 1IIii 6. The aperture of a telescope partially or totally determines its: a. focal length and magnification b. light-gathering power c. focal length d. light-gathering power and magnification e. light-gathering power and diffraction limit ANS: E DIF: Medium REF: Section 6.1 MSC: Factual TOP: 1IIii | 1IIiii | 1Vii 7. Which telescope would collect 100 times more light than a 1-meter telescope? a. 100-meter telescope b. 80-meter telescope c. 50-meter telescope d. 30-meter telescope e. 10-meter telescope ANS: E DIF: Medium REF: Section 6.1 MSC: Applied TOP: 1IIiii

8. Why can you see fainter stars with an 8-inch telescope than you can see with your naked eye? a. The telescope collects light over a larger area. b. The telescope magnifies the field of view. c. The telescope collects light over a wider range of wavelength than your eye. d. The telescope has a wider field of view. e. The telescope has a longer integration time than your eyes.

ANS: A DIF: Easy REF: Section 6.2 MSC: Conceptual TOP: 1IIiii 9. Large reflecting telescopes have mirrors that are _________ in shape. a. spherical b. parabolic c. convex d. hyperbolic e. cylindrical ANS: B DIF: Easy REF: Section 6.1 MSC: Factual TOP: 1IIiv 10. Why do reflecting telescopes usually have a secondary mirror in addition to a primary mirror? a. to increase the light-gathering power b. to make the telescope shorter c. to increase the magnification d. to increase the focal length e. to combat chromatic aberration ANS: B DIF: Medium REF: Section 6.1 MSC: Conceptual TOP: 1IIiv 11. A beam of light passes from air to water at an incident angle of 40°, relative to a plane perpendicular to the boundary between the two. At what angle will it emerge into the water, relative to a plane perpendicular to the boundary? a. less than 40° b. exactly 40° c. more than 40° d. The beam of light does not emerge from the water. e. There is not enough information to answer the question.

ANS: A DIF: Easy REF: Section 6.1 MSC: Applied TOP: 1IIIi

12. As a beam of light travels from one medium to another, the change in direction of the beam of light depends on: a. the wavelength of the light b. the index of refraction of the outgoing medium c. the index of refraction of the incoming medium d. the angle of incidence e. all of the above ANS: E DIF: Medium REF: Section 6.1 MSC: Conceptual TOP: 1IIIi | 1IIIii 13. Which of the following phenomena is shown in the figure below?

a. reflection b. refraction c. magnification d. diffraction e. interference ANS: B DIF: Easy REF: Section 6.1 MSC: Applied TOP: 1IIIi

14. A prism is able to spread white light out into a spectrum of colors based on the property of: a. reflection b. refraction c. magnification d. resolution e. aberration ANS: B DIF: Medium REF: Section 6.1 MSC: Conceptual TOP: 1IIIii 15. Which of the following phenomena is shown in the figure below?

a. reflection b. chromatic aberration c. diffraction d. magnification e. interference ANS: B DIF: Medium REF: Section 6.1 MSC: Applied TOP: 1IIIii 16. Chromatic aberration results from: a. blue light being reflected more than red light b. red light being reflected more than blue light c. red light being refracted more than blue light d. blue light being refracted more than red light e. a lens being polished incorrectly ANS: D DIF: Medium REF: Section 6.1 MSC: Conceptual TOP: 1IIIii

17. Which of the following phenomena is shown in the figure below?

a. reflection b. refraction c. magnification d. diffraction e. interference ANS: A DIF: Easy REF: Section 6.1 MSC: Applied TOP: 1IIIiii 18. According to the law of reflection, if a beam of light strikes a flat mirror at an angle of 30° relative to a plane perpendicular to the surface of the mirror, at what angle will it reflect, relative to a plane perpendicular to the surface of the mirror? a. 0° b. 30° c. 60° d. 90° e. 120° ANS: B DIF: Medium REF: Section 6.1 MSC: Applied TOP: 1IIIiii 19. An object sits at the focal point of a parabolic mirror. At what distance from the mirror will its image be created? a. It will be imaged at half the focal length. b. It will be imaged at the focal length. c. It will be imaged at twice the focal length. d. No image will be created (the beams would be reflected parallel to each other). e. The image is created on the other side of the mirror.

ANS: D DIF: Difficult REF: Section 6.1 MSC: Applied TOP: 1IIIiii

20. Which property of light is responsible for chromatic aberration? a. reflection b. interference c. dispersion d. diffraction e. magnification ANS: C DIF: Medium REF: Section 6.1 MSC: Factual TOP: 1IIIiv 21. In practice, the smallest angular size that one can resolve with a 10-inch telescope is governed by the: a. blurring caused by the Earth’s atmosphere b. diffraction limit of the telescope c. size of the primary mirror d. motion of the night sky e. magnification of the telescope ANS: A DIF: Difficult REF: Section 6.1 MSC: Factual TOP: 1Vi | 1Viii 22. How does the resolution of a telescope depend on its focal length? a. The longer the focal length, the better the resolution. b. The longer the focal length, the worse the resolution. c. There is no relation between resolution and focal length.

ANS: A DIF: Medium REF: Section 6.1 MSC: Applied TOP: 1Vi

23. Which of the following phenomena is shown in the figure below?

a. reflection b. chromatic aberration c. diffraction d. magnification e. interference ANS: C DIF: Medium REF: Section 6.1 MSC: Applied TOP: 1Vii 24. The diffraction limit of a 4-meter telescope is _________ than that of a 2-meter telescope. a. two times larger b. four times larger c. four times smaller d. two times smaller e. It depends on the type of telescope.

ANS: D DIF: Medium REF: Section 6.1 MSC: Applied TOP: 1Vii 25. The angular resolution of the largest single-dish radio telescope in the United States, the 100-m Green Bank Telescope, is _________ when it operates at a wavelength of 20 cm. a. 41 arcmin b. 6.8 arcmin c. 4.1 arcmin d. 6.8 arcsec e. 4.1 arcsec ANS: B DIF: Difficult REF: Section 6.1 MSC: Applied TOP: 1Vii

26. Grote Reber conducted the first radio survey of the sky in the 1930s and 1940s with his 9-meter diameter radio telescope. Why did his telescope need to be so large? a. He needed a large light-collecting area because radio sources are notoriously dim. b. He needed better angular resolution to identify sources because radio waves are so long. c. He needed a higher magnification to identify sources because radio sources are quite small. d. He needed a longer focal length since radio sources are so far away. e. He needed a shorter focal length since radio sources are so far away.

ANS: B DIF: Medium REF: Section 6.1 | Section 6.3 MSC: Conceptual TOP: 1Vii | 3IIii 27. The SETI project’s Allen Telescope Array will have 350 radio dishes, each with an individual diameter of 6 m, spread out over a circle whose diameter is 1 km. What would this array’s spatial resolution be when it operates at 6,000 MHz? a. 10 arcsec b. 0.10 arcsec c. 1 arcsec d. 10 arcmin e. 1.0 arcmin ANS: A DIF: Difficult REF: Section 6.1 | Section 6.3 MSC: Applied TOP: 1Vii | 3IIiv 28. The two Keck 10-m telescopes, separated by a distance of 85 m, can operate as an optical interferometer.

What is its resolution when it observes in the infrared at a wavelength of 2 microns? a. 0.01 arcsec b. 0.005 arcsec c. 0.4 arsec d. 0.06 arcsec e. 0.2 arcsec ANS: B DIF: Difficult REF: Section 6.1 | Section 6.3 MSC: Applied TOP: 1Vii | 3IIiv

29. The angular resolution of a ground-based telescope (without adaptive optics) is typically: a. 30 arcsec b. 1 arcmin c. 10 arcsec d. 1 arcsec e. 30 arcmin ANS: D DIF: Easy REF: Section 6.1 MSC: Factual TOP: 1Viii 30. Cameras that use adaptive optics provide higher spatial resolution images primarily because: a. they operate above the Earth’s atmosphere b. they capture infrared light, which has a longer wavelength than visible light c. deformable mirrors are used to correct the blurring due to the Earth’s atmosphere d. composite lenses correct for chromatic aberration e. they simulate a much larger telescope ANS: C DIF: Medium REF: Section 6.1 MSC: Conceptual TOP: 1Viv 31. What part(s) of the human eye is responsible for detecting light? a. cornea b. lens c. pupil d. rods and cones e. iris ANS: D DIF: Easy REF: Section 6.2 MSC: Factual TOP: 2Ii 32. Typically, video is shot using 24 to 30 frames per second (one frame each 33 to 42 ms). If a filmmaker shot new experimental video at 100 frames per second (one frame each 1 ms), how would it look during playback to the human eye? a. It would look like the video was being fast-forwarded. b. It would look like the video was about the same as normal video. c. It would look like the video was being played back in slow motion. d. It would look like a slideshow, a series of pictures on the screen each for a perceptible amount of time. e. It would look like the video was about the same speed as normal video, but blurry.

ANS: B DIF: Difficult REF: Section 6.2 MSC: Conceptual TOP: 2Iii

33. If we could increase the quantum efficiency of the human eye, it would: a. allow humans to see a larger range of wavelengths b. allow humans to see better at night, or other low-light conditions c. increase the resolution of the human eye d. decrease the resolution of the human eye e. not make a difference in the sight of the human eye ANS: B DIF: Medium REF: Section 6.2 MSC: Conceptual TOP: 2Iiii 34. Before CCDs were invented, what was the device most commonly used for imaging with optical telescopes? a. Polaroid cameras b. photographic glass plates c. 35-mm film d. high-speed film e. video cameras ANS: B DIF: Easy REF: Section 6.2 MSC: Factual TOP: 2IIi 35. Photography provides an improvement over naked-eye observations because: a. it is possible to observe a larger field of view with photographic plates b. the quantum efficiency is higher for photographic plates c. the image resolution is much better for photographic plates d. it is possible to detect fainter objects with the use of photographic plates e. the integration time is much shorter with the use of photographic plates ANS: D DIF: Medium REF: Section 6.2 MSC: Conceptual TOP: 2IIii 36. You are observing the Andromeda Galaxy using both photographic plates and a CCD. If you double the exposure time for both detectors, you: a. double the amount of light collected on both the photographic plate and the CCD b. double the amount of light collected on the CCD, but the photographic plate collects less c. double the amount of light collected on the photographic plate, but the CCD collects less d. double the amount of light collected on the photographic plate, but the CCD collects more e. collect less than twice the amount of light on both the photographic plate and the CCD ANS: B DIF: Medium REF: Section 6.2 MSC: Conceptual TOP: 2IIiii | 2IIIii

37. The major advantage CCDs have over other imaging techniques is that: a. they have a higher quantum efficiency b. they have a linear response to light c. they yield output in digital format d. they operate at visible and near-infrared wavelengths e. All of the above are true.

ANS: E DIF: Easy REF: Section 6.2 MSC: Applied TOP: 2IIIi | 2IIIii | 2IIIiii | 2IIIiv | 2IIIv 38. Most modern spectrographs use a _________ to disperse the light from an object. a. spherical mirror b. lens c. glass prism d. diffraction grating e. parabolic mirror ANS: D DIF: Medium REF: Section 6.2 MSC: Factual TOP: 2IVi | 2IVii 39. Which of the following phenomena is shown in the figure below?

a. reflection b. refraction c. chromatic aberration d. magnification e. interference ANS: E DIF: Medium REF: Section 6.2 MSC: Applied TOP: 2IViii

40. What property of light allows a grating to disperse the light from an object into a spectrum? a. interference b. reflection c. refraction d. aberration e. magnification ANS: A DIF: Medium REF: Section 6.2 MSC: Conceptual TOP: 2IViii | 2IViv 41. Astronomers can use ground-based telescopes to observe in the majority of which of the following parts of the electromagnetic spectrum? a. visible and infrared b. visible and ultraviolet c. visible and radio d. visible, ultraviolet, and infrared e. visible, infrared, and radio ANS: C DIF: Easy REF: Section 6.3 MSC: Factual TOP: 3Ii | 3Iii 42. Which of the following is NOT a reason to put a telescope in space? a. to observe at wavelengths blocked by the Earth’s atmosphere b. to avoid light pollution on Earth c. to avoid weather on Earth d. to avoid atmospheric distortion e. to get closer to the stars ANS: E DIF: Medium REF: Section 6.3 MSC: Conceptual TOP: 3Iii 43. Ultraviolet radiation is hard to observe primarily because: a. the Earth’s atmosphere easily absorbs it b. no space-based telescopes operate at ultraviolet wavelengths c. only the lowest mass stars emit ultraviolet light d. very few objects emit at ultraviolet wavelengths e. the Earth emits too much ultraviolet background light ANS: A DIF: Medium REF: Section 6.3 MSC: Factual TOP: 3Iii

44. The first astronomical radio source ever observed was: a. the Andromeda Galaxy b. the galactic center, in the constellation Sagittarius c. thunderstorms d. the Earth e. Jupiter ANS: B DIF: Medium REF: Section 6.3 MSC: Factual TOP: 3IIi 45. The 305-meter Arecibo radio telescope in Puerto Rico has a resolution that is closest to that of: a. the Hubble Space Telescope (0.1 arcsec) b. a human eye (1 arcmin) c. the Chandra X-ray telescope (0.5 arcsec) d. a 1-meter optical telescope (1 arcsec) e. one of the 10-meter Keck telescopes (0.0133 arcsec) ANS: B DIF: Difficult REF: Section 6.3 MSC: Factual TOP: 3IIiii 46. Arrays of radio telescopes can produce much better resolution than single-dish telescopes because they work based on the principle of: a. reflection b. refraction c. dispersion d. diffraction e. interference ANS: E DIF: Easy REF: Section 6.3 MSC: Conceptual TOP: 3IIiv 47. When we determine the angular resolution of an interferometric array of radio telescopes using the formula

➔

/D, the variable D stands for the:

a. diameter of the telescopes b. separation between the telescopes c. magnification of the telescopes d. number of telescopes e. focal length of the telescopes ANS: B DIF: Easy REF: Section 6.3 MSC: Conceptual TOP: 3IIiv

48. Water vapor in Earth’s atmosphere primarily easily absorbs which type of photons? a. radio b. infrared c. visible d. ultraviolet e. X-ray ANS: B DIF: Easy REF: Section 6.3 MSC: Factual TOP: 3IIIi 49. NASA’s Kuiper Airborne Observatory and SOFIA are two examples of telescopes placed in high-flying aircraft. Why would astronomers put telescopes in airplanes? a. to get the telescopes closer to the stars b. to get the telescopes away from the light-pollution of cities c. to get the telescopes above the majority of the water vapor in the earth’s atmosphere d. to be able to observe one object for more than 24 hours without stopping. e. to allow the telescopes to observe the full spectrum of light ANS: C DIF: Medium REF: Section 6.3 MSC: Conceptual TOP: 3IIIi | 3IIIii 50. Which of the following is the biggest disadvantage of putting a telescope in space? a. Astronomers don’t have as much control in choosing what to observe. b. Astronomers have to wait until the telescopes come back to Earth to get their images. c. Space telescopes can only observe in certain parts of the electromagnetic spectrum. d. Space telescopes don’t last long before they fall back down to Earth. e. Space telescopes are much more expensive than similar ground-based telescopes.

ANS: E DIF: Medium REF: Section 6.4 MSC: Conceptual TOP: 4Iii 51. Remote sensing instruments have been used to: a. map surfaces hidden beneath thick atmospheres b. measure the composition of atmospheres c. identify geological features d. watch weather patterns develop e. all of the above ANS: E DIF: Medium REF: Section 6.5 MSC: Factual TOP: 5Ii

52. The Voyager I spacecraft is currently 18 billion km from Earth and heading out of our Solar System. How long does it take radio messages from Voyager I to reach us? a. 1.7 days b. 17 hours c. 17 days d. 17 weeks e. 17 minutes ANS: B DIF: Medium REF: Section 6.5 MSC: Applied TOP: 5IIi 53. In 2008, the Cassini spacecraft made a flyby of Enceladus, one of the icy moons of Saturn. If the spacecraft’s high resolution camera had an angular resolution of 3 arcsec, and it flew at an altitude of 23 km above Enceladus’s surface, how large an object could be resolved on the surface? a. 3 m b. 30 cm c. 30 km d. 5 cm e. 50 m ANS: B DIF: Difficult REF: Section 6.5 MSC: Applied TOP: 5IIii 54. Landers, rovers, and/or atmospheric probes have visited which object(s) listed below in an effort to gain new information about our Solar System? a. Jupiter b. Titan, Saturn’s moon c. Mars d. Eros, an asteroid e. all of the above ANS: E DIF: Medium REF: Section 6.5 MSC: Factual TOP: 5IIiii

55. Samples of which celestial object(s) have been brought back to Earth to be studied in detail? a. a comet b. the solar wind c. an asteroid d. the Moon e. all of the above ANS: E DIF: Easy REF: Section 6.5 MSC: Factual TOP: 5IIiv 56. Particle accelerators that smash atoms or particles together at high speeds, such as the LHC, are important tools used for simulating conditions in: a. the early universe b. the solar wind c. red giants d. brown dwarf stars e. planetary nebula ANS: A DIF: Easy REF: Section 6.6 | Section 6.7 MSC: Conceptual TOP: 6Ii | 7Ii 57. Neutrino detectors typically capture one out of every _________ neutrinos that pass through them. a. 10 b. 106 (one million) c. 109 (one billion) d. 1012 (one trillion) e. 1022 (10 billion trillion) ANS: E DIF: Difficult REF: Section 6.6 MSC: Factual TOP: 6Iii 58. Which of the following cannot be directly detected using a telescope? a. X-rays b. visible light c. infrared light d. neutrinos e. ultraviolet light ANS: D DIF: Easy REF: Section 6.6 MSC: Factual TOP: 6Iii

59. What type of waves have NOT yet been directly detected by astronomers? a. sound waves b. gravitational waves c. X-ray waves d. gamma-ray waves e. pressure waves ANS: B DIF: Easy REF: Section 6.6 MSC: Factual TOP: 6Iiii 60. High-speed computers have become one of an astronomer’s most important tools. Which of the following does NOT require the use of a high-speed computer? a. analyzing images taken with very large CCDs b. generating and testing theoretical models c. moving a telescope from object to object d. studying the evolution of astronomical objects or systems over time e. correcting for atmospheric distortion ANS: C DIF: Medium REF: Section 6.6 MSC: Conceptual TOP: 6Iiv 61. Telescopes and satellites such as COBE, WMAP, and Planck are designed to detect microwave radiation emitted by: a. galaxies b. black holes c. planets d. the Big Bang e. stars ANS: D DIF: Easy REF: Section 6.7 MSC: Factual TOP: 7Ii | 7Iii

SHORT ANSWER 1. Explain why chromatic aberration is a problem for refracting mirrors but not reflecting mirrors.

ANS: Chromatic aberration occurs because refractors suffer dispersion caused the fact that the index of refraction of the lens depends on the wavelength of light going through it. The result is that different wavelengths of light will focus at different distances from a lens. Because the law of reflection holds for any wavelength, mirrors focus all wavelengths of light to the same focal point.

DIF: Medium REF: Section 6.1 MSC: Conceptual TOP: 1IIi 2. Label the eyepiece, lens, focus, and focal length of the telescope below.

ANS: This telescope is a refracting telescope. A student should label the eyepiece as the lens near the eye, the focus at the point where the light rays cross, the lens as the piece that initially bends the light from the stars, and the focal length as the distance between the lens and the focus.

DIF: Medium REF: Section 6.1 MSC: Factual TOP: 1IIi 3. Explain why the largest telescopes are not refracting telescopes.

ANS: The larger the refracting telescope, the heavier the lens. If the lens is too massive, it will sag under the force of gravity and the image will be distorted.

DIF: Easy REF: Section 6.1 MSC: Conceptual TOP: 1IIi 4. How much larger is the light-gathering power of a 10-inch telescope than the human eye?

ANS: Light-gathering power is proportional to the area of the aperture, which is proportional to the square of the diameter of the aperture. Thus, the light-gathering power of a 10-inch telescope is X times greater than your eye, where X  (10 inch  2.54 cm/1 in)2/(1 cm)2  645.

DIF: Easy REF: Section 6.1 MSC: Applied TOP: 1IIiii 5. Why do reflecting telescopes use curved mirrors instead of flat mirrors?

ANS: The purpose of a telescope is to redirect parallel beams of light from a distant object to converge at a point. A flat mirror would simply redirect them all at the same angle, and they would therefore still travel parallel to each other. A curved mirror reflects the different rays through different angles, so that they all converge at a common focal point.

DIF: Easy REF: Section 6.1 MSC: Conceptual TOP: 1IIiv |1IIIiii

6. Label the eyepiece, primary mirror, secondary mirror, focus, and focal length of the telescope below.

ANS: This telescope is a reflecting telescope. A student should label the eyepiece as the lens near the eye, the focus at the point where the light rays cross, the primary mirror as the curved piece that initially reflects the light from the stars, the secondary mirror as the flat piece that reflects the light from the primary mirror to the eyepiece, and the focal length as the distance between the primary mirror and the focus. (NOTE: In this case, the focal length is not measured in a straight line!)

DIF: Difficult REF: Section 6.1 MSC: Factual TOP: 1IIiv 7. Explain what happens when white light is refracted by a prism.

ANS: When white light is refracted by a prism, the light is bent due to the change in the speed of light in the medium and the incident angle. The amount of bending, or the angle of refraction, also depends on the wavelength of light. For this reason, the white light that enters the prism exits as a rainbow of color.

Blue light is refracted the most, while red light is refracted the least.

DIF: Difficult REF: Section 6.1 MSC: Conceptual TOP: 1IIIi | 1IIIii 8. Explain the difference between dispersion and diffraction. How can both phenomena be used to create a spectrum?

ANS: Dispersion is the wavelength dependence in refraction. Since blue light refracts more than red light, any white light that is refracted through a medium is dispersed into its spectral colors. Diffraction is distortion of a wavefront as it passes the edge of an opaque object. This is also wavelength dependent and can create a spectrum as white light passes through a pair of (or many) narrow slits. The resulting pattern is a mix of constructive and destructive interference patterns. Each wavelength will have its first maxima at a different location along the viewing screen, therefore showing a full spectrum.

DIF: Difficult REF: Section 6.2 MSC: Conceptual TOP: 1IIIiv | 2IViv

9. Where is the best place to put a ground-based optical telescope? Discuss the reasons for your selection.

ANS: Mountaintops away from cities and in dry climates. This location gets your telescope away from light pollution, as high above the atmosphere and water vapor as the Earth’s surface can get. Also, finding a location that fits the above requirements near the equator means you will be able to view the entire sky over the course of the year.

DIF: Medium REF: Section 6.1 MSC: Conceptual TOP: 1IVi 10. Name two reasons why astronomers might use a space telescope over a ground-based telescope.

ANS: (1) To observe at wavelengths other than visible and radio waves.

(2) To avoid dealing with atmospheric distortion.

(3) To avoid light pollution on Earth.

(4) To avoid weather on Earth.

DIF: Medium REF: Section 6.1 | Section 6.3 | Section 6.4 MSC: Factual TOP: 1IVi | 1Viii | 3Ii | 3Iii | 4Ii 11. In 2009, the Cassini spacecraft made repeated orbits around Titan, Saturn’s largest moon. If this spacecraft orbited at an altitude of 1,000 km above Titan’s surface and its high-resolution camera had an angular resolution of 3 arcsec, how large an object could be resolved on Titan’s surface?

ANS: The small angle approximation says ➔  206, 265 arcsec  D/d, where ➔ is the angular resolution of the camera, D is the diameter of the smallest resolvable surface feature on Titan, and d is the altitude of the spacecraft. Therefore, the smallest resolvable surface feature on Titan is D  (➔/206,265 arcsec)  d  (3 arcsec/206, 265 arcsec)  106 m  15 m.

DIF: Difficult REF: Section 6.1 MSC: Applied TOP: 1Vi 12. Calculate the resolution of an interferometric array consisting of five 10-m radio telescopes, each located 1,000 meters apart from each other and observing a distant object at a wavelength of 21 cm.

ANS: ➔  2.06  105  (/D) where   wavelength and D  dish separation.

➔  2.06  105  (/D) arcsec  43 arcsec.

DIF: Difficult REF: Section 6.1 | Section 6.3 MSC: Applied TOP: 1Vi | 3IIiv 13. What is the diffraction limit of a 4-m telescope observing at a wavelength of 650 nm?

ANS: ➔  2.06  105  (6.50  107 m/4m) arcsec  0.033 arcsec.

DIF: Difficult REF: Section 6.1 MSC: Applied TOP: 1Vii

14. What is the angular resolution of a 1-m, ground-based, optical telescope that observes at a wavelength of 600 nm compared to that of a 300-ft, single-dish radio telescope that observes at a wavelength of 21 cm?

ANS: The angular resolution of the 1-m, ground-based telescope is limited by the atmosphere to be approximately 1 arcsec. The angular resolution of the radio dish is given by its diffraction limit, which is ➔  2.06  105  (21 cm/(300  12 in  2.54 cm/1 in)) arcsec  473 arcsec. Therefore, the angular resolution of the optical telescope is about 500 times smaller than that of the radio telescope.

DIF: Difficult REF: Section 6.1 MSC: Applied TOP: 1Vii | 1Viii 15. Explain why stars twinkle when viewed from the ground. Would they twinkle if viewed from outer space?

ANS: Slight differences in air temperature cause light to refract slightly as it passes through different temperature regions. Atmospheric turbulence causes these regions to move over time, so two different beams of light will take slightly different paths over time. This causes a shimmering of objects viewed through Earth’s atmosphere. For telescopes like the Hubble Space Telescope, which lie above Earth’s atmosphere, this does not occur.

DIF: Easy REF: Section 6.1 MSC: Conceptual TOP: 1Viii 16. Explain how adaptive optics help compensate for atmospheric seeing.

ANS: Slight perturbances in the atmosphere can degrade the resolution of an image. Adaptive optics can measure these perturbances and correct for them before the light is imaged by bouncing it off a deformable mirror.

DIF: Medium REF: Section 6.1 MSC: Conceptual TOP: 1Viv 17. Why is it difficult to view low surface brightness objects, such as the Andromeda Galaxy, with the naked eye? Does the view improve with the use of a telescope? What is needed to get a bright, clear view of the Andromeda Galaxy, as commonly seen in pictures?

ANS: The human eye has a low integration time. Using a telescope may increase the light-collecting area, but low surface brightness objects will still look dim. In order to get bright, clear images of such objects, photographic plates or CCDs must be used. With these detectors, the integration time can be increased, allowing more light to be collected for one image.

DIF: Medium REF: Section 6.2 MSC: Conceptual TOP: 2Iii | 2IIii | 2III 18. Explain three major advantages of CCDs over other imaging techniques.

ANS: These are possible answers:

(1) They have much higher quantum efficiency (80 percent).

(2) Their photometric response is linearly proportional to the number of photons they collect.

(3) They yield output in digital format.

(4) They cover a wide spectral range (optical through near-infrared).

DIF: Easy REF: Section 6.2 MSC: Factual TOP: 2IIIi | 2IIIii | 2IIIiii | 2IIIiv | 2IIIv 19. Discuss two tools that modern astronomers use to explore the cosmos that are different from traditional optical telescopes and give an example of how and why each is used.

ANS: These are possible answers: (1) Radio telescopes—to record radio waves and, in interferometric arrays, to increase spatial resolution compared to single-dish radio telescopes.

(2) Adaptive optics—to obtain higher spatial resolution images by correcting for the blurring due to the Earth’s atmosphere.

(3) Space-based telescopes—placed in orbit around the Earth, they provide high spatial resolution images because they are outside the blurring effects of the Earth’s atmosphere.

(4) Airborne or high-flying observatories—can go outside the Earth’s atmosphere and detect wavelengths of light such as infrared or microwave, which are absorbed by molecules in the Earth’s atmosphere and do not reach the ground.

(5) Spacecraft—orbiters and landers can provide images with much better spatial resolution than Earthbased observations, and landers can physically probe the conditions on a planet’s or moon’s surface.

(6) Particle accelerators—they smash atoms or particles together with high energy in order to explore their constituents and probe physical conditions similar to those of the early universe.

(7) Neutrino detectors—are used to probe neutrinos emitted by astronomical objects, including the Sun. (8) Gravitational wave detectors—they measure gravitational waves in order to study changing gravitational fields such as those produced by merging binary stars.

(9) High-speed computers—they are used to make predictions of complex physical processes, such as star formation or the evolution of the universe, that can be compared with observations to test theories.

DIF: Easy REF: Section 6.3 | Section 6.4 | Section 6.5 | Section 6.6 MSC: Factual TOP: 3IIii | 4Ii | 5IIi | 6Ii | 6Iii | 6Iiii | 6Iiv 20. Why don’t astronomers put all telescopes in space?

ANS: Building and launching a telescope into space is much more costly than building one on Earth. Also, the majority of space-based telescopes cannot be repaired when something breaks or updated as

new technology becomes available.

DIF: Medium REF: Section 6.4 MSC: Conceptual TOP: 4Iii 21. Discuss two advantages of flyby missions over orbiters in exploring planets and moons in the Solar System.

ANS: First, flyby missions are relatively inexpensive and are the easiest missions to design and execute.

Second, they can visit several different planets and moons during their travels.

DIF: Easy REF: Section 6.5 MSC: Factual TOP: 5IIi | 5IIii 22. What are gravitational waves? Have astronomers been able to detect them yet?

ANS: Gravitational waves are disturbances in a gravitational field; astronomers have yet to detect them but have strong theoretical evidence that suggests they exist.

DIF: Easy REF: Section 6.6 MSC: Conceptual TOP: 6Iiii

CHAPTER 7

The Birth and Evolution of Planetary Systems

CONCEPT MAP Sec 7.1 1. Planets Form as a By-Product of Star Formation I. Our Solar System i. Constituents: classical planets, dwarf planets, moons, comets ii. All planets orbit in the same direction around the Sun (TF: 1) II. Formation History i. Solar nebula (MC: 1, SA: 1) ii. Meteorites: provide clues on conditions in solar nebula Sec 7.2 2. Theory of Star Formation I. Development of Disks i. Gravitational contraction (MC: 2) ii. Protostar iii. Protoplanetary disks/proplyd (MC: 3) iv. Orbital angular momentum: L  mvr  2r2/P (MC: 4, 5, SA: 2) v. Spin angular momentum: L  4MR2/5P (MC: 6, 7) II. Accretion onto the Star i. Conservation of angular momentum (MC: 8–11, SA: 3) ii. Accretion disk (TF: 2, SA: 4, 5)

iii. Planetesimals collide and either grow or fragment (TF: 3) Sec 7.3 3. Temperature Structure of the Forming Solar System I. What Sets the Temperature Structure of the Disk? i. Potential energy versus kinetic energy (MC: 12, 13) ii. Conservation of energy (MC: 14) iii. Gravitational energy is converted to thermal energy in the cloud collapse (MC: 14) iv. Inner disk is hotter than outer disk because of energy conversion (TF: 4, MC: 15, 16, SA: 6) v. Protostar’s radiation also heats the inner disk more than the outer disk (SA: 6) vi. Refractory materials: rocks, silicates (SA: 7) vii. Volatile materials: water, ammonia, methane (MC: 17, 18, SA: 8) II. Solid Planets Gather Atmospheres i. Primary atmosphere (MC: 19, 20, SA: 9) ii. Secondary atmosphere (TF: 5, SA: 8) Sec 7.4 4. Examples of Solar System Formation I. Our Solar System i. Age of the Solar System is 5 billion years (MC: 21) ii. Terrestrial planets (TF: 6, MC: 22, 23, SA: 10) iii. Gas giants (rocky core; grows due to accretion onto core; mostly H and He) (MC: 24, 25, SA: 9) iv. Gas giant formation: core accretion model versus disk instability model (MC: 26) v. Planet’s maximum mass: 0.08 M (TF: 7, MC: 27) vi. Volatiles ejected from Solar System due to solar wind vii. Remnants of planetesimals: moons, dwarf planets, asteroids, comets (MC: 28, 29) viii. Asteroid belt located between Mars and Jupiter (SA: 11) ix. Comets (SA: 12) x. Collisions with leftover planetesimals lead to cratering of planets and moons (MC: 30) xi. Formation of Earth’s Moon due to a collision with a Mars-sized object (MC: 31, 32, SA: 13) Sec 7.5 5. Planetary Systems I. Our Solar System Is Not Unique

i. There is nothing special about the conditions that led to the formation of our Solar System (TF: 8, SA: 14) II. Exoplanets i. Doppler shift detection of exoplanets; over 330 extrasolar planets discovered to date (MC: 33–39, SA: 15–17) ii. Sun’s velocity due to Jupiter, Saturn, and Earth is 12, 2.5, and 0.09 m/s, respectively, but today’s spectrographic precision limit detection to approximately 1 m/s (MC: 40) iii. Transit detection of exoplanets (TF: 9, MC: 41–43, SA: 17) iv. Transits provide measure of planet’s radius; best method for detecting Earth-like exoplanets (MC: 44, 45) v. Microlensing detection of exoplanets (MC: 46, 48, SA: 17) vi. Direct imaging detection of exoplanets (MC: 47, SA: 17) III. Planetary Systems Are Common i. Hundreds of exoplanets have been found, and thousands of additional candidates are being investigated; multiplanet systems have been found (TF: 10) ii. Exoplanet masses are biased toward the largest masses and smallest orbital distances because those are the easiest to be detected (SA: 18) iii. Most exoplanet masses are mini-Neptunes, 2 to 10MEarth (MC: 49, 50, SA: 18, 19) iv. Hot Jupiters close to their stars; easiest to find using radial velocities (MC: 43) v. Planet migration (MC: 51) vi. Brown dwarfs: 13 MJupiter  M  80 MJupiter  0.08 M(MC: 52) Sec 7.6 6. Kepler’s Search for Earth-like Planets I. The Kepler Mission i. The Kepler satellite was developed by NASA to find Earth-like planets using the transit method (MC: 53–55) ii. Kepler’s transit detections are combined with radial velocity surveys to determine the mass, radius, density, and character of exoplanets (MC: 56, SA: 20) iii. Kepler has found three Earth-sized planets (MC: 56, SA: 20) iv. Habitable zone: range in radii where liquid water can exist (MC: 56, 57)

TRUE/FALSE

1. All planets orbit the Sun in the same direction.

ANS: T DIF: Easy REF: Section 7.1 MSC: Factual TOP: 1Iii 2. In protoplanetary environments, the plane of an accretion disk is perpendicular to the axis of rotation of the interstellar cloud out of which it forms.

ANS: T DIF: Easy REF: Section 7.2 MSC: Factual TOP: 2IIii 3. Inside a solar nebula, as smaller particles collide and stick together, forming larger particles, their selfgravity prevents them from breaking apart, regardless of the strength of the collision.

ANS: F DIF: Medium REF: Section 7.2 MSC: Conceptual TOP: 2IIiii 4. The temperature gradient in the accretion disk surrounding a protostar is due in part to the fact that material closer to the protostar has converted less of its kinetic energy into thermal energy compared to materials farther away.

ANS: F DIF: Medium REF: Section 7.3 MSC: Factual TOP: 3Iiv 5. Astronomers believe that Venus initially formed with the thick atmosphere, primarily made of carbon dioxide, which it currently has.

ANS: F DIF: Medium REF: Section 7.3 MSC: Factual TOP: 3IIii 6. Planets like Earth are found closer to the Sun than planets like Jupiter.

ANS: T DIF: Easy REF: Section 7.4 MSC: Factual TOP: 4Iii 7. The maximum mass of a planet is approximately 0.08 times the mass of the Sun. ANS: T DIF: Medium REF: Section 7.4 MSC: Factual TOP: 4Iv 8. Our Solar System is the only planetary system in our galaxy.

ANS: F DIF: Medium REF: Section 7.5 MSC: Factual TOP: 5Ii 9. Current technology is sensitive enough to detect some extrasolar planets by a dip in brightness of the parent star as the planets pass between us and it.

ANS: T DIF: Easy REF: Section 7.5 MSC: Factual TOP: 5IIiii 10. Hundreds of extrasolar planets have been detected by direct imaging of the planets.

ANS: F DIF: Easy REF: Section 7.5 MSC: Factual TOP: 5IIIi

MULTIPLE CHOICE 1. Which of the following is NOT a characteristic of the early Solar System, based on current observations? a. The early solar nebula must have been flattened. b. The material from which the planets formed was swirling about the Sun in the same average rotational direction. c. The first objects to form started out small and grew in size over time. d. The initial composition of the solar nebula varied between its inner and outer regions. e. Temperatures decreased with increasing distance from the Sun. ANS: D DIF: Medium REF: Section 7.1 MSC: Factual TOP: 1IIi 2. The fact that Jupiter’s radius is contracting at a rate of 1 mm per year results in: a. Jupiter’s rotation rate slowing down with time b. Jupiter’s shape being noticeably oblate c. Jupiter moving slightly farther from the Sun with time d. Jupiter radiating more heat than it receives from the Sun e. Jupiter having a strong magnetic field ANS: D DIF: Difficult REF: Section 7.2 MSC: Applied TOP: 2Ii 3. Approximately how much mass was there in the protoplanetary disk out of which the planets formed, compared to the mass of the Sun? a. 50 percent b. 25 percent c. 10 percent d. 5 percent e.  1 percent ANS: E DIF: Medium REF: Section 7.2 MSC: Factual TOP: 2Iiii

4. What is the ratio of the orbital angular momentum of the Earth compared to its spin angular momentum?

Note that the Earth has a radius of 6  106 m, and 1 AU is 1.5  1011 m. a. 1 b. 70 c. 640 d. 25,000 e. 4.3  106 ANS: E DIF: Difficult REF: Section 7.2 MSC: Applied TOP: 2Iiv 5. What is the ratio of the orbital angular momentum of Jupiter to its spin angular momentum? Jupiter’s orbit has a semimajor axis of 5 AU and period of 12 years, and Jupiter has a rotation period of 0.4 day and a radius of 70,000 km. a. 650,000 b. 26,000 c. 920 d. 38 e. 4.5 ANS: B DIF: Difficult REF: Section 7.2 MSC: Applied TOP: 2Iiv 6. If an interstellar cloud having a diameter of 1016 m and a rotation period of a million years were to collapse to form a sphere that had the diameter of our Solar System, approximately 40 AU, what would its rotation period be? Assume the cloud’s total mass and angular momentum did not change. a. 1 million years b. 600 years c. 1 year d. 6 years e. 4 months ANS: E DIF: Difficult REF: Section 7.2 MSC: Applied TOP: 2Iv

7. Consider a small parcel of gas in the cloud out of which the Sun formed that was initially located in the accretion disk at a distance of 10 AU from the Sun and rotating around it with a speed of 10 km/s. If this parcel of gas eventually found its way to a distance of 1 AU from the Sun without changing its orbital angular momentum, then what would be its new rotation speed? a. 100 km/s b. 0.1 km/s c. 0.001 km/s d. 10 km/s e. 1,000 km/s ANS: A DIF: Difficult REF: Section 7.2 MSC: Conceptual TOP: 2Iv 8. According to the conservation of angular momentum, if an ice skater starts spinning with her arms out wide, then slowly pulls them close to her body, this will cause her to: a. spin faster b. spin slower c. maintain a constant rate of spin d. fall down ANS: A DIF: Easy REF: Section 7.2 MSC: Conceptual TOP: 2IIi 9. If a collapsing interstellar cloud formed only a protostar without an accretion disk around it, what would happen? a. The forming protostar would be significantly less massive than it would have been otherwise. b. The forming protostar would be rotating too fast to hold itself together. c. Only giant planets would form around the protostar. d. Only terrestrial planets would form around the protostar. e. More planets would form around the protostar.

ANS: B DIF: Difficult REF: Section 7.2 MSC: Factual TOP: 2IIi

10. Conservation of angular momentum slows a cloud’s collapse: a. equally in all directions b. only when the cloud is not rotating initially c. mostly along directions perpendicular to the cloud’s axis of rotation d. mostly at the poles that lie along the cloud’s axis of rotation e. to a complete stop ANS: C DIF: Difficult REF: Section 7.2 MSC: Applied TOP: 2IIi 11. Consider the figure shown below. At which point in time does the collapsing cloud have the greatest angular momentum? a. 1 b. 2 c. 3 d. 1 and 2, because the protostar has not yet formed. e. The cloud has the same angular momentum at each point in time.

ANS: E DIF: Medium REF: Section 7.2 MSC: Applied TOP: 2IIi 12. Consider four spheres of equal mass and size. Which has the most potential energy? a. a sphere on the top shelf of a bookshelf b. a sphere rolling on the floor at the base of the bookshelf c. a sphere sitting at rest on the floor at the base of the bookshelf d. a sphere on the middle shelf of a bookshelf e. a sphere that fell from the top shelf to the floor ANS: A DIF: Easy REF: Section 7.3 MSC: Applied TOP: 3Ii 13. When you push your palms together and rub them back and forth, you are demonstrating one way of converting _________ energy into _________ energy. a. potential; thermal b. kinetic; potential c. thermal; kinetic d. kinetic; thermal e. potential; total ANS: D DIF: Easy REF: Section 7.3 MSC: Applied TOP: 3Ii

14. What happens to the kinetic energy of gas as it falls toward and eventually hits the accretion disk surrounding a protostar? a. It is immediately converted into photons, giving off a flash of light upon impact. b. It is converted into thermal energy, heating the disk. c. It is converted into potential energy as the gas plows through the disk and comes out the other side. d. It becomes the kinetic energy of the orbit of the gas in the accretion disk around the protostar. e. It disappears into interstellar space.

ANS: B DIF: Medium REF: Section 7.3 MSC: Applied TOP: 3Iii 15. What sets the temperature of the pocket of gas in a protoplanetary disk? a. its distance from the forming star b. how much kinetic energy was converted to heat c. how much radiation from the forming star shines on the gas d. a combination of all three of the above ANS: D DIF: Medium REF: Section 7.3 MSC: Applied TOP: 3Iiv 16. Whether or not a planet is composed mostly of rock or gas is set by: a. its mass b. its temperature c. its distance from the star when it formed d. a combination of all three of the above ANS: D DIF: Difficult REF: Section 7.3 MSC: Applied TOP: 3Iiv 17. The solid form of a volatile material is generally referred to as a(n): a. metal b. silicate c. ice d. rock e. refractory material ANS: C DIF: Easy REF: Section 7.3 MSC: Factual TOP: 3Ivii

18. Based on the figure shown below, which planet(s) is (are) most likely to have the largest fraction of its (their) mass made of highly volatile materials such as methane and ammonia?

a. Venus, Earth, and Mars b. Earth c. Saturn d. Jupiter e. Uranus ANS: E DIF: Medium REF: Section 7.3 MSC: Applied TOP: 3Ivii 19. The atmosphere of which of these Solar System bodies is primary, as opposed to secondary, in origin? a. Venus b. Earth c. Saturn’s moon Titan d. Saturn e. Mars ANS: D DIF: Easy REF: Section 7.3 MSC: Factual TOP: 3IIi 20. The primary atmospheres of the planets are made mostly of: a. carbon and oxygen b. hydrogen and helium c. oxygen and nitrogen d. iron and nickel e. nitrogen and argon ANS: B DIF: Easy REF: Section 7.3 MSC: Factual TOP: 3IIi

21. What is the age of our Solar System? a. 4.6 billion years b. 4.6 million years c. 13.7 trillion years d. 13.7 billion years e. 13.7 million years ANS: A DIF: Easy REF: Section 7.4 MSC: Factual TOP: 4Ii 22. What is the most important factor in determining whether or not a planet will be rocky like terrestrial planets or gaseous like giant planets? a. the time at which the planet forms b. the planet’s radius c. the planet’s distance from the Sun d. whether the planet has moons e. the planet’s internal temperature ANS: C DIF: Easy REF: Section 7.4 MSC: Applied TOP: 4Iii 23. Why do the terrestrial planets have a much higher fraction of their mass in heavy chemical elements (as opposed to lighter chemical elements) than the giant planets? a. Terrestrial planets are low in mass and high in temperature, thus their lighter chemical elements eventually escaped to the outer reaches of the Solar System. b. The heavier elements in the forming solar nebula sank to the center of the Solar System, thus the inner terrestrial planets formed mostly from heavy chemical elements. c. The giant planets were more massive than terrestrial planets, and the giant planets preferentially pulled the lighter elements from the inner to the outer Solar System. d. Terrestrial planets formed much earlier than giant planets before the hydrogen and helium had a chance to cool and condense onto them. e. Terrestrial planets are colder and thus more massive chemical elements condensed on then than the giant planets.

ANS: A DIF: Difficult REF: Section 7.4 MSC: Applied TOP: 4Iii

24. Why do the outer giant planets have massive gaseous atmospheres of hydrogen and helium while the inner planets do not? a. These gases were more abundant in the outer regions of the accretion disk where the outer planets formed. b. The outer planets grew massive quickly enough to gravitationally hold on to these gases before the solar wind dispersed the accretion disk. c. The inner planets were too close to the Sun, and the solar wind blew away their original gaseous atmospheres. d. Frequent early collisions by comets with the inner planets caused most of their original atmospheres to dissipate. e. Temperatures were too high in the region of the Solar System that contains the inner planets.

ANS: B DIF: Easy REF: Section 7.4 MSC: Conceptual TOP: 4Iiii 25. The difference in composition between the giant planets and the terrestrial planets is most likely caused by the fact that: a. the giant planets are much larger b. only the terrestrial planets have iron cores c. the terrestrial planets are closer to the Sun d. the giant planets are made mostly of carbon e. only small differences in chemical composition existed in the solar nebula ANS: C DIF: Medium REF: Section 7.4 MSC: Applied TOP: 4Iiii 26. Two competing models of the formation of giant gaseous planets suggest they form either from gas accreting onto a rocky core or from: a. fragmentation of the accretion disk that surrounds the protostar b. the merger of two large planetesimals c. planets stolen from another nearby protostar d. materials condensing out of the solar wind e. an eruption of material from the protostar ANS: A DIF: Medium REF: Section 7.4 MSC: Factual TOP: 4Iiv

27. Was it ever possible (or is it currently possible) for Jupiter to become a star? a. Yes, it is in the process of becoming a star in the near future. b. Yes, but it cooled off before it could become a star. c. No, it would have to be at least 13 times more massive. d. No, its composition is too different from stars for it to become one. e. No, it used to be massive enough, but the solar wind has blown off too much of its mass.

ANS: C DIF: Medium REF: Section 7.4 MSC: Applied TOP: 4Iv 28. How much material in an accretion disk goes into forming the planets, moons, and smaller objects? a. most of it b. roughly half of it c. none; these objects were not formed in the accretion disk d. a small amount of it ANS: D DIF: Medium REF: Section 7.4 MSC: Factual TOP: 4Ivii 29. Comets and asteroids are: a. other names for moons of the planets b. primarily located within 1 AU of the Sun c. all more massive than Earth’s Moon d. material left over from the formation of the planets e. other names for meteors ANS: D DIF: Easy REF: Section 7.4 MSC: Factual TOP: 4Ivii 30. Which of the following is NOT considered evidence of cataclysmic impacts in the history of our Solar System? a. Uranus is “tipped over” so that it rotates on its side. b. Valles Marineris on Mars is a huge scar, many times deeper than the Grand Canyon, which spans onefourth the circumference of the planet. c. Mercury has crust that has buckled on the opposite side of an impact crater. d. Mimas has a crater whose diameter is roughly one-third of the Moon’s size. e. Mercury, Earth’s Moon, and many other small bodies are covered with many impact craters.

ANS: B DIF: Medium REF: Section 7.4 MSC: Factual TOP: 4Ix

31. The Moon probably formed: a. out of a collision between the Earth and a Mars-sized object b. when the Earth’s gravity captured a planetesimal c. when the accretion disk around the Earth fragmented d. when planetesimals collided to form a more massive object e. when a piece of Earth broke off and entered orbit ANS: A DIF: Easy REF: Section 7.4 MSC: Applied TOP: 4Ixi 32. What prevented the Moon from maintaining any atmosphere with which it originally formed? a. It repeatedly collided with planetesimals. b. It is too close to the Sun. c. The solar wind blew it away. d. It is not massive enough. e. It is tidally locked to Earth.

ANS: D DIF: Medium REF: Section 7.4 MSC: Applied TOP: 4Ixi 33. Which property of an extrasolar planet CANNOT be determined using the Doppler effect? a. orbital period b. orbital distance c. orbital speed d. mass e. radius ANS: E DIF: Easy REF: Section 7.5 MSC: Factual TOP: 5IIi 34. Why have astronomers using the radial velocity method found more Jupiter-sized planets at a distance of 1 AU around other stars than Earth-sized planets? a. A Jupiter-sized planet occults a larger area than an Earth-sized planet. b. A Jupiter-sized planet exerts a larger gravitational force on the star than an Earth-sized planet, and the Doppler shift of the star is larger. c. A Jupiter-sized planet shines brighter than an Earth-sized planet. d. Earth-sized planets are much rarer than Jupiter-sized planets. e. Actually, the planets found at these distances have all been Earth sized.

ANS: B DIF: Medium REF: Section 7.5 MSC: Conceptual TOP: 5IIi

35. Astronomers have used radial velocity monitoring to discover: a. extrasolar planetary systems that are similar to our own solar system b. Earth-sized planets around other stars c. Earth-sized planets at distances of 10 AU from their parent stars d. extrasolar planetary systems that contain more than one planet e. all of the above ANS: D DIF: Medium REF: Section 7.5 MSC: Applied TOP: 5IIi Figure 1 36. From the data shown in Figure 1, which property of an extrasolar planet CANNOT be determined? a. orbital period b. orbital distance c. radius d. mass e. All of the properties above can be determined.

ANS: C DIF: Difficult REF: Section 7.5 MSC: Applied TOP: 5IIi 37. Using the Doppler effect data shown in Figure 1, determine the approximate orbital period of the extrasolar planet. a. 1 year b. 3 years c. 6 years d. 8 years e. 12 years ANS: C DIF: Medium REF: Section 7.5 MSC: Applied TOP: 5IIi

38. Using the Doppler effect data for a particular star shown in Figure 1, and assuming the star is about the same mass as our Sun, determine the approximate orbital distance of its exoplanet. a. 1.1 AU b. 6.4 AU c. 18 AU d. 36 AU e. 3.3 AU ANS: E DIF: Difficult REF: Section 7.5 MSC: Applied TOP: 5IIi

39. The figure below shows data from Doppler effect studies of three different stars: A, B, and C. Assume that all the stars are similar in mass to the Sun. Which star has the planet with the smallest semimajor axis?

a. A has the smallest. b. B has the smallest. c. C has the smallest. d. A, B, and C are all the same distance from their stars. e. It is impossible to determine from the data given.

ANS: A DIF: Difficult REF: Section 7.5 MSC: Applied TOP: 5IIi

40. An observer located outside our Solar System, who monitors the velocity of our Sun over time, will find that its velocity varies by 12 m/s over a period of 12 years, because of: a. Jupiter’s gravitational pull b. Earth’s gravitational pull c. variations in its brightness d. convection on the Sun’s surface e. the sunspot cycle ANS: A DIF: Medium REF: Section 7.5 MSC: Conceptual TOP: 5IIii 41. Detecting a planet around another star using the transit method is difficult because the: a. planet must pass directly in front of the star b. planet must have a rocky composition c. star must be very dim d. star must be moving with respect to us e. planet’s orbital period is usually longer than 1 month ANS: A DIF: Medium REF: Section 7.5 MSC: Applied TOP: 5IIiii 42. If an astronomer on a planet orbiting a nearby star observed the Sun when Neptune was transiting in front of the Sun, how would the Sun’s brightness change? Note that the radius of Neptune is 2.5  107 m. a. The Sun’s brightness would decrease by 0.1 percent. b. The Sun’s brightness would increase by 0.1 percent. c. The Sun’s brightness would increase by 1 percent. d. The Sun’s brightness would decrease by 1 percent. e. The Sun’s brightness would not change at all.

ANS: A DIF: Difficult REF: Section 7.5 MSC: Applied TOP: 5IIiii

Figure 2 43. In Figure 2, which of the dips in the brightness of the star are caused by the transit of the planet with the largest orbital period? a. A b. B c. C d. A and B e. B and C ANS: C DIF: Medium REF: Section 7.5 MSC: Applied TOP: 5IIiii 44. Figure 2 shows data from the transit study of a star in which three different planets repeatedly transit in front of the star (A, B, and C). Which dips are caused by the transit of the planet with the smallest radius? a. A b. B c. C d. A, B, and C e. impossible to tell from this data ANS: A DIF: Medium REF: Section 7.5 MSC: Applied TOP: 5IIiv 45. Which method can be used to determine the radius of an extrasolar planet? a. Doppler shift can be used. b. Transit can be used. c. Microlensing can be used. d. Direct imaging can be used. e. None of these methods are able to do this.

ANS: B DIF: Easy REF: Section 7.5 MSC: Factual TOP: 5IIiv

46. What is the best method to detect Earth-sized exoplanets with the telescopes and instrumentation that exist today? a. Doppler shift is the best method. b. Transit is the best method. c. Microlensing is the best method. d. Direct imaging is the best method. e. All of the above methods can be used.

ANS: B DIF: Difficult REF: Section 7.5 MSC: Factual TOP: 5IIv 47. Which of the following is FALSE? a. The masses of exoplanets can be determined using the radial velocity technique. b. Most of the exoplanets detected to date have masses that are between 2 and 10 MEarth. c. Some exoplanets have been found in the habitable zone around their stars. d. Using the transit technique, the Kepler satellite has detected rocky planets. e. No images of exoplanets have been obtained because they are too far away.

ANS: E DIF: Difficult REF: Section 7.5 | Section 7.6 MSC: Applied TOP: 5IIi | 5IIvi | 5IIIiii | 6Iiii | 6Iiv 48. When astronomers began searching for extrasolar planets, they were surprised to discover Jupiter-sized planets much closer than 1 AU from their parent stars. Why is this surprising? a. These planets must have formed at larger radii where temperatures were cooler and then migrated inward. b. Jupiter-sized, rocky planets were thought to be uncommon in other solar systems. c. These planets must be the remnants of failed stars. d. Earth-like planets must be rarer than Jupiter-sized planets in other solar systems. e. It is different than in our Solar System.

ANS: A DIF: Medium REF: Section 7.5 MSC: Applied TOP: 5IIv

49. Which of the following is FALSE? a. Hundreds of extrasolar planets have been discovered to date from radial velocity surveys. b. The most common types of extrasolar planets found to date have masses 10 times the mass of Jupiter and lie within 5 AU from their parent star. c. Some planetary systems have been found that contain multiple planets. d. A star can brighten significantly due to gravitational lensing when a planet that orbits it passes directly in front of the star. e. The Kepler mission has begun to find terrestrial planets similar in size to Earth.

ANS: B DIF: Medium REF: Section 7.5 MSC: Applied TOP: 5IIIiii 50. Most planets currently found around other stars are: a. rocky in composition like terrestrial planets b. 2 to 10 MEarth, which is smaller than Neptune c. 2 to 10 MJupiter d. located at distances much larger than Jupiter’s distance from the Sun e. similar in mass to the Earth ANS: B DIF: Easy REF: Section 7.5 MSC: Factual TOP: 5IIIiii 51. Astronomers believe that the “hot Jupiters” found orbiting other stars must have migrated inward over time: a. by slowly accreting large amounts of gas and increasing their gravitational pull b. by losing their gas due to evaporation c. by losing orbital angular momentum d. after colliding with another planet e. after a close encounter between their star and another star ANS: C DIF: Medium REF: Section 7.5 MSC: Applied TOP: 5IIIv 52. The borderline between the most massive planet and the least massive brown dwarf occurs at: a. 4 Jupiter masses b. 13 Jupiter masses c. 120 Jupiter masses d. 80 Jupiter masses e. 45 Jupiter masses ANS: B DIF: Medium REF: Section 7.5 MSC: Factual TOP: 5IIIvi

53. Have astronomers detected any Earth-sized planets around normal stars yet? a. Yes, the Kepler spacecraft is just starting to find them. b. Yes, although the ones detected lie much closer to their stars than we do to ours. c. Yes, although the ones detected lie much farther from their stars than we do from ours. d. No, we do not have the technology to detect such low-mass planets yet. e. No; although we have the technology to detect low-mass planets, we haven’t found any others yet.

ANS: A DIF: Medium REF: Section 7.6 MSC: Applied TOP: 6Ii 54. Which is NOT a scientific goal of NASA’s Kepler mission? a. Finding Earth-sized planets is not a goal. b. Finding rocky planets is not a goal. c. Finding Earth-sized planets that could have liquid water is not a goal. d. Finding intelligent life on other planets is not a goal. e. All the above are goals of the Kepler mission.

ANS: D DIF: Easy REF: Section 7.6 MSC: Applied TOP: 6Ii 55. The Kepler mission is designed to search for extrasolar planets using the _________ method. a. Doppler shift b. transit c. microlensing d. direct imaging ANS: B DIF: Easy REF: Section 7.6 MSC: Factual TOP: 6Ii 56. Earth-sized planets have been found using the _________ method(s). a. Doppler shift b. transit and Doppler shift c. microlensing d. direct imaging e. transit ANS: B DIF: Medium REF: Section 7.6 MSC: Factual TOP: 6Iii | 6Iiii

57. Consider a star that is more massive and hotter than the Sun. For such a star, the habitable zone would: a. be located inside 1 AU b. be located outside 1AU c. not exist at any radii d. exist at every radii ANS: B DIF: Easy REF: Section 7.6 MSC: Applied TOP: 6Iiv

SHORT ANSWER 1. Explain the nebular hypothesis, and describe two observations that support it.

ANS: In the nebular hypothesis, a rotating cloud of interstellar gas collapsed and flattened to form a disk from which the Sun and planets formed. The observation of disks around protostars and young stars provide evidence in support of this idea, as does the fact that all planets orbit the Sun in the same direction.

DIF: Easy REF: Section 7.1 MSC: Conceptual TOP: 1IIi 2. Compare the orbital angular momentum of the Earth and Jupiter. Which is larger and by how much? (Note that Jupiter’s mass is 318 times that of the Earth; the semimajor axis of Jupiter’s orbit is 5.2 AU; and Jupiter’s orbital period is 12 years.)

ANS: The orbital angular momentum is equal to mvr, where m is the mass of the planet, v is its velocity, and r is the semimajor axis of its orbit. The velocity of a planet is v  2r/P, where P is the orbital period. Thus, the orbital angular momentum is proportional to mr2/P. Thus, the ratio of Jupiter’s angular momentum to Earth’s angular momentum is (MJ/ME)  (rJ/rE)2  (PE/PJ)  318  5.22/12  720.

DIF: Difficult REF: Section 7.2 MSC: Applied TOP: 2Iiv 3. What does conservation of angular momentum mean?

ANS: It means the angular momentum of a system cannot be changed via internal forces; it can be changed only by external forces.

DIF: Easy REF: Section 7.2 MSC: Conceptual TOP: 2IIi 4. Explain why an accretion disk forms around a protostar when an interstellar cloud collapses.

ANS: As the cloud collapses, the rate of rotation increases so that it halts the collapse of the cloud toward its axis of rotation, but not parallel to its axis of rotation.

DIF: Medium REF: Section 7.2 MSC: Conceptual TOP: 2IIii 5. What evidence do we have that the accretion disk that formed the Solar System was initially very centrally condensed?

ANS: The Sun contains 99 percent of all the mass of the Solar System.

DIF: Easy REF: Section 7.2 MSC: Applied TOP: 2IIii 6. Explain the two primary reasons why the inner solar nebula was hotter than the outer solar nebula.

ANS: The inner solar nebula was hotter than the outer solar nebula because the inner regions converted more of their potential energy into kinetic energy and heat when the original cloud collapsed to form the Solar System. In addition, when the Sun began to shine, it heated the inner Solar System more than the outer Solar System.

DIF: Medium REF: Section 7.3 MSC: Conceptual TOP: 3Iv 7. What is the difference between refractory and volatile materials?

ANS: Refractory materials are capable of withstanding high temperatures without melting or being vaporized, whereas volatile materials are not.

DIF: Easy REF: Section 7.3 MSC: Factual TOP: 3Ivi 8. Explain why there is a significant amount of methane and ammonia in the atmospheres of Uranus and Neptune but not nearly as much in the atmospheres of Jupiter and Saturn.

ANS: Ammonia and methane are volatile materials that are only found in the far outer Solar System where temperatures are very low. At the radii of Jupiter and Saturn, the nebula was hotter than that at Uranus and Neptune, which are farther from the Sun. DIF: Medium REF: Section 7.3 MSC: Applied TOP: 3Ivii 9. The primordial atmosphere of the Earth consisted of what type of chemical elements and from where did it originate? What chemical elements did the secondary atmosphere of the Earth consist of and from where did it originate?

ANS: The primary atmosphere consisted mostly of hydrogen and helium, similar to the material that formed the solar nebula. The secondary atmosphere of the Earth consisted mostly of carbon dioxide that was outgassed from the interior due to volcanic activity.

DIF: Medium REF: Section 7.3 MSC: Factual TOP: 3IIi

10. How do astronomers explain the basic difference in composition between the inner planets and the outer planets?

ANS: The inner planets (Mercury, Venus, Earth, and Mars) formed in the region of the Solar System where only refractory materials could exist in solid form, and therefore they are composed mostly of rocks and metals. The outer planets (Jupiter, Saturn, Uranus, and Neptune) formed out where even volatile materials could exist in solid form. As a result, in addition to rocks and metals, the Jovian planets and their moons are largely composed of ices as well. The solar wind reinforced these differences by clearing the inner Solar System of light gases during the planetary formation process.

DIF: Medium REF: Section 7.4 MSC: Conceptual TOP: 4Iii 11. Why did the planetesimals in the asteroid belt never coalesce into a planet?

ANS: The gravity of Jupiter kept the material stirred up so it could never pull together.

DIF: Easy REF: Section 7.4 MSC: Conceptual TOP: 4Iviii 12. Why might a newly discovered comet contain clues to the composition of the early solar nebula?

ANS: Comets are believed to be made of ice and dust similar in composition to the early solar nebula. A newly discovered comet might be on its first orbit of the Sun, and, as it heats and melts, the gases it emits can tell us about the chemical composition of the solar nebula.

DIF: Easy REF: Section 7.4 MSC: Conceptual TOP: 4Iix 13. How did the formation of our Moon differ from the formation of the Galilean moons of Jupiter?

ANS: Astronomers believe that the formation of our Moon occurred due to a collision of a Mars-sized object with the early Earth. The remains of the planet eventually coalesced into our Moon. The Galilean moons, on the other hand, are believed to have formed naturally with the rest of the Solar System. Astronomers believe that the Jovian system formed its own mini-accretion disk, out of which the Galilean moons formed around Jupiter, much as the planets formed around the Sun. DIF: Medium REF: Section 7.4 MSC: Applied TOP: 4Ixi 14. Explain why astronomers believe that the formation of planets is a natural by-product of star formation.

ANS: To explain the structure of the Solar System (the direction and inclination of planetary orbits), astronomers believe that planets must form out of disks of material. These disks would bear a striking resemblance to disks that have been observed around a number of young stars.

DIF: Easy REF: Section 7.1 MSC: Conceptual TOP: 5Ii

15. Explain how astronomers use the Doppler effect to detect the presence of extrasolar planets.

ANS: As a planet orbits a star, the gravitational attraction between the planet and star causes them to orbit a common center of mass. To an outside observer, this causes the star to appear to “wobble.” As it does so, it periodically moves toward us and then away from us. These radial motions produce a Doppler effect in the spectra of the star. By measuring the Doppler effect, astronomers can infer the mass of the planet and its distance from the star.

DIF: Medium REF: Section 7.5 MSC: Applied TOP: 5IIi 16. What property of an extrasolar planet can be determined directly from the Doppler effect data shown in the figure below? What other properties of the planet can then be determined?

ANS: The period of repetition of the Doppler shifts is also the orbital period of the planet. The orbital distance and mass of the planet can then be calculated from the maximum orbital velocity observed along with Newton’s generalized version of Kepler’s third law, if the inclination of the orbit can be determined.

DIF: Medium REF: Section 7.5 MSC: Applied TOP: 5IIi 17. Briefly explain the four different observational methods we use to detect extrasolar planets. Which has detected the largest number of extrasolar planets to date?

ANS: The four different observational methods we use to detect extrasolar planets are (1) using the Doppler shift to detect the motion of its parent star, (2) detecting transits when a planet moves in front of its parent star and dims it, (3) detecting microlensing events when the planet moves across the line of sight of its parent star and brightens it, and (4) directly imaging the planet as it orbits its star. The Doppler shift method has been used to find most of the 330 known extrasolar planets.

DIF: Medium REF: Section 7.5 MSC: Applied TOP: 5IIi | 5IIiii | 5IIv | 5IIvi 18. Approximately how massive are most of the extra-solar planets that have been discovered using the Doppler effect, and which planet in our solar system is similar in mass? Why is the Doppler effect method more likely to find massive (rather than low-mass) planets and planets that are close to their stars?

ANS: The planets found are mostly smaller than Neptune, 2 to 10 times the Earth’s mass. The Doppler effect is more likely to find massive planets because the Doppler shift of their parent star will be larger, because the gravitational pull is proportional to the mass. Also, it is easier to find a planet closer in because the force of gravity is stronger as it is inversely proportional to the square of the semimajor axis.

Thus, more massive planets and planets closer to their star are easier to detect with the Doppler shift.

DIF: Easy REF: Section 7.5 MSC: Factual TOP: 5IIIii 19. Explain why most of the extrasolar planets that astronomers first detected were so-called “hot Jupiters.”

ANS: Originally technology did not allow us to detect smaller planets. The easiest planets to detect are massive planets, which cause their parent stars to wobble the most, in close orbits, which cause their parent stars to wobble faster.

DIF: Medium REF: Section 7.5 MSC: Applied TOP: 5IIIii 20. Have any Earth-sized, terrestrial, extrasolar planets been detected? If so, explain what method(s) is (are) used.

ANS: The Kepler mission has been finding Earth-sized planets using the transit method to detect them and using the Doppler shift method to follow up and measure the radii, masses, and densities and to characterize them as either terrestrial or giant planets.

DIF: Medium REF: Section 7.6 MSC: Factual TOP: 6Iii | 6Iiii

CHAPTER 8

The Terrestrial Planets and Earth’s Moon

CONCEPT MAP Sec 8.1 1. Four Main Processes Shape the Inner Planets I. Comparing the Planets i. Terrestrial planets: Mercury, Venus, Earth, Mars (SA: 1) II. Earth’s Surfaces i. Hydrosphere: liquid surface of a planet ii. Lithosphere: solid surface of a planet (MC: 1) iii. Biosphere: vegetation on the surface of a planet III. Four Forces That Reshape the Earth’s Lithosphere i. Plate tectonics; tectonism: the deformation of a planet’s crust (TF: 1, MC: 2, SA: 2, 3) ii. Volcanism; definition of magma (TF: 1, MC: 3, SA: 2, 3) iii. Impacts; definitions of meteor, meteoroid, meteorite (TF: 1, MC: 4, SA: 2, 3) iv. Erosion (TF: 1, MC: 5, SA: 2, 3) Sec 8.2 2. Impacts Help Shape the Evolution of the Planets I. A Planet’s Surface Contains Clues to the Relative Importance of the Four Forces That Shape the Lithosphere i. Earth moves at 30 km/s around the Sun; kinetic energy of impacts: EK  1⁄2 mv2 (MC: 6, 7) ii. Meteor crater (Arizona): NiFe meteorite 50 meters in diameter, 13 km/s, 50,000 years ago, produced crater with 1.2 km diameter (SA: 4) iii Number of impact craters: Earth: 200, Venus: 1,000, Moon: millions (MC: 8, 9)

iv. Plate tectonics, volcanism, and erosion control the number of craters seen today (TF: 2, MC: 10) II. Giant Impacts Can Reshape Planets i. A 100 meter-diameter meteor burns up or breaks up in Earth’s atmosphere (MC: 11) ii. Outflow patterns in Martian craters suggest subsurface water or ice (MC: 12) III. Calibrating the Cosmic Clock i. More cratering means older surface and less geological activity (MC: 13, 14, SA: 5) ii. Average age of Moon’s surface is 3.4 Gyr; but the whiter regions are older (4.4 Gyr), and darker regions, the maria, are younger (3.1 Gyr) (MC: 15–19) iii. Conclusion: most impacts happened in the first 1 Gyr after the Solar System formed (MC: 18, 20, 21) IV. Compute the Ages of Rocks i. Radioactive elements, radioactive decay, isotopes, parent and daughter species ii. Radioactive dating of Moon rocks set the age of the Solar System at 4.6 Gyr (MC: 22) iii. Half life: PF/PO  (1⁄2)n (MC: 23–25, SA: 6) Sec 8.3 3. Interiors of the Planets Tell Their Own Tales I. Seismic Waves Give Information on Density Structure of the Earth i. Primary (P) waves: longitudinal waves that are refracted when traveling through liquid (TF: 3, MC: 26; SA: 7) ii. Secondary (S) waves: traverse waves that cannot travel through liquids (MC: 26, SA: 7) iii. Worldwide seismic monitoring leads to an understanding of Earth’s interior (MC: 26, SA: 7) II. Earth’s Interior i. Hydrostatic equilibrium ii. Two-component core—solid and liquid, highest density (MC: 27, SA: 8) iii. Mantle—liquid, medium density (MC: 28, SA: 8) iv. Crust—solid, lowest density, silicate-rich rock on continents (e.g., granite) and heavier volcanic rock on ocean floors (e.g., basalt) (MC: 29, 30, SA: 8) v. Differentiation (TF: 4, MC: 31, SA: 9, 10) III. Moon Formed from the Earth’s Crust i. Moon’s average density is similar to Earth’s mantle (MC: 32) ii. Moon formed when a Mars-sized protoplanet hit the Earth (MC: 32, 33) IV. Evolution of Planet Interiors Depends on Heating and Cooling

i. Cores melted due to impacts and heat from radioactive decays (TF: 5) ii. Radioactivity keeps the Earth’s interior liquid (TF: 5, MC: 34–36, SA: 11) iii. Earth’s inner core is solid, although T  6,000 K; outer core is liquid (TF: 6, MC: 37) V. Energy Balance i. Heating

R3 and cooling

R2 (MC: 38, 39, SA: 12)

ii. Larger planets take longer to cool and solidify than smaller planets (MC: 40, SA: 12) VI. Planetary Magnetic Fields i. Earth’s magnetic axis NOT the same as its rotational axis (TF: 7) ii. Earth’s magnetic field is NOT due to permanent magnetism in the solid core (TF: 8, MC: 41, SA: 13) iii. Earth’s magnetic field changes over time; reverses direction every 500,000 years (SA: 13, 14) iv. Dynamo: Earth’s magnetic field caused by motion of charged particles (convection) in the liquid outer core converting mechanical energy into magnetic energy (TF: 8, 9, MC: 41–43) v. Moon: little or no magnetic field; implies mostly solid interior vi. Mercury: significant magnetic field; molten core and slow rotation lead to dynamo vii. Venus: no magnetic field; maybe no dynamo due to its slow rotation viii. Mars: no magnetic field now, but did have one in the past Sec 8.4 4. Tectonism, Volcanism, and Erosion I. Tectonism on Earth i. Plate tectonics: continents lie on 13 distinct continental plates (seven major and six minor plates) that move with respect to one another (MC: 44) ii. Spreading centers: where plates move apart and magna solidifies to create new rock (MC: 44, 45, SA: 15) iii. Magnetization bands on ocean floor map Earth’s changing magnetic field (MC: 46) iv. Continental drift: plates move approximately a few cm/yr due to convection in the Earth’s interior (MC: 36, 47, 48, SA: 16) v. Subduction zones: where plates meet and one slides under the other and melts, e.g., Mariana Trench (MC: 44, 45, SA: 15) vi. Oldest seafloor rocks are 200 million years old (TF: 10, SA: 1) vii. Himalayas rising at rate of 0.5 m/century (MC: 49) viii. Transform fault: stick and slip, e.g., San Andreas Fault, where North American and Pacific plates meet (MC: 45, SA: 15)

ix. Most volcanoes, earthquakes, and mountain chains occur along plate boundaries, e.g., Pacific Ring of Fire (TF: 11, MC: 44, 45, 50) II. Tectonism on Other Planets i. All terrestrial planets show evidence of tectonic activity (TF: 12) ii. Mercury: crust cooled rapidly and shrank (MC: 33, SA: 17) iii. Mars: Valles Marineris, a giant crack in the lithosphere (MC: 51, 52, 53, SA: 17) iv. Venus (Earth’s twin): Magellan mapping shows a young surface,  1 Gyr old, many volcanoes, but no sign of plate motion (SA: 1, 17) III. Volcanism on Earth i. Friction between moving plates generates earthquakes and thermal energy, which leads to volcanism (MC: 45, SA: 1) ii. Types of volcanoes: shield, composite, hot spots leading to chains of volcanoes/islands (MC: 54, SA: 18) IV. Volcanism on Other Planets and Moons i. Moon: maria, dark regions of more recent volcanic activity, 3 Gyr old (TF: 13, MC: 54) ii. Mars: many shield volcanoes (no plate tectonics); Olympus Mons, largest volcano in Solar System (TF: 14, MC: 19, 54–57, SA: 1, 18) iii. Venus: highest number of volcanoes, many different types (MC: 33, 54, SA: 18) V. Erosion Levels Surfaces i. Erosion is caused by water, ice, or wind on planets with atmosphere and by radiation on Moon and Mercury (MC: 19, 38, 39, 58, SA: 1, 4) ii. Wind streaks, sand dunes, dust storms Sec 8.5 5. The Geological Evidence for Water I. Water on Mars? i. Water once flowed on surface of Mars; dry riverbeds seen today and impact craters show evidence of liquid outflows (MC: 19, 59, SA: 19) ii. Mars rovers (Spirit and Opportunity) see signs of ancient seas in the mineral content of Martian rocks (TF: 15, SA: 19) iii. Mars polar caps mostly carbon dioxide; most water is probably frozen under the surface of Mars (TF: 16, MC: 33, 60, SA: 19) iv. Small amount of water ice, mostly at poles and in shaded craters, probably exists on the Moon and

Mercury Sec 8.6 6. Origins: The Death of the Dinosaurs I. Extinction Event i. K-T boundary (Cretaceous–Tertiary boundary): mass extinction, 65 million years ago, more than 50 percent of all living species died; mammals dominated since then (TF: 17, MC: 33, 61, SA: 4) ii. Traces of iridium and soot mark the boundary (MC: 61, 62, SA: 18) iii. Asteroid (10 km diameter) hit near Mexico’s Yucatan Peninsula; crater identified (TF: 17, MC: 61, SA: 20) iv. Debris thrown into atmosphere, tidal wave, firestorms, dust and smoke blocking sunlight lead to mass extinctions

TRUE/FALSE 1. There are two main forces that shape a planet’s surface: impacts and plate tectonics.

ANS: F DIF: Easy REF: Section 8.1 MSC: Conceptual TOP: 1IIIi | 1IIIii | 1IIIiii | 1IIIiv 2. The smoother, less cratered surfaces of the Moon resulted from those areas being protected by the Moon’s proximity to Earth.

ANS: F DIF: Medium REF: Section 8.2 MSC: Conceptual TOP: 2Iiv 3. Primary waves are seismic longitudinal waves whose paths are deflected when they travel through the Earth’s liquid core.

ANS: T DIF: Easy REF: Section 8.3 MSC: Factual TOP: 3Ii 4. The interior of the Earth is differentiated because it cooled very quickly.

ANS: F DIF: Medium REF: Section 8.3 MSC: Factual TOP: 3IIiv 5. The interior of the Earth is hot mainly because of the pressure induced by the weight of the material above it.

ANS: F DIF: Difficult REF: Section 8.3 MSC: Factual TOP: 3IVi | 3IVii 6. The center of Earth has approximately the same temperature as the surface of the Sun. ANS: T DIF: Medium REF: Section 8.3 MSC: Factual TOP: 3IViii 7. Earth’s magnetic axis and its spin axis are not aligned.

ANS: T DIF: Easy REF: Section 8.3 MSC: Factual TOP: 3VIi 8. Permanent magnetism in the Earth’s solid iron core is the source of Earth’s magnetic field.

ANS: F DIF: Easy REF: Section 8.3 MSC: Factual TOP: 3VIii | 3VIiv 9. Convection cannot take place in the Earth’s mantle because it is not molten.

ANS: F DIF: Easy REF: Section 8.3 MSC: Factual TOP: 3VIiv 10. The oldest rocks on the Earth are approximately 4 billion years old.

ANS: T DIF: Medium REF: Section 8.4 MSC: Factual TOP: 4Ivi

11. Volcanoes on Earth typically occur in the middle of oceans where two tectonic plates are spreading apart.

ANS: F DIF: Easy REF: Section 8.4 MSC: Factual TOP: 4Iix 12. All the terrestrial planets show evidence of disruptions due to tectonics occurring now or in the past.

ANS: T DIF: Medium REF: Section 8.4 MSC: Factual TOP: 4IIi 13. The maria, the dark regions of the Moon’s surface, are approximately 1 billion years younger than the rest of its surface.

ANS: T DIF: Medium REF: Section 8.4 MSC: Factual TOP: 4IVi 14. Mars has many large shield volcanoes because it does not have plate tectonics.

ANS: T DIF: Medium REF: Section 8.4 MSC: Applied TOP: 4IVii 15. The Mars rovers named Spirit and Opportunity found evidence in the chemical makeup of the rocks that proves Mars had liquid water on its surface in the past.

ANS: T DIF: Easy REF: Section 8.5 MSC: Factual TOP: 5Iii 16. On Mars, water ice is present primarily at its polar ice caps.

ANS: F DIF: Easy REF: Section 8.5 MSC: Factual TOP: 5Iiii 17. An asteroid or comet impact near the Yucatan Peninsula approximately 20,000 years ago resulted in the extinction of more than 50 percent of all living species, including the dinosaurs.

ANS: F DIF: Medium REF: Section 8.6 MSC: Factual TOP: 6Ii | 6Iiii

MULTIPLE CHOICE 1. The lithosphere of a planet is: a. the molten layer under the crust b. the layer of the atmosphere in which clouds form c. the upper layer of its atmosphere d. its solid surface e. its frozen surface ANS: D DIF: Easy REF: Section 8.1 MSC: Factual TOP: 1IIii

Figure 1

2. Which picture in Figure 1 illustrates an example of plate tectonics occurring on Earth? a. A b. B c. C d. D ANS: B DIF: Medium REF: Section 8.1 MSC: Applied TOP: 1IIIi 3. Which picture in Figure 1 illustrates an example of volcanism occurring on Earth? a. A b. B c. C d. D ANS: C DIF: Easy REF: Section 8.1 MSC: Applied TOP: 1IIIii 4. Which picture in Figure 1 is an example of impacts occurring on Earth? a. A b. B c. C d. D ANS: A DIF: Easy REF: Section 8.1 MSC: Applied TOP: 1IIIiii 5. Which picture in Figure 1 is an example of erosion occurring on Earth? a. A b. B c. C d. D ANS: D DIF: Easy REF: Section 8.1 MSC: Applied TOP: 1IIIiv

6. Which object would have the LOWEST impact on our planet if it were to strike Earth? a. a 1-kg asteroid traveling at 30 km/s b. a 5-kg asteroid traveling at 10 km/s c. a 100-kg comet traveling at 10 km/s d. a 1,000-kg Mini Cooper car traveling at 100 miles/hr, which is 0.05 km/s e. a 3,000-kg truck traveling at 35 miles/hr, which is 0.02 km/s ANS: E DIF: Medium REF: Section 8.2 MSC: Applied TOP: 2Ii 7. Which object would have the LARGEST impact if it were to strike the Earth? a. a 1-m diameter asteroid moving at 100 m/s b. a 1-m diameter comet moving at 100 m/s c. a 10-m diameter comet moving at 10 m/s d. a 10-m diameter asteroid moving at 10 m/s e. a 1-m diameter comet moving at 50 m/s ANS: A DIF: Difficult REF: Section 8.2 MSC: Applied TOP: 2Ii 8. Based on the number of impact craters observed per square meter on their surface, place these terrestrial planets in order of youngest to oldest surface: a. Earth, Venus, Mercury b. Venus, Earth, Mercury c. Mercury, Venus, Earth d. Earth, Mercury, Venus e. Venus, Mercury, Earth ANS: A DIF: Easy REF: Section 8.2 MSC: Applied TOP: 2Iiii 9. The smallest number of craters per square meter are found on the surface of: a. Mercury b. Mars c. Venus d. Earth ANS: D DIF: Medium REF: Section 8.2 MSC: Factual TOP: 2Iiii

10. Which of the following is NOT a factor that helps explain Earth’s lack of craters compared to the Moon? a. wind erosion b. larger atmosphere c. higher density interior d. liquid water on surface e. active tectonics and volcanism ANS: C DIF: Easy REF: Section 8.2 MSC: Conceptual TOP: 2Iiv 11. To survive passage through the Earth’s atmosphere without burning or breaking up before it hits the ground, an asteroid must be: a. at least 1 meter in size b. at least 10 meters in size c. at least 100 meters in size d. at least 1 kilometer in size e. at least 1,000 kilometers in size ANS: C DIF: Difficult REF: Section 8.2 MSC: Factual TOP: 2IIi 12. Flows of material surrounding Martian craters suggest: a. volcanism in its interior b. the presence of water in surface rocks c. active plate tectonics at the time of impact d. a very thin crust e. the presence of ice ANS: B DIF: Easy REF: Section 8.2 MSC: Factual TOP: 2IIii 13. Mars, Venus, and Earth are much less heavily cratered than Mercury and the Moon. This is explained by the fact that: a. the rate of cratering in the early Solar System was strongly dependent on location b. Mars, Venus, and Earth have thicker atmospheres c. Earth and Venus were shielded from impacts by the Moon, and Mars was protected by the asteroid belt d. Mars, Venus, and Earth were geologically active for a longer period of time than Mercury and the Moon e. Mars, Venus, and Earth are much larger in size than Mercury and the Moon ANS: D DIF: Medium REF: Section 8.2 MSC: Conceptual TOP: 2IIIi

14. According to studies of impact cratering, which of these terrestrial objects has, on average, the oldest surface? a. Mercury b. Venus c. Earth d. Mars e. the Moon ANS: A DIF: Easy REF: Section 8.2 MSC: Factual TOP: 2IIIi 15. Compared to the dark-colored regions of the surface of the Moon, the light-colored regions are approximately: a. 1 billion years older b. 1 billion years younger c. 1 million years older d. 1 million years younger e. a few thousand years younger ANS: A DIF: Easy REF: Section 8.2 MSC: Factual TOP: 2IIIii

Figure 2

16. Which of these three lunar surfaces shown in Figure 2 is the oldest? a. A b. B c. C d. A and C are probably about the same age and are older than B. e. It is impossible to tell without radioactive dating.

ANS: A DIF: Medium REF: Section 8.2 MSC: Applied TOP: 2IIIii

17. Which of the three lunar surfaces shown in Figure 2 is the youngest? a. A b. B c. C d. A and C are probably about the same age and are younger than B. e. It is impossible to tell without radioactive dating.

ANS: B DIF: Medium REF: Section 8.2 MSC: Applied TOP: 2IIIii 18. Based on the age of the light- and dark-colored regions of the Moon and the number of craters observed in these regions, we know that impacts in the inner Solar System: a. rapidly decreased approximately 1 billion years ago b. rapidly decreased approximately 3 billion years ago c. were very rare in the last 4.6 billion years d. occurred at approximately a constant rate throughout most of the age of the Solar System e. never occur anymore ANS: B DIF: Difficult REF: Section 8.2 MSC: Applied TOP: 2IIIii | 2IIIiii 19. Which of the following statements is FALSE? a) The surface of Venus has very few craters primarily because asteroids burn up in its thick atmosphere. b) Geological features and the chemical composition of some rocks on Mars suggest liquid water flowed on the surface in the past, but not at the present time. c) Darker regions of the Moon’s surface have less craters and are approximately 1 billion years younger than the lighter regions. d) Volcanoes on Mars are larger, on average, than the Earth’s volcanoes because Mars does not have moving continental plates. e) Impact craters on the Earth are erased over time because of erosion due to water and the recycling of its crust.

ANS: A DIF: Difficult REF: Section 8.2 | Section 8.4 | Section 8.5 MSC: Factual TOP: 2IIIii | 4IVii | 4Vi | 5Ii

20. Studies of the amount of cratering at different locations on the Moon indicate that: a. the rate of cratering in the Solar System has changed dramatically over time b. the younger lunar surfaces are hundreds of billions of years younger than the oldest surfaces c. the Moon has never been geologically active at any point in its history d. most of the heavy cratering in the Solar System occurred before Earth formed e. that cratering is no longer occurring in the Solar System ANS: A DIF: Difficult REF: Section 8.2 MSC: Applied TOP: 2IIIiii 21. Which list below gives the correct order of the age of the three lunar surfaces shown in the figure below going from youngest to oldest?

a. A, B, C b. A, C, B c. B, A, C d. B, C, A e. C, B, A ANS D DIF: Medium REF: Section 8.2 MSC: Applied TOP: 2IIIiii

22. Of the following methods, the age of the Solar System can be determined most accurately by: a. measuring the number of craters per square meter on Mercury b. radioactive dating of rocks retrieved from the Moon c. carbon dating of rocks from mountains on the Earth d. measurement of the magnetic field variations in rocks under the Earth’s oceans e. measuring the rate of energy production in the Sun ANS: B DIF: Easy REF: Section 8.2 MSC: Applied TOP: 2IVii 23. If a radioactive element has a half-life of 10,000 years, what fraction of it is left in a rock after 40,000 years? a. 1⁄2 b. 1⁄4 c. 1⁄8 d. 1⁄16 e. 1⁄32 ANS: D DIF: Difficult REF: Section 8.2 MSC: Applied TOP: 2IViii 24. If you obtained a sample of a meteorite and determined the abundances of uranium (238U) and lead (207Pb) in it, and found that for every 1 uranium atom there were 15 lead atoms, then what would the age of this rock be? Note that this form of uranium decays to this form of lead with a half-life of 700 million years. For simplicity, you can assume that there was no lead in the rock when it originally formed. a. 1.4 billion years b. 2.8 billion years c. 4.0 billion years d. 10.5 billion years e. 3.6 billion years ANS: B DIF: Difficult REF: Section 8.2 MSC: Applied TOP: 2IViii

25. If you obtained a sample of rock from Venus and determined the abundances of uranium (238U) and lead (207Pb) in it, and found that for every one uranium atom there were three lead atoms, then what would the age of this rock be? Note that this form of uranium decays to this form of lead with a half-life of 700 million years. For simplicity, you can assume that there was no lead in the rock when it originally formed. a. 1.4 billion years b. 2.8 billion years c. 4.0 million years d. 10.5 billion years e. 3.6 billion years ANS: A DIF: Difficult REF: Section 8.2 MSC: Applied TOP: 2IViii 26. Suppose an earthquake occurs on an imaginary planet. Scientists on the other side of the planet detect primary waves but not secondary waves after the quake occurs. This suggests that: a. part of the planet’s interior is liquid b. all of the planet’s interior is solid c. the planet has an iron core d. the planet’s interior consists entirely of rocky materials e. The planet’s mantle is liquid ANS: A DIF: Medium REF: Section 8.3 MSC: Applied TOP: 3Ii | 3Iii | 3Iiii

Figure 3

27. Which layer in Figure 3 represents the Earth’s liquid core? a. A b. B c. C d. D ANS: C DIF: Medium REF: Section 8.3 MSC: Applied TOP: 3IIii 28. Which layer in Figure 3 represents the Earth’s liquid mantle? a. A b. B c. C d. D ANS: B DIF: Easy REF: Section 8.3 MSC: Applied TOP: 3IIiii 29. In the Earth’s crust, lower density igneous rock such as _________ make up the continents, and higher density volcanic rock such as _________ make up the ocean floor. a. limestone; granite b. granite; iron-rich silicates c. granite; basalt d. limestone; sandstone e. marble; basalt ANS: C DIF: Difficult REF: Section 8.3 MSC: Applied TOP: 3IIiv 30. The fact that Earth’s interior is differentiated suggests that: a. it formed first from denser material and then afterward accreted lighter material b. it has both a liquid and solid core c. it was entirely liquid at some point in the past d. only the crust is solid; the rest of Earth’s interior is liquid e. it formed first from lighter material, then afterward accreted heavier material ANS: C DIF: Medium REF: Section 8.3 MSC: Conceptual TOP: 3IIiv

31. Differentiation refers to materials that are separate based on their: a. weight b. mass c. volume d. density e. heat capacity ANS: D DIF: Easy REF: Section 8.3 MSC: Factual TOP: 3IIv 32. The observation that the Moon’s average density is similar to the density of the Earth’s _________ supports the collision theory of the Moon’s origin. a. oceans b. average density c. core d. atmosphere e. mantle ANS: E DIF: Easy REF: Section 8.3 MSC: Conceptual TOP: 3IIIi | 3IIIii 33. Which of the following statements is FALSE? a) Approximately 65 million years ago, a 10-km wide asteroid struck Earth and wiped out more than 50 percent of all living species. b) The Moon probably was formed by a collision between a Mars-sized body and the Earth. c) During summer in the northern hemisphere of Mars, the polar ice cap melts and liquid water flows outward from it in rivers. d) The surface of Venus is relatively young with an estimated age of less than 1 billion years. e) Mercury has many fractures and faults on its surface that probably arose when it cooled very rapidly and shrank.

ANS: C DIF: Easy REF: Section 8.3 | Section 8.4 | Section 8.5 | Section 8.6 MSC: Factual TOP: 3IIIii | 4IIii | 4IViii | 5Iiii | 6Ii

34. What is the main reason that the Earth’s interior is liquid today? a. tidal force of the Moon on the Earth b. seismic waves that travel through Earth’s interior c. decay of radioactive elements d. convective motions in the mantle e. pressure on the core from Earth’s outer layers.

ANS: C DIF: Medium REF: Section 8.3 MSC: Conceptual TOP: 3IVii 35. Which of the following will NOT be a consequence of Earth’s consumption of the bulk of its radioactive “fuel” in the future? a. Earth will spin more slowly on its axis. b. The interior of the planet will solidify. c. Volcanic activity will cease. d. Continental drift will no longer occur. e. Earth’s mass will decrease.

ANS: A DIF: Medium REF: Section 8.3 MSC: Conceptual TOP: 3IVii 36. What will eventually happen to the Earth when radioactive decays in its interior cease? a. The Earth’s core will solidify. b. Continental drift will cease. c. Earthquakes will cease. d. The strength of the Earth’s magnetic field will decrease. e. All of the above will happen.

ANS: E DIF: Medium REF: Section 8.3 | Section 8.4 MSC: Applied TOP: 3IVii | 4Iiv 37. Earth’s innermost core is solid, not liquid, because: a. the core temperature is too low to melt iron b. differentiation caused all of the heavy, solid material to sink to the bottom while Earth was forming c. all the liquid has moved up into the mantle via convection d. the pressure is too high for the material to be in a liquid state e. Iron does not melt ANS: D DIF: Difficult REF: Section 8.3 MSC: Conceptual TOP: 3IViii

38. The Moon has a diameter that is approximately one-fourth that of Earth. If these objects’ interiors are heated by radioactive decays, and the total amount of energy in decays is proportional to the object’s volume, then how does the amount of heat the Moon has compare to that of Earth? a. The Moon has 0.016 times that of Earth’s. b. The Moon’s heating rate is 8 times that of the Earth’s. c. The Moon’s heating rate is 0.5 times that of Earth’s. d. The Moon’s heating rate is 4 times that of the Earth’s. e. The heating rates are about the same.

ANS: A DIF: Medium REF: Section 8.3 MSC: Applied TOP: 3Vi 39. Mars has a diameter that is approximately half that of Earth’s. If the interiors of these planets are heated by radioactive decays, how does the heating rate of Mars’ interior compare to that of Earth’s? a. Mars’s heating rate is 0.125 times that of Earth’s. b. Mars’s heating rate is 8 times that of the Earth’s. c. Mars’s heating rate is 0.5 times that of Earth’s. d. Mars’s heating rate is 4 times that of the Earth’s. e. The heating rates are about the same.

ANS: A DIF: Difficult REF: Section 8.3 MSC: Applied TOP: 3Vi 40. Consider an external solar system in which there are three terrestrial planets. All are located far from other objects so tidal forces aren’t significant. If planet A has a radius of 1 Earth radius, and planet B has a radius of 2 Earth radii, and planet C has a radius of 3 Earth radii, which planet has the highest chance of having at least a partially liquid core and a detectable magnetic field? a. Planet A b. Planet B c. Planet C d. They all have the same likelihood of having a liquid core. e. None of these planets should have a liquid core, because they all should have completely solidified.

ANS: C DIF: Medium REF: Section 8.3 MSC: Applied TOP: 3Vii

41. Which of the following is NOT a requirement for a planetary magnetic dynamo? a. rapid rotation b. solid iron core c. convective motions d. charged particles in the interior e. liquid interior ANS: B DIF: Medium REF: Section 8.3 MSC: Applied TOP: 3VIii | 3VIiv 42. The dynamo theory says that a planet will have a strong magnetic field if it has: a. fast rotation and a solid core b. slow rotation and a liquid core c. fast rotation and a liquid core d. slow rotation and a solid core e. fast rotation and a gaseous core ANS: C DIF: Easy REF: Section 8.3 MSC: Applied TOP: 3VIiv 43. Based on the assumption that a liquid, conducting core and rapid rotation are both required for a magnetic dynamo to operate, which terrestrial planets would you expect to have magnetic fields? a. only Earth b. only Earth, Venus, and Mars c. only Earth and Mars d. only Earth and Mercury e. Earth, Venus, Mars, and Mercury ANS: C DIF: Difficult REF: Section 8.3 MSC: Applied TOP: 3VIiv

44. Examine the figure below that shows the continental plates of the Earth and the locations of volcanoes and earthquakes. Which statement is FALSE?

a) The Earth’s crust is broken up into 13 separate continental plates. b) Volcanoes occur more often where two plates are coming together rather than spreading apart. c) Earthquakes happen where two plates come together and when they spread apart. d) The Atlantic Ocean is getting smaller with time. e) Southern California in the United States and Baja in Mexico are sliding northeastward relative to the rest of the North American Plate.

ANS: D DIF: Easy REF: Section 8.4 MSC: Factual TOP: 4Ii | 4Iii | 4Iv | 4Iix 45. Which of the following are NOT sites of frequent volcanic and earthquake activity on Earth? a. local hot spots b. spreading centers c. subduction zones d. transform faults e. inactive faults ANS: E DIF: Easy REF: Section 8.4 MSC: Factual TOP: 4Iii | 4Iv | 4Iviii | 4Iix | 4IIIi

46. What would you study in order to determine the timescale on which the Earth’s magnetic field reverses direction? a. a spreading center on the sea floor b. a volcano in the middle of a continental plate c. a fault at the border between two plates d. a subduction zone on the sea floor e. the rate of motion of tectonic plates ANS: A DIF: Easy REF: Section 8.4 MSC: Applied TOP: 4Iiii 47. Continental drift occurs at a typical rate of a few: a. mm/year b. cm/year c. m/year d. km/year e. nm/year ANS: B DIF: Easy REF: Section 8.4 MSC: Factual TOP: 4Iiv 48. The North American plate and the Pacific plate are sliding past one another at a rate of approximately 3 cm/year. San Francisco, which is located on the edge of the North American plate, is sliding southward toward Los Angles, which is located on the Pacific Plate. If they are currently separated by a distance of 600 km, how many years will it take for the two cities to meet? a. 3 million years b. 300,000 years c. 20 million years d. 20,000 years e. 600 years ANS: C DIF: Difficult REF: Section 8.4 MSC: Applied TOP: 4Iiv

49. If the Himalaya mountain range is presently 8,000 meters in height and is rising at a rate of 0.5 meters per century because of the convergence of two continental plates, how long did it take to create this mountain range? a. 1,600 years b. 160,000 years c. 1.6 million years d. 160 million years e. 1.6 billion years ANS: C DIF: Difficult REF: Section 8.4 MSC: Conceptual TOP: 4Ivii 50. Plate tectonics is NOT responsible for: a. mountain ranges b. canyons c. volcanoes d. ocean trenches e. continental drift ANS: B DIF: Easy REF: Section 8.4 MSC: Conceptual TOP: 4Iix

Figure 4 51. The large feature spanning the surface of Mars in Figure 4 is _________ and probably was created by _________. a. an impact crater; an asteroid or comet b. a dry riverbed; flowing water c. a canyon; a rapid cooling of the crust d. a canyon; flowing water e. a highway; an extinct civilization ANS: C DIF: Medium REF: Section 8.4 MSC: Factual TOP: 4IIiii 52. The large feature spanning the planet _________ in Figure 4 is called_________. a. Mars; Olympus Mons b. Venus; Valles Marineris c. Venus; Olympus Mons d. Mars; Valles Marineris e. Mercury; Caloris Basin ANS: D DIF: Easy REF: Section 8.4 MSC: Applied TOP: 4IIiii

53. The feature in the figure shown below is a(n) _________ , the largest one of its kind in the Solar System, and is located on the planet _________.

a. impact crater; Mercury b. mountain; Venus c. mountain; Earth d. volcano; Mars e. impact crater; the Moon ANS: D DIF: Easy REF: Section 8.4 MSC: Applied TOP: 4IIiii 54. Which terrestrial object shows the least evidence of recent volcanic activity? a. Mercury b. Venus c. Earth d. Mars e. the Moon ANS: A DIF: Easy REF: Section 8.4 MSC: Factual TOP: 4IIIii | 4IVi | 4IVii | 4IViii 55. The largest volcanic mountains in the Solar System are found on a. Mercury b. Venus c. Earth d. Mars e. the Moon ANS: D DIF: Easy REF: Section 8.4 MSC: Factual TOP: 4IVii

56. Which is NOT a reason for the large size of volcanoes on Mars as compared to Earth’s smaller volcanoes? a. absence of plate tectonics b. lack of atmosphere; therefore no erosion c. less gravity than other terrestrial planets d. many repeated eruptions e. All of these are reasons.

ANS: B DIF: Medium REF: Section 8.4 MSC: Applied TOP: 4IVii 57. The feature on Mars shown in the figure below is _________ named _________.

a. an impact crater; Meteor Crater b. a volcano; Olympus Mons c. a canyon ; Valles Marineris d. a canyon: Caloris Basin e. a mountain; Mount Neil Armstrong ANS: B DIF: Medium REF: Section 8.4 MSC: Applied TOP: 4IVii 58. Present-day erosion on the surface of the Moon is primarily caused by: a. flowing water b. wind c. solar radiation d. dust storms e. tectonic shifts ANS: C DIF: Easy REF: Section 8.4 MSC: Factual TOP: 4Vi

59. Which is NOT a reason that we suspect Mars once had liquid water on its surface? a. Mapping satellites have detected dry river beds. b. Rovers have detected minerals that must have formed in the presence of liquid water. c. Mapping satellites have detected outflow channels coming from impact craters. d. The observed presence of water ice in Mars’s polar icecaps. e. All of the above are reasons.

ANS: D DIF: Easy REF: Section 8.5 MSC: Factual TOP: 5Ii 60. We have direct evidence for the current existence of water on the surface of which terrestrial object? a. Mercury b. Venus c. Mars d. Ganymede e. Callisto ANS: C DIF: Easy REF: Section 8.5 MSC: Factual TOP: 5Iiii 61. Which is NOT a reason that we suspect that the extinction of the dinosaurs was caused by a large impact by a large object? a. Many dinosaur fossils are found below the K-T boundary, but none above it. b. The material in the K-T boundary is rich in iridium. c. Soot is found in the material in the K-T boundary, which probably came from fires caused by the impact. d. An impact crater has been found near Mexico’s Yucatan Peninsula. e. The remaining meteorite has been identified on the bottom of the Gulf of Mexico.

ANS: E DIF: Medium REF: Section 8.6 MSC: Factual TOP: 6Ii | 6Iii | 6Iiii 62. The rovers named Spirit and Opportunity that recently explored the surface of Mars discovered: a. tiny streams of flowing water too small to be detected by orbiting satellites b. minerals that must have formed in an environment rich in liquid water c. dust storms that rapidly erode the surfaces of most geological formations d. the northern polar ice cap is made primarily of frozen water ice e. the presence of methane that arises from biological life ANS: B DIF: Medium REF: Section 8.5 MSC: Factual TOP: 6Iii

SHORT ANSWER 1. Name the terrestrial planets in order of increasing distance from the Sun. What are the terrestrial planets in order of increasing geologic age of their surface?

ANS: In order of increasing distance from the Sun, they are Mercury, Venus, Earth, and Mars. In order of increasing geologic age of their surface, they are Earth, Venus, Mars, and Mercury.

DIF: Easy REF: Section 8.1 | Section 8.4 MSC: Factual TOP: 1Ii | 4Ivi | 4IIIi | 4IVii | 4Vi 2. What are the four main processes that shape the surfaces of the terrestrial planets?

ANS: The four main processes are impact cratering, plate tectonics, volcanism, and erosion.

DIF: Easy REF: Section 8.1 MSC: Factual TOP: 1IIIi | 1IIIii | 1IIIiii |1IIIiv 3. List the main process that shaped the planetary surface features illustrated in each of the four pictures shown below.

ANS: The important process for each of the figures are (a) impact cratering, (b) tectonism or plate tectonics, (c) volcanism or igneous activity, and (d) erosion.

DIF: Medium REF: Section 8.1 MSC: Applied TOP: 1IIIi | 1IIIii | 1IIIiii |1IIIiv 4. Give a specific example of a historical impact of an asteroid or comet that hit the Earth. Why are impact craters rare on the surface of Earth but plentiful on the Moon?

ANS: Examples of impacts on Earth include the (1) Meteor Crater in Arizona, or (2) the impact of a com-

et or asteroid on the Yucatan peninsula 65 million years ago that led to a mass extinction, including the dinosaurs, found in the fossil record at the K-T boundary. Impact craters on the Moon have been preserved because there is very little erosion and no plate tectonics, but those have erased the evidence of past cratering on the surface of the Earth.

DIF: Medium REF: Section 8.2 | Section 8.4 | Section 8.6 MSC: Factual TOP: 2Iii | | 4Vi | 6Ii 5. List the three areas of the lunar surface shown in the figure below in the order of their age from youngest to oldest. Explain your reasoning.

ANS: B, C, A. Reason: the more heavily cratered, the older the surface is.

DIF: Medium REF: Section 8.2 MSC: Applied TOP: 2IIIi 6. If you obtained a sample of Martian rock, determined the abundances of 230U and 207Pb in it, and found that for every one uranium atom there were seven lead atoms, then what would the age of this rock be?

Note that 230U decays to 207Pb with a half-life of 700 million years. Assume that there was no 207Pb in the rock when it originally formed.

ANS: The ratio of the original amount of uranium to final amount of uranium is 1/(1  7)  1/8. The number of half lives that have passed, n, is equal to (1/2)x  1/8, and n  3. Therefore, the age of this Martian rock is 3  half life  3  700 million yr  2.1 billion yr.

DIF: Difficult REF: Section 8.2 MSC: Applied TOP: 2IViii

7. Describe the difference between seismic primary and secondary waves and why this difference makes them useful in probing the structure of the Earth’s interior.

ANS: Secondary waves are transverse waves. They propagate because rock springs back after being bent. However, liquids do not spring back; therefore, secondary waves cannot travel through a liquid.

Primary waves are longitudinal waves, and they can be transmitted through liquid, although their path of propagation is bent toward the lesser dense regions. Because the two waves propagate differently, they can be used to probe the interior of the Earth. Primary waves can go through the liquid regions and travel long distances through the Earth’s interior while secondary waves probe the upper region of the mantle and crust.

DIF: Medium REF: Section 8.3 MSC: Conceptual TOP: 3Ii | 3Iii | 3Iiii 8. List the names of the four layers of Earth’s interior shown in the figure below going from the outer layer to the innermost layer, and designate whether they are solid or liquid.

ANS: A—crust/solid, B—mantle/liquid, C—outer core/liquid, and D—inner core/solid.

DIF: Easy REF: Section 8.3 MSC: Factual TOP: 3IIii | 3IIiii | 3IIiv 9. Which is denser—the mantle or crust of the Earth? Explain why.

ANS: Because Earth’s mantle is liquid, differentiation has occurred with heavier elements sinking to the core and lighter elements floating to the surface, where they have cooled and formed the crust. Thus the crust is less dense than the mantle.

DIF: Easy REF: Section 8.3 MSC: Conceptual TOP: 3IIv 10. Suppose that two planets of the same size formed from the same material. If planet A had differentiated and planet B had not, how would samples of their surface rock differ? Explain why.

ANS: Planet A’s surface rock would be lower in density than planet B because in planet A, not B, the material was liquid and heavier material fell to the center of the planet and lighter material rose to the top and formed the crust. In planet B, all the material was mixed together evenly.

DIF: Easy REF: Section 8.3 MSC: Applied TOP: 3IIv 11. How did the radioactive heating of Earth vary from when it was first formed 4.6 billion years ago until today?

ANS: Radioactivity in the rocks on Earth (or in any rocky object) decreases with time as more and more of the radioactive elements decay to daughter elements, which are not radioactive. On Earth, this meant that heating from radioactivity was at its highest when Earth was young and heated by short-lived radioactive isotopes. As these were used up, long-lived radioactive isotopes took their place as the main heat source. These are sufficient to keep Earth’s mantle in a molten state, although in general the interior is much cooler than it was at its formation. As time continues, the long-lived radioactive fuel supply decreases, and Earth’s interior continues to cool.

DIF: Medium REF: Section 8.3 MSC: Conceptual TOP: 3IVii 12. Which planet would you expect to have a larger molten core, a planet of Earth’s size or a planet that had half the radius of Earth? Explain why.

ANS: Because planets’ interiors are kept molten by the energy released by radioactivity, the heating rate of a planet’s core is proportional to the volume of the planet (V portion to their surface area (A

R3) But planets lose their heat in pro-

R2) Therefore, a planet with a larger radius will have a larger molten

core. The Earth-sized planet will have a larger molten core compared to a planet that has only half its radius.

DIF: Difficult REF: Section 8.3 MSC: Applied TOP: 3Vi | 3Vii 13. Describe two reasons why we know that the Earth’s magnetic field cannot be a result of permanent magnetism in a solid iron core.

ANS: First, Earth’s interior temperature is too high (iron loses its magnetism at high temperatures). Second, Earth’s field has been shown to change with time (the positions of the magnetic poles change along with entire field reversals).

DIF: Medium REF: Section 8.3 MSC: Conceptual TOP: 3VIii | 3VIiii 14. How do we know that Earth’s magnetic field has flipped its polarity many times in the past?

ANS: Because the direction of Earth’s magnetic field becomes imprinted on iron in the magma as it rises up and cools to form new rock, where two plates are moving apart from one another on the seafloor.

This results in a magnetic striping of alternate magnetic field directions that we observe in rocks on the seafloor.

DIF: Easy REF: Section 8.4 MSC: Conceptual TOP: 3VIiii 15. Three different things can happen when two continental plates meet. What is the name given to each, and briefly explain what happens in each.

ANS: (1) Subduction can happen when one plate dips down below the other one and melts; (2) two plates can be moving away from one another and form a spreading center; or (3) the two plates can be sliding past one another but stick due to friction, which is called a transform or stick-slip fault.

DIF: Easy REF: Section 8.4 MSC: Factual TOP: 4Iii | 4Iv | 4Iviii 16. The American and African/European continents are now separated by the Atlantic Ocean, which is approximately 4,000 km wide. Assuming a continental drift rate of 2 cm/year, how long has it been since they were one land mass?

ANS: If we assume a constant drift rate of 2 cm/year, we can set up the relationship 2 cm/year  4,000 km/X years, where X is the amount of time since the two landmasses were one. Rearranging, X  4,000 km/(2 cm/year)  4  108 cm/(2 cm/year)  200 million years ago.

DIF: Medium REF: Section 8.4 MSC: Applied TOP: 4Iiv 17. Describe one example of tectonic disruption on Mercury, Venus, and Mars, respectively, and explain how they formed.

ANS: Mercury has evidence of faults due to the core rapidly cooling and shrinking. Venus has circular fractures, which may be due to upwelling plumes of hot mantle material deforming the surface, and many volcanoes. Mars has Valles Marineris, a giant canyon, which may have been created as mantle convection split the lithosphere apart in the past.

DIF: Easy REF: Section 8.4 MSC: Conceptual TOP: 4IIii | 4IIiii | 4IIiv

18. What is one major difference between the volcanoes on Venus and Mars and the volcanoes on the Earth?

What might explain this difference?

ANS: Venus and Mars have many more shield volcanoes than the Earth, and these volcanoes are much higher than the Earth’s. This is probably due to the fact that the hot spots on the Earth’s crust move around because of continental drift, while they stay in the same place on Venus and Mars because those planets currently do not have moving plates.

DIF: Medium REF: Section 8.4 MSC: Conceptual TOP: 4IIIii | 4IVii | 4IViii 19. If there is water on Mars today, where is it likely to be? Name two separate pieces of evidence we have that Mars once had flowing water on its surface and how this evidence was obtained.

ANS: Water on Mars today is likely to be frozen in the ground and in its polar ice caps, although most of the ice caps are frozen carbon dioxide. Two pieces of evidence that Mars once had flowing water on its surface are (1) observations of dry river beds and outflows from impact craters on the Martian surface come from satellites used to make high spatial resolution map its surface; and (2) the Martian rovers

Spirit and Opportunity have found evidence for certain types of rock that must form in the presence of liquid water.

DIF: Medium REF: Section 8.5 MSC: Factual TOP: 5Ii | 5Iii | 5Iiii 20. What are two materials present in the K-T boundary that support the idea that a 10 km-wide asteroid or comet hit the Yucatan peninsula and caused or accelerated the extinction of more than 50 percent of all living species on the Earth? Explain where these two materials came from. How long ago did this happen?

ANS: Iridium and soot mark the K-T boundary. The iridium came from the asteroid or comet itself, and the soot came from the widespread fires generated by the impact and explosion. This happened approximately 65 million years ago.

DIF: Difficult REF: Section 8.6 MSC: Factual TOP: 6Iiii

CHAPTER 9

Atmospheres of the Terrestrial Planets

CONCEPT MAP Sec 9.1 1. Atmospheres Are Oceans of Air I. Primary Atmospheres i. Primary atmospheres consist of the gas in the solar nebula; primarily H and He (SA: 1) ii. Average velocity of particles in a gas: v  (3kT/m) 1⁄2 (MC: 1–5) iii. Equipartition of kinetic energy, E  1⁄2 mv2, means heavier molecules move slower than lighter molecules (MC: 6, SA: 2) iv. Sun heats the atmosphere, thermal equilibrium is established, and atoms or molecules in the gas escape the planet’s atmosphere if v  1⁄6  vescape (MC: 1–5, 7, SA: 2) II. Secondary Atmospheres i. Secondary atmospheres arise from accretion, volcanism, and comet impacts (MC: 8, SA: 3) ii. Cometary impacts bring ammonia (NH4) to an atmosphere, and UV light breaks it up and releases the nitrogen (TF: 1, MC: 8, 9, SA: 3, 4) Sec 9.2 2. Evolution of Secondary Atmospheres I. Similarities and Differences i. Mass of Earth and Venus similar, but Mars has only 0.1  MEarth (MC: 10) ii. Atmospheres of Mars and Venus: 95 percent CO2, few percent N2 (TF: 2) iii. Earth’s atmosphere: 78 percent N2, 21 percent O2, and traces of CO2 and H2O iv. Atmospheric pressures: Mars (6  10 3), Earth (1), Venus (92) (TF: 3, 11, MC: 11, 20) II. Explaining the Differences in Atmospheric Compositions

i. CO2 and H2O outgas from volcanoes and N2 deposited by comets (MC: 12, 13) ii. Greenhouse effect: greenhouse gases like carbon dioxide, water vapor, and methane trap IR radiation, retain heat, and warm the planet (MC: 14, 15) iii. Temperature increase due to greenhouse effect: Mars: 5 K, Earth: 35 K; Venus: 400 K (TF: 2, MC: 16—18, SA: 5) iv. Moon and Mars: lost atmospheres (MC: 1) v. Venus: runaway greenhouse effect (TF: 4, MC: 13) vi. Earth: surface water is liquid rather than frozen due to the greenhouse effect (TF: 5, MC: 17, 20, 21) vii. Earth: CO2 locked up in rock (e.g., limestone) lessens greenhouse effect (TF: 4, MC: 22, 23, SA: 6) Sec 9.3 3. Earth’s Atmosphere I. Important Facts i. Earth’s atmosphere: thickness of several  100 km, mass of 10 6  MEarth (SA: 7, 8) ii. Pressure: 1 bar  the pressure at sea level (MC: 24, SA: 9,10) iii. Equipartition of kinetic energy: E  1⁄2 mv2 (MC: 6, 25, 26) iv. Ideal gas law: P  T (MC: 27, SA: 11–12) II. Life Controls the Composition of the Earth’s Atmosphere i. Oxygen is very chemically reactive ii. Earth’s oxygen is constantly replenished by plant life/photosynthesis (MC: 28—31) iii. Bacteria and algae started producing significant amounts of O2 in the Earth’s atmosphere only 2.5 billion years ago; O2 became a large component Earth’s atmosphere (80 percent of today’s abundance) only 0.5 billion years ago (TF: 6, MC: 32–35, SA: 13) iv. Life on other planets can be detected by oxygen and methane absorption (TF: 7, MC: 36) III. Global Changes i. Carbon: sources (fossil fuel burning) and sinks (plant life, coral reefs, shells) (MC: 37) ii. Global warming caused by increase in CO2 since Industrial Revolution (MC: 37–38, SA: 4) iii. Ozone (O3): photodissociation, recombination, absorbs UV radiation (SA: 15) iv. Ozone depletion and ozone hole (TF: 8) IV. Layers in the Atmosphere i. Atmospheric layers are denoted by the change in temperature as a function of altitude (MC: 39, 40) ii. Troposphere: Earth’s lower atmosphere, 90 percent by mass, weather occurs here, pressure and temperature decrease with increasing altitude (MC: 41–43, SA: 16)

iii. Convection carries energy upward iv. Condensation leads to cloud formation v. Tropopause: temperature no longer decreases with increasing altitude vi. Stratosphere: temperature increases with increasing altitude due to UV absorption (MC: 44, SA: 17) vii. Mesosphere: temperature lowers with increasing altitude (MC: 43) viii. Thermosphere: high temperature due to ionization by solar wind ix. Magnetosphere (MC: 45) x. Van Allen radiation belt, synchrotron radiation, and aurorae (TF: 9, MC: 46) V. Why the Wind Blows i. Winds and weather patterns caused by a combination of nonuniform solar heating, Hadley circulation, and the Coriolis effect (TF: 10, MC: 47, SA: 18) ii. Hadley circulation versus zonal winds (TF: 11, MC: 48) iii. Rotation of high- and low-pressure systems iv. Runaway convection leads to violent weather (MC: 49) v. Lightning carries powerful electric currents vi. Hurricanes are powered by the heat of vaporization of water (MC: 50) vii. Tornadoes arise because of thermal updrafts and angular momentum conservation VI. Climate i. Climate: average conditions over long time intervals ii. Few degrees variation in Earth’s temperature can cause climate change (SA: 14) iii. Temperature of Earth has risen by 1 C over the last 50 years Sec 9.4 4. Venus: Earth’s Sister Planet I. Atmosphere i. Hot and dense; large greenhouse effect (TF: 12, MC: 51, SA: 19) ii. Conditions of surface of Venus: T  737 K, sulfuric acid rain iii. Scattering: occurs when  of light is approximately the size of intervening particles iv. Earth’s sky is blue and sunsets are red due to scattering (MC: 52) II. Rotation i. Venus’s sidereal rotation period: 243 day and retrograde (MC: 53) ii. Solar day on Venus: 117 day iii. Venus’s atmospheric circulation: mostly Hadley circulation (TF: 11, 13, MC: 54)

Sec 9.5 5. Mars I. Atmosphere and Weather i. Thin atmosphere; cold planet (20°C in daytime, 100°C in nighttime); poles colder than equator (MC: 55) ii. Water sublimates on Martian surface (TF: 14, MC: 56, SA: 20) iii. No oxygen, no ozone, and copious UV radiation on surface of Mars iv. Mars’s seasons more dramatic than Earth’s due to its elliptical orbit (SA: 20) Sec 9.6 6. Mercury and the Moon I. Atmosphere and Weather i. No atmosphere; no weather; large swings in day- and nighttime temperatures (TF: 15, MC: 57) ii. Very small amounts of gas captured from solar wind or released by impacts

TRUE/FALSE 1. The major chemical component of the air we breathe today was deposited on Earth primarily by cometary impacts.

ANS: T DIF: Medium REF: Section 9.1 MSC: Factual TOP: 1IIii 2. The atmosphere of Mars is mostly composed of carbon dioxide; therefore the greenhouse effect makes the average temperature 35 degrees warmer than it would be without its atmosphere.

ANS: F DIF: Medium REF: Section 9.2 MSC: Applied TOP: 2Iii | 2IIiii 3. All the terrestrial planets have atmospheres as dense or denser than Earth’s atmosphere.

ANS: F DIF: Easy REF: Section 9.2 MSC: Factual TOP: 2Iiv 4. If the carbon dioxide in Earth’s rocks were suddenly released into its atmosphere, Earth could possibly undergo a runaway greenhouse effect.

ANS: T DIF: Medium REF: Section 9.2 MSC: Applied TOP: 2IIv | 2IIvii 5. In the absence of a greenhouse effect, water on the surface of Earth would be frozen.

ANS: T DIF: Easy REF: Section 9.2 MSC: Applied TOP: 2IIvi

6. The fraction of oxygen in the Earth’s atmosphere has always remained at about the same as it is today.

ANS: F DIF: Easy REF: Section 9.3 MSC: Factual TOP: 3IIiii 7. The best way to look for life on other planets is to search for absorption from nitrogen in their atmospheres in the infrared region of the spectrum.

ANS: F DIF: Difficult REF: Section 9.3 MSC: Factual TOP: 3IIiv 8. Each halogen atom, such as chlorine, fluorine, and bromine, in the Earth’s atmosphere catalyzes the destruction of ozone for a long time ranging from decades to centuries.

ANS: T DIF: Medium REF: Section 9.3. MSC: Factual TOP: 3IIIiv 9. Aurorae are produced only near the northern and southern magnetic poles of a planet because charged particles arriving in the solar wind cannot cross the magnetic field lines.

ANS: T DIF: Easy REF: Section 9.3 MSC: Factual TOP: 3IVx 10. Winds are generated on Earth primarily because the Sun unevenly heats our rotating planet.

ANS: T DIF: Easy REF: Section 9.3 MSC: Conceptual TOP: 3Vi 11. The planet-wide flow of air from a warmer equator to the colder poles is called Hadley circulation, and an example of this effect is seen on the planet Venus.

ANS: T DIF: Medium REF: Section 9.3 MSC: Factual TOP: 3Vii | 4IIiii 12. Venus’s atmospheric clouds are so thick that the surface of the planet is not seen when observing it in visible light.

ANS: T DIF: Easy REF: Section 9.4 MSC: Factual TOP: 4Ii 13. Venus rotates so rapidly that the dominant form of atmospheric circulation is powered by hurricanes moving from its equator to its poles.

ANS: F DIF: Medium REF: Section 9.4 MSC: Applied TOP: 4IIiii 14. When the Martian summer occurs and the daytime temperature is 20°C, water on the surface melts and forms small pools of liquid.

ANS: F DIF: Difficult REF: Section 9.5 MSC: Applied TOP: 5Iii

15. The temperature differences between night and day on the Moon and Mercury are very small because they have so little atmosphere.

ANS: F DIF: Difficult REF: Section 9.6 MSC: Applied TOP: 6Ii

MULTIPLE CHOICE 1. What is the reason Mercury has so little gas in its atmosphere? a. Its mass is small. b. It has a high temperature. c. It is close to the Sun. d. Its escape velocity is low. e. All of the above are reasons.

ANS: E DIF: Easy REF: Section 9.1 MSC: Applied TOP: 1Iii | 1Iiv 2. Why did the terrestrial planets lose the majority of the gas in their primary atmospheres? a. They were too hot and their escape velocities too low to hold onto them. b. The solar wind was too strong and blew these gasses off the planets. c. Their high surface temperatures made the gas chemically react with the rock. d. The centrifugal force from the planets’ fast rotation rates made them fly off. e. The initial gases were so heavy when the planet differentiated that they sank to the core.

ANS: A DIF: Easy REF: Section 9.1 MSC: Conceptual TOP: 1Iii | 1Iiv 3. Would a nitrogen atom in Venus’s atmosphere, whose temperature is 740 K, eventually escape into outer space? Note that a nitrogen atom has a mass that is 14 times that of a hydrogen atom. Recall that atoms eventually will escape if their average velocity is greater than 1/6 times the escape velocity of the planet.

The escape velocity of Venus is 10 km/s. For comparison, a hydrogen atom has an average velocity of 2.5 km/s at a temperature of 300 K. a. The average velocity of nitrogen atoms is 0.4 km/s, and nitrogen does not escape. b. The average velocity of nitrogen atoms is 1.0 km/s, and nitrogen does not escape. c. The average velocity of nitrogen atoms is 1.0 km/s, and nitrogen escapes. d. The average velocity of nitrogen atoms is 4.5 km/s, and nitrogen does not escape. e. The average velocity of nitrogen atoms is 4.5 km/s, and nitrogen escapes.

ANS: B DIF: Medium REF: Section 9.1 MSC: Applied TOP: 1Iii | 1Iiv

4. Would water molecules in Venus’s atmosphere, whose temperature is 740 K, eventually escape into outer space? Note that a water molecule has a mass that is 18 times that of a hydrogen atom. The escape velocity of Venus is 10 km/s. For comparison, a hydrogen atom has an average velocity of 2.5 km/s at a temperature of 300 K. a. No, the average velocity of water molecules is 0.9 km/s. b. Yes, the average velocity of water molecules is 0.9 km/s. c. Yes, the average velocity of water molecules is 2.1 km/s. d. No, the average velocity of water molecules is 2.1 km/s. e. Yes, the average velocity of water molecules is 19 km/s.

ANS: A DIF: Difficult REF: Section 9.1 MSC: Applied TOP: 1Iii | 1Iiv 5. If sunlight broke up water molecules in Venus’s atmosphere, would the hydrogen atoms escape into outer space? Note that Venus’s temperature is 740 K. Recall that gas eventually will escape if the average velocity of its atoms is greater than 1/6 times the escape velocity of the planet. The escape velocity of Venus is 10 km/s. a. No, the average velocity of hydrogen atoms would be 0.8 km/s. b. No, the average velocity of hydrogen atoms would be 3.9 km/s. c. Yes, the average velocity of hydrogen atoms would be 3.9 km/s. d. Yes, the average velocity of hydrogen atoms would be 25 km/s. e. No, the average velocity of hydrogen atoms would be 25 km/s.

ANS: C DIF: Medium REF: Section 9.1 MSC: Applied TOP: 1Iii | 1Iiv 6. If an average hydrogen atom in Earth’s atmosphere has a velocity of 2.5 km/s, what would be the average velocity of an oxygen molecule in the Earth’s atmosphere? Note that the atomic mass of an oxygen atom is 16 times that of a hydrogen atom. a. 0.16 km/s b. 2.5 km/s c. 0.62 km/s d. 0.44 km/s e. 0.25 km/s ANS: D DIF: Medium REF: Section 9.1 | Section 9.3 MSC: Applied TOP: 1Iiii | 3Iiii

7. A gas eventually will escape from a planet’s atmosphere if the average velocity of the atoms exceeds 1/6 times the escape velocity of the planet. If the average velocity of water vapor in Venus’s atmosphere is 0.9 km/s, would it eventually escape into outer space? Note that Venus’s mass is 5  1024 kg and its radius is 6,050 km. a. Water vapor would escape because 1⁄6 times the escape velocity is 0.51 km/s. b. Water vapor would not escape because 1⁄6 times the escape velocity is 1.7 km/s. c. Water vapor would escape because 1⁄6 times the escape velocity is 0.42 km/s. d. Water vapor would not escape because 1⁄6 times the escape velocity is 2.6 km/s. e. Water vapor would escape because 1⁄6 times the escape velocity is 1.3 km/s.

ANS: B DIF: Difficult REF: Section 9.1 MSC: Applied TOP: 1Iiv 8. Which of the following processes did NOT contribute gas to Earth’s secondary atmosphere? a. Volcanism b. Accretion c. Oxidation d. Comet impacts e. All of the above contributed gasses to Earth’s secondary atmosphere.

ANS: C DIF: Medium REF: Section 9.1 MSC: Conceptual TOP: 1IIi | 1IIii 9. The nitrogen in the Earth’s atmosphere primarily came from: a. ammonia delivered by comet impacts b. photosynthesis done by algae and plants c. oxidation of silicate rich minerals d. rock delivered by asteroid impacts e. its primary atmosphere ANS: A DIF: Easy REF: Section 9.1 MSC: Factual TOP: 1IIii

10. Based solely on mass and distance from the Sun, which of the following terrestrial planets would you expect to retain the densest secondary atmosphere? a. Mercury b. Venus c. Mars d. the Moon e. Earth ANS: E DIF: Medium REF: Section 9.2 MSC: Applied TOP: 2Ii 11. Earth has roughly _________ times more atmospheric pressure than Mars and _________ times less than Venus. a. 10; 10 b. 200; 100 c. 2,000; 2 d. 2; 10 e. 1,000; 200 ANS: B DIF: Difficult REF: Section 9.2 MSC: Factual TOP: 2Iiv 12. The presence of gases like carbon dioxide and water vapor in a planet’s atmosphere is direct evidence of _________ in a planet’s history. a. high surface temperatures b. volcanic activity c. cometary impacts d. a lack of asteroid impacts e. the greenhouse effect ANS: B DIF: Medium REF: Section 8.2 MSC: Applied TOP: 2IIi

13. The main greenhouse gases in the atmosphere of the terrestrial planets are: a. oxygen and nitrogen b. methane and ozone c. carbon dioxide and water vapor d. hydrogen and helium e. methane and ammonia ANS: C DIF: Easy REF: Section 8.2 MSC: Factual TOP: 2IIi 14. Earth releases the energy it receives from the Sun by emitting _________ radiation. a. infrared b. visible c. ultraviolet d. radio e. microwave ANS: A DIF: Easy REF: Section 9.2 MSC: Factual TOP: 2IIii 15. What makes carbon dioxide a highly effective greenhouse gas? a. It easily absorbs UV radiation. b. It easily absorbs visible light. c. It easily absorbs infrared radiation. d. It easily reacts chemically with rock. e. It easily photodissociates in the upper atmosphere.

ANS: C DIF: Medium REF: Section 9.2 MSC: Conceptual TOP: 2IIii 16. The greenhouse effect raises Earth’s surface temperature by roughly: a. 0 K b. 0.35 K c. 3.5 K d. 35 K e. 350 K ANS: D DIF: Medium REF: Section 9.2 MSC: Factual TOP: 2IIiii

17. If it were not for the greenhouse effect on Earth: a. there would be no liquid water on Earth b. life as we know it would not have developed on Earth c. it would be a much colder planet d. there would be no oxygen in the Earth’s atmosphere e. All of the above are results of the greenhouse effect.

ANS: E DIF: Difficult REF: Section 9.2 MSC: Applied TOP: 2IIiii | 2IIvi 18. If water vapor were released from Venus’s surface due to tectonic activity into its upper atmosphere, what would most likely happen to it? a. The water vapor would relieve the greenhouse effect and decrease Venus’s surface temperature. b. Water droplets would condense into rain and form lakes on Venus’s surface. c. The water vapor would chemically react with carbon dioxide and form acid rain. d. UV light would break apart the water molecules, and the hydrogen would be lost into space. e. It would rise into the atmosphere and form hurricane-like storms.

ANS: D DIF: Difficult REF: Section 9.2 MSC: Applied TOP: 2IIiii 19. When learning about light, we predicted that Venus should have a temperature of 250 K based on its albedo and distance from the Sun. Why is Venus’s observed average surface temperature equal to 740 K, which is hot enough to melt lead? a. Venus has slow, retrograde rotation, and its seasons are very long. b. Venus has many active volcanoes that release heat into its atmosphere. c. Venus has a very thin atmosphere, and more sunlight falls onto its surface. d. Venus has a strong greenhouse effect. e. Venus has a highly eccentric orbit and is sometimes much closer to the Sun than other times.

ANS: D DIF: Easy REF: Section 9.2 MSC: Applied TOP: 2IIiii

20. In the absence of the greenhouse effect, the water on the surface of the Earth would: a. escape into outer space b. remain in liquid form c. vaporize and form clouds in the atmosphere d. freeze e. be absorbed into rocks ANS: D DIF: Easy REF: Section 9.2 MSC: Factual TOP: 2IIvi 21. By examining the images shown below, what can you conclude?

a. Venus is covered with clouds. b. Earth has a large amount of liquid water. c. Some form of ice does exist on Mars, but it does not have large amounts of liquid water. d. The planets in order from the least to most dense atmospheres are Venus, Earth, and Mars. e. All of the above are valid conclusions.

ANS: E DIF: Easy REF: Section 9.2 MSC: Applied TOP: 2IIvi 22. Like Mars and Venus, Earth originally had a significant amount of carbon dioxide in its atmosphere. Where is the majority of the carbon now? a. It has escaped into outer space. b. It is bound up in the plant life on Earth. c. It is bound up in rocks. d. It is dissolved into the oceans. e. It is still in the atmosphere in the form of complex molecules.

ANS: C DIF: Medium REF: Section 9.2 MSC: Factual TOP: 2IIvii

23. Venus and Earth probably formed with similar amounts of carbon dioxide in their secondary atmospheres.

Which of the following is TRUE? a. The majority of Earth’s carbon dioxide escaped into space because of its hotter temperature, while Venus’s carbon dioxide remains gravitationally bound to Venus. b. The majority of Earth’s carbon is now bound up in rock while Venus’s remains in its atmosphere. c. Earth lost more of its secondary atmosphere because it was bombarded by more planetesimals than Venus. d. The majority of Earth’s carbon was absorbed by plants during photosynthesis. e. Earth and Venus still have equal amounts of carbon dioxide in their atmospheres.

ANS: B DIF: Medium REF: Section 9.2 MSC: Conceptual TOP: 2IIvii 24. Which molecule moves with the fastest average speed while being bound in the Earth’s atmosphere in thermal equilibrium? a. Water, H2O (atomic mass  18) b. Carbon dioxide, CO2 (atomic mass  44) c. Nitrogen (atomic mass  28) d. Oxygen (atomic mass  32) e. Hydrogen, H2 (atomic mass  2) ANS: E DIF: Easy REF: Section 9.3 MSC: Applied TOP: 3Iii 25. According to the Ideal Gas law, if you blow up a balloon, seal it, and then lower its temperature, the balloon will: a. contract b. expand c. remain the same size d. explode ANS: A DIF: Easy REF: Section 9.3 MSC: Applied TOP: 3Iiii

Figure 1

26. According to Figure 1, what effect does doubling the temperature of a gas while keeping its volume constant have on the pressure of the gas? a. The pressure doubles. b. The pressure is cut in half. c. The pressure remains the same. d. The volume quadruples. e. The volume is cut to one-third its original value.

ANS: A DIF: Medium REF: Section 9.3 MSC: Applied TOP: 3Iiii 27. According to Figure 1, what must be done to the volume of the gas in order to double its pressure while keeping its temperature constant? a. The volume should be doubled. b. The volume should be cut in half. c. The volume should remain the same. d. The volume should be tripled. e. The volume should be cut to one-fourth its original value.

ANS: B DIF: Medium REF: Section 9.3 MSC: Applied TOP: 3Iiv 28. The major difference in the composition of Earth’s atmosphere compared to the atmospheres of Venus and Mars is a direct consequence of: a. life on Earth b. Earth’s plate tectonics c. differences in the greenhouse effect d. the presence of liquid water e. differing distances from the Sun ANS: A DIF: Medium REF: Section 8.3 MSC: Applied TOP: 3IIii

29. According to the figure, about how long ago did oxygen reach its current abundance in Earth’s atmosphere?

a. 3 billion years ago b. 1 billion years ago c. 0.5 billion years ago d. 0.25 billion years ago e. 0.1 billion years ago ANS: D DIF: Medium REF: Section 9.3 MSC: Applied TOP: 3IIii 30. _________ in our atmosphere is a direct consequence of the emergence of life. a. Carbon dioxide b. Water vapor c. Nitrogen d. Oxygen e. Helium ANS: D DIF: Easy REF: Section 9.3 MSC: Applied TOP: 3IIii 31. If photosynthesis were to disappear on Earth: a. the atmosphere would become less dense b. oxygen would disappear from the atmosphere c. the atmosphere would become hotter d. nitrogen would disappear from the atmosphere e. the amount of water vapor in the atmosphere would decrease ANS: B DIF: Easy REF: Section 9.3 MSC: Applied TOP: 3IIii

32. Approximately _________ years ago, _________ began producing oxygen in enough amounts to be a significant fraction in the Earth’s atmosphere. a. 100 million; trees and plants b. 1 billion; trees and plants c. 250 million; bacteria and algae d. 2.5 billion; bacteria and algae e. 2,000; animals and humans ANS: C DIF: Medium REF: Section 9.3 MSC: Factual TOP: 3IIiii 33. Approximately how long after the Solar System formed did it take for oxygen to get to within 80 percent of its present abundance in the Earth’s atmosphere? a. 4 billion years b. 1 billion years c. 400 million years d. 1 million years e. Oxygen was always a primary component of the Earth’s atmosphere.

ANS: A DIF: Difficult REF: Section 9.3 MSC: Factual TOP: 3IIiii 34. For the first 1 billion years of the Earth’s evolution, the fraction of oxygen in its atmosphere was approximately: a. zero b. 1⁄2 of what it is today c. 2 times what it is today d. 10 times what it is today e. the same as it is today ANS: A DIF: Easy REF: Section 9.3 MSC: Factual TOP: 3IIiii

35. According to the figure shown below, approximately how many years ago did oxygen finally get to half its current abundance in Earth’s atmosphere?

a. 3 billion years ago b. 1 billion years ago c. 0.6 billion years ago d. 0.25 billion years ago e. 0.1 billion years ago ANS: C DIF: Medium REF: Section 9.3 MSC: Applied TOP: 3IIiii 36. If you found absorption rising from _________ in the spectrum of a planet, you could conclude that it might contain some form of life. a. oxygen b. methane c. water vapor d. oxygen, methane, or water vapor ANS: D DIF: Easy REF: Section 9.3 MSC: Factual TOP: 3IIiv 37. The amount of carbon dioxide in the Earth’s atmosphere has been increasing over the last 50 years because of: a. global warming b. the growth of the ozone hole c. the burning of fossil fuels d. increased energy output from the Sun e. increased magnetic activity in the Sun ANS: C DIF: Easy REF: Section 9.3 MSC: Factual TOP: 3IIIi | 3IIIii

38. Without the ozone layer, life on Earth would be in danger from increased levels of _________ radiation. a. ultraviolet b. X-ray c. gamma ray d. infrared e. microwave ANS: A DIF: Easy REF: Section 9.3 MSC: Factual TOP: 3IIIii 39. According to the figure shown below, the different layers of Earth’s atmosphere are defined by:

a. how the temperature varies with altitude b. how the pressure varies with altitude c. how the density varies with altitude d. different temperature ranges e. different pressure ranges ANS: A DIF: Easy REF: Section 9.3 MSC: Applied TOP: 3IVi

40. According to the way the layers of Earth’s atmosphere are defined in the figure shown below, the atmosphere of Venus has only _________ distinct layer(s).

a. one b. two c. three d. four e. five

ANS: B DIF: Medium REF: Section 9.3 MSC: Applied TOP: 3IVi 41. All weather and wind on Earth are a result of convection in the: a. troposphere b. stratosphere c. mesosphere d. ionosphere e. thermosphere ANS: A DIF: Medium REF: Section 9.3 MSC: Factual TOP: 3IVii 42. According to the figure shown below, as you increase in altitude in the Earth’s lower atmosphere, the atmospheric pressure _________ dramatically at a(n) _________ rate.

a. increases; increasing b. increases; decreasing c. decreases; decreasing d. decreases; increasing e. decreases; constant ANS: D DIF: Medium REF: Section 9.3 MSC: Applied TOP: 3IVii

43. The only two layers of Earth’s atmosphere that have temperature gradients that allow convection to take place are: a. the troposphere and the thermosphere b. the mesosphere and the stratosphere c. the thermosphere and the stratosphere d. the troposphere and the mesosphere e. the troposphere and the stratosphere ANS: D DIF: Difficult REF: Section 9.3 MSC: Factual TOP: 3IVii | 3IVvii 44. Heating from _________ causes the top of the Earth’s stratosphere to be warmer than the bottom. a. higher-energy particles in the solar wind b. convection c. the ozone layer absorbing UV light d. charged particles trapped by magnetic fields e. the greenhouse effect ANS: C DIF: Medium REF: Section 9.3 MSC: Factual TOP: 3IVvi 45. The shape of the Earth’s magnetosphere is modified by: a. the Moon’s tidal force b. the solar wind c. Earth’s own gravity d. asymmetries in the shape of Earth’s core e. Earth’s elliptical orbit ANS: B DIF: Medium REF: Section 9.3 MSC: Conceptual TOP: 3IVix 46. Auroras are caused by: a. gases fluorescing in the atmosphere due to collisions with solar wind particles b. the magnetosphere of Earth touching its atmosphere c. the ozone layer being destroyed by UV light d. a product of the atmospheric greenhouse effect e. scattering of sunlight from particles in the Earth’s stratosphere ANS: A DIF: Easy REF: Section 9.3 MSC: Factual TOP: 3IVx

47. In the Southern hemisphere, hurricanes _________ compared to hurricanes in the Northern hemisphere due to the Coriolis effect. a. rotate in the same direction b. rotate in the opposite direction c. move from east to west d. have larger wind speeds e. cause more damage ANS: B DIF: Easy REF: Section 9.3 MSC: Conceptual TOP: 3Vi 48. What is the main reason Hadley circulation in a planet’s atmosphere breaks up into zonal winds? a. convection driven by solar heating b. heating from the solar wind c. hurricanes developing along the planet’s equator d. a planet’s rapid rotation e. heating from the greenhouse effect ANS: D DIF: Difficult REF: Section 9.3 MSC: Conceptual TOP: 3Vii 49. Runaway convection in the Earth’s atmosphere can lead to: a. snow b. destruction of ozone c. aurorae d. acid rain e. violent storms ANS: E DIF: Easy REF: Section 9.3 MSC: Factual TOP: 3Viv 50. Hurricanes are powered by: a. Hadley circulation b. the Coriolis effect c. the heat of vaporization of water d. electrical conductivity of water e. the greenhouse effect ANS: C DIF: Medium REF: Section 9.3 MSC: Factual TOP: 3Vvi

51. Given the thickness and chemical composition of Venus’s atmosphere, by how much would you expect its average surface temperature to change between day and night? a. There should be almost no change in temperature. b. by tens of K (like Earth) c. by hundreds of K (like Mercury) d. The answer depends on where Venus is in its orbit around the Sun. ANS: A DIF: Medium REF: Section 9.4 MSC: Applied TOP: 4Ii 52. Earth’s sky is blue because: a. blue light from the sun is more readily scattered by molecules in the atmosphere than red light b. of reflected light from the oceans c. red light from the sun is more readily scattered by molecules in the atmosphere than blue light d. molecules that make up the Earth’s atmosphere radiate preferentially at blue wavelengths e. the Sun radiates more blue light than other wavelengths ANS: A DIF: Easy REF: Section 9.4 MSC: Applied TOP: 4Iiv 53. Venus has an unusual rotation rate because: a. it is very slow b. it is very slow and retrograde c. its obliquity is 90 degrees d. it is very fast e. it is very fast and retrograde ANS: B DIF: Easy REF: Section 9.4 MSC: Applied TOP: 4IIi 54. Venus’s surface temperature is fairly uniform from the equator to the poles because: a. Venus rotates very rapidly, which causes strong zonal winds b. Venus is covered by a thick cloud layer that absorbs most of the sunlight that falls on it c. the carbon dioxide in Venus’s atmosphere efficiently emits infrared radiation d. Venus rotates slowly so Coriolis forces do not disrupt Hadley circulation e. Venus’s orbit is nearly perfectly circular ANS: D DIF: Difficult REF: Section 9.4 MSC: Applied TOP: 4IIiii

55. Humans cannot survive on the surface of Mars for long periods of time because: a. there is not enough oxygen in the atmosphere b. the range in temperature between day and night is too large c. the flux of ultraviolet radiation reaching the surface is too high d. the atmospheric pressure would be too low e. all of the above are valid reasons ANS: E DIF: Easy REF: Section 9.5 MSC: Applied TOP: 5Ii 56. When frozen water on the surface of Mars heats up during summer time, the water: a. melts and forms liquid pools on the surface b. boils off the surface and escapes into outer space c. sublimates and goes directly into the gaseous phase d. remains frozen because the temperature remains below the freezing point e. melts and creates flowing rivers that erode the landscape ANS: C DIF: Medium REF: Section 9.5 MSC: Applied TOP: 5Iii 57. Which of the following contributes most to the large difference in the average daytime and nighttime temperatures on the Moon? a. the lack of CO2 in its atmosphere b. the lack of a magnetosphere c. the lack of an atmosphere d. the lack of geologic activity e. its slow rotation rate ANS: C DIF: Medium REF: Section 9.6 MSC: Conceptual TOP: 6Ii

SHORT ANSWER 1. The primary atmospheres of the terrestrial planets formed from hydrogen and helium. Why? What happened to this gas?

ANS: The primary atmospheres were composed of hydrogen and helium because those were the two dominant elements in the interstellar gas cloud out of which the Sun condensed. When the Sun heated the terrestrial planets, the velocity of these light elements was high enough to exceed the escape velocity from the planets and they escaped the planets’ atmospheres.

DIF: Easy REF: Section 9.1 MSC: Conceptual TOP: 1Ii | 1Iiv 2. A gas eventually will escape from a planet’s atmosphere if the average velocity of its atoms exceeds 1⁄6 times the escape velocity of the planet. If the average velocity of water vapor in Venus’s atmosphere is 0.5 km/s, what would be the average velocity of a single hydrogen atom? If Venus’s escape velocity is 11 km/s, will hydrogen atoms eventually escape?

ANS: Because of equipartition of energy, This exceeds 1⁄6  11 km/s  1.8 km/s, thus hydrogen will escape from Venus’s atmosphere.

DIF: Difficult REF: Section 9.1 MSC: Applied TOP: 1Iiii | 1Iiv 3. Most of Earth’s present-day atmosphere comes from a combination of what three sources?

ANS: Volcanoes, comets, and life contribute to the Earth’s atmosphere.

DIF: Easy REF: Section 9.1 MSC: Factual TOP: 1IIi | 1IIii 4. If the average CO2 molecule in Venus’s atmosphere has a velocity of 0.6 km/s, what would the velocity be for a hydrogen atom in Venus’s atmosphere? Note the mass of a CO2 molecule is 44 times that of a hydrogen atom.

ANS: Because of equipartition of energy, DIF: Medium REF: Section 9.1 MSC: Applied TOP: 1IIii 5. List the three planets shown in the figure below in order of decreasing surface temperature, and cite evidence that can be seen in the figure that supports your choice.

ANS: Venus, Earth, Mars: Venus is covered with clouds, which suggests a runaway greenhouse effect; Earth has much liquid water, so is likely cooler; and Mars has no evidence of liquid water but much ice on its surface.

DIF: Easy REF: Section 9.2 MSC: Applied TOP: 2IIiii

6. Where is most of Earth’s supply of carbon dioxide today?

ANS: Atmospheric CO2 reacted with exposed minerals in surface rocks, creating limestone beds. Most of the CO2 dissolved directly in the oceans, where it precipitated out as calcite or was removed by organisms such as coral in the manufacture of their shells.

DIF: Easy REF: Section 9.2 MSC: Factual TOP: 2IIvii 7. Earth’s atmosphere is a (seemingly) enormous blanket roughly 250 km thick. What percentage of Earth’s radius, which is 6,400 km, does this represent? How does it compare to the average depth of the oceans, which is 3 km?

ANS: The Earth’s atmosphere is 250/6, 400  0.39 or 3.9 percent of Earth’s radius. Thus it is dwarfed by the planet’s size. The atmosphere is 250/3  83 times higher than the oceans are deep. The atmosphere is really a very thin layer covering the planet; the hydrosphere represents an even thinner layer.

Yet both are vital to life on the planet.

DIF: Medium REF: Section 9.3 MSC: Applied TOP: 3Ii 8. If there are 104 kg of air above every square meter of the surface of Earth, and Earth is modeled as a sphere of radius 6.4  106 m, what is the mass of Earth’s atmosphere and what fraction is it of the total mass of the Earth? Show your calculation.

ANS: The surface area of Earth is 4   (6.4  106 m)2  5.15  1014 m2. Therefore the mass of Earth’s atmosphere is 5.15  1014 m2  104 kg/m2  5.15  1018 kg. The fraction of the Earth’s total mass that is in the atmosphere is 5.15  1018 kg/5.97  1024 kg  8  10 7, or approximately one millionth.

DIF: Difficult REF: Section 9.3 MSC: Applied TOP: 3Ii 9. Suppose you go out hiking in the snow on a mountain top on a cold winter day when the temperature outside is 0 C  273 K and the pressure is 0.75 bar. If you brought along a package of potato chips that was sealed at sea level when the temperature was 24 C  297 K, what would have happened to the volume of the bag of chips? By how much will the volume have changed?

ANS: The ideal gas law says that P

T

T/V, therefore, V

T/P. Thus, the volume of the bag on the

mountain top is equal to the original volume times (Tmountian /Tsea level)  (Psea level / Pmountain)  (273 K / 297 K)  (1 bar / 0.75 bar)  1.23. Thus, the volume of the bag has increased and is 23 percent larger than the original volume.

DIF: Difficult REF: Section 9.3 MSC: Applied TOP: 3Iii

10. You take a sealed plastic bag of snacks on an airline flight where the atmospheric pressure is reduced to 0.8 bar, but the cabin is heated so the temperature is approximately the same as when you sealed the bag.

What will happen to the volume of the bag? By how much will it have changed?

ANS: The ideal gas law says P

T

T/V. If the temperature does not change, then P

1/V. Thus, the

volume of the bag on the plane is the volume when it was sealed  (Psealed/Pplane)  (1/0.75)  1.33.

The bag’s volume increased by 25 percent.

DIF: Medium REF: Section 9.3 MSC: Applied TOP: 3Iii

Figure 2

11. According to Figure 2, what effect does doubling the temperature of a gas while keeping its volume constant have on the pressure of the gas?

ANS: Because the ideal gas law says P

T

T/V, if the volume is constant and the temperature dou-

bles then the pressure will double.

DIF: Easy REF: Section 9.3 MSC: Applied TOP: 3Iiv 12. According to Figure 2, what effect does doubling the pressure on a gas while keeping its temperature constant have on the volume of the gas?

ANS: Because the ideal gas law says P

T

T/V, if the temperature is constant and the pressure

doubles, then the volume must decrease by a factor of two.

DIF: Easy REF: Section 9.3 MSC: Applied TOP: 3Iiv

13. According to the figure shown below, about how long ago did oxygen first appear in Earth’s atmosphere?

About how long ago did oxygen reach 50 percent of its current abundance in Earth’s atmosphere?

ANS: 3 Billion years ago; 0.6 billion years ago.

DIF: Medium REF: Section 9.3 MSC: Applied TOP: 3IIiii 14. Carbon dioxide levels in Earth’s atmosphere have been rising by about 4 percent per decade due to the use of fossil fuels. If this trend continues, what could happen to Earth?

ANS: Increased concentration of carbon dioxide is acidifying the oceans, directly affecting the carbon cycle. Higher temperatures are also associated with an increase in the severity of extreme weather events and changing climate zones. As the surface temperature increases, polar ice caps of Antarctica and Greenland could melt. This, along with increased temperatures, would lead to sea level rise and coastlines moving inland with flooding as the intermediate result. Eventually the carbon dioxide could cause a runaway greenhouse effect.

DIF: Medium REF: Section 9.3 MSC: Applied TOP: 3IIIii | 3VIii 15. Over the last century, why has the ozone hole over the Earth grown larger? How long might it take to revert to its former state?

ANS: Because chemicals released into the atmosphere—mainly halogens like chlorine, fluorine, and bromine—work as catalysts to break up ozone; they do not settle out of the atmosphere for decades or centuries, so we have a long time to wait for the problem to be reversed.

DIF: Medium REF: Section 9.3 MSC: Conceptual TOP: 3IIIiii 16. Why does the temperature decrease as you go higher up in altitude in the troposphere on Earth?

ANS: The temperature decreases as you go higher up because you are farther from the lower atmosphere’s major heat source, the Earth’s surface, which is itself heated by absorbing sunlight.

DIF: Easy REF: Section 9.3 MSC: Conceptual TOP: 3IVii 17. In the stratosphere of the Earth’s atmosphere, how does the temperature vary with increasing altitude and what causes this variation?

ANS: Ozone in the stratosphere absorbs ultraviolet light coming from the Sun and heats the stratosphere. The higher you go, the more UV light there is to absorb, and thus the temperature rises with altitude in the stratosphere.

DIF: Difficult REF: Section 9.3 MSC: Applied TOP: 3IVvi 18. The global winds on Earth are the result of a combination of what three things?

ANS: Globular winds result from an uneven solar heating across the surface of the planet (the greatest amount of heating at the equator and least at the poles), Hadley circulation, and the Coriolis effect.

DIF: Medium REF: Section 9.3 MSC: Applied TOP: 3Vi 19. If sunlight cannot penetrate Venus’s cloud layer efficiently, why does the temperature of the planet remain so high?

ANS: Venus cannot cool because of the highly efficient greenhouse effect due to its thick atmosphere, which is composed mostly of carbon dioxide.

DIF: Medium REF: Section 9.4 MSC: Applied TOP: 4Ii 20. On Mars, water could exist in what form(s): solid, liquid, or gas? How does this vary with the seasons on Mars? Why are the seasonal variations on Mars different in its northern and southern hemispheres?

ANS: Any water on Mars would either be in the solid form, frozen in the ground, or in the vapor form in the thin atmosphere of Mars. Even when the temperature rises during the daytime or summertime, water sublimates on Mars, meaning it goes straight from being frozen to being a vapor without passing through the liquid phase, because the atmospheric pressure is so low on Mars. The seasonal variations in the southern hemisphere on Mars are larger than in the northern hemisphere because, besides having an axial tilt, Mars also has an elliptical orbit and Mars is farther from the Sun when the southern hemisphere is pointed more directly toward the Sun. DIF: Difficult REF: Section 9.5 MSC: Applied TOP: 5Iii | 5Iiv

CHAPTER 10

Worlds of Gas and Liquid— The Giant Planets

CONCEPT MAP Sec 10.1 1. Giant Planets—Distant Worlds, Different Worlds I. Physical Properties of the Giant Planets i. Chemical composition: H, He, and water (TF: 1, MC: 1, 2) ii. Distance, solar flux from these distances (MC: 3, SA: 1) II. Planet Discovery i. Uranus: William Herschel; accidental discovery with 6-inch telescope (MC: 4) ii. Neptune: predicted to exist based on differences between observed orbit and predicted orbit (Le Verrier and Adams predicted the position; Galle first observed Neptune) (MC: 5–7, SA: 2) Sec 10.2 2. How Giant and Terrestrial Planets Differ I. Giant Planets Are Large and Massive i. Stellar occultation: used to measure diameters of planets (MC: 8, 9, SA: 3) ii. Orbiting moons and Newton’s laws: used to measure masses of planets (MC: 10, SA: 4) iii. Large masses and diameters (TF: 2, 3, MC: 11, SA: 5) II. Atmospheres i. Visible surfaces are the atmospheres of the giant planets (MC: 12) ii. Seamless variation of gas to liquid to solid in core (MC: 13, 14) III. Chemical Composition of the Giant Planets

i. Low densities; Saturn 0.7 times the density of water (MC: 15, 16) ii. Jupiter: only 2% heavy elements; Saturn slightly more; ice giants much more (MC: 17) IV. Days and Seasons on the Giant Planets i. Fast rotation rates (10–17 hrs) lead to short days/nights (MC: 18, SA: 6) ii. Oblateness: flattening due to fast rotation (~10 percent for Saturn) (MC: 19, 20) iii. Obliquity: inclination of the planet’s equatorial plane relative to its orbital plane; sets the prominence of the planet’s seasons; Uranus: ~98°, each season lasts 42 years with alternating darkness and sunshine (MC: 21–23, SA: 7, 8) Sec 10.3 3. A View of the Clouds I. Jupiter i. Dark bands  “belts,” light bands  “zones,” vortices storms (TF: 4, MC: 24) ii. Great Red Spot: large size  2 DEarth, long existence, anticyclonic (MC: 25, 26, SA: 9) II. Clouds on Saturn, Uranus, and Neptune i. Saturn: More muted colors and bands than Jupiter (TF: 4, MC: 27) ii. Uranus and Neptune: Even more muted banding, Great Dark Spot on Neptune (TF: 4, MC: 28, 29) III. A Journey into the Clouds i. Temperature, density, and pressure decrease with increasing altitude (MC: 30) ii. Each cloud layer composed of different chemical substances because different volatiles condense at different temperatures and pressures (TF: 5, MC: 31, 32) iii. Clouds colored by chemical impurities (MC: 33, 34, SA: 10) iv. Uranus and Neptune are bluish due to scattering of blue light by methane (MC: 35) Sec 10.4 4. Weather on the Giant Planets I. Winds and Storms i. Rapid rotation and Coriolis effect create strong winds (up to 1000 km/hr) (MC: 36, SA: 11) ii. Jupiter: Alternating easterly and westerly winds, vortex storms between them (TF: 6, MC: 37) iii. Saturn: Jet streams, periods of fair and stormy weather similar to Earth iv. Uranus: Poles warmer than equator due to large tilt (MC: 38) v. Cloud motion used to measure wind speed (MC: 39, 40) II. Weather from the Sun and from Gravity

i. Convection from temperature difference fuels weather (MC: 41) ii. Rain droplets lead to lightning (MC: 42) iii. Jupiter, Saturn, and Neptune radiate more energy than they receive from Sun (TF: 7, MC: 43, 44, SA: 12) iv. Gravitational potential energy converted to heat (MC: 45) Sec 10.5 5. Interiors of Giant Planets Are Hot and Dense I. Properties of the Interiors of Jupiter and Saturn i. Hydrogen at high temperature and pressure: liquid metal (MC: 46, 47) ii. Thick layers of molecular hydrogen and metallic hydrogen (MC: 48) iii. Dense liquid core made of heavier materials (water, rocks, metals) (MC: 49) iv. Layers of interiors: molecular H, metallic H, ices, rock (SA: 13) v. Cores formed first, then captured H and He from protoplanetary disk (MC: 50) II. Uranus and Neptune Differ from Jupiter and Saturn i. Higher density than Jupiter and Saturn (MC: 51, SA: 14) ii. Made of mostly water: ice giants with deep, salty oceans (MC: 2, SA: 14) III. Difference Are Clues to Origins i. Jupiter and Saturn: more massive cores captured more mass (MC: 52, 53, SA: 15, 16) ii. Ice giants: smaller cores formed later, capturing less mass (TF: 8, MC: 52, 53, SA: 17) Sec 10.6 6. Giant Planets Have Strong Magnetic Fields I. Characteristics of Magnetic Fields i. Origins: Gas giants: metallic hydrogen; ice giants: salty oceans (MC: 54) ii. Strong magnetic fields (Jupiter: 20,000 times stronger than Earth’s) (MC: 55) iii. Magnetic axes can be tilted relative to planet’s rotational axis and displaced from the center of the planets: Jupiter (slightly), Uranus and Neptune (strongly) (SA: 18) II. Giant Planets Have Giant Magnetospheres i. Magnetosphere: many times larger than the planet’s radii (MC: 56) ii. Divert solar wind (MC: 57) III. Magnetospheres Produce Synchrotron Radiation i. Radio emission from charged particles moving in the magnetospheres (TF: 9, MC: 58, 59, SA: 19)

IV. Radiation Belts and Auroras i. Collisions between particles heat plasma to millions of degrees (SA: 20) ii. Charged particles concentrated in radiation belts; potentially fatal (MC: 60) iii. Powerful auroras in giant planets from solar wind particles (TF: 10, MC: 61) Sec 10.7 7. Origins: Giant Planet Migration and the Inner Solar System I. Computer Models of Solar System Formation i. Planets migrate throughout Solar System due to other planets’ gravity (MC: 62)

TRUE/FALSE 1. The giant planets are made primarily of water and carbon dioxide.

ANS: F DIF: Easy REF: Section 10.1 MSC: Factual TOP: 1Ii 2. Jupiter’s mass is more than twice the mass of all the other planets in our Solar System combined.

ANS: T DIF: Medium REF: Section 10.2 MSC: Factual TOP: 2Iiii 3. Jupiter is approximately 20 times more massive than the Earth.

ANS: F DIF: Easy REF: Section 10.2 MSC: Factual TOP: 2Iiii 4. All the giant planets, not just Jupiter, have atmospheric bands and storms.

ANS: T DIF: Easy REF: Section 10.3 MSC: Factual TOP: 3Ii | 3IIi | 3IIii 5. The different cloud layers seen in Jupiter’s bands represent clouds at different altitudes in Jupiter’s atmosphere.

ANS: T DIF: Difficult REF: Section 10.3 MSC: Factual TOP: 3IIIii 6. On the giant planets, the atmospheric vortices that occur almost always lie between oppositely directed zonal winds.

ANS: T DIF: Medium REF: Section 10.4 MSC: Factual TOP: 4Iii 7. Jupiter radiates more energy than it receives from the Sun, mostly because it is still contracting under its own gravity.

ANS: T DIF: Medium REF: Section 10.4 MSC: Factual TOP: 4IIiii 8. Uranus and Neptune are less massive than Jupiter and Saturn, probably because they formed earlier than Jupiter or Saturn.

ANS: F DIF: Medium REF: Section 10.5 MSC: Conceptual TOP: 5IIIii 9. When charged particles oscillate around magnetic field lines of a planet, they emit radiation in the microwave region of the spectrum.

ANS: F DIF: Difficult REF: Section 10.6 MSC: Factual TOP: 6IIIi

10. Jupiter’s strong aurorae result from particles ejected by Io’s volcanoes.

ANS: T DIF: Medium REF: Section 10.6 MSC: Factual TOP: 6IViii

MULTIPLE CHOICE 1. Jupiter and Saturn are composed primarily of: a. hydrogen b. helium c. water d. ammonia e. carbon ANS: A DIF: Easy REF: Section 10.1 MSC: Factual TOP: 1Ii 2. The compositions of Uranus and Neptune differ primarily from that of Jupiter and Saturn in that the outer two planets contain more: a. hydrogen b. helium c. water d. carbon dioxide e. iron ANS: C DIF: Easy REF: Section 10.1 MSC: Factual TOP: 1Ii | 5IIii 3. Which planet receives the least amount of energy from the Sun? a. Jupiter b. Earth c. Neptune d. Saturn e. Uranus ANS: C DIF: Easy REF: Section 10.1 MSC: Applied TOP: 1Iii

4. Which of the giant planets was discovered by accident by William Herschel? a. Jupiter b. Saturn c. Uranus d. Neptune ANS: C DIF: Easy REF: Section 10.1 MSC: Factual TOP: 1IIi 5. What is the angular diameter of Neptune if its diameter is 50,000 km and its distance is 30 AU? a. 45 arcseconds b. 30 arcseconds c. 20 arcseconds d. 10 arcseconds e. 2 arcseconds ANS: E DIF: Medium REF: Section 10.1 MSC: Applied TOP: 1IIii 6. Which of the giant planets was predicted to exist mathematically before it was ever seen through a telescope? a. Jupiter b. Saturn c. Uranus d. Neptune ANS: D DIF: Easy REF: Section 10.1 MSC: Factual TOP: 1IIii 7. Why were Adams and Le Verrier acknowledged as the discoverers of the planet Neptune? a. They were the first to see it through a telescope. b. They took time-lapsed photos to show its motion across the sky. c. They predicted its position based on the observed discrepancies in the orbit of Uranus. d. They were the directors of the observatory at which it was discovered. e. They paid for the observations that discovered it.

ANS: C DIF: Medium REF: Section 10.1 MSC: Factual TOP: 1IIii

8. You observe Neptune as it occults a background star when the relative velocity between Neptune and the Earth is 30 km/s, and the star crosses through the middle of the planet and disappears for 27.6 minutes.

What is Neptune’s diameter? a. 5  104 km b. 800 km c. 4,000 km d. 9  103 km e. 3  106 km ANS: A DIF: Medium REF: Section 10.2 MSC: Applied TOP: 2Ii 9. Assume you want to deduce the radius of a planet in our Solar System as it occults a background star when the relative velocity between the planet and the Earth is 30 km/s. If the star crosses through the middle of the planet and disappears for a total of 26 minutes, what is the planet’s radius? a. 3,000 km b. 23,000 km c. 15,000 km d. 5,000 km e. 31,000 km ANS: B DIF: Medium REF: Section 10.2 MSC: Applied TOP: 2Ii 10. Which of these observations would allow you to measure the mass of a planet? a. the planet’s orbital period b. the planet’s rotational period c. the planet’s distance from the Sun d. the orbit of one of that planet’s moons e. the planet’s temperature ANS: D DIF: Easy REF: Section 10.2 MSC: Applied TOP: 2Iii

11. You could fit roughly _________ Jupiters across the diameter of the Sun and roughly _________ Earths across Jupiter’s diameter. a. 10; 100 b. 100; 10 c. 10; 10 d. 100; 100 e. 1,000; 1,000 ANS: C DIF: Difficult REF: Section 10.2 MSC: Factual TOP: 2Iiii 12. When you look at the visible surface of a gas giant planet, you are looking at that planet’s _________. a. oceans b. core c. atmosphere d. metallic hydrogen e. solid surface ANS: C DIF: Easy REF: Section 10.2 MSC: Factual TOP: 2IIi 13. The figure below shows a drawing of some of the layers inside the atmosphere of Jupiter. The layer at the top of the figure is closest to the outer surface of Jupiter, while the bottom layer is closer to the core.

Which of these is a possible list of what the layers contain, starting with layer 1 and moving to layer 3? a. gas, solid, liquid b. gas, smooth transition from gas to solid, solid c. gas, distinct line between gas and solid, solid d. gas, smooth transition from gas to liquid, liquid e. gas, distinct line between gas and liquid, liquid ANS: D DIF: Easy REF: Section 10.2 MSC: Applied TOP: 2IIii

14. How is the atmosphere of Saturn similar to the atmosphere of Earth? a. They are both made of mostly hydrogen and helium. b. They both create magnetic fields. c. They both have jet streams and periods of stormy and calm weather. d. They both rotate in less than 11 hours. e. They both have a seamless transition between gas and liquid.

ANS: C DIF: Easy REF: Section 10.4 MSC: Applied TOP: 2IIii | 4Iiii 15. If you could find a large enough ocean, which one of these planets would float in it? a. Uranus b. Saturn c. Neptune d. Mars e. Earth ANS: B DIF: Easy REF: Section 10.2 MSC: Applied TOP: 2IIIi 16. Assume that you discovered a new planet in the Solar System. To study it, you measured the orbital period and semimajor axis of one of its moons and deduced that the planet’s mass was 4  1025 kg (7 MEarth), then you observed the planet occult a background star and deduced that its radius is 12,000 km (2 REarth).

What is this planet’s average density? Is this planet’s chemical composition more similar to a rocky terrestrial planet or a giant planet? For comparison, the density of iron, rock, and water are approximately 9,000 kg/m3, 3,000 kg/m3, and 1,000 kg/m3, respectively. a. The planet’s average density is 1,200 kg/m3, and its composition is similar to that of giant planets. b. The planet’s average density is 1,200 kg/m3, and its composition is similar to that of terrestrial planets. c. The planet’s average density is 3,100 kg/m3, and its composition is similar to that of terrestrial planets. d. The planet’s average density is 5,500 kg/m3, and its composition is similar to that of giant planets. e. The planet’s average density is 5,500 kg/m3, and its composition is similar to that of terrestrial planets.

ANS: E DIF: Medium REF: Section 10.2 MSC: Applied TOP: 2IIIi

17. Which of these planets has a composition that is most like the Sun? a. Uranus b. Saturn c. Neptune d. Jupiter e. Earth ANS: D DIF: Easy REF: Section 10.2 MSC: Factual TOP: 2IIIii 18. As a group, the giant planets all rotate _________ terrestrial planets. a. faster than b. slower than c. the same as d. retrograde compared to e. sideways compared to ANS: A DIF: Easy REF: Section 10.2 MSC: Factual TOP: 2IVi 19. Why are Jupiter and Saturn not perfectly spherical? a. They formed from the collision of two large planetesimals. b. They rotate rapidly. c. They have storms that develop preferentially along their equators. d. They have very active aurorae that heat the atmospheres along the poles. e. They have so much more gravity that the poles get pulled harder than the equators.

ANS: B DIF: Easy REF: Section 10.2 MSC: Conceptual TOP: 2IVii 20. Suppose you attach a weight to one end of a spring and then hold the other end of the spring and spin it above your head. The faster you spin the spring, the farther away the weight will move from your hand.

This example illustrates: a. why the Great Red Spot exhibits anticyclonic motion b. why winds blow in an easterly direction on Neptune c. why the poles rotate faster than the equator on Uranus d. why Saturn is the most oblate of the giant planets e. why the giant planets are farther from the Sun than the terrestrial planets ANS: D DIF: Difficult REF: Section 10.2 MSC: Conceptual TOP: 2IVii

21. All the giant planets except _________ experience seasons. a. Jupiter b. Saturn c. Uranus d. Neptune ANS: A DIF: Easy REF: Section 10.2 MSC: Applied TOP: 2IViii 22. _________ has the most extreme seasons of any planet in the Solar System. a. Jupiter b. Saturn c. Uranus d. Neptune e. Earth ANS: C DIF: Medium REF: Section 10.2 MSC: Factual TOP: 2IViii 23. Each season is on Uranus lasts approximate 21 Earth years because: a. Uranus takes a very long time to orbit around the Sun b. Uranus rotates very slowly c. Uranus’s rotational axis is tipped by 45 degrees relative to it orbital axis d. Hadley circulation is ineffective in transferring heat in Uranus’ atmosphere e. Uranus has many strong storms ANS: A DIF: Difficult REF: Section 10.2 MSC: Applied TOP: 2IViii 24. A planet will have bands in its atmosphere like Jupiter and Saturn if: a. the planet is more than 3 AU from the Sun b. the planet rotates slowly c. the wind speeds vary greatly with latitude d. the planet has a high temperature e. the planet has a large mass ANS: C DIF: Medium REF: Section 10.3 MSC: Applied TOP: 3Ii

25. The Great Red Spot, Jupiter’s most prominent storm system, has a diameter that is _________ times the Earth’s diameter. a. two b. five c. 10 d. 50 e. 100 ANS: A DIF: Medium REF: Section 10.3 MSC: Factual TOP: 3Iii

26. If you tracked the motion of the clouds near Jupiter’s Great Red Spot, which of the following diagrams shows the correct motion you would observe?

a. A b. B c. C d. D e. E ANS: D DIF: Easy REF: Section 10.3 MSC: Applied TOP: 3Iii 27. Which giant planet has the most prominent band structures? a. Jupiter b. Saturn c. Uranus d. Neptune ANS: A DIF: Easy REF: Section 10.3 MSC: Factual TOP: 3IIi

28. Uranus and Neptune do not have bands as distinct as those on Jupiter and Saturn, because Uranus and Neptune: a. have wind speeds that vary more smoothly from the equator to the poles b. are composed entirely of hydrogen and helium and lack more complex molecules c. are much closer to the Sun and much colder d. rotate 10 times slower e. have larger masses ANS: A DIF: Medium REF: Section 10.3 MSC: Applied TOP: 3IIii 29. Band systems on Saturn, Uranus, and Neptune are most prominent when viewed in which wavelength regime? a. visible b. infrared c. ultraviolet d. X-ray e. microwave ANS: B DIF: Difficult REF: Section 10.3 MSC: Applied TOP: 3IIii 30. As you move from the top atmospheric layer toward the center of a gas planet, the temperature _________ and the pressure _________. a. increases; decreases b. increases; increases c. decreases; decreases d. decreases; increases e. increases; stays the same ANS: B DIF: Medium REF: Section 10.3 MSC: Factual TOP: 3IIIi

31. Why do we find methane clouds above water clouds in the atmosphere of Saturn? a. Methane clouds are less dense than water clouds. b. Methane is far more plentiful than water on Saturn. c. Methane is in a liquid/gas state at lower temperatures than water. d. We can’t observe the methane clouds that are deeper in the atmosphere. e. All of the above are good reasons.

ANS: C DIF: Difficult REF: Section 10.3 MSC: Applied TOP: 3IIIii 32. If we measure the spectrum of radiation coming from different clouds in Jupiter’s atmosphere, and we find that a cloud that appears white in visible light emits the largest number of photons at a wavelength of 3  10 5 m, while a cloud that appears brown in visible light emits the largest number of photons at a wavelength of 1.9  10 5 m, how do the temperatures of the clouds compare? a. The white cloud is 1.6 times hotter than the brown cloud. b. The brown cloud is 1.6 times hotter than the white cloud. c. The brown cloud is 3 times hotter than the white cloud. d. The white cloud is 3 times hotter than the brown cloud. e. Both clouds are the same temperature.

ANS: B DIF: Difficult REF: Section 10.3 MSC: Applied TOP: 3IIIii 33. Why aren’t all clouds on Jupiter white, like on Earth? a. Jupiter’s clouds are made of methane. b. Jupiter’s clouds are made of carbon dioxide. c. There are chemical impurities in the ice crystals in Jupiter’s clouds. d. The Sun is not as bright when viewed from Jupiter compared to what it looks like from Earth. e. For the same reason that we see colors in rainbows on Earth.

ANS: C DIF: Medium REF: Section 10.3 MSC: Conceptual TOP: 3IIIiii

34. The colors of the cloud bands on Jupiter and Saturn are due primarily to differences in their: a. wind speeds b. chemical compositions c. altitudes d. temperatures e. densities ANS: B DIF: Medium REF: Section 10.3 MSC: Applied TOP: 3IIIiii 35. Uranus and Neptune are bluish green in color because they contain large amounts of: a. ammonia b. methane c. water vapor d. hydrocarbons e. oxygen ANS: B DIF: Easy REF: Section 10.3 MSC: Factual TOP: 3IIIiv 36. The Jovian atmospheric vortices are created by a combination of the Coriolis effect and: a. rapid rotation b. convection c. their strong magnetic fields d. solar radiation e. gravity ANS: B DIF: Easy REF: Section 10.4 MSC: Conceptual TOP: 4Ii

37. The figure below shows a drawing of bands in the atmosphere of Jupiter, and the arrows indicate the direction the winds are blowing in those bands.

At which of the labeled locations would you be most likely to find a vortex storm? a. A b. B c. C d. D e. E ANS: C DIF: Easy REF: Section 10.4 MSC: Applied TOP: 4Iii 38. The poles of Uranus can have a higher temperature than its equator because Uranus: a. has a large axial tilt relative to its equator b. has a high mass c. is mostly made of water d. is far from the Sun e. has large storms on the surface ANS: A DIF: Easy REF: Section 10.4 MSC: Applied TOP: 4Iiv

39. If you monitor Saturn’s atmosphere and you see a storm near its equator at a longitude of 0° west on one day and at a longitude of 90° west three days later, what is the average wind speed on Saturn at this storm’s latitude? Note that these positions are measured on a coordinate system that rotates with the planet’s interior, and the radius of Saturn is 6  107 m. a. 720 m/s b. 120 m/s c. 360 m/s d. 540 m/s e. 1,440 m/s ANS: C DIF: Medium REF: Section 10.4 MSC: Applied TOP: 4Iv 40. If you monitor Jupiter’s atmosphere and you see a storm near the equator move from a longitude of 60° west to a longitude of 80° west over six days, what is the wind speed at this storm’s latitude on Jupiter?

Note that these positions are measured on a coordinate system that rotates with the planet’s interior, and the radius of Jupiter is 7.2  107 m . a. 700 m/s b. 300 m/s c. 100 m/s d. 50 m/s e. 500 m/s ANS: D DIF: Difficult REF: Section 10.4 MSC: Applied TOP: 4Iv 41. If convection on Jupiter got weaker, what would happen to the storms in the upper atmosphere? a. They would get stronger. b. They would get weaker. c. They would stay the same strength but become larger. d. They would begin to rotate the opposite direction. e. They would move deeper into the planet.

ANS: B DIF: Easy REF: Section 10.4 MSC: Applied TOP: 4IIi

42. Which of these things happens because of rain droplets falling through the atmosphere of gas giant planets? a. banding b. aurora c. magnetic fields d. cyclonic motion e. lightning ANS: E DIF: Easy REF: Section 10.4 MSC: Applied TOP: 4IIii 43. Which giant planet does NOT radiate more energy into space than it receives from the Sun? a. Jupiter b. Saturn c. Uranus d. Neptune ANS: C DIF: Medium REF: Section 10.4 MSC: Factual TOP: 4IIiii 44. If the flux of sunlight on a planet suggested its temperature should be 200 K, but its actual temperature was 220 K, then how much more energy does this planet emit relative to the energy it receives from its parent star? a. 5.3 times more energy b. 2.1 times more energy c. 2.9 times more energy d. 1.1 times more energy e. 1.5 times more energy ANS: E DIF: Difficult REF: Section 10.4 MSC: Applied TOP: 4IIiii 45. The fact that Jupiter’s radius is contracting at a rate of 1 mm/year results in: a. differential convection that powers Jupiter’s Great Red Spot b. Jupiter’s rotation rate slowing down with time c. Jupiter’s shape being less oblate d. Jupiter radiating more heat than it receives from the Sun e. Jupiter’s orbit around the Sun getting smaller ANS: D DIF: Medium REF: Section 10.4 MSC: Conceptual TOP: 4IIiv

46. We refer to some of the inner regions of Jupiter and Saturn as metallic hydrogen because they: a. are as dense as lead b. are solid c. provide support for the upper layers of hydrogen and helium d. efficiently conduct electricity e. are found in the core like iron is found at the core of the Earth ANS: D DIF: Easy REF: Section 10.5 MSC: Conceptual TOP: 5Ii 47. Despite the high temperatures deep in the interior of giant planets, their cores remain liquid because: a. they are under very high pressures b. gravitational potential energy is being converted into thermal energy in the cores c. they are composed of heavy materials like rock and water d. their rotations are rapid compared to those of the terrestrial planets e. the giant planets have strong magnetic fields ANS: A DIF: Medium REF: Section 10.5 MSC: Conceptual TOP: 5Ii 48. Of the giant planets, only Jupiter and Saturn have thick inner layers of: a. liquid rock b. solid rock c. metallic hydrogen d. liquid methane e. water ANS: C DIF: Easy REF: Section 10.5 MSC: Factual TOP: 5Iii 49. Each giant planet has a core made of _________ that is five to 10 times the mass of the Earth. a. hydrogen b. rocky material c. water d. hydrocarbons e. methane ANS: B DIF: Easy REF: Section 10.5 MSC: Factual TOP: 5Iiii

50. If you could watch Saturn form starting from the beginning of the Solar System, which of these features of Saturn would come together first? a. magnetosphere b. metallic hydrogen c. molecular hydrogen d. rocky core e. ammonia ice ANS: D DIF: Easy REF: Section 10.5 MSC: Applied TOP: 5Iv 51. What measurement tells us that the interiors of Uranus and Neptune are made of mostly water? a. their mass b. their distance from the sun c. their average densities d. their temperatures e. their colors ANS: C DIF: Easy REF: Section 10.5 MSC: Applied TOP: 5IIi 52. Neptune and Uranus probably took longer to form than Jupiter and Saturn, because the solar nebula was _________ at the radius of Neptune and Uranus. a. rotating faster b. composed of rockier planetesimals c. not as dense d. hotter e. colder ANS: C DIF: Easy REF: Section 10.5 MSC: Applied TOP: 5IIIi | 5IIIii

53. Uranus and Neptune contain smaller percentages of hydrogen and helium than Jupiter and Saturn probably because Uranus and Neptune _________ than Jupiter and Saturn. a. are much smaller in radius b. are much warmer c. are much colder d. formed later e. formed earlier ANS: D DIF: Medium REF: Section 10.5 MSC: Applied TOP: 5IIIi | 5IIIii 54. Where do Uranus’s and Neptune’s strong magnetic fields originate? a. molten rocky cores b. salty oceans c. large magnetospheres d. metallic hydrogen layers e. methane clouds ANS: B DIF: Easy REF: Section 10.6 MSC: Factual TOP: 6Ii 55. The strongest magnetic fields in the Solar System are found on which planet? a. Jupiter b. Saturn c. Uranus d. Neptune e. Earth ANS: A DIF: Easy REF: Section 10.6 MSC: Factual TOP: 6Iii 56. If you were to fly to Jupiter from the Earth, which of these parts of Jupiter would you hit first? a. magnetosphere b. metallic hydrogen c. molecular hydrogen d. rocky materials e. stratosphere ANS: A DIF: Easy REF: Section 10.6 MSC: Applied TOP: 6IIi

57. Why would a satellite orbiting close to Jupiter have a very hard time detecting solar wind particles? a. Jupiter’s strong gravity pulls them into the planet. b. Jupiter is too far away from the Sun to get any solar wind. c. The satellite would be moving too fast in its orbit to catch any of them. d. The Great Red Spot pushes them away from Jupiter. e. Jupiter’s magnetosphere deflects them.

ANS: E DIF: Medium REF: Section 10.6 MSC: Conceptual TOP: 6IIii 58. What would you observe in order to accurately measure the rotational period of a giant planet? a. clouds in the atmosphere b. bands of storms on the equator c. stellar occultations d. synchrotron emission e. the orbit of its moons ANS: D DIF: Difficult REF: Section 10.6 MSC: Applied TOP: 6IIIi 59. Jupiter emits a large amount of radio emission because: a. charged particles blasted off of Io’s surface move through Jupiter’s magnetic field b. violent storms in its atmosphere produce a lot of lightening c. Jupiter is so cold that its blackbody radiation peaks at radio wavelengths d. Jupiter’s thick inner shell of metallic hydrogen is electrically conductive e. Jupiter’s core has a very high temperature and pressure ANS: A DIF: Difficult REF: Section 10.6 MSC: Applied TOP: 6IIIi 60. The radiation belts around Jupiter are much stronger than those found around the Earth because: a. Jupiter has larger storms than the Earth. b. Jupiter is colder than the Earth. c. Jupiter rotates faster than the Earth. d. Jupiter is farther from the Sun than the Earth. e. Jupiter has a stronger magnetic field than the Earth.

ANS: E DIF: Easy REF: Section 10.7 MSC: Applied TOP: 6IVii

61. The figure below shows a picture of Saturn taken by the Hubble Space Telescope.

What is causing the circle of light seen near the Saturn’s pole? a. Solar wind particles are being trapped by Saturn’s magnetic field, causing an aurora. b. Strong storms on Saturn are causing lightning strikes. c. Saturn’s tilt is causing that area of the planet to be warmer, so it gives off bluer light. d. Metallic hydrogen is being released from the surface of Saturn. e. Saturn is giving off energy because it is shrinking.

ANS: A DIF: Difficult REF: Section 10.6 MSC: Applied TOP: 6IViii 62. What could have caused the planets to migrate through the Solar System? a. gravitational pull from the Sun b. interaction with the solar wind c. accreting gas from the solar nebula d. gravitational pull from other planets e. differentiation of their interiors ANS: D DIF: Medium REF: Section 10.7 MSC: Applied TOP: 7Ii

SHORT ANSWER 1. Compare the flux of sunlight at the Earth’s orbit to that at Saturn’s orbit. Note that Saturn’s average distance from the Sun is 9.5 AU. ANS: The flux of sunlight is proportional to d 2 where d is the distance. Thus, the flux of sunlight at Saturn’s orbit is (1 AU/9.5 AU)2  0.01 times the flux of sunlight at Earth’s orbit. Thus, the Sun is only 1 percent as bright at Saturn’s distance compared to Earth’s distance.

DIF: Difficult REF: Section 10.1 MSC: Applied TOP: 1Iii

2. How does the discovery of Neptune relate to the discovery of extrasolar planets?

ANS: Neptune was discovered by measuring a wobble in Uranus’s orbit and using Newton’s laws to predict the size and orbit of the disturber. This is the same basic method used by astronomers today to find extrasolar planets—except that in other solar systems, the wobble is usually in the parent sun. This method works well but is much more difficult to do in other solar systems because of the distances involved in the calculations.

DIF: Difficult REF: Section 10.1 MSC: Conceptual TOP: 1IIii 3. Suppose Neptune moves with an average orbital speed of 3.5 km/s. If it takes Neptune four hours to pass directly in front of a star, what is Neptune’s diameter? Give Neptune’s radius in units of Earth diameters, where the diameter of Earth is 12,800 km.

ANS: The diameter of Neptune is equal to its speed times the duration of the occultation: Diameter  3.5 km/s  14,400 seconds  50,400 km. In terms of Earth diameters:

DNeptune  50,400 km  (Earth diameter/ 12,800 km)  3.9 Earth diameters.

DIF: Medium REF: Section 10.2 MSC: Applied TOP: 2Ii 4. Calculate Jupiter’s mass (in Earth masses) based on its gravitational pull on its moon, Io, using Newton’s version of Kepler’s third law: P2  A3/MJ. In order to do so, you will need the following information: Io’s period  1.769 days; Io’s semimajor axis  422,000 km; the mass of the Sun  2  1030 kg; the mass of Earth  6  1024 kg (also, 1 AU  1.5  108 km).

ANS: First convert Io’s period and semimajor axis into the appropriate units:

PIo  1.769 days  (1 year/365.25 days)  0.00485 years.

AIo  422,000 km  (1 AU/1.5  108 km)  0.00281 AU. Using Newton’s version of Kepler’s third law:

MJ  A3/P2  (0.00281 AU)3/(0.00485 years)2  9.5  10 4 solar masses.

Converting this to Earth masses:

MJ  9.5  10 4 solar masses  (2  1030 kg/ solar mass)  (1 Earth mass/6  1024 kg)  315.6 Earth masses.

DIF: Difficult REF: Section 10.2 MSC: Applied TOP: 2Iii

5. What is the ratio of Jupiter’s volume to that of Earth’s if both planets can be modeled as spheres and Jupiter’s radius is 11 times that of the Earth’s?

ANS: The volume of a sphere is 4/3r3 The ratio of Jupiter’s radius to that of Earth’s gives the volume ratio of [4/3(rJ)3]/[4/3(rE)3]  (rJ)3/(rE)3  1,331.

DIF: Medium REF: Section 10.2 MSC: Applied TOP: 2Iiii 6. If Saturn’s rotational period is 11 hours and its radius is 6  107 m, what is the average speed of a cloud in its atmosphere that is rotating with Saturn? (Neglect differential speeds due to winds.)

ANS: The cloud will complete one orbit and travel the circumference of a circle of radius r  60,000 in a period, P  11 hr. Thus, its average speed is

v  2 r/P  2   6  107 m/(11 hr  3,600 sec/1 hr)  9,500 m/s.

DIF: Medium REF: Section 10.2 MSC: Applied TOP: 2IVi 7. If Saturn’s orbital period is 30 years and the obliquity is 26 degrees, how long is it from the first day of spring to the first day of autumn on Saturn?

ANS: The tilt of Saturn’s orbital axis is similar to that of Earth’s, so it should have seasonal changes similar to Earth’s as well. This means that the time between the vernal and autumnal equinoxes on Saturn will be about half the orbital period of Saturn (that is, 15 years), just as they are approximately half of Earth’s orbit (6 months) apart on Earth.

DIF: Medium REF: Section 10.2 MSC: Applied TOP: 2IViii 8. Uranus has an orbital period of 84 Earth years, a rotation period of 17.2 hours, and an obliquity of 98°.

Explain what solar days are like near the north pole of Uranus, and how long they last.

ANS: Each solar day will last 84 Earth years for locations near the north pole of Uranus. Since Uranus has an orbital tilt of nearly 90°, as the planet spins, the Sun will not rise or set. If it is summer in Uranus’s northern hemisphere, then the Sun will be above the horizon until Uranus moves far enough along its orbit for the season to change, and the Sun will not be in the same position in the sky until Uranus completes its entire orbit. So, solar days will be as long as a full year on Uranus.

DIF: Difficult REF: Section 10.2 MSC: Conceptual TOP: 2IViii 9. Define cloud cannibalism. Where is it observed in the Solar System?

ANS: Cloud cannibalism occurs when smaller storm systems are overtaken by and assimilated into larger storm systems. We have observed this taking place with the Great Red Spot on Jupiter.

DIF: Medium REF: Section 10.3 MSC: Conceptual TOP: 3Iii 10. What causes the horizontal bands on Jupiter and Saturn to have different colors? How can they be used to probe different altitudes in their atmospheres?

ANS: The different bands in Jupiter’s and Saturn’s atmospheres are caused by differences in chemical composition. Different chemicals, such as ammonia, methane, hydrogen sulfide, and so on, will condense and form clouds at different temperatures and pressures. Thus, by examining the chemical composition of a cloud, we can learn about the temperature and pressure at that particular cloud’s altitude.

DIF: Medium REF: Section 10.3 MSC: Applied TOP: 3IIIiii 11. Why are winds on the giant planets far faster than those on Earth?

ANS: Their surfaces are rotating far faster, creating a much stronger Coriolis effect.

DIF: Medium REF: Section 10.4 MSC: Conceptual TOP: 4Ii 12. Based on the flux of sunlight that it gets, Jupiter should have a temperature of 109 K. However its temperature is observed to be 124 K. How much more energy is Jupiter radiating out into space compared to what it gets from the Sun? ANS: Stefan-Boltzmann law says that the rate of energy radiated or absorbed over an object’s surface area is proportional to T4, where T is temperature. Thus, Jupiter radiates (124/109)4  1.7 times the energy it gets from the Sun or 70 percent more than it gets from the Sun. DIF: Difficult REF: Section 10.4 MSC: Applied TOP: 4IIiii

13. A diagram of the interior of Jupiter is shown below, with layers labeled A–D.

Describe what each of the four labeled layers is made of.

ANS: A: molecular hydrogen and helium, in gaseous form B: liquid metallic hydrogen C: ices: water, methane, and so on D: rocky materials DIF: Difficult REF: Section 10.5 MSC: Factual TOP: 5Iiv 14. On which of the giant planets do we think we can find deep oceans of water? Why do we think this, when we can’t directly see inside the giant planets?

ANS: Uranus and Neptune probably have deep oceans of water because they are of higher average density than Saturn or Jupiter. With their lower masses, they should be less dense than Jupiter and Saturn because their masses aren’t sufficient to compress their hydrogen and helium as much as Jupiter and Saturn do. This means that Uranus and Neptune must be made of denser material; their densities aren’t high enough to be rock, so they must be mostly compressed water and other low-density ices.

DIF: Medium REF: Section 10.5 MSC: Conceptual TOP: 5IIi | 5IIii 15. How does the structure of the solar nebula help explain why Jupiter is so much larger than the other giant planets?

ANS: The farther from the protosun a planet formed, the lower the density of the gas available for accretion. Gas density decreased with distance in the solar nebula. Saturn may be less massive than Jupiter

because less gas was available at its distance from the Sun. Uranus and Neptune were at an even greater disadvantage.

DIF: Medium REF: Section 10.5 MSC: Conceptual TOP: 5IIIi 16. Explain why the densest materials in Jupiter are found in the core of the planet. How does this differ from the formation of the Earth’s dense core?

ANS: The core of Jupiter formed first, and then the gravity of the core allowed Jupiter to collect gasses from the solar nebula. The Earth’s core, however, contains the densest materials because of differentiation, where the denser materials sunk through the molten Earth.

DIF: Difficult REF: Section 10.5 MSC: Applied TOP: 5IIIi 17. Explain why the ice giants likely formed at a different time than the gas giant planets, and describe how this led to their different compositions.

ANS: The cores of Uranus and Neptune likely formed later than those of Jupiter and Saturn because the ice giants formed at larger distances from the protosun where the temperatures were the coldest. The larger distance to the ice giants means there would have been more space between planetesimals, meaning it would take longer to accumulate a core large enough to hold on to an atmosphere. By the time Uranus and Neptune came together, the Sun would have blown away most of the hydrogen and helium from the solar nebula, leaving little for the ice giants to collect.

DIF: Medium REF: Section 10.5 MSC: Conceptual TOP: 5IIIii 18. What causes the large magnetic fields of Uranus and Neptune? How does this source help explain why the axis of their magnetic fields are misaligned and significantly offset from their rotational axes?

ANS: Uranus and Neptune have very deep, salty oceans that are electrically conductive. Motions of charged particles in the oceans produce these planets’ large magnetic fields. Because the oceans are shells around the core, and the motions may not be centered around the center of the planet, this can explain why each of these planets are unique in that their magnetic axes are offset from the planet’s center and not aligned with their rotational axes.

DIF: Medium REF: Section 10.6 MSC: Applied TOP: 6Iiii 19. In addition to the visible light that we can see with our own eyes, Jupiter emits a large amount of radio waves. Explain the processes that allow Jupiter to give off each of these types of light.

ANS: The visible light we see from Jupiter is mostly reflected sunlight. The radio waves come from syn-

chrotron radiation, created by charged particles moving around the strong magnetic field of Jupiter.

DIF: Medium REF: Section 10.6 MSC: Applied TOP: 6IIIi 20. When Voyager I passed through Jupiter’s magnetosphere, it flew through a plasma 20 times hotter than the surface of the Sun. Why did the low density of the plasma save the spacecraft from melting?

ANS: You only “feel” temperatures when high-energy particles collide with you, imparting energy to your skin. In the extreme “vacuum” conditions of the plasma, there were simply too few hot particles for collisions to increase the spacecraft’s temperature significantly. This is analogous to the difference between being outside on an 85° day and taking an 85° bath. In the lower density air, you don’t feel as hot as you do in the higher density water.

DIF: Difficult REF: Section 10.6 MSC: Conceptual TOP: 6IVi

CHAPTER 11

Planetary Adornments— Moons and Rings

CONCEPT MAP Sec 11.1 1. Moons in the Solar System I. Galileo’s Discovery of the Four Galilean Moons of Jupiter i. 1610: Galileo Galilei discovered the four brightest moons of Jupiter; strengthened support for heliocentric model of the Solar System (MC: 1) ii. Over 170 known moons in the Solar System and more probably exist (TF: 1) iii. Giant planets have more moons than terrestrial planets (MC: 2) iv. Moons are mixtures of rock and ice, like small terrestrial planets (MC: 3, SA: 1) II. Regular Moons i. Regular moons: formed alongside their planets and grew by accretion (MC: 4, SA: 2, 3) ii. Orbit in the same direction the planet rotates and in equatorial plane (TF: 2, MC: 3, 5, SA: 3) iii. Tidal locking is common (MC: 5, SA: 3) III. Irregular Moons i. Irregular moons: captured asteroids or comets (MC: 6 SA: 4) ii. Irregular moons can have retrograde orbits (TF: 3, MC: 6, 7, SA: 4) iii. Irregular moons can have distant and unstable orbits (MC: 6, SA: 4) Sec 11.2 2. Geological Activity of Moons I. Age of the Moon’s Surface i. Generally, dark surface  older; bright surface  younger (TF: 4, MC: 8, 9, 10, 11, SA: 5, 6) ii. Older surfaces have more craters (MC: 8, 9, SA: 6)

iii. Erupting volcanoes are direct evidence of geological activity (MC: 8, 12, SA: 6) II. Geologically Active Moons: Io, Enceladus, and Triton (SA: 2) i. Io: extremely volcanically active due to Jupiter’s tidal force (TF: 5, MC: 13–15, SA: 7) ii. Io’s surface is brightly colored due to sulfur and sulfur compounds (MC: 9, 16) iii. Cyrovolcanism (MC: 12, 17) iv. Enceladus: geysers shoot particles into space forming Saturn’s E ring (TF: 6, MC: 12, 15, 18, 19, SA: 8) v. Triton has two types of surfaces, “cantaloupe terrain” (MC: 15) III. Possibly Active Moons: Europa and Titan (SA: 2) i. Europa: Jupiter’s moon that is smaller than our Moon (MC: 20) ii. Water is frozen on Europa, and the surface is covered with fractures (MC: 21, SA: 9) iii. The tidal effect of Jupiter may heat the interior enough to have liquid oceans under Europa’s ice surface (TF: 7, MC: 15, 22, SA: 9) iv. Titan: about the size of Mercury; density likely half water and half rock (MC: 23) v. Titan: dense atmosphere of mostly nitrogen, but also methane, ammonia, and carbon (MC: 15, 24– 26, SA: 10) vi. 2004: Cassini used radar to map Titan’s surface, Huygens probe landed on surface (MC: 27) vii. Titan has lakes of methane, ethane, and other hydrocarbons on the surface (TF: 8) viii. Titan resembles the primordial Earth but at a lower temperature (MC: 28) IV. Formerly Active Moons: Ganymede and Some Moons of Saturn and Uranus (SA: 2) i. Ganymede: dark terrain with many impact craters; bright terrain is younger (MC: 10) ii. Ganymede’s geologic activity stopped when differentiation ended (SA: 11) iii. Tethys: Jupiter’s moon with an impact crater covering 40 percent of the moon iv. Mimas: Saturn’s moon also has a huge impact crater V. Geologically Dead Moons (SA: 2) i. Callisto (Jupiter’s moon), Umbriel (Uranus’s moon), other irregular moons ii. Callisto: unclear whether or not it is differentiated (MC: 11, 29) iii. Umbriel: puzzle as to why it’s geologically dead but other moons of Uranus are not Sec 11.3 3. The Discovery of Rings around the Giant Planets I. Introduction to Rings i. Galileo saw rings, Christiaan Huygens “discovered” them (TF: 9, MC: 30)

ii. Stellar occultations measure width and density of rings (MC: 31, SA: 12) iii. Planetary spacecraft used to get close up view of rings (MC: 32) iv. Neptune has ring arcs, segments of higher density material (SA: 13) II. Saturn’s Magnificent Rings—A Closer Look i. All giant planets have rings, and each ring system is unique (TF: 10, MC: 33, 34) ii. Cassini Division is wider than Mercury but not empty, just low density (MC: 35) iii. Each ring is composed of hundreds or thousands of ringlets (MC: 36) iv. Tilt of Saturn’s rings time variable (appear to disappear every 15 years) (MC: 30, 37) v. Saturn’s rings are extremely thin, some diffuse (MC: 38, 39) III. Other Planets, Other Rings i. Other rings are very narrow with a low particle density, therefore fainter (MC: 34, 40) ii. Rings detected when backlit by sunlight (TF: 11, MC: 41, SA: 14, 15) iii. Jupiter’s rings made of fine dust from its inner moons by meteorite impacts (MC: 42) iv. Uranus has 13 rings that are relatively narrow and widely spaced (MC: 43) v. Some of Neptune’s six rings have arclets due to orbital resonances (SA: 13) Sec 11.4 4. Composition of Ring Material I. Similarities and Differences i. Origin of ring material (TF: 12, MC: 44–46) ii. Saturn’s rings are bright because they are mostly water ice (MC: 47–49, SA: 16) iii. Uranus’s and Neptune’s rings are made of dark organic material (MC: 50, SA: 16) iv. Jupiter’s rings are probably made of dark silicate material (SA: 16) Sec 11.5 5. Gravity and Ring Systems I. Moons Maintain Order in the Rings and Create Gaps i. Size of ring particles (MC: 51) ii. Resonances with orbiting moons (MC: 52, 53, SA: 17) iii. The mass of all Saturn’s bright rings is comparable to a small icy moon (MC: 54) II. Strange Things among the Rings i. Shepherd moons interact with the rings (TF: 12, MC: 52, 53, SA: 13) ii. Spokes (MC: 52, SA: 18)

III. Planetary Rings Are Ephemeral i. Ring systems do not have long-term stability (TF: 12–13, MC: 46, 55, 56, SA: 19) Sec 11.6 6. Origins: Extreme Environments and an Organic Deep Freeze I. Necessities of Life i. Extremophiles (TF: 14, MC: 57) ii. Chemosynthesis (MC: 58) iii. Requirements: liquid water, energy source, organic compounds (MC: 59, SA: 20) iv. Conditions needed to create and support life possible on some moons (MC: 24)

TRUE/FALSE 1. There are over 170 known moons in the Solar System.

ANS: T DIF: Easy REF: Section 11.1 MSC: Factual TOP: 1Iii 2. Regular moons orbit in the same direction as their parent planets rotate.

ANS: T DIF: Easy REF: Section 11.1 MSC: Factual TOP: 1IIii 3. Moons in retrograde orbits are called irregular moons.

ANS: T DIF: Easy REF: Section 11.1 MSC: Factual TOP: 1IIIii 4. In terms of the surfaces of moons, brighter surfaces usually indicate older surfaces.

ANS: F DIF: Difficult REF: Section 11.2 MSC: Applied TOP: 2Ii 5. Io is one of the most geologically active moons in the Solar System.

ANS: T DIF: Easy REF: Section 11.2 MSC: Applied TOP: 2IIi 6. Eruptions of water-powered geysers have been seen on the surface of Saturn’s moon Titan.

ANS: F DIF: Medium REF: Section 11.2 MSC: Factual TOP: 2IIiv 7. The oceans under Europa’s icy crust could be very deep and could contain more water than all the oceans on the Earth.

ANS: T DIF: Medium REF: Section 11.2 MSC: Applied TOP: 2IIIiii 8. Lakes of methane, ethane, and other hydrocarbons can be found on the surface of Saturn’s moon Titan.

ANS: T DIF: Easy REF: Section 11.2 MSC: Factual TOP: 2IIIvii 9. Galileo discovered Saturn’s rings and correctly concluded that they were formed by moons that had been tidally ripped apart.

ANS: F DIF: Easy REF: Section 11.3 MSC: Factual TOP: 3Ii 10. Of the giant planets, only Saturn and Jupiter have ring systems.

ANS: F DIF: Easy REF: Section 11.3 MSC: Factual TOP: 3IIi

11. To be effective, backlighting should be done with wavelengths of light that are large compared to the size of the object.

ANS: F DIF: Medium REF: Section 11.3 MSC: Applied TOP: 3IIIii 12. It is theoretically possible for Earth to someday have a temporary ring structure.

ANS: T DIF: Medium REF: Section 11.5 MSC: Applied TOP: 4Ii | 5IIi | 5IIIi 13. The presence of shepherd moons keeps the giant planets’ ring systems completely stable and the rings will last forever.

ANS: F DIF: Medium REF: Section 11.5 MSC: Applied TOP: 5IIIi 14. Extremophiles exist on Earth in both extremely cold and hot environments, but only in places where sunlight allows photosynthesis to occur.

ANS: F DIF: Difficult REF: Section 11.6 MSC: Factual TOP: 6Ii

MULTIPLE CHOICE 1. Who first discovered moons around a planet in our Solar System other than the Earth? a. Newton b. Kepler c. Galileo d. Huygens e. Einstein ANS: C DIF: Easy REF: Section 11.1 MSC: Factual TOP: 1Ii 2. The only planet(s) without a moon is (are): a. Mercury b. Venus c. Mars d. Mercury and Venus e. Mercury, Venus, and Mars ANS: D DIF: Easy REF: Section 11.1 MSC: Factual TOP: 1Iiii

3. Which of the following is NOT a characteristic of regular moons? a. They revolve around their planets in the same direction as the planets rotate. b. They have orbits that lie nearly in the planets’ equatorial plane. c. They are usually tidally locked to their parent planets. d. They are much smaller than all of the known planets. e. They formed in an accretion disk around their parent planet.

ANS: D DIF: Medium REF: Section 11.1 MSC: Factual TOP: 1Iiv | 1IIii 4. Large regular moons probably formed: a. when passing asteroids were captured by the gravitational field of their planet b. at the same time as their planets and grew by accretion c. after a collision between a planet and a large asteroid fractured off a piece of the planet d. after the period of heavy bombardment in the early Solar System e. after a planet got kicked out of its orbit and was gravitationally captured by another planet ANS: B DIF: Easy REF: Section 11.1 MSC: Factual TOP: 1IIi 5. Which property of a moon might lead you to believe it was a captured asteroid? a. It is tidally locked. b. Its orbital axis is tilted by 5° compared to the planet’s rotational axis. c. It rotates in the opposite direction than its planet rotates. d. Its surface is very smooth and lacks craters. e. It is roughly the size of Earth’s moon.

ANS: C DIF: Medium REF: Section 11.1 MSC: Applied TOP: 1IIii | 1IIiii

6. Assume that we discover a new moon of Jupiter. It orbits Jupiter at a large distance and in the opposite direction that Jupiter rotates. It is much smaller than most of Jupiter’s other moons and has a density close to that of Earth rocks. Therefore, this moon is most likely: a. a regular moon that formed with Jupiter in the early solar system b. an irregular moon that is most likely a captured asteroid c. an irregular moon that is most likely a captured comet d. an irregular moon that is most likely a protoplanet which collided with Jupiter in the early solar system and then was caught in orbit by Jupiter’s gravity e. more information is needed before any conclusion can be made ANS: B DIF: Medium REF: Section 11.1 MSC: Conceptual TOP: 1IIIi | 1IIIii | 1IIIiii 7. If a moon has a retrograde orbit then it: a. orbits in the opposite direction than its planet rotates b. orbits in the opposite direction than its planet revolves around the Sun c. orbits in a clockwise direction as viewed from the planet’s north pole d. A and C e. all of the above ANS: D DIF: Difficult REF: Section 11.1 MSC: Conceptual TOP: 1IIIii 8. Which of the following can be used as an indicator of the age of a moon’s surface? a. color of the surface b. crater density c. volcanic activity d. radioactive dating e. all of the above ANS: E DIF: Easy REF: Section 11.2 MSC: Applied TOP: 2Ii | 2Iii | 2Iiii

9. Based on the photo shown below, this moon:

a. is geologically active b. is possibly geologically active c. was geologically active in the past, but is no longer active d. is geologically dead e. More information is needed before any conclusion can be made.

ANS: A DIF: Medium REF: Section 11.2 MSC: Applied TOP: 2Ii | 2Iii | 2IIii 10. Based on the photo shown below, this moon:

a. is geologically active b. is possibly geologically active c. was geologically active in the past but is no longer active d. is geologically dead e. More information is needed before any conclusion can be made.

ANS: C DIF: Difficult REF: Section 11.2 MSC: Applied TOP: 2Ii | 2IVi 11. Based on the photo shown below, this moon:

a. is geologically active b. is possibly geologically active c. was geologically active in the past but is not longer active d. is geologically dead e. More information is needed before any conclusion can be made.

ANS: D DIF: Medium REF: Section 11.2 MSC: Applied TOP: 2Ii | 2Vii

12. Based on the photo shown below, this moon:

a. is geologically active b. is possibly geologically active c. was geologically active in the past but is no longer active d. is geologically dead e. More information is needed before any conclusion can be made.

ANS: A DIF: Easy REF: Section 11.2 MSC: Applied TOP: 2Iiii | 2IIiii | 2IIiv 13. Which object has turned itself inside out numerous times, leading to a situation where lighter elements have escaped, sulfur compounds compose the crust, and primarily heavier elements make up its core? a. Mercury b. Titan c. Callisto d. Pluto e. Io ANS: E DIF: Easy REF: Section 11.2 MSC: Applied TOP: 2IIi 14. Io has the most volcanic activity in the Solar System because: a. it is continually being bombarded with material in Saturn’s E ring b. it is one of the largest moons and its interior is heated by radioactive decays c. of gravitational friction caused by the moon Enceladus d. its interior is tidally heated as it orbits around Jupiter e. the ice on the surface creates a large pressure on the water below ANS: D DIF: Easy REF: Section 11.2 MSC: Factual TOP: 2IIi

15. Which of the following moons is NOT geologically active? a. Callisto b. Triton c. Europa d. Enceladus e. Io ANS: A DIF: Easy REF: Section 11.2 MSC: Applied TOP: 2IIi | 2IIiv | 2IIv | 2IIIiii | 2Vi 16. The varied colors found on Io’s surface are due to the presence of various molecules containing: a. sulfur b. silicon c. iron d. mercury e. magnesium ANS: A DIF: Medium REF: Section 11.2 MSC: Factual TOP: 2IIii 17. Cryovolcanism occurs when: a. molten lava freezes when it reaches the surface because of extremely low temperatures b. volcanoes erupt underwater c. an icy moon has volcanoes emitting molten lava from deep underground d. low-temperature liquids explode through the surface because of increasing pressure underground e. a comet hits an object and causes volcanic eruptions ANS: D DIF: Medium REF: Section 11.2 MSC: Factual TOP: 2IIiii 18. How do particles from the moon Enceladus wind up in Saturn’s E ring? a. Volcanoes erupt and expel silicates into space. b. Water geysers erupt from the surface and expel them into space. c. Cosmic rays bombard the surface rock on Enceladus and expel them into space. d. A collision with a co-orbiting moon knocked rocky debris into orbit around Saturn. e. Strong winds from Saturn blow material off of Enceladus’s surface.

ANS: B DIF: Medium REF: Section 11.2 MSC: Conceptual TOP: 2IIiv

19. Which moon gives rise to the particles that make up Saturn’s E ring? a. Titan b. Triton c. Callisto d. Enceladus e. Thethys ANS: D DIF: Medium REF: Section 11.2 MSC: Factual TOP: 2IIiv 20. What is the escape velocity from Europa, whose radius is 1,600 km and mass is 5  1022 kg? a. 27 km/s b. 7.0 km/s c. 2.0 km/s d. 0.5 km/s e. 15 km/s ANS: C DIF: Difficult REF: Section 11.2 MSC: Applied TOP: 2IIIi 21. Based on the photo shown below, this moon:

a. is geologically active b. is possibly geologically active c. was geologically active in the past but is no longer active d. is geologically dead e. More information is needed before any conclusion can be made.

ANS: B DIF: Medium REF: Section 11.2 MSC: Applied TOP: 2IIIii

22. Which of the following moons is thought to have a vast ocean of water beneath its frozen surface? a. Triton b. Europa c. Ganymede d. Io e. Callisto ANS: B DIF: Easy REF: Section 11.2 MSC: Factual TOP: 2IIIiii 23. What leads astronomers to believe that many large moons associated with the giant planets have compositions that are roughly half water? a. Spectroscopic analysis indicates the presence of large bodies of water. b. They have average densities midway between water and rock. c. Space probes have drilled into the surfaces of many of the moons and detected water. d. Rocks and other features that form only in the presence of water have been observed. e. Astronomers have observed the gravitational effects of tides on those moons.

ANS: B DIF: Medium REF: Section 11.2 MSC: Applied TOP: 2IIIiv 24. Titan is a high-priority candidate for the search for life outside Earth primarily because it has: a. liquid water b. a dense atmosphere like Earth’s c. warm temperatures d. active volcanoes e. organic material ANS: E DIF: Medium REF: Section 11.2 MSC: Applied TOP: 2IIIv | 6Iiv

25. Titan’s thick atmosphere is believed to have been created when ultraviolet photons broke apart methane molecules, ultimately creating the observed smog-like conditions. This process, though, would likely remove all of the atmospheric methane in roughly 10 million years, yet we still see its presence today.

What is the most likely cause? a. Cometary impacts periodically bring new methane to Titan. b. Ethane rains down out of the atmosphere, combines with surface rocks, and creates new methane. c. Infrared photons give atmospheric molecules enough energy to recombine into methane. d. Volcanoes on Titan periodically release new methane into the atmosphere. e. Bacteria on Titan constantly replenish the methane in the atmosphere.

ANS: D DIF: Medium REF: Section 11.2 MSC: Applied TOP: 2IIIv 26. From where does Titan’s thick, nitrogen-rich atmosphere likely arise? a. photodissociation of methane and ammonia in its atmosphere b. emitted by frequent volcanic eruptions c. deposited by ongoing cometary impacts over the age of the Solar System d. photosynthesis of algae in oceans that lie beneath its icy surface e. released from underground reservoirs from early impacts.

ANS: A DIF: Difficult REF: Section 11.2 MSC: Applied TOP: 2IIIv 27. On which of Saturn’s moons did the Cassini-Huygens probe land in 2004? a. Callisto b. Io c. Europa d. Enceladus e. Titan ANS: E DIF: Easy REF: Section 11.2 MSC: Factual TOP: 2IIIvi

28. Which of the following moons do scientists believe most closely represents the primordial Earth, although at a much lower temperature? a. Titan b. Europa c. Callisto d. Io e. Ganymede ANS: A DIF: Easy REF: Section 11.2 MSC: Factual TOP: 2IIIviii 29. Which of the following moons is geologically dead? a. Callisto b. Io c. Europa d. Enceladus e. Titan ANS: A DIF: Easy REF: Section 11.2 MSC: Factual TOP: 2Vii 30. Two years after first being observed, astronomers reported that Saturn’s rings vanished. What happened to them? a. The old ring system dissipated, and since then a new one has formed. b. The orbital plane of the rings was seen edge-on, and the rings were too thin to be visible. c. Most telescopes used hundreds of years ago couldn’t adequately resolve the ring system. d. Astronomers were looking at the wrong planet, leading to the chance discovery of Uranus. e. They were hidden behind some of Saturn’s many moons.

ANS: B DIF: Easy REF: Section 11.3 MSC: Applied TOP: 3Ii | 3IIiv 31. The density of particles in a planet’s rings can be measured using: a. infrared light b. the Doppler shift c. shadows cast by nearby moons d. light from background stars e. their proper motions ANS: D DIF: Easy REF: Section 11.3 MSC: Applied TOP: 3Iii

32. How do astronomers take such detailed, close-up pictures of ring systems? a. They send satellites to the outer planets to take pictures for us. b. They take them using backyard telescopes, just like Galileo did. c. They take them using the largest optical telescopes on Earth. d. They have astronauts in space take pictures of them. e. They wait until the planet is closest to the Earth and use the Hubble Space Telescope.

ANS: A DIF: Easy REF: Section 11.3 MSC: Conceptual TOP: 3Iiii 33. Which of the giant planets does NOT have rings? a. Jupiter b. Saturn c. Uranus d. Neptune e. All of the giant planets have rings.

ANS: E DIF: Easy REF: Section 11.3 MSC: Applied TOP: 3IIi 34. Which of the following planets has the most complex and magnificent ring system? a. Mars b. Jupiter c. Saturn d. Uranus e. Neptune ANS: C DIF: Easy REF: Section 11.3 MSC: Factual TOP: 3IIi | 3IIIi 35. Astronomers originally planned to have the Pioneer 11 space probe pass through the Cassini Gap in Saturn’s rings. Would this mission have been successful? a. Yes, but they decided that it was more important to observe Saturn’s moons. b. Yes, but they decided to land on the rings instead. c. No, because the Cassini Gap turns out to be too narrow. d. No, because the Cassini Gap is not completely empty. e. No, because the same gravitational influences that create the Cassini Gap would have destroyed the probe.

ANS: D DIF: Medium REF: Section 11.3 MSC: Applied TOP: 3IIii

36. Of what are Saturn’s brightest rings primarily made? a. a thin, solid surface of rock and ice b. an orbiting cloud of high-density gas c. hundreds to thousands of smaller ringlets d. a very diffuse collection of dust e. house-sized rocks ANS: C DIF: Easy REF: Section 11.3 MSC: Factual TOP: 3IIiii 37. Saturn’s rings disappear from sight every: a. 40 years b. 25 years c. 15 years d. 8 years e. 6 months ANS: C DIF: Difficult REF: Section 11.3 MSC: Factual TOP: 3IIiv 38. How does the thickness of Saturn’s bright ring system compare to its diameter? a. It’s about 10 times thinner. b. It’s about 1,000 times thinner. c. It’s about 10,000 times thinner. d. It’s about 100,000 times thinner. e. It’s about 10 million times thinner.

ANS: E DIF: Difficult REF: Section 11.3 MSC: Applied TOP: 3IIv

39. Saturn’s G ring, as shown in the figure below, is known as:

a. a ringlet b. an arclet c. a diffuse ring d. a spoke e. a crepe ring ANS: C DIF: Easy REF: Section 11.3 MSC: Applied TOP: 3IIv 40. If a planetary ring had an inner diameter of 100,000 km, an outer diameter of 120,000 km, a thickness of 10 meters, and a density of 100 kg/m3, what would be the total mass of material in this ring? a. 6  1020 kg b. 5  1023 kg c. 4  1015 kg d. 2  1021 kg e. 3  1018 kg ANS: E DIF: Difficult REF: Section 11.3 MSC: Applied TOP: 3IIIi

41. If you wanted to search for faint rings around a giant planet by sending a spacecraft on a flyby, it would be best to make your observations: a. as the spacecraft approached the planet b. after the spacecraft passed the planet c. while orbiting the planet d. during the closest flyby e. while orbiting one of its moons ANS: B DIF: Medium REF: Section 11.3 MSC: Applied TOP: 3IIIii 42. Jupiter’s rings are made of material from: a. its innermost moons b. its upper atmosphere c. its outermost moons d. only Io e. only its retrograde moons ANS: A DIF: Difficult REF: Section 11.3 MSC: Factual TOP: 3IIIiii 43. How do Uranus’s rings differ from the ring systems of the other giant planets? a. Uranus has only one ring made up of fine dust. b. Uranus has the most spectacular ring system with many bright, wide rings. c. Uranus has 13 rings that are narrow and widely spaced. d. Uranus has rings that are clumped into several arc-like segments. e. Uranus has rings that are solid enough to land on.

ANS: C DIF: Difficult REF: Section 11.3 MSC: Applied TOP: 3IIIiv 44. If the Moon had active volcanoes: a. the Moon would have a thick hydrogen atmosphere b. the Earth might have a ring c. the Moon’s surface would have more craters than it currently does d. life could not exist on Earth e. the Moon would have different phases than we see today ANS: B DIF: Medium REF: Section 11.4 MSC: Applied TOP: 4Ii

45. Which of the following is NOT a way to renew particles in a ring system? a. shredding an object that came within a planet’s Roche limit b. a collision between moons or other objects near the ring system c. eruptions on a nearby moon, sending particles into space d. a planet’s gravity drawing particles from the nearby interstellar medium e. impacts on a nearby moon, sending particles into space ANS: D DIF: Easy REF: Section 11.4 MSC: Applied TOP: 4Ii 46. Which of the following is FALSE? a. The sizes of planetary ring material ranges from tiny grains to house-sized boulders. b. Some rings around giant planets are made from particles that are ejected by its moons. c. Planetary rings can be made when a moon is torn apart by tidal forces. d. The material in planetary rings orbit the planet while obeying Kepler’s third law. e. Planetary rings around the giant planets usually remain for tens of billions of years.

ANS: E DIF: Medium REF: Section 11.4, 11.5 MSC: Factual TOP: 4Ii | 5IIIi 47. What is the most likely reason that a planet’s rings would reflect more than 50 percent of the sunlight they receive? a. They are made of ice. b. They are made of silicate rock. c. They are made of liquid. d. They are made of iron. e. They are very old.

ANS: A DIF: Easy REF: Section 11.4 MSC: Conceptual TOP: 4Iii 48. Of what are Saturn’s ring primarily made? a. water ice b. methane c. nitrogen d. dark organic material e. dark silicate material ANS: A DIF: Easy REF: Section 11.4 MSC: Factual TOP: 4Iii

49. Saturn’s rings are much brighter than the rings of the other giant planets because: a. Saturn is closer to the Sun and receives a higher flux of sunlight b. the material in Saturn’s rings is made mostly of ice rather than rock c. Saturn’s rings have over 100 times more material in them d. Saturn’s rings are tilted by a larger angle relative to our line of sight e. the material in Saturn’s rings is much hotter than material in other ring systems ANS: B DIF: Difficult REF: Section 11.4 MSC: Factual TOP: 4Iii 50. Particles that make up the rings of Uranus and Neptune are composed of: a. rocky material from tidally disrupted moons b. organic material that has darkened due to bombardment by high-energy, charged particles c. icy material from tidally disrupted comets d. magma from volcanic eruptions on the surfaces of their moons e. all of the above ANS: B DIF: Difficult REF: Section 11.4 MSC: Conceptual TOP: 4Iiii 51. Ring particles range in size from tiny grains to: a. house-sized boulders b. basketball-sized boulders c. city-sized chunks d. tennis ball–sized rocks e. fingernail-sized pebbles ANS: A DIF: Difficult REF: Section 11.5 MSC: Factual TOP: 5Ii 52. All of the following ring structures are known to be created by shepherd moons EXCEPT: a. braided rings b. spokes c. scalloped edges d. ring gaps e. knots and kinks ANS: B DIF: Difficult REF: Section 11.5 MSC: Applied TOP: 5Iii | 5IIi | 5IIii

53. Rings that project to look like they are intertwined (but are not) are caused by: a. new laws of physics b. ring material on highly elliptical orbits c. the gravitational influence of small moons d. electromagnetic interaction of the rings with Saturn’s magnetic field e. meteoroid impacts ANS: C DIF: Medium REF: Section 11.5 MSC: Applied TOP: 5Iii | 5IIi 54. The mass of all of Saturn’s bright rings is comparable to the mass of: a. a small comet b. a small icy moon c. the Earth’s Moon d. Mars e. Venus ANS: B DIF: Medium REF: Section 11.5 MSC: Applied TOP: 5Iiii 55. Ring material: a. is made primarily of fine dust b. has always orbited the giant planets c. reflects more than 75 percent of the light that falls on them d. must constantly be renewed e. is made primarily of kilometer-sized rocks ANS: D DIF: Medium REF: Section 11.5 MSC: Applied TOP: 5IIIi 56. Rings of giant planets are very thin compared to their diameters mainly because: a. of collisions between ring particles b. moons that tidally disrupt have small diameters c. energy is conserved when a moon tidally disrupts d. the planets have large tidal forces e. shepherd moons force them to be extremely thin ANS: A DIF: Difficult REF: Section 11.5 MSC: Conceptual TOP: 5IIIi

57. Extremophiles on Earth have been found: a. in the scalding waters of Yellowstone’s hot springs b. in the bone-dry oxidizing environment of Chile’s Atacama Desert c. in the Dead Sea d. in the deep subsurface ice of the Antarctic ice sheet e. all of the above ANS: E DIF: Easy REF: Section 11.6 MSC: Factual TOP: 6Ii 58. Through what process do some living organisms find energy to survive deep under the ocean? a. electrolysis b. photosynthesis c. plasmosynthesis d. chemosynthesis e. magnetosynthesis ANS: D DIF: Difficult REF: Section 11.6 MSC: Conceptual TOP: 6Iii 59. By examining extremophiles on Earth, which of the following appears NOT to be a key ingredient for life? a. liquid water b. an energy source c. oxygen d. organic compounds e. All of the above are key ingredients.

ANS: C DIF: Medium REF: Section 11.6 MSC: Applied TOP: 6Iiii

SHORT ANSWER 1. What are the two basic materials of which the moons in the solar system are composed? For each type of material, name an example of a moon whose surface is composed primarily of that material.

ANS: Rocky material and ices. Some examples of moons with rocky surfaces: Io, Ganymede, Callisto.

Some examples of icy moons: Europa, Enceladus.

DIF: Medium REF: Sections 11.1, 11.2 MSC: Factual TOP: 1Iiv | 2II | 2III | 2IV | 2V

2. Explain how a planet obtains a regular moon orbiting it.

ANS: Regular moons are formed from an accretion disk surrounding the parent planet as the parent planet itself is forming.

DIF: Easy REF: Sections 11.1 MSC: Factual TOP: 1IIi 3. What are the orbital characteristics of a regular moon?

ANS: Regular moons orbit in the same direction as their parent planet rotates. Regular moons also orbit in the equatorial plane of their parent planet. Many orbital moons are tidally locked, although that is not a requirement.

DIF: Easy REF: Sections 11.1 MSC: Factual TOP: 1IIi | 1IIii | 1IIiii 4. What are three characteristics of the orbits of irregular moons, and how are irregular moons formed?

ANS: Irregular moons are probably captured asteroids. Three characteristics of irregular moons are (1) retrograde orbits, (2) large distances from their planet, and (3) chaotic orbits or orbital axes that are misaligned with the planet’s rotational axis.

DIF: Medium REF: Sections 11.1 MSC: Factual TOP: 1IIIi | 1IIIii | 1IIIiii 5. The color of a moon’s surface contains clues as to its age. What is the typical relationship between surface color and surface age, and why does this relationship exist?

ANS: Darker surfaces are typically older and brighter surfaces are typically younger. This is because water ice is a common surface material among the moons of the outer solar system. Water ice reflects the majority of light that hits its surface making it very bright. Over time, meteorite dust darkens a moon’s surface. So, a bright surface means that some activity has recently refreshed the surface with new water ice.

DIF: Medium REF: Sections 11.2 MSC: Applied TOP: 2Ii 6. Name three characteristics of a geologically active moon.

ANS: A geologically active moon would have a (1) relatively bright surface that is (2) free of many impact craters and is likely to have (3) volcanic activity.

DIF: Easy REF: Sections 11.2 MSC: Applied TOP: 2Ii | 2Iii | 2Iiii 7. Why is Io, a moon that is smaller and farther from the Sun than our own Moon, still geologically active?

ANS: Tidal stresses from Jupiter continually cause Io’s interior to flex, keeping it heated and stopping it

from completely cooling off.

DIF: Easy REF: Section 11.2 MSC: Conceptual TOP: 2IIi 8. What material has been seen erupting from the surface of the icy moon Enceladus, and why?

ANS: Geysers of water erupt from the surface of Enceladus because tidal stresses from Saturn heat up the interior and melt water below its icy surface.

DIF: Medium REF: Section 11.2 MSC: Conceptual TOP: 2IIiv 9. Europa is a very interesting moon that scientists are considering visiting with a spacecraft in order to search for signs of life. What is it about this moon that makes it so interesting, and what surface features give us clues about its interior?

ANS: Europa has an icy surface riddled with cracks. It appears that liquid or slush rises up from the cracks and solidifies. Saturn’s tidal force may keep Europa’s interior liquid, and deep oceans filled with water may exist under its icy surface, which might contain extreme forms of life.

DIF: Medium REF: Section 11.2 MSC: Conceptual TOP: 2IIIii | 2IIIiii 10. If ultraviolet photons destroy methane, why do scientists think Titan has so much of it in its atmosphere?

ANS: Internal heating drives cryovolcanism on Titan, constantly releasing methane into Titan’s atmosphere.

DIF: Medium REF: Section 11.2 MSC: Applied TOP: 2IIIv 11. Ganymede is one of the largest moons in the Solar System. It shows some terrain that is ancient and heavily cratered, younger terrain with less craters, but no terrain that is free of craters. Why would Ganymede’s geologic activity stop?

ANS: Ganymede’s geologic activity probably stopped because its interior solidified after differentiation stopped releasing energy.

DIF: Medium REF: Section 11.2 MSC: Applied TOP: 2IVii 12. Explain how Uranus’s rings were first discovered.

ANS: Uranus’s rings were first discovered through stellar occultation, which consists of observing how starlight is dimmed as a ring passes in front of a background star.

DIF: Easy REF: Section 11.3 MSC: Applied TOP: 3Iii

13. Describe some of the effects that moons can have on nearby ring systems.

ANS: Shepherd moons can create gaps, sharp edges, knots, twists, and rope-like features in the rings.

Moons are also responsible for changing the density of rings, creating arclets and ring arcs, and creating gaps, via orbital resonances.

DIF: Medium REF: Sections 11.3, 11.5 MSC: Applied TOP: 3Iiv | 3IIIv | 5IIi 14. Explain why it was difficult for the Voyager space probe to detect Jupiter’s ring system as it was approaching the planet, but easy to detect the rings once it passed behind Jupiter.

ANS: Jupiter’s ring system is composed mostly of tiny dust grains. Particles this small tend to scatter light in the direction in which the light was originally traveling. As the space probe approached Jupiter, the Sun and the probe were on the same side of the ring system, so all the light scattered off the ring was directed away from the probe. As the probe passed behind Jupiter, the Sun was now on the opposite side of the ring system from the probe, and all of the light scattered off the ring was directed toward the probe.

DIF: Medium REF: Section 11.3 MSC: Conceptual TOP: 3IIIii 15. Explain how pictures such as the one shown below are taken. Where must the camera be in relation to the planet and the Sun? Why do the rings appear so bright from this direction?

ANS: This picture was taken using the technique of backlighting. The camera must be on the opposite of the planet from the Sun. Backlighting happens when light falls on very small objects, such as the particles in Saturn’s rings. Since very little of the light is scattered backward or to the sides of the particles, they appear much brighter from this angle, making it easier to see the small particles in the diffuse rings.

DIF: Difficult REF: Sections 11.3 MSC: Conceptual TOP: 3IIIii

16. Rank the four giant planets’ ring systems from brightest to darkest.

ANS: Saturn’s rings are the brightest, followed by Jupiter’s ring. Uranus’s and Neptune’s ring systems are the darkest (consider them tied).

DIF: Easy REF: Sections 11.4 MSC: Factual TOP: 4Iii | 4Iiii | 4Iiv 17. Why do we suspect that the inner planets do not have rings?

ANS: They lack small moons to act as shepherds of the ring material, which lends stability to a ring system and allows them to last over long periods of time.

DIF: Difficult REF: Section 11.5 MSC: Applied TOP: 5Iii 18. What do astronomers believe causes the spoke-like features associated with Saturn’s B ring?

ANS: Meteoroid impacts with larger ring particles send dust above the ring plane. These particles become ionized and Saturn’s magnetic field causes them to drift outward.

DIF: Difficult REF: Section 11.5 MSC: Factual TOP: 5IIii 19. Do a planet’s rings last forever? Why or why not?

ANS: Because ring particles collide over time, they lose energy and angular momentum and eventually will fall into the planet. They do not last forever.

DIF: Medium REF: Section 11.5 MSC: Applied TOP: 5IIIi 20. Looking at the life forms found to exist in extreme environments on Earth suggests that there are probably three things needed for life. What are they?

ANS: The three things needed for life appear to be liquid water, an energy source (sunlight, geothermal energy, or chemical energy) and organic, carbon-based compounds.

DIF: Difficult REF: Section 11.6 MSC: Conceptual TOP: 6Iiii

CHAPTER 12

Dwarf Planets and Small Solar System Bodies

CONCEPT MAP Sec 12.1 1. Leftover Material: From the Small to the Tiniest I. Classes of Debris i. Debris: dwarf planets, KBOs, asteroids, comets, meteoroids, zodiacal dust (MC: 1) ii. IAU definition of a planet (MC: 2) Sec 12.2 2. Dwarf Planets: Pluto and Others I. Pluto (SA: 1) i. Discovered by Clyde Tombaugh in 1930 (MC: 3, SA: 19) ii. Mass of Pluto system  1/400  mass of Earth (TF: 1, MC: 3) iii. Pluto has five moons (MC: 3) iv. Pluto has very eccentric orbit, which crosses that of Neptune (TF: 2, MC: 2, 3) v. Density roughly equal to twice that of water (TF: 3, MC: 4) vi. IAU voted to demote Pluto from a planet to a dwarf planet in 2006 (MC: 39) II. Eris (SA: 1) i. Eris is very similar to Pluto but more massive (TF: 2, 4, MC: 5, 6, SA: 2) ii. Eris has a high albedo due to a pristine methane ice surface (MC: 7) III. Haumea and Makemake (SA: 1) i. Both are smaller than Pluto and further from the Sun (SA: 19)

ii. Haumea: rapid rotation, oblate, two icy moons IV. Ceres (SA: 1) i. Largest asteroid (MC: 4, SA: 5) Sec 12.3 3. Asteroids—Pieces of the Past I. Asteroids i. Most asteroids orbit between the orbits of Mars and Jupiter (TF: 5, MC: 8) ii. 107 asteroids larger than 1 km; total mass is ~(1/3) MMoon (TF: 6, MC: 9, 10) iii. Trojan asteroids and near-Earth asteroids (MC: 11) II. Asteroids Are Fractured Rocks i. Rocky fragments that are pieces of planetesimals (TF: 7, MC: 12) ii. Asteroids are irregularly shaped and tumble through space (MC: 13) iii. C-type asteroids are nearly pristine Solar System material (Mathilde) (MC: 14, SA: 3) iv. S- and M-type asteroids are highly processed (MC: 15, 16, 17, SA: 4, 5, 7, 22) III. Asteroids Viewed Up Close i. Galileo flyby of Gaspra, irregularly shaped and pitted by impact craters (MC: 13, 17) ii. Fractures show asteroids can be solid rock iii. Some asteroids have moons (TF: 8, MC: 18) iv. Hayabusa sample return mission (MC: 19) v. Vesta is large protoplanet and has volcanic activity (MC: 20) Sec 12.4 4. Comets: Clumps of Ice I. Origin i. Icy planetesimals ii. Active comets: those that come close to the Sun and melt and shed debris (MC: 21, 22) iii. Location: Kuiper Belt (flattened disk) or Oort Cloud (spherical) (TF: 9, 23, MC: 19, 29) II. Orbits of Comets i. Short-period comets: P  200 years (Kuiper Belt) (TF: 10, MC: 24, SA: 5, 6) ii. Long-period comets: P  200 years (Oort Cloud) (MC: 22, 23, 25, SA: 5, 6, 7) iii. Sungrazers: comets that come very close to the surface of the Sun (SA: 6) iv. Orbits are usually very elliptical (MC: 26)

v. Short-period comets: prograde (MC: 22, 26) vi. Long-period comets: prograde and retrograde (MC: 23, 26) vii. A spectacularly active, visible comet appears about every 10 years (MC: 27, 49, SA: 7) viii. Comet Halley: first comet whose return was predicted (MC: 28, 29, SA: 8) III. An Anatomy of an Active Comet i. Nucleus (MC: 30, 31) ii. Sublimation (MC: 32, SA: 9) iii. Coma iv. Head/tail (SA: 10, 11) IV. Comets Have Two Types of Tails i. Ion tail (SA: 11, 12) ii. Dust tail (MC: 33, SA: 11, 12) iii. Composed of very low-density gas and dust (SA: 7) V. Visits to Comets (MC: 34) i. Halley: many jets, nucleus very black (MC: 35, 50, SA: 13, 14, 15) ii. Wild 2: visited by Stardust mission in 2004 (MC: 50, SA: 15, 16) iii. Tempel 1: Deep Impact collision (MC: 50, SA: 11) iv. Hartley 2: water has same ratio of hydrogen isotopes as Earth’s oceans (MC: 50) VI. Collisions Still Happen Today i. Comet Shoemaker-Levy 9 (MC: 36, SA: 15) ii. Tunguska explosion (SA: 16) iii. Impacts of large objects are rare but could be very dangerous (MC: 37, SA: 17) Sec 12.5 5. Solar System Debris I. Meteors/Meteorites/Meteoroids (SA: 18) i. Meteoroids (larger than a fist) that survive descent become meteorites (MC: 38, 39) ii. Meteor shower (MC: 40–44) iii. Radiant (TF: 11, MC: 45) II. Types of Meteoriods i. Antarctica is the best place to find meteorites (MC: 46) ii. Three categories of meteorites: stony, iron, and stony-iron mixture (TF: 12, MC: 47, 48) iii. Stony meteorites: most common, many contain chondrules (TF: 13, MC: 49, 50, SA: 19)

iv. Chondrites versus achondrites (MC: 47) v. Iron meteorites: melted and pitted appearance (MC: 47) vi. Martian meteorites (TF: 7, MC: 51) III. Zodiacal Dust i. Traces the ecliptic plane, very faint, but visible from a very dark location (MC: 52, 53) ii. Zodiacal dust contains about as much mass as found in a large comet (MC: 54) Sec 12.6 6. Origins: Comets, Asteroids, and Life I. Importance of Impacts i. Comets might have contributed to some of the water on Earth (MC: 55, SA: 20) ii. Impacts threaten to disrupt and/or end life on Earth (dinosaurs, 65 million years ago) iii. Comets seed Earth with organic elements that are the chemical origins of life (TF: 14)

TRUE/FALSE 1. The mass of Pluto is approximately 100 times less than that of Earth.

ANS: F DIF: Medium REF: Section 12.2 MSC: Factual TOP: 2Iii 2. Pluto is the most massive member of the Kuiper Belt.

ANS: F DIF: Easy REF: Section 12.2 MSC: Factual TOP: 2Iiv | 2IIi 3. Pluto is composed primarily of rock.

ANS: F DIF: Easy REF: Section 12.2 MSC: Factual TOP: 2Iv 4. Eris is classified as an asteroid even though its mass is 28 percent larger than Pluto’s mass.

ANS: F DIF: Medium REF: Section 12.2 MSC: Factual TOP: 2IIi 5. Most asteroids are found in orbit around the Sun between the orbits of Jupiter and Saturn.

ANS: F DIF: Easy REF: Section 12.3 MSC: Applied TOP: 3Ii 6. Although asteroids are small individually, when combined they make up about a quarter of all the mass in the Solar System, excluding the Sun. ANS: F DIF: Easy REF: Section 12.3 MSC: Factual TOP: 3Iii 7. All meteorites are remnants of planetesimals that never coalesced to form a planet.

ANS: F DIF: Difficult REF: Section 12.3 | Section 12.5 MSC: Applied TOP: 3IIi | 5IIvi 8. Some asteroids have moons just like planets do.

ANS: T DIF: Easy REF: Section 12.3 MSC: Factual TOP: 3IIIiii 9. Like meteoroids and asteroids, cometary nuclei in the Oort Cloud undergo frequent collisions that often send them into the inner Solar System.

ANS: F DIF: Medium REF: Section 12.4 MSC: Applied TOP: 4Iiii 10. All short-period comets have periods less than or equal to 20 years.

ANS: F DIF: Easy REF: Section 12.4 MSC: Factual TOP: 4IIi 11. Astronomers can use the speed and direction of a cometary meteor’s flight to identify its parent comet.

ANS: T DIF: Difficult REF: Section 12.5 MSC: Applied TOP: 5Iiii

12. Meteorites are more likely to come from asteroids than comets.

ANS: T DIF: Medium REF: Section 12.5 MSC: Applied TOP: 5IIii 13. The most common type of meteorites found are iron meteorites.

ANS: F DIF: Easy REF: Section 12.5 MSC: Factual TOP: 5IIiii 14. Cometary nuclei provide evidence that the ingredients necessary for the creation of life were present in the early solar nebula.

ANS: T DIF: Difficult REF: Section 12.6 MSC: Applied TOP: 6Iiii

MULTIPLE CHOICE 1. Which of the following types of Solar System debris were NOT discovered until the age of telescopes? a. comets b. meteoroids c. zodiacal dust d. asteroids e. all of the above ANS: D DIF: Medium REF: 12.1 MSC: Factual TOP: 1Ii 2. Pluto is classified as a dwarf planet because: a. it has not cleared out other bodies from its orbit b. it is over 1,000 times smaller than the Earth’s moon c. it has no moons of its own d. it has a unique chemical composition that is very different from other planets e. it orbits just outside the Solar System ANS: A DIF: Easy REF: Section 12.1 |12.2 MSC: Applied TOP: 1Iii | 2Ivi

3. Which of following is FALSE? a. Pluto has five moons. b. Pluto has a mass that is 10 times less than the Earth’s mass. c. Pluto’s orbit sometimes brings it closer to the Sun than Neptune. d. Pluto was discovered by Clyde Tombaugh in 1930. e. Pluto has a thin atmosphere.

ANS: B DIF: Medium REF: Section 12.2 MSC: Factual TOP: 2Ii | 2Iii | 2Iiii | 2Iiv 4. Pluto has a density that is roughly equal to two times that of: a. a feather b. water c. lead d. a rock e. air ANS: B DIF: Easy REF: Section 12.2 MSC: Factual TOP: 2Iv 5. The dwarf planet Eris has a moon called Dysomia, which is much smaller in mass than Eris. If Dysomia has an orbital period of 16 days and orbits Eris at a distance of 40,000 km, then what is the mass of Eris? a. 2  1013 kg b. 2  1022 kg c. 2  1028 kg d. 2  1032 kg e. 2  1035 kg ANS: B DIF: Difficult REF: Section 12.2 MSC: Applied TOP: 2IIi 6. Eris, Ceres, and Haumea are examples of: a. asteroids b. dwarf planets c. meteoroids d. comets e. meteor showers ANS: B DIF: Easy REF: Section 12.2 MSC: Factual TOP: 2IIi | 2IVi

7. Currently the surface of the dwarf planet Eris is covered with _________, which makes it have the highest albedo of any object in the Solar System. a. methane ice b. water ice c. nitrogen ice d. sulfur dioxide ice e. carbon dioxide ice ANS: A DIF: Difficult REF: Section 12.2 MSC: Factual TOP: 2IIii 8. Most asteroids are located between the orbits of: a. Earth and Mars b. Mars and Jupiter c. Jupiter and Saturn d. Neptune and Pluto e. the Kuiper Belt and the Oort Cloud ANS: B DIF: Easy REF: Section 12.3 MSC: Factual TOP: 3Ii 9. Most asteroids are: a. very large ( 100 km) b. large (30–100 km) c. medium (10–30 km) d. small (1–10 km) e. very small ( 1 km) ANS: D DIF: Difficult REF: Section 12.3 MSC: Factual TOP: 3Iii 10. The mass of all the known asteroids combined is approximately equal to: a. half the mass of Earth b. three times the mass of Earth c. twice the mass of Mars d. the mass of Mars e. one-third the mass of the Moon ANS: E DIF: Easy REF: Section 12.3 MSC: Factual TOP: 3Iii

11. Which group of asteroids regularly crosses Earth’s orbit and thus might possibly collide with our planet? a. the Amors b. the Atens c. the Kuiper Belt objects d. the Trojans e. all of the above ANS: B DIF: Medium REF: Section 12.3 MSC: Applied TOP: 3Iiii 12. Asteroids are primarily composed of: a. hydrogen and helium b. ice and dust c. rock d. iron e. methane ANS: C DIF: Easy REF: Section 12.3 MSC: Factual TOP: 3IIi 13. Most asteroids are closest in shape to: a. a potato b. a banana c. a hot dog d. a stick e. a baseball ANS: A DIF: Easy REF: Section 12.3 MSC: Factual TOP: 3IIii | 3IIIi 14. The darkest asteroids are: a. M-type b. S-type c. C-type d. A-type e. Q-type ANS: C DIF: Difficult REF: Section 12.3 MSC: Factual TOP: 3IIiii

15. Iron meteorites are fragments of which type of asteroid? a. A-type b. C-type c. M-type d. Q-type e. S-type ANS: C DIF: Difficult REF: Section 12.3 MSC: Applied TOP: 3IIiv 16. Carbonaceous chondrite meteorites are fragments of which type of asteroid? a. A-type b. C-type c. M-type d. Q-type e. S-type ANS: B DIF: Medium REF: Section 12.3 MSC: Applied TOP: 3IIiv 17. Until spacecraft flew by asteroids, scientists did not have a good idea of what they looked like. Which of the following missions was the first to fly by an asteroid? a. NEAR Shoemaker b. Rosetta c. Galileo d. Dawn e. Stardust ANS: C DIF: Medium REF: Section 12.3 MSC: Factual TOP: 3IIIi 18. The most straightforward way to determine the mass of an asteroid is if it has: a. a rocky composition b. a moon c. an orbit that lies between the Earth and Mars d. carbonaceous chondrites e. a magnetic field ANS: B DIF: Easy REF: Section 12.3 MSC: Applied TOP: 3IIIiii

19. In November 2005, the Japanese spacecraft Hayabusa brought back sample from which type of object for the first time? a. comet b. asteroid c. moon d. terrestrial planet e. gas giant planet ANS: B DIF: Medium REF: Section 12.3 MSC: Factual TOP: 3IIIiv 20. Remnants of volcanic activity on the asteroid Vesta indicate that members of the asteroid belt: a. were once part of a single protoplanet that was shattered by collisions b. have all undergone significant chemical evolution since formation c. occasionally grow large enough to become differentiated and geologically active d. were once a part of a young Mars e. used to be volcanic moons orbiting other planets ANS: C DIF: Medium REF: Section 12.3 MSC: Conceptual TOP: 3IIIv 21. Identify the object in the picture below.

a. an active comet b. a meteor shower c. a meteorite d. an asteroid e. zodiacal dust ANS: A DIF: Easy REF: Section 12.4 MSC: Applied TOP: 4Iii

22. Suppose we discover a comet whose orbit was very highly eccentric, retrograde, had a very large tilt with respect to the ecliptic plane, and a period of 2,000 years. Where is the most likely place of origin for this comet? a. the Kuiper Belt b. the Oort Cloud c. the asteroid belt d. the Jovian family e. outside the Solar System ANS: B DIF: Easy REF: Section 12.4 MSC: Applied TOP: 4Iii | 4IIii | 4IIv 23. Which of the following does NOT describe comets in the Oort Cloud? a. long period b. pristine condition c. cold temperatures d. randomly directed orbits e. flattened distribution ANS: E DIF: Easy REF: Section 12.4 MSC: Applied TOP: 4Iiii | 4IIii | 4IIvi 24. A comet having an orbit of 50 years would likely have come from the: a. Atens family b. Oort Cloud c. Trojan family d. zodiacal zone e. Kuiper Belt ANS: E DIF: Medium REF: Section 12.4 MSC: Applied TOP: 4IIi 25. Most comets originate: a. near Earth and Venus in the early Solar System b. far from the planets, many thousands of AU from the Sun c. from the region between the orbits of Jupiter and Neptune d. between the Sun and Mercury e. between the orbits of Mars and Jupiter ANS: B DIF: Easy REF: Section 12.4 MSC: Applied TOP: 4IIii

26. The one orbital characteristic both short- and long-period comets share is: a. mostly prograde orbits b. orbits with completely random tilts c. mostly retrograde orbits d. orbital periods longer than any planet e. highly eccentric orbits ANS: E DIF: Medium REF: Section 12.4 MSC: Applied TOP: 4IIiv | 4IIv | 4IIvi 27. Approximately how often does a spectacularly active, visible comet appear? a. once a year b. once every 5 years c. once every 10 years d. once every 50 years e. once every 1,000 years ANS: C DIF: Medium REF: Section 12.4 MSC: Applied TOP: 4IIvii 28. Comet Halley is unique because: a. it was the first comet whose return was predicted b. it is a member of the Jovian family, but has a retrograde orbit c. its period is less than a human lifetime d. it was successfully visited by a spacecraft e. it was the brightest comet ever observed by humans ANS: A DIF: Medium REF: Section 12.4 MSC: Applied TOP: 4IIviii 29. With a semimajor axis of 18 AU, Comet Halley has a period of: a. 7 years b. 16 years c. 32 years d. 67 years e. 76 years ANS: E DIF: Easy REF: Section 12.4 MSC: Factual TOP: 4IIviii

30. The nucleus of the typical comet is approximately _________ in size. a. 10 km b. 1,000 km c. 100 m d. 10 m e. 1 cm ANS: A DIF: Medium REF: Section 12.4 MSC: Factual TOP: 4IIIi 31. The nuclei of a comet is mostly: a. solid ice b. solid rock c. liquid water d. a porous mix of ice and dust e. frozen carbon dioxide ANS: D DIF: Medium REF: Section 12.4 MSC: Applied TOP: 4IIIi 32. When a comet comes close to the Sun, its volatile ice sublimates and transforms directly from the solid to _________ phase. a. liquid b. crystalline c. energized d. gas e. ionized ANS: D DIF: Medium REF: Section 12.4 MSC: Conceptual TOP: 4IIIii 33. Why does the dust tail separate from the ion tail? a. The dust is not ionized, so it is not affected by the solar wind. b. Dust cannot sublimate as ice can, so it cannot form a tail as easily. c. The dust tail forms on the leading side of the nucleus, whereas the gas tail forms on the opposite side. d. Dust particles are more massive than ions, so their accelerations are less. e. The dust tail has the opposite charge as the ion tail.

ANS: D DIF: Difficult REF: Section 12.4 MSC: Conceptual TOP: 4IVii

34. Which of the following comets has NOT been visited by spacecraft? a. Halley b. Wild 2 c. Tempel 1 d. Hartley 2 e. Shoemaker-Levy 9 ANS: E DIF: Easy REF: Section 12.4 MSC: Applied TOP: 4V 35. Comet nuclei, absent their tails, are very dark because: a. they are made of water ice b. they have iron and nickel mixed with ice c. they have organic molecules mixed with ice d. they are covered in rock e. they are too cold to emit any light ANS: C DIF: Easy REF: Section 12.4 MSC: Factual TOP: 4Vi 36. In 1994, dozens of fragments of Comet Shoemaker-Levy 9 collided with: a. Jupiter b. the Earth c. Neptune d. the Moon e. Saturn ANS: A DIF: Easy REF: Section 12.4 MSC: Factual TOP: 4VIi 37. Consider a meteoroid with a diameter of 10 cm and a mass of 2 kg that hits the Earth head-on while traveling at a speed of 25,000 m/s. How many times larger or smaller is the meteoroid’s kinetic energy compared to that of a typical train whose mass is 2  106 kg and speed is 25 m/s? a. The meteoroid’s kinetic energy is equal to that of the train. b. The meteoroid’s kinetic energy is 1,000 times less than that of the train. c. The meteoroid’s kinetic energy is 1,000 times greater than that of the train. d. The meteoroid’s kinetic energy is 106 times greater than that of the train. e. The meteoroid’s kinetic energy is 109 times greater than that of the train.

ANS: A DIF: Difficult REF: Section 12.5 MSC: Applied TOP: 4VIiii

38. The minimum size of a meteoroid that is capable of surviving its passage through the Earth’s atmosphere and hitting the ground is about as big as: a. a car b. a house c. a basketball d. a grain of sand e. your fist ANS: E DIF: Medium REF: Section 12.5 MSC: Factual TOP: 5Ii 39. A recent estimate finds that approximately 800 meteorites with mass greater than 0.1 kg strike the surface of the Earth each day. If a house covers an area of roughly 100 m2, then what is the probability that your house will be struck by a meteorite in your 100-year lifetime? Note that the radius of the Earth is 6,400 km. a. 1 in 1  104 b. 1 in 2  105 c. 1 in 4  106 d. 1 in 6  107 e. 1 in 8  108 ANS: B DIF: Difficult REF: Section 12.5 MSC: Applied TOP: 5Ii 40. The Perseid meteor shower will occur: a. every month b. every year c. every 4 years d. every 76 years e. every 132 years ANS: B DIF: Easy REF: Section 12.5 MSC: Factual TOP: 5Iii

41. The meteoroids in the Leonids meteor shower, which occurs every November, come from: a. dust in the star-forming Leo nebula b. dust melted off Comet Tempel-Tuttle c. debris from the collision of Comet Shoemaker-Levy 9 d. zodiacal dust e. dust blown off of Earth’s surface ANS: B DIF: Medium REF: Section 12.5 MSC: Applied TOP: 5Iii 42. The Lyrid meteor shower occurs every year on approximately April 21 because: a. the Lyrae constellation is directly overhead at midnight b. the Earth passes through a cloud of debris left behind by Comet Thatcher c. the Earth passes through a cloud of debris left over from the Solar System’s formation d. the Earth undergoes a periodic volcanic eruption every April e. the sun is located in the Lyrae constellation at noon ANS: B DIF: Medium REF: Section 12.5 MSC: Factual TOP: 5Iii 43. A large meteor shower will often occur once a year because: a. Earth typically has one large volcanic eruption every year b. the Earth’s orbit passes through the Apollo asteroid belt c. the Sun goes through a yearly solar cycle d. Jupiter routinely disturbs the orbits of asteroids in the Jovian belt e. the Earth passes through the debris left behind by a specific comet ANS: E DIF: Easy REF: Section 12.5 MSC: Conceptual TOP: 5Iii

44. Identify the object in the picture below.

a. an active comet b. a meteor shower c. a meteorite d. an asteroid e. zodiacal dust ANS: B DIF: Easy REF: Section 12.5 MSC: Applied TOP: 5Iii 45. Meteor showers appear as if they are coming from one particular place in the sky because: a. that is the direction in which the comet is coming toward us b. that is the direction in which the comet is moving away from us c. that is the direction toward which Earth is traveling d. that is the direction Earth just passed e. that is the location in the sky from which the meteors originate ANS: C DIF: Medium REF: Section 12.5 MSC: Conceptual TOP: 5Iiii

46. Antarctica is the best hunting ground for meteorites for all of the following reasons EXCEPT: a. the ground is covered with ice b. more meteorites fall there than on other locations on Earth c. few native rocks are found on the glaciers d. meteorites are protected from weathering and contamination there e. by searching at different depths in the ice you can determine the history of impacts over time ANS: B DIF: Medium REF: Section 12.5 MSC: Conceptual TOP: 5IIi 47. Identify the object in the picture below.

a. a meteor b. a chondrite meteorite c. an achondrite meteorite d. an iron meteorite e. an asteroid ANS: B DIF: Easy REF: Section 12.5 MSC: Applied TOP: 5IIii | 5IIiv | 5IIv 48. Meteorites contain clues to all of the following EXCEPT: a. the age of the Solar System b. the temperature in the early solar nebula c. changes in the rate of cratering in the early Solar System d. the composition of the primitive Solar System e. the physical processes that controlled the formation of the Solar System ANS: C DIF: Difficult REF: Section 12.5 MSC: Conceptual TOP: 5IIii

49. The most common type of meteorites are: a. stony meteorites b. iron meteorites c. achondrite meteorites d. stony-iron meteorites e. carbonaceous chondrite meteorites ANS: A DIF: Easy REF: Section 12.5 MSC: Factual TOP: 5IIiii 50. Which group of meteorites represents the conditions in the earliest stages of the formation of the Solar System? a. chondrites b. achondrites c. icy meteorites d. iron meteorites e. stony-iron meteorites ANS: A DIF: Difficult REF: Section 12.5 MSC: Applied TOP: 5IIiii 51. While most meteorites have ages around 4.5 billion years, a small subset have ages around 1.3 billion years. What caused the substantial difference in age between these two populations of meteorites? a. These meteorites just happened to form later than most meteorites. b. Not all meteorites hit the Earth in the early Solar System. We should expect to find younger meteorites as more meteors pass through the atmosphere. c. The younger meteorites were created when a protoplanet collided with Earth, creating the Moon. The leftover fragments became meteorites. d. These meteorites were thrown into space after an impact with Mars and afterward some happened to collide with Earth. e. The younger ones are the result of comets repeatedly passing close to the Sun, melting their surfaces and making them appear younger.

ANS: D DIF: Medium REF: Section 12.5 MSC: Applied TOP: 5IIvi

52. Identify the object in the picture below.

a. an active comet b. a meteor shower c. a meteorite d. an asteroid e. zodiacal dust ANS: E DIF: Medium REF: Section 12.5 MSC: Applied TOP: 5IIIi 53. Which of the following likely helps replenish the zodiacal dust in the vicinity of Earth the most? a. comets b. asteroids c. the Moon d. volcanoes on Earth e. tornados on Earth ANS: A DIF: Medium REF: Section 12.5 MSC: Applied TOP: 5IIIi

54. All of the zodiacal dust in the Solar System combined is roughly equal in mass to: a. a meteoroid b. a comet c. Jupiter d. the Moon e. a terrestrial planet ANS: B DIF: Easy REF: Section 12.5 MSC: Factual TOP: 5IIIii 55. In the early universe, when the Solar System had yet to be cleared of the debris out of which it formed, which type of object would have been most likely to deposit water onto Earth’s surface? a. comets b. asteroids c. a Mars-sized protoplanet d. a rogue moon e. none, because water is not a major component of any of the objects above ANS: A DIF: Easy REF: Section 12.6 MSC: Conceptual TOP: 6Ii

SHORT ANSWER 1. List the names of the known dwarf planets and their approximate location in the Solar System.

ANS: As of printing, there are five dwarf planets.

(1) Pluto—Kuiper Belt (2) Eris—Kuiper Belt (3) Haumea—Kuiper Belt (4) Makemake—Kuiper Belt (5) Ceres—asteroid belt DIF: Medium REF: Section 12.2 MSC: Factual TOP: 2I | 2II | 2III | 2IV 2. Name three properties of the dwarf planets Pluto and Eris that are similar.

ANS: They have relatively similar masses and densities, and are made primarily of rock and ice. They are similar in size and both have moons. They also have large distances from the Sun and elliptical orbits.

DIF: Easy REF: Section 12.2 MSC: Applied TOP: 2IIi

3. Give examples of a C-type asteroid and an S-type asteroid that have been observed by spacecraft. What did we learn about each type?

ANS: An example of a C-type asteroid would be Mathilde. It is nonreflective (due to its composition of carbon compounds), has a low density (indicating that it is porous), and has many craters (therefore dating back to the early Solar System). Some examples of an S-type asteroid would be Gaspra (which is made up of different types of rock, is cratered, and is irregularly shaped), Ida (which is composed of relatively solid rock, shows evidence of past landslides and therefore the presence of soil, and—based on the number of craters it has—suggests an age of around 1 billion years old), and Eros (which is composed of solid rock, has a cratered surface, and has similar composition to primitive meteorites).

DIF: Medium REF: Section 12.3 MSC: Applied TOP: 3IIiii | 3IIvi 4. What does the existence of M-type asteroids tell us about their origin?

ANS: They were once the metallic cores of larger, differentiated objects.

DIF: Easy REF: Section 12.3 MSC: Factual TOP: 3IIiv 5. Comets have highly eccentric orbits, with eccentricities of 0.95 0.99 being common. Suppose a certain comet has an eccentricity of 0.99. If the semimajor axis of its orbit is 2,500 AU, what will be its distance at perihelion and at aphelion? Is this most likely a Kuiper Belt object or an Oort Cloud comet? (Note: For an ellipse, a(1  e) is the distance from one focus to the farther edge of the long axis and a(1

e) is the

distance from the same focus to the closer edge of the long axis.)

ANS: Aphelion is the point of farthest approach, which equals a(1  e)  2,500 AU (1  0.99)  4,975 AU. Perihelion is the point of closest approach, which equals a(1

e)2,500 AU (1 – 0.99)  25 AU.

This comet would probably be from the Oort Cloud because the aphelion distance is too large for the Kuiper Belt.

DIF: Difficult REF: Section 12.4 MSC: Applied TOP: 4IIi | 4IIii 6. Consider three comets that have orbital periods of 10, 100, and 1,000 years. Where would each of these comets likely originate, in the Oort Cloud or the Kuiper Belt? If you wanted to study material that was the best example of pristine Solar System material, which would you study?

ANS: For all objects, including comets, that orbit the Sun, Kepler’s law (P2

A3) applies and thus the

longer the period the farther from the Sun the object orbits. Thus, the comet with a period of 1,000 years

would be the one that is farthest from the Sun during most of its orbit and might contain more pristine Solar System material. The 10- and 100-year comets are short-period comets and probably come from the Kuiper Belt, and the 1,000-year comet is a long-period comet and probably came from the Oort Cloud.

DIF: Medium REF: Section 12.4 MSC: Applied TOP: 4IIi | 4IIii | 4IIiii 7. Why do long-period comets usually put on a much more visually spectacular display than short-period comets?

ANS: Long-period comets have not passed perihelion as often as short-period comets, so their volatile ices remain close to the surface. Short-period comets have burned off much of their volatile ices on previous passages, leaving a nucleus coated in a (relatively) thick layer of dust and organic material.

DIF: Easy REF: Section 12.4 MSC: Conceptual TOP: 4IIii | 4IIvii 8. In its 1986 trip around the Sun, it was estimated that Comet Halley lost approximately 100 billion kg of material. The total mass of the nucleus was estimated to be 3  1014 kg. Assuming the mass loss rate is constant with each passage, and assuming the nucleus remains intact until there is nothing left, how many more times will we see Comet Halley? Explain why your answer is an upper limit.

ANS: The number of trips equals the total mass of the comet divided by the mass lost each trip. # trips  3  1014 kg/1011 kg/trip  3,000 trips. This is an upper limit because as more of the nucleus is eaten away, the surface area/volume ratio will (eventually) change significantly, which can alter the amount of material lost with each trip. Also, it is unlikely that a porous object will remain intact until the last of its material is gone. The comet will probably break apart before it completely loses all its material.

DIF: Difficult REF: Section 12.4 MSC: Conceptual TOP: 4IIviii 9. Do icy cometary nuclei melt and move from solid to liquid phase as they are warmed by the radiation from the Sun? ANS: No, they sublimate. Their ices change from solids directly into gases without ever going through the liquid stage because the material is under very low pressure.

DIF: Medium REF: Section 12.4 MSC: Conceptual TOP: 4IIIii

10. Assume the larger circle shown below is the Sun, and the smaller circle is the head of a comet. If the comet is moving AWAY from the Sun, draw and label the two tails onto the comet.

ANS: Both tails should be pointing away from the Sun, with the dust tail slightly curved and the ion tail moving directly away from the Sun. DIF: Medium REF: Section 12.4 MSC: Applied TOP: 4IIIiv 11. How is it possible for the tail of a comet to actually move ahead of the comet itself?

ANS: The tail is blown outward by the solar wind, which points radially away from the Sun in all directions. As the comet passes perihelion and begins to move back toward the outer Solar System, its tails are blown ahead of the nucleus itself.

DIF: Medium REF: Section 12.4 MSC: Conceptual TOP: 4IIIiv | 4IVi | 4IVii | 4Viii 12. Looking at the figure below, identify the two tails.

ANS: The top tail is pointing directly away from the sun and therefore is the ion tail. The curved tail on the bottom is the dust tail, as the dust particles are heavier and are being pushed away from the comet more gently.

DIF: Easy REF: Section 12.4 MSC: Applied TOP: 4IVi | 4IVii

13. If you can model the mass in Comet Halley as a sphere 5 km in radius, what is its density if it has a mass of 1014 kg? How does that density compare to that of water (1,000 kg/m3)?

ANS: The volume of a sphere 5 km in radius is 4/3   (5  103 m)3  5.236  1011 m3. So the density of Comet Halley is then 1014kg/(5.235  1011 m3)  191 kg/m3, which is far less than that of water.

DIF: Difficult REF: Section 12.4 MSC: Applied TOP: 4Vi 14. Let’s say that you discovered a comet in the outer Solar System that had an average albedo of 0.6. If its surface was composed of a mixture of organic substances, which had an albedo of 0, and ice, which had an albedo of 1.0, then what percent of its surface is covered by organic substances?

ANS: Let X be the fraction of the surface that is covered by organic substances. The average albedo would be equal to (X  0  (1

X)  1)  (1 X). If (1

X)  0.6, then X  0.4 and 40 percent of the

surface is covered by organic substances.

DIF: Difficult REF: Section 12.4 MSC: Applied TOP: 4Vi 15. Why does a comet usually have two tails, one that is straight and one that is curved? What materials compose each tail, and why do they have different shapes?

ANS: The straight tail is made of ionized gas and is called the ion tail. It points directly away from the Sun because the charged particles in the tail interact with the particles in the solar wind. The curved tail is the dust tail, and it is not as straight as the dust tail because dust particles are more massive than the particles in the ion tail, and thus the dust tail is less affected by the solar wind.

DIF: Medium REF: Section 12.4 MSC: Conceptual TOP: 4Vi | 4Vii 16. Describe two modern-day (within the past 150 years) events when comets or asteroids collided with a planet. Cite the planet, and describe the major consequences of the collision.

ANS: Shoemaker-Levy 9 crashed into Jupiter’s atmosphere in 1994. Fireballs were seen in the Jovian atmosphere as a result. An asteroid exploded in Earth’s atmosphere above Tunguska, Siberia, in 1908.

Trees were burned or flattened over an area equal to 2,150 km2. In 2013, an asteroid exploded in the atmosphere over Russia’s Ural mountains injuring hundreds and damaging thousands of buildings in the area.

DIF: Easy REF: Section 12.4 MSC: Factual TOP: 4Vii | 4VIii

17. Consider a small comet nucleus, whose diameter is 1 km and mass is 5  1011 kg, hitting the Earth headon, traveling at a speed of 1,000 m/s. How many times larger or smaller is the comet’s kinetic energy compared to that of a typical train pulling 20 boxcars whose total mass is 2  106 kg and speed is 25 m/s?

ANS: Kinetic energy is equal to 1/2mv2. Therefore, the ratio of the kinetic energy of the comet to that of the train equals (Mcomet/Mtrain)  (vcomet/vtrain)2  (5  1011/2  106)  (1,000/25)2  4  108.

DIF: Difficult REF: Section 12.4 MSC: Applied TOP: 4VIiii 18. Give the definitions of meteoroid, meteor, and meteorite, and clearly explain how they differ.

ANS: Meteoroids are rocks too small to be asteroids in orbit around the Sun. Meteors are the bright streaks in the sky made when a meteoroid enters the atmosphere. Meteorites are the rocks that survive meteoroids’ passage through Earth’s atmosphere and land on the ground.

DIF: Medium REF: Section 12.5 MSC: Factual TOP: 5I 19. You find a blackened rock lying on top of the snow. You find that it is fairly dense and suspect it might be a meteorite. You take it to a lab, and they cut it open to reveal many small spherical, glassy particles set into the surrounding rock. Is this a meteorite? Why, or why not?

ANS: Yes, it is likely that you have found a stony meteorite with chondrules. Meteorites are typically denser than earth rocks and have a blackened appearance after falling through Earth’s atmosphere. The small spherical, glassy particles are the chondrules that often range in size from sand grains to marbles.

DIF: Medium REF: Section 12.5 MSC: Applied TOP: 5IIiii 20. How might impacts have helped increase Earth’s water supply in the early history of the Solar System?

ANS: Icy planetesimals formed near the orbits of the giant planets. As the giant planets formed and grew, they produced strong gravitational interactions with nearby planetesimals. These interactions sent some icy planetesimals were thrown outward to form the Kuiper Belt and Oort Cloud and others were thrown inward toward the Sun. It is likely that some of the objects thrown in toward the Sun hit Earth.

Since most of the mass in comet nuclei is water ice, the impact and temperature of Earth would have melted the ice, thus giving new water to Earth’s surface.

DIF: Medium REF: Section 12.6 MSC: Applied TOP: 6Ii

CHAPTER 13

Taking the Measure of Stars

CONCEPT MAP Sec 13.1 1. Measuring the Distance, Brightness, and Luminosity of Stars I. Parallax i. Stereoscopic vision: different viewing angles allow us to judge distances (MC: 1) ii. Parallax: p  1/d (TF: 1, 2, MC: 2–6, SA: 1, 2) iii. Angular measurements: radian, degree, arcminute, and arcsecond (MC: 7) iv. Distance measurements: parsec and light-year (TF: 2, SA: 3) II. Distance and Brightness Yield Luminosity i. Brightness: B  L/4d2 (TF: 3, MC: 8–11, 16, SA: 12, 19) ii. Luminosity: L (MC: 10) III. Magnitude System i. Logarithmic scale; each 1 magnitude is a factor of 2.512 (MC: 12, SA: 4, 5) ii. Apparent magnitude: m, depends on luminosity and distance (MC: 12, 13) iii. Absolute magnitude: M, depends on luminosity (TF: 4, MC: 13) iv. Color Index: B V Sec 13.2 2. The Temperature, Size, and Composition of Stars I. Wien’s Law Revisited i. Wien’s law: T  2900 m K / peak (MC: 14, SA: 13) ii. Bluer stars are hotter; redder stars are cooler (MC: 11, 14–16) iii. Filters are used to measure the color and surface temperatures of stars (MC: 14, SA: 6, 7, 19)

II. Stars Are Classified According to Surface Temperature i. Stars are hotter in the interior than the gas on the surface and exhibit an absorption spectrum on top of a blackbody spectrum (MC: 17) ii. Amount of absorption depends on the temperature and composition of the star (SA: 8) iii. Spectral types: O, B, A, F, G, K, M (from hottest to coolest), with finer divisions 0 (hottest) to 9 (coolest) inside each spectral type (TF: 5, MC: 18–22, 26, 54, SA: 19) iv. A-type stars have strongest hydrogen absorption lines; hydrogen is ionized in the hottest stars (TF: 6, MC: 23, SA: 8) v. Coolest stars have molecular absorption in spectra (TF: 7) III. Stars Consist Mostly of Hydrogen and Helium i. Heavy elements: all elements with larger masses than hydrogen and helium (MC: 24) ii. Composition of Sun by mass: 74.5% H, 23.7% He, 2% heavy elements (MC: 25, 26, SA: 9) IV. Stefan-Boltzmann Law Revisited: Finding the Sizes of Stars i. Stefan-Boltzmann law: L/4R2  T 4; larger stars and hotter stars are more luminous (MC: 11, 16, 21, 27–31, SA: 10–13, 19) ii. Smaller mass stars are far more common than larger mass stars (MC: 32) Sec 13.3 3. Measuring Stellar Masses I. Binary Stars Orbit a Common Center of Mass i. Each star in a binary system orbits the center of mass (MC: 33) ii. Lower mass star has larger orbit and moves faster: v1/v2  m2/m1 (MC: 34–37, SA: 14, 15) iii. Doppler shifts used to measure radial velocities (MC: 38) II. Kepler’s Third Law Gives the Total Mass of a Binary System i. Newton’s version of Kepler’s third law: G(m1  m2)P2  42A3 (TF: 8, MC: 36, 37, SA: 15) ii. Visual binary: the two stars are resolved individually (MC: 39) iii. Eclipsing binary: stars that move directly in front of one another as they orbit, leading to a periodic dip in their combined brightness (MC: 40, SA: 16) iv. Spectroscopic binary: unresolved, but periodic changes observed in the stars’ Doppler shifts (SA: 16) v. Range of stellar masses: 0.08 M to 150 M (MC: 41) Sec 13.4

4. The H-R Diagram Is the Key to Understanding Stars I. H-R Diagram i. Hertzsprung-Russell (H-R) diagram: plot of luminosity versus temperature of stars (MC: 42) ii. Temperature is plotted logarithmically on x-axis of H-R diagram, with hotter stars to the left of the diagram (TF: 10, MC: 43, 44, SA: 20) iii. Luminosity is plotted logarithmically on y-axis of H-R diagram, with more luminous stars toward the top of the diagram (MC: 45, SA: 20) iv. Lines of constant radius run from the upper left to the lower right (MC: 46, 47, SA: 20) II. The Main sequence Is a Grand Pattern in Stellar Properties i. Main sequence: Area of H-R diagram running from lower right to upper left that contains about 90 percent of all stars (MC: 48–50) ii. Spectroscopic parallax: determining a star’s distance using its color and apparent magnitude, because for main sequence stars temperature determines luminosity (SA: 18, 19) iii. Mass of main-sequence stars increases from bottom right to top left. Mass determines luminosity: L

M3.5 (TF: 9, 10, MC: 51–53, SA: 20) iv. For stars with similar chemical composition, mass of a main-sequence star determines all other characteristics (TF: 12, MC: 54, SA: 17) III. Not All Stars Are Main-Sequence Stars i. Luminosity class: related to the star’s radius (TF: 9–11) ii. Luminosity classes—I: supergiants; II: bright giants; III: giants; IV: subgiants; V: main-sequence; VI: white dwarf (TF: 11, MC: 22, 55, 56) Sec 13.5 5. Origins: Habitable Zones I. Stellar Properties Determine Where Life Could Exist i. Habitable zone: area around a star where a planet orbiting in that area could have liquid water on its surface ii. Solar System habitable zone: 0.9 to 1.4 AU (MC: 57) iii. More massive stars have higher temperatures, and habitable zones are wider and farther from the stars (MC: 50)

TRUE/FALSE

1. With today’s advanced technology, we can measure the parallax for any star inside the Milky Way. ANS: F DIF: Easy REF: Section 13.1 MSC: Applied TOP: 1Iii 2. A parsec is a unit of time.

ANS: F DIF: Easy REF: Section 13.1 MSC: Factual TOP: 1Iii | 1Iiv 3. Stars with a larger brightness must be closer to us than fainter stars.

ANS: F DIF: Easy REF: Section 13.1 MSC: Factual TOP: 1IIi 4. The absolute magnitude of a star is a measure of its luminosity.

ANS: T DIF: Medium REF: Section 13.1 MSC: Factual TOP: 1IIIiii 5. Stars that have spectral type B are lower in temperature than spectral type M. ANS: F DIF: Medium REF: Section 13.2 MSC: Factual TOP: 2IIiii 6. Stars of spectral class O have the strongest hydrogen absorption lines, whereas stars of spectral class M have the weakest hydrogen absorption lines.

ANS: F DIF: Medium REF: Section 13.2 MSC: Applied TOP: 2IIiv 7. Spectra of the least massive main-sequence stars show the largest amount of absorption from molecules like TiO and CN. ANS: T DIF: Difficult REF: Section 13.2 | 13.4 MSC: Applied TOP: 2IIv | 4IIiii 8. Binary star systems are extremely useful in studying stars because they allow us to determine stars’ masses.

ANS: T DIF: Easy REF: Section 13.3 MSC: Conceptual TOP: 3IIi 9. A star’s mass is the only characteristic that determines its position in the H-R diagram.

ANS: F DIF: Difficult REF: Section 13.4 MSC: Conceptual TOP: 4IIiii | 4IIIi 10. A star’s mass determines where it lies on the main sequence of an H-R diagram.

ANS: T DIF: Medium REF: Section 13.4 MSC: Conceptual TOP: 4IIiv | 4IIIi 11. The stars that have the largest radii are classified as supergiants.

ANS: T DIF: Easy REF: Section 13.4 MSC: Conceptual TOP: 4IIIi | 4IIIii

MULTIPLE CHOICE 1. What advantage do you gain by having two eyes that are separated on your face, rather than being very close together? a. better collecting area, which allows you to see dimmer objects b. double vision, which allows you to see multiple objects at once c. color vision, which allows you to determine temperatures d. stereoscopic vision, which allows you to determine distances e. better magnification, which allows you to see smaller objects ANS: D DIF: Medium REF: Section 13.1 MSC: Applied TOP: 1Ii 2. To measure the parallax of the most distant stars measurable, we would make two measurements of the star’s position on the sky separated by: a. 6 hours b. 12 hours c. 24 hours d. 6 months e. 12 months ANS: D DIF: Easy REF: Section 13.1 MSC: Conceptual TOP: 1Iii 3. Parallax is used to measure a star’s: a. distance b. velocity c. luminosity d. mass e. radius ANS: A DIF: Easy REF: Section 13.1 MSC: Factual TOP: 1Iii

4. How is the distance to a star related to its parallax? a. Distance is directly proportional to parallax. b. Distance is inversely proportional to parallax. c. Distance is directly proportional to parallax squared. d. Distance is inversely proportional to parallax squared. e. Distance and parallax are not related to each other at all.

ANS: B DIF: Medium REF: Section 13.1 MSC: Applied TOP: 1Iii 5. If a star’s measured parallax is 0.2 arcsec, what is its distance? a. 2 pc b. 5 pc c. 20 pc d. 40 pc e. 50 pc ANS: B DIF: Medium REF: Section 13.1 MSC: Applied TOP: 1Iii 6. If a star’s distance is 10 pc, what is its parallax? a. 0.01 arcsec b. 0.05 arcsec c. 0.1 arcsec d. 0.5 arcsec e. 1 arcsec ANS: C DIF: Medium REF: Section 13.1 MSC: Applied TOP: 1Iii 7. How many arcseconds are there in 1 degree? a. 60 b. 360 c. 3,600 d. 6,000 e. 36,000 ANS: C DIF: Easy REF: Section 13.1 MSC: Factual TOP: 1Iiii

8. Stars A and B appear equally bright, but star A is twice as far away from us as star B. Which of the following is true? a. Star A is twice as luminous as star B. b. Star A is four times as luminous as star B. c. Star B is twice as luminous as star A. d. Star B is four times as luminous as star B. e. Star A and star B have the same luminosity because they have the same brightness.

ANS: B DIF: Medium REF: Section 13.1 MSC: Applied TOP: 1IIi 9. Two main-sequence stars have the same temperature. If star A is four times brighter than star B, then: a. star B is two times farther away than star A b. star B is four times farther away than star A c. star B is eight times farther away than star A d. star B and star A lie at the same distance from us e. it is impossible to determine their relative distances from the information given ANS: A DIF: Difficult REF: Section 13.1 MSC: Applied TOP: 1IIi 10. What is the difference between brightness and luminosity? a. These are different names for the same property. b. Luminosity is how much light we see from a star; brightness is how much light it emits. c. Brightness is how much light we see from a star; luminosity is how much light it emits. d. Luminosity measures size; brightness measures temperature. e. Brightness measure size; luminosity measures temperature.

ANS: C DIF: Easy REF: Section 13.1 MSC: Conceptual TOP: 1IIi | 1IIii 11. Star A is a red star. Star B is a blue star. You are able to determine that both stars are the same size.

Which star is brighter? a. Star A is brighter. b. Star B is brighter. c. They have the same brightness. d. We also need to know the distance of the stars to determine their brightness. e. Color is not related to brightness at all.

ANS: D DIF: Medium REF: Section 13.1 | 13.2 MSC: Applied TOP: 1IIi | 2Iii | 2IVi

12. The star named Capella has an apparent magnitude of 0, while the star named Polaris has an apparent magnitude of 2, which means that Capella is _________ than Polaris. a. 18 times fainter b. 6 times fainter c. 2 times fainter d. 2 times brighter e. 6 times brighter ANS: E DIF: Medium REF: Section 13.1 MSC: Applied TOP: 1IIIi | 1IIIii 13. Star A and star B both have the same temperature but different sizes and distances. As a result, star A is more luminous than star B, but star B is brighter than star A. Which of these statements about the absolute and apparent magnitudes of the two stars is correct? a. Star A has a larger apparent magnitude and a larger absolute magnitude. b. Star A has a larger apparent magnitude, while star B has a larger absolute magnitude. c. Star B has a larger apparent magnitude and a larger absolute magnitude. d. Star B has a larger apparent magnitude, while star A has a larger absolute magnitude. e. Both stars have the same apparent and absolute magnitudes.

ANS: B DIF: Medium REF: Section 13.1 MSC: Applied TOP: 1IIIii | 1IIIiii 14. You observe two stars in a visual binary system using a blue filter that is centered at a wavelength of 550 nm and a red filter that is centered at a wavelength of 650 nm. Star A has a temperature of 10,000 K, while star B has a temperature of 4000 K, and you know that both stars are the same size. Which star will be the brightest in each filter? a. Star A is the brightest in the blue filter, and star B is the brightest in the red filter. b. Star B is the brightest in the blue filter, and star A is the brightest in the red filter. c. Star A is the brightest in both filters. d. Star B is the brightest in both filters. e. Both stars have the same brightness in each filter.

ANS: C DIF: Difficult REF: Section 13.2 MSC: Applied TOP: 2Ii | 2Iii | 2Iiii

15. Star A is a red star. Star B is a blue star. Which star is hotter? a. Star A is hotter. b. Star B is hotter. c. They are the same temperature. d. We also need to know the luminosities of the stars to determine their temperatures. e. Color is not related to temperature at all.

ANS: B DIF: Easy REF: Section 13.2 MSC: Applied TOP: 2Iii 16. Star A is a red star. Star B is a blue star. You are able to determine that both stars are the same size.

Which star is more luminous? a. Star A is more luminous. b. Star B is more luminous. c. They have the same luminosities. d. We also need to know the distance of the stars to determine their luminosity. e. We cannot tell because color is not related to luminosity.

ANS: B DIF: Easy REF: Section 13.2 MSC: Applied TOP: 2Iii | 2IVi 17. What type of spectrum do most stars produce? a. an absorption spectrum on top of a blackbody spectrum b. an emission spectrum on top of a blackbody spectrum c. an absorption spectrum on top of an emission spectrum d. a pure emission spectrum e. a pure blackbody spectrum ANS: A DIF: Easy REF: Section 13.2 MSC: Factual TOP: 2IIi 18. Which sequence correctly lists the spectral classes of stars in order from hottest to coolest? a. A B F G K M O b. O A B G F M K c. A F O B M G K d. O B A F G K M e. M K G F A B O ANS: D DIF: Medium REF: Section 13.2 MSC: Factual TOP: 2IIiii

19. The spectral class of a star is related to its: a. luminosity b. brightness c. radius d. mass e. temperature ANS: E DIF: Easy REF: Section 13.2 MSC: Conceptual TOP: 2IIiii 20. What spectral class is the Sun? a. A0 b. B7 c. F5 d. M3 e. G2 ANS: E DIF: Easy REF: Section 13.2 MSC: Factual TOP: 2IIiii 21. Two stars with similar temperatures but different sizes will have: a. similar spectral types but different luminosities b. similar luminosities but different brightnesses c. similar brightnesses but different distances d. similar distances but different masses e similar masses but different spectral types ANS: A DIF: Medium REF: Section 13.2 MSC: Applied TOP: 2IIiii | 2IVi 22. A star classified as a K0III star is: a. a giant that is cooler than the Sun b. a supergiant that is hotter than the Sun c. a main-sequence star that is hotter than the Sun d. a subgiant that is cooler than the Sun e. a dwarf that is hotter than the Sun ANS: A DIF: Difficult REF: Section 13.2| 13.4 MSC: Factual TOP: 2IIiii | 4IIIii

23. Why do O- and B-type stars have weaker hydrogen absorption lines than A-type stars? a. O- and B-type stars are cooler than A-type stars. b. O- and B-type stars are smaller than A-type stars. c. A larger fraction of hydrogen atoms in O- and B-type stars is ionized. d. O- and B-type stars have converted much more of their hydrogen into heavier elements. e. A-type stars have a higher mass than O- and B-type stars, so they have more hydrogen.

ANS: C DIF: Difficult REF: Section 13.2 MSC: Factual TOP: 2IIiv 24. When astronomers refer to “heavy elements,” which elements are they talking about? a. all elements b. all elements more massive than hydrogen c. all elements more massive than helium d. all elements more massive than carbon e. all elements more massive than iron ANS: C DIF: Easy REF: Section 13.2 MSC: Factual TOP: 2IIIi 25. Stars are made mostly of: a. helium b. oxygen c. hydrogen d. nitrogen e. carbon ANS: C DIF: Easy REF: Section 13.2 MSC: Factual TOP: 2IIIii 26. The fraction of the Sun’s mass that is made of heavy elements is: a. 0.5 percent b. 2 percent c. 10 percent d. 20 percent e. 50 percent ANS: B DIF: Medium REF: Section 13.2 MSC: Factual TOP: 2IIIii

27. If we know the temperature and luminosity of a star, we can also calculate its: a. radius b. mass c. chemical composition d. brightness e. all of the above ANS: A DIF: Easy REF: Section 13.2 MSC: Applied TOP: 2IVi 28. Star C is a red star. Star D is a blue star. Which has a larger radius? a. Star C has a larger radius. b. Star D has a larger radius. c. Stars C and D have the same radius. d. We also need to know the luminosities of the stars to determine their radii. e. We cannot determine the radii because color is not related to the radius.

ANS: D DIF: Medium REF: Section 13.2 MSC: Applied TOP: 2IVi 29. Star E is the same temperature as star F, but star E is four times as luminous as star F. How do the radii of the stars compare? a. Star E has twice the radius of star F. b. Star E has four times the radius of star F. c. Star F has twice the radius of star E. d. Star F has four times the radius of star E. e. They are the same size.

ANS: A DIF: Difficult REF: Section 13.2 MSC: Applied TOP: 2IVi 30. If star A has a temperature that is twice as hot as the Sun but has the same luminosity as the Sun, the diameter of star A must be _________ times the diameter of the Sun. a. 16 b. 4 c. 2 d. 1/2 e. 1/4 ANS: E DIF: Difficult REF: Section 13.2 MSC: Applied TOP: 2IVi

31. The bright star named Rigel has a luminosity of 66,000 L and a temperature of 11,000 K. What is its radius? Note that the temperature of the Sun is 5,800 K. a. 5 R b. 30 R c. 70 R d. 135 R e. 190 R ANS: C DIF: Difficult REF: Section 13.2 MSC: Applied TOP: 2IVi 32. Which stars are the most common? a. Stars with a larger mass and a larger radius than the Sun’s are the most common. b. Stars with a smaller mass and a smaller radius than the Sun’s are the most common. c. Stars with a larger mass and a smaller radius than the Sun’s are the most common. d. Stars with a smaller mass and a larger radius than the Sun’s are the most common. e. All of the above are equally common.

ANS: B DIF: Easy REF: Section 13.2 MSC: Factual TOP: 2IVii 33. Stars X and Y are 5 AU apart from each other. Star X is four times as massive as star Y. The center of mass of this system is _________ AU away from star X and _________ AU away from star Y. a. 3; 2 b. 2; 3 c. 2.5; 2.5 d. 1; 4 e. 4; 1 ANS: D DIF: Difficult REF: Section 13.3 MSC: Applied TOP: 3Ii 34. The faster-moving star in a binary is: a. the less massive star b. the more massive star c. the smaller radius star d. the larger radius star e. the lower temperature star ANS: A DIF: Medium REF: Section 13.3 MSC: Applied TOP: 3Iii

35. In a binary star system that contains stars with 2M and 1M , the velocity of the 2M star will be _________ times the velocity of the 1M star. a. 0.2 b. 0.5 c. 1 d. 2 e. 3 ANS: B DIF: Medium REF: Section 13.3 MSC: Applied TOP: 3Iii 36. Which of the following properties are NOT useful in determining the masses of stars in a typical binary system? a. The period of the orbits of the two stars is not useful. b. The average separation between the two stars is not useful. c. The radii of the two stars is not useful. d. The velocities of the two stars is not useful. e. All of the above are useful for determining the masses of stars in a binary.

ANS: C DIF: Medium REF: Section 13.3 MSC: Applied TOP: 3Iii | 3IIi 37. You discover a binary star system in which star A has a velocity of 10 km/s and star B has a velocity of 30 km/s. If you study the system further and find out that the orbital period is 30 days and the orbital separation is a constant 0.3 AU, then what are the masses of stars A and B? a. Star A is 3M, and Star B is 1M. b. Star A is 1M, and Star B is 0.3M. c. Star A is 6M, and Star B is 2M. d. Star A is 2M, and Star B is 0.5M. e. Star A is 0.3M, and Star B is 1M. ANS: A DIF: Difficult REF: Section 13.3 MSC: Applied TOP: 3Iii | 3IIi

38. Astronomers can measure the speed of the stars in a binary system by measuring the _________ of the stars. a. temperatures b. luminosities c. distance d. colors e. spectra ANS: E DIF: Easy REF: Section 13.3 MSC: Factual TOP: 3Iiii 39. For which type of binary system are astronomers able to resolve each of the two stars individually? a. eclipsing binary b. spectroscopic binary c. visual binary d. binaries where the two stars have the same mass e. binaries where the two stars have the same luminosity ANS: C DIF: Easy REF: Section 13.3 MSC: Factual TOP: 3IIii 40. Eclipsing binary systems: a. orbit in the plane of the sky b. exhibit large radial velocity shifts c. contain equal mass stars d. contain stars that pass in front of one another during their orbit e. contain stars that can be resolved as two separate stars ANS: D DIF: Medium REF: Section 13.3 MSC: Factual TOP: 3IIiii 41. Main-sequence stars range in mass from approximately: a. 0.5 to 10 M b. 0.08 to 150 M c. 1 to 100 M d. 0.1 to 75 M e. 5 to 50 M ANS: B DIF: Easy REF: Section 13.3 MSC: Factual TOP: 3IIv

42. The Hertzsprung-Russell diagram is a graph of _________ for stars. a. mass versus brightness b. size versus mass c. luminosity versus temperature d. mass versus spectral type e. luminosity versus brightness ANS: C DIF: Easy REF: Section 13.4 MSC: Factual TOP: 4Ii 43. Which of the following properties would not be plotted on the horizontal axis of an H-R diagram? a. Color would not be plotted. b. Luminosity would not be plotted. c. Temperature would not be plotted. d. Spectral class would not be plotted. e. All of the above are plotted on the horizontal axis of an H-R diagram.

ANS: B DIF: Easy REF: Section 13.4 MSC: Factual TOP: 4Iii Figure 1 44. Figure 1 shows an H-R diagram, with five stars labeled A through E. Which star has the largest temperature? a. A b. B c. C d. D e. E ANS: A DIF: Easy REF: Section 13.4 MSC: Applied TOP: 4Iii 45. Figure 1 shows an H-R diagram, with five stars labeled A through E. Which star has the largest luminosity? a. A b. B c. C d. D e. E ANS: B DIF: Easy REF: Section 13.4 MSC: Applied TOP: 4Iiii

46. Figure 1 shows an H-R diagram, with five stars labeled A through E. Which star has the smallest radius? a. A b. B c. C d. D e. E ANS: D DIF: Medium REF: Section 13.4 MSC: Applied TOP: 4Iiv 47. On a typical H-R diagram, where are the stars with the largest radii located? a. in the upper left corner b. in the upper right corner c. in the center d. in the lower left corner e. in the lower right corner ANS: B DIF: Medium REF: Section 13.4 MSC: Applied TOP: 4Iiv 48. What type of star is most common in the solar neighborhood? a. subgiants b. supergiant c. white dwarf d. giant e. main-sequence ANS: E DIF: Easy REF: Section 13.4 MSC: Factual TOP: 4IIi 49. Roughly what percentage of stars in our galaxy are main-sequence stars? a. 10 percent b. 25 percent c. 50 percent d. 75 percent e. 90 percent ANS: E DIF: Medium REF: Section 13.4 MSC: Factual TOP: 4IIi

50. In which region of an H-R diagram would you find the main-sequence stars with the widest habitable zones? a. upper left b. upper right c. center d. lower left e. lower right ANS: A DIF: Medium REF: Section 13.4 | 13.5 MSC: Applied TOP: 4IIi | 5Iiii 51. The picture below shows an H-R diagram, with five stars labeled A through E. Which of the main-sequence stars has the smallest mass? a. A b. B c. C d. D e. E ANS: E DIF: Medium REF: Section 13.4 MSC: Applied TOP: 4IIiii 52. What is the approximate luminosity of a 5 M main-sequence star? a. 50 L b. 80 L c. 150 L d. 280 L e. 510 L ANS: D DIF: Medium REF: Section 13.4 MSC: Applied TOP: 4IIiii 53. What is the approximate luminosity of a 0.5 M main-sequence star? a. 0.09 L b. 0.01 L c. 0.2 L d. 0.5 L e. 0.7 L ANS: A DIF: Medium REF: Section 13.4 MSC: Applied TOP: 4IIiii

54. The one property of a main-sequence star that determines all its other properties is its: a. luminosity b. mass c. temperature d. spectral type e. brightness ANS: B DIF: Easy REF: Section 13.4 MSC: Conceptual TOP: 4IIiv 55. The stars that have the largest radii are classified as: a. giants b. ultragiants c. supergiants d. megagiants e. supernovae ANS: C DIF: Easy REF: Section 13.4 MSC: Factual TOP: 4IIIii 56. The brightest stars in the sky also tend to be: a. the highest mass stars b. the hottest stars in the sky c. very near to us d. very luminous e. all of the above ANS: D DIF: Difficult REF: Section 13.4 MSC: Applied TOP: 4IIIii 57. The habitable zone for the Sun covers the area that is between _________ from the Sun. a. 0 to 0.8 AU b. 0.5 to 10 AU c. 1.2 to 4.2 AU d. 0.9 to 1.4 AU e. 0.2 to 10.2 AU ANS: D DIF: Medium REF: Section 13.5 MSC: Factual TOP: 5Iii

SHORT ANSWER

1. If a star’s parallax is measured using identical telescopes, one on Earth and the other on Mars, which planet’s telescope would measure the biggest parallax? Explain your answer.

ANS: The telescope on Mars would measure a larger parallax. Because Mars has a larger orbit than the Earth, it will have a bigger distance between the two parallax observations. This larger distance between observations for the telescope on Mars will lead to a larger apparent motion of a star.

DIF: Difficult REF: Section 13.1 MSC: Conceptual TOP: 1Iii 2. A star with a stellar parallax of 0.025 arcseconds has a distance of how many parsecs?

ANS: The inverse of stellar parallax given in arcseconds is its distance in parsecs: 1/0.025 arcseconds  40 parsecs.

DIF: Medium REF: Section 13.1 MSC: Applied TOP: 1Iii 3. How is the unit of length known as a parsec defined?

ANS: The parsec is defined such that an object at a distance of 1 parsec has a parallax exactly equal to 1 arcsecond.

DIF: Easy REF: Section 13.1 MSC: Factual TOP: 1Iiv 4. Rigel is a star with an apparent magnitude of 0.1, and Betelgeuse is a star with an apparent magnitude of 0.4. Which star appears brighter, and what is the ratio of their brightnesses?

ANS: Rigel is brighter than Betelgeuse by a factor of 2.512(0.4–0.1)  1.32. Thus, Rigel is 32 percent brighter than Betelgeuse.

DIF: Difficult REF: Section 13.1 MSC: Applied TOP: 1IIIi 5. If the Hubble space telescope can see stars as faint as magnitude 27, how much fainter are these stars than the faintest ones you can see in a very dark night sky, which have magnitude 6?

ANS: The Hubble space telescope can see objects that are 2.512(27–6)  2.5  108  250 million times fainter than the stars you can see in a dark night sky.

DIF: Medium REF: Section 13.1 MSC: Applied TOP: 1IIIi 6. Explain how astronomers can use the blue and visual filters to determine the temperatures of stars.

ANS: Astronomers compare the relative intensities of light measured through each filter. Stars with more blue than visual light are hotter, whereas stars with more visual than blue light are cooler.

DIF: Easy REF: Section 13.2 MSC: Conceptual TOP: 2Iiii

7. The blackbody spectra of a star with a temperature of 6000 K and a star with a temperature of 4000 K are shown below.

An astronomer uses a telescope to observe each of these two stars in both the blue and red filters. The blue filter is centered at 450 nm, while the red filter is centered at 660 nm. For each of the two stars, state through which filter that star will be the brightest. Explain your answer.

ANS: Looking at the blackbody curves, the 6000 K star emits more light at 450 nm than it does at 660 nm, so it will be brighter when using the blue filter than it will be when using the red filter. For the 4000 K star, the opposite is true, so it will appear brighter through the red filter than it will be through the blue filter.

DIF: Medium REF: Section 13.1 MSC: Applied TOP: 2Iiii 8. What is the spectral type of stars that have the strongest hydrogen absorption lines? Why do stars that are hotter than these have weaker hydrogen lines?

ANS: A-type stars have the strongest hydrogen absorption lines in their spectra. O- and B-type stars are hotter than A stars, so the hydrogen in O and B stars becomes ionized. Electrons not in atoms do little absorbing, so the hydrogen absorption lines in O and B stars are weaker than those in A stars.

DIF: Difficult REF: Section 13.2 MSC: Applied TOP: 2IIii | 2IIiv 9. What are the two main chemical elements that make up the Sun? How much of the mass of the Sun is composed of elements other than these two?

ANS: By mass, the Sun is made up of 74.5 percent hydrogen and 23.7 percent helium. All the other elements in the periodic table only make up about 2 percent of the mass of the Sun. DIF: Medium REF: Section 13.2 MSC: Factual TOP: 2IIIii 10. If we measure a star’s luminosity and temperature, what other property of the star can we calculate?

Explain how.

ANS: If we measure the luminosity L and temperature T of a star, then we can use the StefanBoltzmann law that says L/4 R2  T4 to calculate the star’s radius R. DIF: Medium REF: Section 13.2 MSC: Applied TOP: 2IVi 11. The bright star Arcturus has a luminosity of 210 L and a temperature of 4300 K. What is its radius? Note that the Sun has a temperature of 5800 K. ANS: Using the Stefan-Boltzmann law and solving for the radius we get . Comparing Arcturus to the

Sun, we find  26. Thus the radius of Arcturus is 26 R. DIF: Difficult REF: Section 13.2 MSC: Applied TOP: 2IVi 12. Star A is exactly the same color as star B and appears equally bright. Through stellar parallax measurements, we find that star B is twice as far away from us as star A. Determine which star has the largest radius and how much larger it is.

ANS: For stars of equal brightness, luminosity is directly proportional to their distance squared. If star B is twice as far away, then it must be four times as luminous as star A. Second, if the two stars are exactly the same color, then they are also the same temperature. For stars of the same temperature, luminosity is directly proportional to the square of the radius. If star B is four times as luminous, it must be twice as big as star A. DIF: Difficult REF: Section 13.1 | 13.2 MSC: Applied TOP: 1IIi | 2IVi 13. Star A emits its peak energy at a wavelength of 500 nm, and star B emits its peak energy at a wavelength of 750 nm. If both stars have the same radii, which star is hotter and by how much?

ANS: By Wien’s law, the temperature of a star is inversely proportional to the wavelength of its peak emission: peakA/peakB  500/750  2/3. This means that star A is 3/2 = 1.5 times hotter than star B. DIF: Difficult REF: Section 13.2 MSC: Applied TOP: 2Ii | 2IVi 14. You observe a binary star system and find that star 1 has a velocity of 10 m/s while star 2 has a velocity of 35 m/s. What is the ratio of masses of the two stars (M1/M2)?

ANS: The ratio of masses of stars in a binary system is inversely proportional to the ratio of velocities:

M1/M2  v2/v1  (35 m/s)/(10 m/s)  3.5. Therefore, star 1 is 3.5 times as massive as star 2.

DIF: Medium REF: Section 13.3 MSC: Applied TOP: 3Iii 15. You observe a binary star system and find that star 1 has a velocity of 20 m/s while star 2 has a velocity of 40 m/s. What is the ratio of masses of the two stars (M1/M2)? If you find that the separation of the two stars is 0.5 AU and the orbital period is 70 days, then what are the individual masses of the two stars?

ANS: The ratio of masses of stars in a binary system is simply inversely proportional to the ratio of velocities: M1/M2  v2/v1  40 m/s / 20 m/s  2. Therefore, M1  2M2. Using Kepler’s third law, we can calculate the sum of the masses:

P  70 days  24 hr/day  3,600 s/hr  6.0  106 s (M1  M2)  4 2A3/GP2  4 2 (0.5  1.5 

1011 m)3 / (6.7  10 11 Nm2/kg  (6.0  106 s)2) (M1  M2)  6.9  1030 kg  1 M/2  1030 kg  3.5 M. Solving for the individual masses gives (2M2  M2)  3M2  3.5M or M2  1.1 M, and M1  2.2 M. DIF: Difficult REF: Section 13.3 MSC: Applied TOP: 3Iii | 3IIi 16. What is the physical difference between an eclipsing binary system and a spectroscopic binary system?

ANS: The only real difference is the tilt of the stars’ orbits relative to the Earth’s position (also known as the inclination angle). For an eclipsing binary system, the stars are aligned in a way so that one star passes directly between the Earth and the other star. For a spectroscopic binary, the stars’ orbits do not line up exactly with the Earth’s position.

DIF: Easy REF: Section 13.3 MSC: Conceptual TOP: 3IIiii | 3IIiv 17. What is the main property of a main-sequence star that determines all its other properties?

ANS: The star’s mass has the most effect on all its other properties.

DIF: Easy REF: Section 13.4 MSC: Factual TOP: 4IIiv 18. Explain how we can use spectroscopic parallax to determine the distance to a star farther away than a few hundred light-years.

ANS: First, we can determine the temperature of the star based on its absorption line spectra, as well as determine whether the star is a main-sequence star. If the star is a main-sequence star, and if we know the temperature of the star, we can simply read off its luminosity from the diagram. We then measure how bright the star appears and use the inverse square law of radiation to determine its distance.

DIF: Medium REF: Section 13.4 MSC: Conceptual TOP: 4IIii 19. Imagine you are observing a nearby star. You know that it is a main-sequence star but don’t know anything else about it. If you had access to any telescope equipment you wanted, explain how you would determine this star’s temperature, luminosity, distance, and radius.

ANS: You could measure the temperature either by determining its color using different filters, or by taking a spectrum to determine its spectral type. Once you know the color of a main-sequence star, you can use an H-R diagram to read off the luminosity of that temperature star. Then, since you know the luminosity, measuring the brightness of this star tells you the distance, using the equation B  L/(4d2).

Alternatively, if the star was relatively nearby, you could measure the distance to it using parallax, then use the brightness equation to determine the luminosity. Finally, since you already know the temperature and luminosity of the star, you can use the Stefan-Boltzmann equation L  4R2T4 to calculate

its radius.

DIF: Difficult REF: Section 13.1 | 13.2 | 13.4 MSC: Applied TOP: 1IIi | 2Iiii | 2IIiii | 2IVi | 4IIii 20. Along the main sequence, how do the luminosity, temperature, radius, and mass of stars change as you go from the upper-left to the lower-right corners of the H-R diagram?

ANS: Stars near the upper-left end of the main sequence are very luminous, hot, large, massive stars.

Stars near the lower-right end of the main sequence are low-luminosity, cool, small, low-mass stars.

DIF: Medium REF: Section 13.4 MSC: Conceptual TOP: 4Iii | 4Iiii | 4Iiv | 4IIiii

CHAPTER 14

Our Star—The Sun

CONCEPT MAP Sec 14.1 1. The Structure of the Sun I. The Sun i. The Sun is a typical star, special only because it is close to us (MC: 1, 2) ii. We observe the exterior of the Sun and use physics to determine the properties of its interior (SA: 1) II. The Sun Is in Balance i. Hydrostatic equilibrium: outward pressure and inward pull of gravity are equal at all radii in the Sun (TF: 1, MC: 3, SA: 2) ii. Density, pressure, temperature increase toward the center of the Sun (MC: 4, 5, SA: 2) iii. Energy balance: amount of energy produced is equal to amount radiated (MC: 6) Sec 14.2 2. The Sun Is Powered by Nuclear Fusion I. Nuclear Fusion i. Protons and neutrons held together by strong nuclear force (TF: 2, MC: 7) ii. Hydrogen burning: the nuclear fusion of H into He that powers the Sun (MC: 8, 9, 10) iii. Energy comes from mass loss; Einstein’s E  mc2 (MC: 11, 12, SA: 3, 4) iv. Fusion requires overcoming repulsive electrical forces, so it only happens at high temperatures (MC: 13, SA: 5) v. Only the core of the Sun, the inner 25 percent in radius, is hot enough for fusion to occur (TF: 3, MC: 14, SA: 5) vi. Hydrogen burning is most efficient form of fusion and occurs at lowest temperatures (MC: 15, SA: 6)

vii. Hydrogen burns mostly via the proton-proton chain (TF: 4, MC: 16) II. Energy Produced in the Sun’s Core Must Find Its Way to the Surface i. Thermal conduction: heat traveling through a solid material; does not occur in the Sun (MC: 17, 18, SA: 7) ii. Radiative transfer: energy transported by photons (MC: 19, SA: 7) iii. Radiative zone: inner 71 percent of the Sun, where radiation is dominant energy transport method (MC: 20, 21, SA: 8) iv. Takes approximately 100,000 years for energy to travel from core to surface of the Sun (TF: 5, 6, MC: 22, 23) v. Convection: moving gas (or liquid) carries energy (MC: 24, SA: 7) vi. Convective zone: outer area of the Sun (MC: 20, SA: 8) III. What If the Sun Were Different? i. If Sun were different, it would not be in equilibrium, and it would change until equilibrium was reached (MC: 13, SA: 9) Sec 14.3 3. The Interior of the Sun I. Astronomers Use Neutrinos to Observe the Heart of the Sun i. Neutrinos interact weakly with matter, flow freely out of the Sun (TF: 7, MC: 25) ii. Proton-proton chain produces large numbers of neutrinos (MC: 26) iii. Solar neutrino problem: only one-third to one-half as many neutrinos are detected as were predicted (MC: 27) iv. Neutrinos have small amount of mass and oscillate between three flavors (electron, muon, and tau), and this solves the solar neutrino problem (TF: 8, MC: 28, SA: 10) II. Helioseismology Can Be Used to Probe the Sun’s Interior i. Helioseismology: observe the motions of the Sun’s surface to study its interior (TF: 9, MC: 29, 30) Sec 14.4 4. The Atmosphere of the Sun I. Photosphere i. Sun has no solid surface or defined boundary; density decreases as you move upward in radius (MC: 31) ii. Photosphere: apparent surface of the Sun; T  5780 K (MC: 31)

iii. Limb darkening: Sun is fainter at its edges compared to its center (MC: 32, SA: 11) II. The Solar Spectrum Is Complex i. Sun’s spectrum: black body spectrum of T  6,000 K with absorption features (MC: 33) III. The Sun’s Outer Atmosphere: Chromosphere and Corona i. Chromosphere: region above the photosphere seen best in hydrogen emission lines (red light) (MC: 34, 35, 36) ii. Corona: region above the photosphere; temperature is a few million degrees, heated by magnetic fields; best seen at X-ray wavelengths (TF: 10, MC: 37, 38, 39) IV. Solar Activity Is Caused by Magnetic Fields i. Coronal loops: corona held in by magnetic field (MC: 40, SA: 12) ii. Solar wind escapes the Sun via coronal holes in the Sun’s magnetic field that cover about 20 percent of the Sun’s surface (TF: 11) iii. Solar wind causes comet tails, shapes magnetospheres, and fuels auroras (MC: 40) iv. Sunspots: regions of low temperature on the surface of the Sun where gas is held by the Sun’s magnetic field and radiates away its energy (MC: 42, 43, 44, 45, 50, SA: 13) v. Anatomy of a sunspot: dark umbra surrounded by a lighter penumbra (TF: 12, MC: 46) vi. Sun experiences differential rotation; material at the poles rotates slower than material at the equator (MC: 47, 48, SA: 14) vii. Sunspot cycle: 11-year pattern of latitude and frequency of sunspots (MC: 49) viii. Maunder Minimum: a near total lack of sunspot activity that occurred from 1645–1715 (MC: 50, SA: 15) ix. Solar cycle is half of a magnetic cycle in the Sun, in which the polarity of its magnetic field changes direction over a period of 22 years (TF: 13, MC: 51) x. Solar prominences: magnetic flux tubes containing relatively cool gas that extend out into the corona (MC: 36, 45) xi. Solar flare: eruptions on the surface of the sun that eject material into space (MC: 45, 51, SA: 16) xii. Coronal mass ejection: bursts of solar wind rising from the corona (MC: 45, 52: SA: 17) V. Solar Activity Affects Earth i. Sun is 0.1 percent brighter during peak of the solar cycle (MC: 53, SA: 15, 18) ii. X-rays given off by active regions cause Earth’s atmosphere to expand, creating extra drag on satellites (MC: 41, 54, SA: 19) iii. Solar storms can cause auroras and disrupt electric power grids (MC: 41, SA: 19)

Sec 14.5 5. Origins: Solar Wind and Life I. Solar Wind Can Affect Life i. Earth’s magnetic field protects us from solar wind (MC: 55) ii. Solar wind clears out the heliosphere, protecting the Solar System from galactic high-energy particles known as cosmic rays (SA: 20)

TRUE/FALSE 1. In our Sun, hydrostatic equilibrium exists only in the core, where energy production via fusion can balance gravity.

ANS: F DIF: Easy REF: Section 14.1 MSC: Conceptual TOP: 1IIi 2. Nuclei of atoms are held together by gravity.

ANS: F DIF: Easy REF: Section 14.2 MSC: Factual TOP: 2Ii 3. At the center of the Sun, the temperature is roughly 15 million K. ANS: T DIF: Easy REF: Section 14.2 MSC: Factual TOP: 2Iv 4. The net result of the proton-proton chain of nuclear reactions is that four protons are converted into one helium nucleus and energy, electrons, and neutrinos are released.

ANS: F DIF: Difficult REF: Section 14.2 MSC: Factual TOP: 2Ivii 5. In the radiative zone inside the Sun, photons are transported from the core to the convective zone in a matter of seconds.

ANS: F DIF: Medium REF: Section 14.2 MSC: Factual TOP: 2IIiv 6. If the Sun stopped nuclear fusion in its core, it would take 1,000 years for its luminosity to change.

ANS: F DIF: Medium REF: Section 14.2 MSC: Applied TOP: 2IIiv 7. Neutrinos are particles with small masses that interact easily with normal matter.

ANS: F DIF: Easy REF: Section 14.3 MSC: Factual TOP: 3Ii 8. The solar neutrino problem was solved by postulating that neutrinos have a small mass and oscillate between three different types of neutrino.

ANS: T DIF: Medium REF: Section 14.3 MSC: Factual TOP: 3Iiv 9. The fact that the surface of the Sun rings like a bell lets us understand its interior better.

ANS: T DIF: Medium REF: Section 14.3 MSC: Conceptual TOP: 3IIi 10. The temperature of the corona is much hotter than any other layer in the solar atmosphere.

ANS: T DIF: Easy REF: Section 14.4 MSC: Factual TOP: 4IIIii

11. If coronal holes covered a larger fraction of the Sun’s surface, the solar wind would contain a higher density of particles.

ANS: T DIF: Easy REF: Section 14.4 MSC: Applied TOP: 4IVii 12. In a sunspot, the umbra is cooler than the penumbra.

ANS: T DIF: Difficult REF: Section 14.4 MSC: Applied TOP: 4IVv 13. The solar magnetic field switches polarity every 11 years.

ANS: T DIF: Easy REF: Section 14.4 MSC: Factual TOP: 4IVix

MULTIPLE CHOICE 1. Our Sun is unique compared to the other stars in our galaxy because of its: a. temperature b. size c. evolutionary stage d. proximity e. mass ANS: D DIF: Easy REF: Section 14.1 MSC: Factual TOP: 1Ii 2. The Sun has a mass of: a. 2  1010 kg b. 2  1025 kg c. 2  1030 kg d. 2  1035 kg e. 2  1045 kg ANS: C DIF: Easy REF: Section 14.1 MSC: Factual TOP: 1Ii

3. Hydrostatic equilibrium is a balance between: a. heat and centrifugal force b. core temperature and surface temperature c. pressure and gravity d. radiation and heat e. centrifugal force and gravity ANS: C DIF: Medium REF: Section 14.1 MSC: Conceptual TOP: 1IIi 4. Density, temperature, and pressure increase as you move inward in the interior of the Sun. This means that the force of gravity, (Fg  GMm/r2), _________ as you move inward toward the core. a. increases b. decreases c. stays the same d. There is not enough information to answer.

ANS: A DIF: Difficult REF: Section 14.1 MSC: Applied TOP: 1IIii

5. Which of the following curves best matches the shape of a graph of the density of material inside the Sun (in thousands of kg/m3) as you move further away from the center?

a. A b. B c. C d. D e. E ANS: E DIF: Medium REF: Section 14.1 MSC: Applied TOP: 1IIii 6. The balance of energy in the solar interior means that: a. energy production rate in the core equals the rate of radiation escaping the Sun’s surface b. the source of energy in the core is stable and will sustain the Sun for millions of years c. the outer layers of the Sun absorb and re-emit the radiation from the core at increasingly longer wavelengths d. radiation pressure balances the weight of the overlying solar layers e. the core of the Sun has higher pressure than the outer layers ANS: A DIF: Medium REF: Section 14.1 MSC: Applied TOP: 1IIiii

7. Which force is responsible for holding the protons and neutrons in the nucleus of an atom together? a. gravity b. strong nuclear force c. electric force d. magnetic force e. Electrons push them together.

ANS: B DIF: Medium REF: Section 14.2 MSC: Factual TOP: 2Ii 8. The majority of the Sun’s energy comes from: a. gravitational contraction b. nuclear fission of uranium c. hydrogen fusion d. helium burning e. burning material as in a fire ANS: C DIF: Easy REF: Section 14.2 MSC: Factual TOP: 2Iii 9. What do astronomers mean when they say that the Sun makes energy by hydrogen burning? a. The Sun is combusting hydrogen in a fire and releasing energy. b. The Sun is fusing hydrogen into uranium and releasing energy. c. The Sun is made of mostly hydrogen at very high temperature. d. The Sun is fusing hydrogen into helium and releasing energy. e. The Sun is accumulating hydrogen from the solar wind and releasing energy.

ANS: D DIF: Medium REF: Section 14.2 MSC: Conceptual TOP: 2Iii 10. When two atomic nuclei come together to form a new species of atom, this is called: a. nuclear fission b. nuclear recombination c. nuclear splitting d. nuclear fusion e. ionization ANS: D DIF: Easy REF: Section 14.2 MSC: Factual TOP: 2Iii

11. If the Sun converts 5  1011 kg of H to He per second and the mass of a single hydrogen nucleus is 1.7  10 27 kg, how many net proton-proton reactions go on per second in the Sun? What is the luminosity produced if the mass difference between a single helium nucleus and four hydrogen nuclei is 4  10 29 kg?

Note that 1 Watt  1 m2 kg/s3. a. 7  1037 reactions per sec; 3  1026 Watt b. 3  1038 reactions per sec; 1027 Watt c. 3  1038 reactions per sec; 4  1026 Watt d. 7  1037 reactions per sec; 5  1025 Watt e. 3  1037 ractions per sec; 6  1024 Watt ANS: A DIF: Difficult REF: Section 14.2 MSC: Applied TOP: 2Iiii 12. If the Sun converts 5  1011 kg of H to He per second and 10 percent of the Sun’s total mass is available for nuclear burning, how long might we expect the Sun to live? a. 104 years b. 108 years c. 1010 years d. 1011 years e. 1014 years ANS: C DIF: Difficult REF: Section 14.2 MSC: Applied TOP: 2Iiii 13. If the core of the Sun were hotter than it is now, how would the Sun’s energy production change? a. It would produce less energy per second than it does now. b. It would produce more energy per second than it does now. c. Its energy production would vary more than it does now. d. Its energy production would be more stable than it is now. e. The Sun’s energy production would not change.

ANS: B DIF: Easy REF: Section 14.2 MSC: Applied TOP: 2Iiv | 2IIIi

14. The energy that fuels the Sun is generated: a. only on its surface b. only in its core c. only in the solar wind d. both in its core and on its surface e. in its core, on the surface, and in the solar wind ANS: B DIF: Easy REF: Section 14.2 MSC: Factual TOP: 2Iv 15. Why is hydrogen burning the main energy source for main-sequence stars? a. Hydrogen is the most common element in stars. b. Hydrogen nuclei have the smallest positive charge. c. Hydrogen burning is the most efficient of all fusion or fission reactions. d. Hydrogen can fuse at temperatures lower than other elements. e. All the above are valid reasons.

ANS: E DIF: Medium REF: Section 14.2 MSC: Conceptual TOP: 2Ivi 16. The net effect of the proton-proton chain is that four hydrogen nuclei are converted to one helium nucleus and _________ are released. a. visible wavelength photons b. gamma ray photons, positrons, and neutrinos c. ultraviolet photons and neutrinos d. X-ray photons, electrons, and neutrinos e. infrared photons and positrons ANS: B DIF: Medium REF: Section 14.2 MSC: Factual TOP: 2Ivii 17. Which of the following method(s) is (are) NOT used to transport energy from the core of the Sun to its surface? a. radiation b. convection c. conduction d. All of the above are important in the solar interior. e. None of the above are important in the solar interior.

ANS: C DIF: Medium REF: Section 14.2 MSC: Factual TOP: 2IIi

18. If you hold on to one end of a metal spoon while placing the other end in a pot of boiling water, you will burn your hand. This is an example of energy being transported by: a. radiation b. convection c. conduction d. convection and radiation e. radiation and conduction ANS: C DIF: Easy REF: Section 14.2 MSC: Applied TOP: 2IIi 19. Some restaurants place food under infrared heat lamps so that it stays warm after it has been cooked. This is an example of energy being transported by: a. radiation b. convection c. conduction d. convection and conduction e. radiation and conduction ANS: A DIF: Easy REF: Section 14.2 MSC: Applied TOP: 2IIii 20. The interior zones of the Sun are distinguished by: a. jumps in density between zones b. their temperature profiles c. pressure differences inside each zone d. their modes of energy transport e. all of the above ANS: D DIF: Difficult REF: Section 14.2 MSC: Applied TOP: 2IIiii | 2IIvi 21. Which of the following layers of the Sun makes up the majority of its interior? a. the core b. the radiative zone c. the convective zone d. the photosphere e. the chromosphere ANS: B DIF: Medium REF: Section 14.2 MSC: Factual TOP: 2IIiii

22. Approximately how long does it take the photons released in nuclear reactions in the core of the Sun to exit the photosphere? a. 8 minutes b. 16 hours c. 1,000 years d. 100,000 years e. 4.6 billion years ANS: D DIF: Medium REF: Section 14.2 MSC: Factual TOP: 2IIiv 23. Light from the Sun’s photosphere reaches Earth approximately _________ times faster than photons released in fusion in the core. a. 1,000 b. 600,000 c. 1 million d. 6 billion e. 10 billion ANS: D DIF: Difficult REF: Section 14.2 MSC: Applied TOP: 2IIiv 24. When you turn on the heater in a car, the passengers in the front seat warm up first, then eventually the warm air gets to the passengers in the back seat. This is an example of energy being transported by: a. radiation b. convection c. conduction d. convection and conduction e. radiation and conduction ANS: B DIF: Easy REF: Section 14.2 MSC: Applied TOP: 2IIv

25. Which of these can travel directly from the center of the Sun to Earth in about 8 minutes? a. photons b. electrons c. protons d. neutrons e. neutrinos ANS: E DIF: Easy REF: Section 14.3 MSC: Factual TOP: 3Ii 26. The detection of solar neutrinos confirms that: a. the Sun’s core is powered by proton-proton fusion b. energy transport by radiation occurs throughout much of the solar interior c. magnetic fields are responsible for surface activity on the Sun d. convection churns the base of the solar atmosphere e. sunspots are cooler than the rest of the photosphere ANS: A DIF: Easy REF: Section 14.3 MSC: Conceptual TOP: 3Iii 27. If neutrinos oscillated between five different types of neutrino during their transit from the Sun to Earth, then how many neutrinos would we have detected compared to what was emitted by the Sun? a. one-half as many b. one-third as many c. one-fourth as many d. one-fifth as many e. We would detect no neutrinos.

ANS: D DIF: Easy REF: Section 14.3 MSC: Applied TOP: 3Iiii 28. The solar neutrino problem was solved by: a. adjusting the rate of hydrogen burning in solar models b. improving detector efficiencies so more neutrinos were observed c. postulating that neutrinos had mass and oscillated between three different types d. lowering the percentage of helium in models of solar composition e. correctly measuring the density of the Sun’s interior ANS: C DIF: Difficult REF: Section 14.3 MSC: Factual TOP: 3Iiv

29. By studying how the surface of the Sun vibrates like a struck bell we can determine its: a. age b. interior density c. total mass d. size e. temperature ANS: B DIF: Easy REF: Section 14.3 MSC: Applied TOP: 3IIi 30. We can determine how the density changes with radius in the Sun using: a. radar observations b. neutrino detections c. high-energy (gamma ray) observations d. helioseismology e. infrared observations ANS: D DIF: Medium REF: Section 14.3 MSC: Conceptual TOP: 3IIi 31. The surface of the Sun appears sharp when we look at it in visible light because: a. the photosphere is cooler than the layers below it b. the photosphere is thin compared to the other layers in the Sun c. the photosphere is much less dense than the convection zone d. the photosphere is transparent to radiation e. the Sun has a distinct surface ANS: B DIF: Difficult REF: Section 14.4 MSC: Applied TOP: 4Ii | 4Iii

32. Imagine that you observed the Sun and measured the brightness of the face of the Sun at the locations marked in this image:

At which of these locations would you measure the lowest brightness? a. A b. B c. C d. D e. They would all have the same brightness.

ANS: D DIF: Medium REF: Section 14.4 MSC: Applied TOP: 4Iiii 33. The solar spectrum is an example of a(n) _________ spectrum. a. emission b. absorption c. continuum d. blackbody e. X-ray ANS: B DIF: Medium REF: Section 14.4 MSC: Factual TOP: 4IIi 34. The Sun’s chromosphere appears red because: a. it is hotter than the photosphere b. as the Sun rotates, the chromosphere appears to move away from us radially c. it has a higher concentration of heavy metals d. it is made of mostly helium e. its spectrum is dominated by H emission ANS: E DIF: Difficult REF: Section 14.4 MSC: Applied TOP: 4IIIi

35. The image below shows the Sun during a solar eclipse at visible wavelengths.

Which part of the Sun is visible around the shadow of the Moon? a. chromosphere b. photosphere c. radiative zone d. convective zone e. corona ANS: A DIF: Easy REF: Section 14.4 MSC: Applied TOP: 4IIIi 36. The best wavelength to use to observe a solar prominence is: a. 550 nm, green visible light b. 656 nm, a red hydrogen emission line c. 16 mm, an ultraviolet emission line d. 21 cm, microwave emission e. 0.02 nm, X-ray emission ANS: B DIF: Medium REF: Section 14.3 | 14.4 MSC: Factual TOP: 4IIIi | 4IVx 37. The Sun’s corona has a temperature of approximately 1 million degrees. At what wavelength and in what part of the electromagnetic spectrum does its radiation peak? a. 550 nm, visible b. 2  10 5 m, infrared c. 4  10 7 m, ultraviolet d. 3  10 9 m, X-rays e. 6 m, radio ANS: D DIF: Easy REF: Section 14.4 MSC: Applied TOP: 4IIIii

38. Which of the layers of the Sun is located the furthest from the center of the Sun? a. chromosphere b. photosphere c. radiative zone d. convective zone e. corona ANS: E DIF: Easy REF: Section 14.4 MSC: Factual TOP: 4IIIii 39. We know the Sun’s corona is very hot because: a. we observe it emitting radiation at visible wavelengths b. the chromosphere and the photosphere are that hot, too c. we observe absorption from highly ionized atoms of iron and calcium in its spectrum d. the gas emits most of its radiation at radio wavelengths e. all of the above ANS: C DIF: Medium REF: Section 14.4 MSC: Applied TOP: 4IIIii 40. What keeps the gas in the Sun’s corona from flying away from the Sun? a. gravity b. strong nuclear force c. the Sun’s magnetic field d. the solar wind e. sunspots ANS: C DIF: Medium REF: Section 14.4 MSC: Factual TOP: 4IVi 41. Which of the following is NOT a result of an increase in solar activity? a. The altitudes of orbiting satellites decrease. b. Airplanes have trouble navigating. c. Stronger auroras are seen. d. Power grids can be damaged. e. All of the above can be caused by increased solar activity.

ANS: E DIF: Easy REF: Section 14.4 MSC: Applied TOP: 4IViii | 4Vii | 4Viii

42. The image below taken at visible wavelengths shows a section of the Sun with sunspots visible.

Which of the labeled regions is the lowest temperature? a. region A b. region B c. region C d. They are all the same temperature. e. There is not enough information to determine their relative temperatures.

ANS: C DIF: Easy REF: Section 14.4 MSC: Applied TOP: 4IViv 43. Sunspots appear dark because they have _________ than the surrounding gas. a. higher densities b. lower densities c. higher pressures d. lower temperatures e. higher temperatures ANS: D DIF: Easy REF: Section 14.4 MSC: Factual TOP: 4IViv

44. If a sunspot appears one-quarter as bright as the surrounding photosphere, and the average temperature of the photosphere is 5800 K, what is the temperature of the gas in this sunspot? a. 3625 K b. 4100 K c. 4500 K d. 5200 K e. 5500 K ANS: B DIF: Difficult REF: Section 14.4 MSC: Applied TOP: 4IViv 45. Which of the following are created by solar magnetic activity? a. sunspots b. prominences c. coronal mass ejections d. solar flares e. all of the above ANS: E DIF: Easy REF: Section 14.4 MSC: Applied TOP: 4IViv | 4IVx | 4IVxi | 4IVxii 46. The darkest part of a sunspot is called the: a. penumbra b. umbra c. granule d. photosphere e. magnetic field ANS: B DIF: Medium REF: Section 14.4 MSC: Factual TOP: 4IVv 47. The magnetic field of the Sun is continuously produced and deformed by: a. its differential rotation b. the solar wind c. changes in the rate of nuclear fusion in the core d. a liquid conducting layer in the interior e. This is a trick question. The solar magnetic field is primordial.

ANS: A DIF: Difficult REF: Section 14.4 MSC: Applied TOP: 4IVvi

48. The Sun’s internal magnetic field becomes tangled up over time because of: a. coronal holes b. coronal mass ejections c. differential rotation d. temperature changes in the Sun’s core e. all of the above ANS: C DIF: Medium REF: Section 14.4 MSC: Applied TOP: 4IVvi 49. If you observe a maximum number of sunspots right now, how long would you have to wait to see the next solar maximum? a. 24 hours b. 6 months c. 1 year d. 11 years e. 22 years ANS: D DIF: Medium REF: Section 14.4 MSC: Applied TOP: 4IVvii 50. The Maunder Minimum was a 60-year period when: a. debris thrown up in a comet collision blanketed the Sun b. almost no sunspots occurred on the Sun c. the Voyager 2 spacecraft traversed the heliopause d. very few dust storms occurred on Mars e. very few volcanic eruptions occurred on Mars ANS: B DIF: Difficult REF: Section 14.4 MSC: Factual TOP: 4IVviii 51. The Sun’s magnetic field reverses direction every: a. 24 hours b. 27 days c. 12 months d. 11 years e. 22 years ANS: D DIF: Medium REF: Section 14.4 MSC: Factual TOP: 4IVix

52. If a coronal mass ejection occurs on the Sun that expels material at a speed of 800 km/s, how long will it take these charged particles to reach the Earth? a. 0.7 day b. 1.4 days c. 1.8 days d. 2.2 days e. 3.5 days ANS: D DIF: Medium REF: Section 14.4 MSC: Applied TOP: 4IVxii 53. When is the Sun most luminous? a. when there are a maximum number of sunspots b. when there are a average number of sunspots c. where there a minimum number of sunspots d. the Sun’s luminosity does not change e. the Sun’s luminosity changes, but it has no relation to the number of sunspots ANS: A DIF: Medium REF: Section 14.4 MSC: Applied TOP: 4Vi 54. When solar activity is very high, the Earth’s atmosphere will: a. expand b. contract c. remain approximately the same d. repel charged particles e. block out sunlight ANS: A DIF: Difficult REF: Section 14.4 MSC: Applied TOP: 4Vii 55. Solar wind particles hit the surface of the Moon, but they don’t make it to the surface of the Earth because the Earth: a. is larger than the Moon b. is warmer than the Moon c. has an atmosphere while the Moon does not d. has a magnetic field while the Moon does not e. is further from the Sun than the Moon ANS: D DIF: Medium REF: Section 14.5 MSC: Applied TOP: 5Ii

SHORT ANSWER 1. In addition to the laws of physics and chemistry, what information do we need to know about our Sun to calculate its internal structure and radius?

ANS: We need to know its mass and its chemical composition.

DIF: Easy REF: Section 14.1 MSC: Factual TOP: 1Iii 2. Explain why hydrostatic equilibrium results in the center of the Sun having the highest pressure and temperature.

ANS: Hydrostatic equilibrium means that, at any radius in the Sun, the gas pressure of the material interior to this radius must balance the weight of all the material above it. At the center, the weight of the material on top is the largest, so that is where the pressure must be the greatest. The ideal gas law says that pressure is proportional to temperature, thus the temperature must be highest in the center, too.

DIF: Medium REF: Section 14.1 MSC: Conceptual TOP: 1IIi | 1IIii 3. Calculate the amount of energy released by converting four hydrogen atoms into one helium atom. The mass of a hydrogen atom is 1.67  10 24g; the mass of a helium atom is 6.65  10 24 g. The speed of light is 3  108 m/s.

ANS: First, we need to calculate the amount of mass which is converted into energy: 4MH

MHe  4(1.67  10 24 g)

6.65  10 24 g  3.0  10 26 g.

Next, we must convert this mass into energy, using the equation E  mc2:

E  (3.0  10 26 g)  (1 kg/1,000 g)  (3  108 m/s)2  2.7  10 12 J. DIF: Medium REF: Section 14.2 MSC: Applied TOP: 2Iiii 4. Through hydrogen fusion, the Sun loses approximately 4 million tons of mass each second. If it burns hydrogen at this rate for 10 billion years, what percentage of its original mass will it lose in all? (Note: The mass of the Sun is 1.99  1030 kg, and 1 ton  1,000 kg.)

ANS: The total amount of mass lost is: 4000  106 kg/s  (60 s/min)  (60 min/hr)  (24 hr/day)  (365 days/yr)  (10  109 years)  1.26  1027 kg total mass lost, or 1.26  1027 kg/1.99  1030 kg  6.34  10 4  0.063 percent of its mass.

DIF: Medium REF: Section 14.2 MSC: Applied TOP: 2Iiii

5. Why is hydrogen burning the main energy source for main-sequence stars? Give at least two reasons.

ANS: (1) Hydrogen is the most abundant element in stars. It is the largest fuel source available to mainsequence stars.

(2) Hydrogen fusion occurs at the lowest temperature of all the elements because its force of mutual repulsion is the smallest. This means conditions in the interiors of stars are right for hydrogen fusion before they are right for any other kind of fusion.

(3) Hydrogen fusion converts a larger fraction of mass to energy than any other fusion process, so it’s the most efficient source.

DIF: Easy REF: Section 14.2 MSC: Conceptual TOP: 2Iiv | 2Iv 6. In the proton-proton chain, the net reaction is that 4 protons are converted into 1 helium nucleus. What other byproducts are released in this reaction, and why?

ANS: Energy is released in the form of gamma rays because a small amount of mass is converted to energy. In addition, positrons are released so that charge is conserved and neutrinos are released so that lepton number is conserved.

DIF: Difficult REF: Section 14.2 MSC: Conceptual TOP: 2Ivi 7. List three methods of energy transport in nature and explain how the energy is being transferred in each of those methods. Which two are means by which energy is transported inside the Sun? ANS: Radiative—via electromagnetic radiation; convective—via the motions of molecules and atoms from one place to another; and conductive—by the transference of motion from one set of atoms/molecules to another set of atoms/molecules. Radiative and convective transport are important in the Sun. DIF: Medium REF: Section 14.2 MSC: Conceptual TOP: 2IIi | 2IIii | 2IIv

8. The picture below shows a diagram of the Sun with zones labeled A, B, and C.

Explain how energy is being transferred in each of the three regions.

ANS: Region A is the radiative zone, where energy is being carried by photons. Region B is the convective zone, where moving gas carries the energy. Region C is outside the Sun, where the energy is again being carried by photons.

DIF: Easy REF: Section 14.2 MSC: Applied TOP: 2IIiii | 2IIvi 9. In the text we considered the case of a “too-large” Sun. Show that a star with the same mass, composition, radius, and luminosity as the Sun, but with a higher temperature (that is, a “too-hot” Sun), also leads to a contradiction.

ANS: If the temperature is higher, more energy will be produced in the core because the proton–proton chain runs faster at higher temperatures. This means that to have equilibrium, the gravity at the center must be larger, so the matter must be denser (we must increase mass or decrease radius). At the same time, more energy production will lead to a higher luminosity at the surface unless the radius increases.

So the only way to keep mass and luminosity constant is for the radius to both increase and decrease simultaneously.

DIF: Difficult REF: Section 14.2 MSC: Applied TOP: 2IIIi

10. Explain why the solution to the solar neutrino problem is an excellent example of how observations drive the evolution of science.

ANS: The standard model of the Sun predicted three times more neutrinos should arrive from the Sun than were observed, which led theorists to alter their ideas about neutrinos and postulate that they had a small mass and could oscillate between three different types of neutrinos. Later, particle physics experiments confirmed this idea was correct.

DIF: Difficult REF: Section 14.3 MSC: Conceptual TOP: 3Iiv 11. What is “limb darkening”? Explain why limb darkening occurs in the Sun. ANS: Limb darkening means that the edges of the Sun are fainter than the center. It occurs because, at the edges of the Sun, we are not looking as deeply down into the interior of the Sun as we are at the center.

DIF: Easy REF: Section 14.4 MSC: Conceptual TOP: 4Iiii 12. Explain why magnetic fields trap coronal gas over much of the solar surface but allow it to escape in coronal holes.

ANS: Most of the solar magnetic field consists of closed field loops. Coronal material can move along these loops, but it can’t cross them. This traps the material. Coronal holes are regions where field lines are open and stream away from the solar surface. Coronal material can follow these lines outward and into the Solar System.

DIF: Medium REF: Section 14.4 MSC: Conceptual TOP: 4IVi 13. If a sunspot is half as bright as the surrounding photosphere of the Sun, what is the approximate temperature of the gas in the sunspot if the photosphere’s average temperature is 5800 K? ANS: Using the Stefan-Boltzmann law, F

T4, if the sunspot is half as bright as the surrounding photo-

sphere, then its temperature is 5800 K  0.50.25  4880 K. DIF: Difficult REF: Section 14.4 MSC: Applied TOP: 4IViv 14. The Sun exhibits differential rotation. Explain what differential rotation is. Which planets also do this? Why don’t the others?

ANS: Differential rotation means that the equator of an object rotates more rapidly than the higher latitude regions. In addition to the Sun, the giant planets all do this. The terrestrial planets can’t have differential rotation because they all have solid surfaces that must rotate together.

DIF: Medium REF: Section 14.4 MSC: Conceptual TOP: 4IVvi 15. When, during its 11-year cycle, is the Sun most luminous? What might this have to do with the Maunder Minimum?

ANS: The Sun is most luminous during solar maxima, when the number of sunspots on its surface is at maximum. During the Maunder Minimum, few sunspots were seen on the Sun, implying that it was a bit less luminous than today and caused Earth to be a bit colder than it is today.

DIF: Medium REF: Section 14.4 MSC: Factual TOP: 4IVviii | 4Vi 16. Astronauts in space could be harmed by the high-energy particles given off during a solar flare. So, when a solar flare is observed, a warning can be given to astronauts to tell them to get inside the space station for protection. Explain why there is enough time between the first observation of a flare and the arrival of the harmful particles for this system to work.

ANS: The first sign of a solar flare to arrive to us will be the photons that are given off. These travel at the speed of light, while any particle given off would travel slower than the speed of light. This means there will be a delay between the arrival of the photons and the arrival of the harmful particles.

DIF: Easy REF: Section 14.4 MSC: Applied TOP: 4IVxi 17. If a coronal mass ejection occurred on the Sun and ejected particles toward the Earth that traveled at the speed of 1,000 km/s, how long would it take them to reach Earth?

ANS: Using   d/t and d  1 AU  1.5  1011 m, the time it would take particles to travel to the Earth is t  d/v  1.5  1011 m/106 m/s  15  105 s  1 hr/3,600 s  1 day/24 hr  1.7 day.

DIF: Easy REF: Section 14.4 MSC: Applied TOP: 4IVxii 18. If the Sun went through a period where there were many sunspots for a number of decades straight, what would happen to the climate of the Earth?

ANS: Because the luminosity of the Sun increases during periods of high solar activity, the Earth would receive more sunlight during this high-sunspot time. This would raise the temperature of the Earth.

DIF: Medium REF: Section 14.4 MSC: Applied TOP: 4Vi 19. How do periods of strong solar activity affect near-Earth orbiting spacecraft?

ANS: At solar maxima, the amount of extreme ultraviolet and X-ray radiation reaching Earth’s atmosphere increases significantly. This causes extra heating in the upper atmosphere, which swells in response. The swelling of the upper atmosphere greatly increases the drag on near-Earth satellites and

can cause their orbits to decay.

DIF: Medium REF: Section 14.4 MSC: Applied TOP: 4Vii | 4Viii 20. Explain what the heliosphere is and how it helps protect life on Earth.

ANS: The heliosphere is the area around the Sun where the interstellar medium has been cleared out by the solar wind. The heliosphere stops high-energy particles (cosmic rays) that are given off by supernova explosions. These particles are harmful to living things, so stopping them can help make the Earth habitable.

DIF: Medium REF: Section 14.5 MSC: Applied TOP: 51ii

CHAPTER 15

Star Formation and the Interstellar Medium

CONCEPT MAP Sec 15.1 1. The Interstellar Medium I. Characteristics of the Interstellar Medium i. Star formation takes 104 years (high-mass) to 108 years (low-mass) (MC: 1) ii. Interstellar medium (ISM): the gas and dust that pervade the space between stars in a galaxy (MC: 2) iii. Ninety-nine percent of the ISM is very low-density gas (TF: 1, MC: 3, SA: 1, 2) iv. ISM chemical composition is similar to that of the Sun II. The Interstellar Medium Is Dusty i. One percent of the ISM is in solid grains of interstellar dust similar to soot (SA: 2) ii. Dust forms in cool, dense environments (TF: 2) iii. Half of all heavy elements in the ISM are in interstellar grains (MC: 4) iv. Dust causes (wavelength-dependent) extinction and reddening (MC: 5–10) v. Dust is heated by starlight and emits in far infrared (MC: 11, SA: 3) vi. The infrared sky looks much different than the visible sky (MC: 10) III. Interstellar Gas Has Different Temperatures and Densities i. The ISM is concentrated into relatively dense, cold clouds, not uniformly distributed throughout the galaxy; half the ISM is 2 percent of the volume; the other half is intercloud gas that is very tenuous and spread over 98 percent of the volume (TF: 3) ii. Intercloud gas can be hot but has extremely low density (MC: 12) iii. Half of intercloud gas is very hot, with a temperature of a million K and emitting at X-ray wavelengths that arise from supernovae explosions (TF: 4, MC: 10, 13, SA: 4–5)

iv. Interstellar gas can be found in emission (H-alpha) or absorption against background stars (MC: 14, 15) v. H II regions are gas-emitting H-alpha emission because it’s ionized by hot, luminous, blue O- and Btype stars (MC: 16, SA: 4–7) vi. H II regions trace regions of currently active star formation (MC: 16, 17) vii. Neutral hydrogen gas emits 21-cm radiation, generated when an electron flips its spin in a hydrogen atom (lowest energy state is when the electron’s and proton’s spins are aligned; spontaneous spin flip happens after about 11 million years) (TF: 5, MC: 17–19, SA: 5) IV. Clouds Are Regions of Cool, Dense Gas i. Typical interstellar gas clouds: T  100 K,   1 to 100 atoms/cm3 (SA: 8) ii. Molecules are easily broken up in collisions and from starlight so they only survive for long periods of time in the densest, coldest clouds, called molecular clouds, that are self-shielded from external starlight (TF: 6, MC: 20) iii. Molecular clouds: T  10 K,   10 to 1,000 atoms/cm3 (TF: 4, 7, MC: 21, 22, SA: 9) iv. Molecular clouds are mostly composed of H2, with trace amounts of H2O, CO, CN, CH, and even more chemically complex molecules (MC: 23) v. H2 does not emit readily so we infer its presence by observing CO at radio wavelengths, assuming that the ratio of H2 to CO is about constant in various clouds (TF: 8, MC: 24, SA: 5) vi. Molecular clouds: masses range from a few to 107 M with the largest called giant molecular clouds (GMC) vii. GMCs are on average 120 ly  40 pc across and our galaxy has about 4,000 GMCs viii. Explosions from supernovae and light from stars stirs the ISM (MC: 25) Sec 15.2 2. Molecular Clouds Are the Cradles of Star Formation I. Gravitational Collapse i. In hydrostatic equilibrium, the pressure due to the motion of the cloud’s atoms and molecules balances the inward pull of gravity (self-gravity) (MC: 26, SA: 10) ii. Normally the pressure in clouds is even greater than the self-gravity, and they are held together by the external pressure of the ISM (SA: 10) iii. Some molecular clouds are massive enough to collapse under their own weight (SA: 10, 11) iv. Angular momentum must be conserved as the cloud collapses (MC: 27)

v. Angular momentum, turbulence, and magnetic fields slow the collapse of a molecular cloud (TF: 9, MC: 28–30, SA: 12) II. Molecular Clouds Fragment as They Collapse (SA: 12) i. Stars form in molecular-cloud cores (TF: 10, MC: 21, 23, SA: 13, 14) ii. Angular momentum: a rotating accretion disk forms during the collapse (MC: 31) Sec 15.3 3. A Protostar Becomes a Star I. Protostars i. Protostar is heated by the kinetic energy deposited by the infalling material (MC: 32) ii. The protostar is large and bright, even without nuclear fusion (MC: 33, 34, SA: 15) iii. Dust and gas from the molecular cloud heavily obscures protostars (MC: 35, SA: 6) II. A Shifting Balance: The Evolving Protostar (SA: 2) i. A protostar collapses as it radiates away its energy (TF: 11, MC: 36–38, SA: 17) ii. Eventually nuclear reactions begin in the protostar’s core, and it becomes a star (MC: 49) iii. Minimum mass for nuclear fusion to occur in the core is 0.08 M (MC: 46) iv. Brown dwarfs (MC; 41, 42, SA: 18) III. Evolving Stars and Protostars Follow “Evolutionary Tracks” in the H-R Diagram i. Convection is the dominant form of energy transport in protostars ii. H ion as temperature regulator (TF: 12, MC: 43, SA: 19) iii. Protostars follow “Hayashi tracks” in the H-R diagram (MC: 44–49) Sec 15.4 4. Not All Stars Are Created Equal I. Protostar Differences i. Star formation is very inefficient (MC: 50, 51) ii. Bipolar outflows and jets: seen from young, forming stars iii. Herbig-Haro objects (TF: 13, MC: 16, SA: 20) iv. T Tauri stars (TF: 14, MC: 17, SA: 20) v. Star clusters vi. Star formation timescales vary with the mass of the star; low mass stars take longer to form than high mass stars (TF: 15, MC: 52–54) Sec 15.5

5. Origins: Star Formation, Planets, and Life I. A Planet for Every Star? i. Water, organic molecules, even O2 have been detected in molecular clouds ii. Planets have been found in binary systems (MC: 55) iii. There may be isolated planets that have been ejected from their star systems (MC: 55)

TRUE/FALSE 1. The average density of the interstellar medium is many times less dense than the best vacuum on Earth.

ANS: T DIF: Easy REF: Section 15.1 MSC: Factual TOP: 1Iiii 2. The dust in the interstellar medium comes primarily from the stellar winds of main-sequence stars.

ANS: F DIF: Difficult REF: Section 15.1 MSC: Factual TOP: 1IIii 3. The lowest-density gas in the interstellar medium is also the coldest.

ANS: F DIF: Easy REF: Section 15.1 MSC: Factual TOP: 1IIIi 4. The interstellar medium is divided up into three different kinds of gas clouds: cold gas at 10 K, warm gas at 8000 K, and hot gas at about 1 million K. ANS: T DIF: Medium REF: Section 15.1 MSC: Factual TOP: 1IIIiii | 1IViii 5. We observe neutral hydrogen gas using 21-cm emission.

ANS: T DIF: Easy REF: Section 15.1 MSC: Factual TOP: 1IIIvii 6. Molecular hydrogen atoms are found only inside dense clouds where they are shielded from stellar radiation.

ANS: T DIF: Medium REF: Section 15.1 MSC: Factual TOP: 1IVii 7. The coldest molecular clouds in our galaxy have temperatures of approximately 1000 K. ANS: F DIF: Easy REF: Section 15.1 MSC: Factual TOP: 1IViii 8. Electronic transitions from the H2 molecule are easily seen at radio wavelengths.

ANS: F DIF: Difficult REF: Section 15.1 MSC: Applied TOP: 1IVv 9. Star formation in a molecular cloud can be slowed by the strength of its magnetic field and turbulence caused by supernovae and stellar winds from massive stars.

ANS: T DIF: Easy REF: Section 15.2 MSC: Applied TOP: 2Iv 10. Stars forming in molecular clouds tend to form first in the low-density periphery.

ANS: F DIF: Easy REF: Section 15.2 MSC: Applied TOP: 2IIi

11. A protostar is usually in hydrostatic equilibrium as its collapses.

ANS: T DIF: Medium REF: Section 15.3 MSC: Factual TOP: 3IIi 12. The H– atom is important in protostars because it acts as a powerful temperature regulator.

ANS: T DIF: Difficult REF: Section 15.3 MSC: Factual TOP: 3IIIii 13. Herbig-Haro objects are almost always found in pairs on either side of a young protostar.

ANS: T DIF: Difficult REF: Section 15.4 MSC: Factual TOP: 4Iiii 14. When winds blow the gas away from a forming protostar it becomes visible as a T Tauri star.

ANS: T DIF: Medium REF: Section 15.4 MSC: Factual TOP: 4Iiv 15. When a molecular cloud fragments and stars form, the least massive stars are the first to form while the most massive stars take longer to form.

ANS: F DIF: Medium REF: Section 15.4 MSC: Factual TOP: 4Ivi

MULTIPLE CHOICE 1. If you could watch stars forming out of a gas cloud, which stars would form first? a. low-mass stars b. medium-mass stars c. high-mass stars d. stars with low temperatures e. stars with more heavy elements ANS: C DIF: Easy REF: Section 15.1 MSC: Applied TOP: 1Ii 2. When looking at the space between stars, what might you see? a. Nothing; it is empty. b. spacecraft c. gas d. dark matter e. none of the above ANS: C DIF: Easy REF: Section 15.1 MSC: Applied TOP: 1Iii

3. The average density of the interstellar medium is: a. 1 atom/cm3 b. 1,000 atom/cm3 c. 104 atom/cm3 d. 106 atom/cm3 e. 1012 atom/cm3 ANS: A DIF: Medium REF: Section 15.1 MSC: Factual TOP: 1Iiii 4. If you wanted to observe heavy elements in the interstellar medium, where would be the best place to look? a. dust grains b. cold gas c. hot gas d. warm gas ANS: A DIF: Medium REF: Section 15.1 MSC: Applied TOP: 1IIiii 5. When radiation from an object passes through the interstellar medium: a. the object appears dimmer b. the object appears bluer c. the object appears bluer and dimmer d. the object appears redder and dimmer e. the object’s apparent velocity changes ANS: D DIF: Easy REF: Section 15.1 MSC: Factual TOP: 1IIiv 6. Dust in the ISM appears dark in _________ wavelengths and bright in _________ wavelengths. a. visible; ultraviolet b. infrared; radio c. infrared; visible d. radio; ultraviolet e. visible; infrared ANS: E DIF: Medium REF: Section 15.1 MSC: Applied TOP: 1IIiv

7. Dust reddens starlight because: a. it re-emits the light it absorbs at red wavelengths b. it emits mostly in the infrared due to its cold temperature c. it is made mostly of hydrogen, which produces the red H-alpha emission line d. it preferentially affects light at visible and shorter wavelengths e. it primarily moves away from Earth ANS: D DIF: Difficult REF: Section 15.1 MSC: Conceptual TOP: 1IIiv 8. What is the most likely explanation for the dark area in the image below?

a. It is a region where there are no stars. b. It is a region with lots of dark matter. c. It is a super-massive black hole. d. It is a region with thick dust blocking the starlight coming from behind. e. It is a dark star cluster.

ANS: D DIF: Easy REF: Section 15.1 MSC: Applied TOP: 1IIiv

9. The figures below show the spectrum of a star, along with five other spectra labeled A through E.

Which one of the labeled spectra shows what the spectrum of that star would look like if it were viewed through a significant amount of interstellar dust? a. A b. B c. C d. D e. E ANS: D DIF: Medium REF: Section 15.1 MSC: Applied TOP: 1IIiv

10. Below are three pictures of the disk of the Milky Way, taken in three different wavelength ranges.

Put the three pictures in order from shortest to longest wavelength. a. I, II, III b. II, III, I c. I, III, II d. II, I, III e. III, I, II ANS: B DIF: Difficult REF: Section 15.1 MSC: TOP: 1IIiv | 1IIvi | 1IIIiii

11. Dust that is heated to 30 K will emit a blackbody spectrum that peaks at: a. 1 m b. 30 m c. 50 m d. 100 m e. 500 m ANS: D DIF: Medium REF: Section 15.1 MSC: Applied TOP: 1IIv 12. Sitting in a 100°F hot tub feels much hotter than standing outside on a 100°F day. This analogy illustrates why: a. interstellar dust is dark at optical wavelengths, but bright in the infrared b. supernovae can heat their shells to such high temperatures c. an astronaut would feel cold standing in the 106 K intercloud gas d. the Solar System is immersed in a hot bubble of gas e. fusion only occurs in the cores of stars ANS: C DIF: Difficult REF: Section 15.1 MSC: Applied TOP: 1IIIii 13. Which of the following is responsible for heating the bulk of the very hot intercloud gas? a. high-energy radiation from stars b. supernovae c. young O and B stars d. planetary nebulae e. The heating is an even mix of all of the sources above.

ANS: B DIF: Medium REF: Section 15.1 MSC: Factual TOP: 1IIIiii 14. Warm ionized gas in the interstellar medium appears _________ when imaged in the optical region of the electromagnetic spectrum. a. red b. yellow c. white d. blue e. dark ANS: A DIF: Medium REF: Section 15.1 MSC: Applied TOP: 1IIIiv

15. The red emission in the photo below is due to:

a. carbon monoxide (CO) b. warm, neutral hydrogen c. molecular hydrogen (H2) d. ionized hydrogen (H II region) e. dust ANS: D DIF: Easy REF: Section 15.1 MSC: Applied TOP: 1IIIiv 16. An H II region signals the presence of: a. newly formed stars b. young stars c. ionized hydrogen gas d. O- and B-type stars e. all of the above ANS: E DIF: Difficult REF: Section 15.1 MSC: Applied TOP: 1IIIv | 1IIIvi

17. If you wanted to study regions where star formation is currently happening you could use: a. H-alpha emission to look for O and B stars b. 21 cm radiation to find neutral hydrogen clouds c. radio emission from carbon monoxide (CO) to find molecular cloud cores d. infrared emission to identify T Tauri stars e. all of the above ANS: E DIF: Easy REF: Section 15.1 | Section 15.4 MSC: Applied TOP: 1IIIvi | 1IIIvii | 4Iiv 18. 21-cm radiation is important because: a. it allows us to study the deep interiors of stars b. it allows us to image magnetic fields directly c. it allows us to study neutral hydrogen in the interstellar medium d. it is produced by every object in the universe e. it is the longest wavelength of light that can naturally be produced ANS: C DIF: Medium REF: Section 15.1 MSC: Factual TOP: 1IIIvii 19. We detect neutral gas in the interstellar medium by looking for radiation at 21 cm that arises when: a. an electron moves from the n  1 to n  2 state in a hydrogen atom b. an electron is ionized from a hydrogen atom c. carbon monoxide (CO) gas is excited by stellar radiation d. the spin of an electron flips and aligns with the spin of a proton in a hydrogen atom e. an electron combines with a proton to make a hydrogen atom ANS: D DIF: Difficult REF: Section 15.1 MSC: Factual TOP: 1IIIvii 20. In the interstellar medium, molecules survive only in regions with: a. low temperatures b. high densities c. lots of dust d. all of the above ANS: D DIF: Medium REF: Section 15.1 MSC: Factual TOP: 1IVii

21. Interstellar clouds are: a. hydrogen gas, condensed out of the interstellar medium, like water clouds in the Earth’s atmosphere b. regions where hydrogen tends to be denser than the surrounding gas c. regions where water condenses out of the interstellar medium d. oxygen gas, condensed out of the interstellar medium, like water clouds in the Earth’s atmosphere e. regions where hydrogen combines with oxygen to create water molecules ANS: B DIF: Medium REF: Section 15.1 MSC: Factual TOP: 1IViii 22. A typical molecular cloud has a temperature of approximately: a. 0.3 K b. 10 K c. 80 K d. 300 K e. 1000 K ANS: B DIF: Medium REF: Section 15.1 MSC: Factual TOP: 1IViii 23. Molecular cloud cores are places where you might find: a. protostars b. Herbig-Haro objects c. molecular hydrogen (H2) d. carbon monoxide (CO) e. all of the above ANS: E DIF: Medium REF: Section 15.1 MSC: Applied TOP: 1IViv | 2IIi 24. Molecular clouds, which have temperatures of around 10 K, are best observed at _________ wavelengths. a. X-ray b. ultraviolet c. optical d. infrared e. radio ANS: E DIF: Easy REF: Section 15.1 MSC: Applied TOP: 1IVv

25. “Weather” in the interstellar medium is produced: a. only by supernovae b. by supernovae and strong winds from luminous stars c. by supernovae, strong winds from luminous stars, and fast-moving stars d. by supernovae, strong winds from luminous stars, fast-moving stars, and exploding planets e. by supernovae, strong winds from luminous stars, fast-moving stars, exploding planets, and black holes ANS: B DIF: Difficult REF: Section 15.1 MSC: Applied TOP: 1IVviii 26. For an object in hydrostatic equilibrium, if the temperature inside the object were to increase, the object would: a. expand b. contract c. implode d. remain the same size e. explode ANS: A DIF: Easy REF: Section 15.2 MSC: Applied TOP: 2Ii 27. Because angular momentum must be conserved, as a gas cloud contracts due to gravity it will also: a. spin slower b. spin faster c. increase in temperature d. decrease in temperature e. stay the same temperature ANS: B DIF: Easy REF: Section 15.2 MSC: Applied TOP: 2Iiv 28. Of the following processes at work in molecular clouds, which is the one that inevitably dominates the clouds’ evolution? a. magnetic fields b. conservation of angular momentum c. pressure d. gravity e. turbulence ANS: D DIF: Medium REF: Section 15.2 MSC: Applied TOP: 2Iv

29. Magnetic fields inside a molecular cloud act to: a. inhibit gravitational collapse b. fragment the cloud into numerous cores c. modulate the temperature of the molecules d. increase the formation of dust grains e. increase the formation of protostars ANS: A DIF: Difficult REF: Section 15.2 MSC: Applied TOP: 2Iv 30. Which of the following traits does NOT help slow or prevent the collapse of a gas cloud? a. high mass b. high temperature c. turbulence d. magnetic fields e. angular momentum ANS: A DIF: Easy REF: Section 15.2 MSC: Applied TOP: 2Iv 31. An accretion disk forms around a collapsing protostar because infalling material must conserve: a. energy b. centrifugal force c. gravity d. velocity e. angular momentum ANS: E DIF: Easy REF: Section 15.2 MSC: Applied TOP: 2IIii 32. As a protostar evolves, its temperature: a. decreases because it is radiating b. decreases because of gravitational contraction c. decreases because of angular momentum d. increases because of nuclear fusion e. increases due to the kinetic energy of infalling material ANS: E DIF: Medium REF: Section 15.3 MSC: Applied TOP: 3Ii

33. A surprising fact about a 1 M protostar is that, even though nuclear reactions have not yet started in their cores, they are _________ than the Sun. a. hotter b. rotating faster c. smaller d. denser e. more luminous ANS: E DIF: Medium REF: Section 15.3 MSC: Factual TOP: 3Iii 34. A young protostar is _________ than the Sun even though its surface temperature is _________. a. less luminous; hotter b. larger; cooler c. smaller; the same d. more luminous; cooler e. smaller; hotter ANS: D DIF: Medium REF: Section 15.3 MSC: Factual TOP: 3Iii 35. What primarily makes it difficult to observe protostars? a. They occur in dusty regions. b. They have low luminosities. c. They do not shine at any wavelength until they become T Tauri stars. d. The star formation process happens so quickly. e. They are too small to be seen.

ANS: A DIF: Easy REF: Section 15.3 MSC: Factual TOP: 3Iiii 36. The entire process of star formation is really just an evolving balance between: a. heat and rotation b. core temperature and surface temperature c. pressure and gravity d. radiation and heat e. luminosity and distance ANS: C DIF: Easy REF: Section 15.2 MSC: Conceptual TOP: 3IIi

37. The source of energy for a contracting protostar comes from: a. thermonuclear energy b. kinetic energy c. chemical energy d. gravitational potential energy e. radiation energy ANS: D DIF: Medium REF: Section 15.3 MSC: Conceptual TOP: 3IIi 38. What happens as a protostar contracts? a. Its density rises. b. Its temperature rises. c. Its radius decreases. d. Its pressure rises. e. All of the above are true.

ANS: E DIF: Medium REF: Section 15.3 MSC: Applied TOP: 3IIi 39. What critical event transforms a protostar into a normal main-sequence star? a. Planets form in the accretion disk. b. The star grows suddenly larger in radius. c. Triple alpha reactions begin in the core. d. Nuclear fusion begins in the core. e. Convection begins throughout the star’s interior.

ANS: D DIF: Easy REF: Section 15.3 MSC: Applied TOP: 3IIii 40. Stars with a mass from 0.01 M to 0.08 M are very different from the Sun because they: a. do not have strong enough gravity to form planets b. have much higher temperatures than the Sun c. cannot successfully execute the proton-proton chain reactions d. form much faster than the Sun did e. do not exhibit sunspots ANS: C DIF: Medium REF: Section 15.3 MSC: Applied TOP: 3IIiii

41. A _________ is a failed star that shines primarily because of energy derived from its gravitational collapse rather than nuclear burning. a. black hole b. brown dwarf c. Herbig-Haro object d. protostar e. T Tauri star ANS: B DIF: Easy REF: Section 15.3 MSC: Factual TOP: 3IIiv 42. Brown dwarfs are considered failed stars because: a. they never reach masses larger than 50 Jupiter masses b. hydrogen fusion never begins in their cores c. convection never plays a role in their energy transport d. they primarily shine at infrared wavelengths e. they are never as luminous as the Sun ANS: B DIF: Medium REF: Section 15.3 MSC: Conceptual TOP: 3IIiv 43. The H– ion is very important in protostars because it: a. reacts with oxygen to produce water b. undergoes fusion and produces energy c. helps make the protostars denser d. acts as a temperature regulator e. reduces angular momentum ANS: D DIF: Difficult REF: Section 15.3 MSC: Factual TOP: 3IIIii 44. A protostar’s evolutionary “track” in the H-R diagram traces out: a. only how the protostar’s radius changes with time b. how the protostar’s luminosity, temperature, and radius change with time c. only how the protostar’s luminosity changes with time d. only how the protostar’s spectral type changes with time e. the protostar’s location in the molecular cloud ANS: B DIF: Medium REF: Section 15.3 MSC: Factual TOP: 3IIIiii

45. The Hayashi track of a low-mass protostar in the H-R diagram is a path of approximately constant: a. density b. luminosity c. age d. temperature e. radius ANS: D DIF: Easy REF: Section 15.3 MSC: Factual TOP: 3IIIiii

Figure 1 46. Use Figure 1 to complete the following statement. A high-mass protostar remains roughly constant in _________ and increases in _________ as it follows its evolutionary track. a. temperature; luminosity b. radius; temperature c. luminosity; radius d. luminosity; temperature e. radius; luminosity ANS: D DIF: Easy REF: Section 15.3 MSC: Applied TOP: 3IIIiii

47. Use Figure 1 to complete the following statement. A low-mass protostar remains roughly constant in _________ and decreases in _________ as it follows its evolutionary track. a. temperature; luminosity b. radius; temperature c. luminosity; radius d. luminosity; temperature e. radius; luminosity ANS: A DIF: Easy REF: Section 15.3 MSC: Applied TOP: 3IIIiii 48. Use Figure 1 to complete the following statement. At the start of the evolution of a protostar, the radius of a 60 M protostar is roughly _________ a 1 M main-sequence star. a. 10 times bigger than b. 100 times bigger than c. 10 times smaller than d. 100 time smaller than e. the same as ANS: B DIF: Easy REF: Section 15.3 MSC: Applied TOP: 3IIIiii 49. Use Figure 1 to complete the following statement. As a protostar contracts: a. the luminosity decreases b. the luminosity increases c. the temperature increases d. the temperature decreases e. either the luminosity decreases or the temperature increases ANS: E DIF: Easy REF: Section 15.3 MSC: Applied TOP: 3IIIiii

50. Given the low efficiency of the star formation process, the initial mass of a molecular cloud fragment that formed a 2 M star was probably close to: a. 10 M b. 50 M c. 100 M d. 500 M e. 1,000 M ANS: C DIF: Difficult REF: Section 15.4 MSC: Applied TOP: 4Ii 51. If a 1 Mprotostar starts out on the Hayashi track with a temperature of 3300 K and a luminosity of 320 L, what is its approximate radius? a. 25 R b. 55 R c. 75 R d. 105 R e. 125 R ANS: B DIF: Difficult REF: Section 15.4 MSC: Applied TOP: 4Ii 52. Which of the following stars spend the longest time on their Hayashi tracks? a. 100 M stars b. 10 M stars c. 1 M stars d. 0.1 M stars e. 0.08 M stars ANS: E DIF: Easy REF: Section 15.4 MSC: Factual TOP: 4Ivi

53. How long does it typically take for a protostar to form a 1 M star? a. 3  107 years b. 3  105 years c. 3,000 years d. 300 years e. 30 years ANS: A DIF: Medium REF: Section 15.4 MSC: Factual TOP: 4Ivi 54. The most common types of stars in our galaxy are: a. high-mass stars b. low-mass stars c. an equal mix of high- and low-mass stars d. low-mass stars near the Sun and high-mass stars far away e. We do not yet know which types of stars are most common in our galaxy.

ANS: B DIF: Easy REF: Section 15.4 MSC: Factual TOP: 4Ivi 55. Where have astronomers observed the existence of planets? a. in our Solar System b. orbiting stars other than the Sun c. orbiting stars in binary systems d. traveling on their own through the Milky Way, not orbiting a star e. all of the above ANS: E DIF: Easy REF: Section 15.5 MSC: Factual TOP: 5Iii | 5Iiii

SHORT ANSWER 1. Compare the volume of the Sun with the volume of interstellar space it occupies. Is the occupied percentage large or small? Consider the volume around the Sun to be a sphere whose radius is equal to the distance to the nearest star, which is equal to 5 light-years. (Note: the radius of the Sun is 7  105 km, and 1 light-year  9.5  1012 km.)

ANS: The volume occupied by the Sun is given by: 4/3 R3  4/3   (7  105 km)3  1.4  1018 km3.

The volume of the neighboring space is given by: 4/3  R3  4/3   (5  9.5  1012 km)3  4.5  1041 km3. Thus the percentage of space occupied by the Sun is: (1.4  1018 km3) / (4.5  1041 km3) 

100 percent  3  10 22 percent, which is very, very small!

DIF: Difficult REF: Section 15.1 MSC: Applied TOP: 1Iiii 2. What is the interstellar medium made of? Give rough percentages of each.

ANS: The ISM is 99 % gas and 1% dust.

DIF: Easy REF: Section 15.1 MSC: Applied TOP: 1Iiii | 1IIi 3. Why can we see dust in the interstellar medium better at far-infrared wavelengths than we can at optical wavelengths?

ANS: Because the dust is cold at temperatures of 10 to 100 K, it radiates more in the far-infrared than the optical.

DIF: Easy REF: Section 15.1 MSC: Conceptual TOP: 1IIv 4. How are H II regions and the hot intercloud gas heated?

ANS: Hot intercloud gas is heated by supernovae, and H II regions are ionized by O and B stars, which release lots of ultraviolet light.

DIF: Medium REF: Section 15.1 MSC: Applied TOP: 1IIIiii | 1IIIv 5. How are each of the following types of ISM detected by astronomers: hot intercloud gas, H II regions, neutral hydrogen gas, and molecular clouds.

ANS: The hot intercloud gas is detected by X-rays. HII regions are detected primarily from H-alpha emission. Neutral hydrogen gas is detected in the radio via 21-cm emission. Molecular clouds are often detected by CO (or sometimes H2) emission.

DIF: Medium REF: Section 15.1 MSC: Applied TOP: 1IIIiii | 1IIIv | 1IIIvii | 1IVv 6. At what wavelength are H II regions most clearly visible, and why do H II regions mark the regions where new stars are currently being formed?

ANS: H II regions are best seen at H-alpha with a wavelength of 656 nm, where hydrogen easily emits in the visible region of the spectrum. H II regions contain ionized gas that is heated by young, massive O and B stars. Because these stars do not live very long, H II regions trace the current, active sites of star formation.

DIF: Medium REF: Section 15.1 MSC: Conceptual TOP: 1IIIv

7. Why do H II regions mark the regions where new stars are currently being formed?

ANS: H II regions contain ionized gas that is heated by young, massive O and B stars. Because these stars do not live very long (less than a few hundred million years), H II regions trace the current, active sites of star formation.

DIF: Medium REF: Section 15.1 MSC: Conceptual TOP: 1IIIv 8. How are typical interstellar gas clouds different from the clouds that we see in the Earth’s sky?

ANS: ISM gas clouds are mostly made of hydrogen, not water vapor, they are much colder (T  100 K) and have a much lower density than clouds in the sky.

DIF: Medium REF: Section 15.1 MSC: Applied TOP: 1IVi 9. In the densest molecular clouds, the average density is approximately 300 atoms/cm3. If a cube of molecular cloud gas with this density contained 100 M of material (the amount needed to make a 1 M star), what would be the length of a side of the cube in units of AU? For reference, the mass of the Sun is 2  1030 kg, the mass of a hydrogen atom is 1.7  10 27 kg, and 1 AU  1.5  1011 m.

ANS: If the one side of the cube has length X, then the volume is v  X3, and the density is P  M/V 

M/X3. Solving for X gives us : X  (M/p)1⁄3  (100  2  1030/300  1.7  10 27 kg/cm3)1⁄3  7.3  1018 cm  7.3  1016 m(1 AU/1.5  1011 m)  5  105 AU. DIF: Difficult REF: Section 15.1 MSC: Applied TOP: 1IViii 10. Why is it possible for self-gravity to dominate pressure in molecular clouds but not in most interstellar clouds?

ANS: Molecular clouds are denser (more mass in a small volume) so they have higher self-gravity. They are also very cool, so internal pressures are very low.

DIF: Medium REF: Section 15.2 MSC: Conceptual TOP: 2Ii | 2Iii | 2Iiii 11. Some molecular clouds have so much internal pressure that it exceeds their self-gravity. What keeps them from expanding and dissipating?

ANS: The pressure of the external interstellar medium surrounding the molecular cloud will keep it from expanding and dissipating.

DIF: Medium REF: Section 15.2 MSC: Factual TOP: 2Iiii

12. Describe the general process of how the interstellar medium can create a star.

ANS: Deep inside of a cold, molecular cloud, gravity begins to collapse the gas. As the cloud collapses, it fragments, making many smaller, denser cloud cores, each destined to become a star. Slightly denser regions in the cloud collapse first, as the gravity is able to overcome opposing forces such as turbulence, angular momentum, and magnetic fields. As the cloud core collapses, angular momentum flattens the cloud into a disk with the center being the densest part. We call the center of the disk a protostar and the rest of the solar system forms from the gas in the disk. The protostar continues to collapse turning gravitational energy into thermal energy. Once the core of the protostar is able to sustain hydrogen fusion, it has become a star.

DIF: Medium REF: Section 15.2 |15.3 MSC: Applied TOP: 2Iv | 2II | 3II 13. Why do many stars form from a single molecular cloud?

ANS: Small density fluctuations in the parent cloud lead to localized areas where self-gravity is stronger, and the collapse occurs faster in these regions than in the surrounding gas. Each of these regions, or fragments, will become a separate star.

DIF: Easy REF: Section 15.2 MSC: Conceptual TOP: 2IIi 14. Why do stars form most often within molecular clouds?

ANS: Because that is where the interstellar medium is both the coldest and the densest.

DIF: Easy REF: Section 15.2 MSC: Conceptual TOP: 2IIi 15. When a 3 M protostar forms, it starts out at the top of the Hayashi track with a luminosity of 4,000 L and a temperature of 3600 K. What is its radius at this point (give the answer in units of R), and how many times larger is it at this stage compared to its radius as a main-sequence star, which is about 2.5 R? For reference, the Sun’s temperature is 5800 K. ANS: The Stefan-Boltzmann law says that , and thus the protostar’s radius is During its formation, the star contracts by a factor of 164/2.5  66.

DIF: Difficult REF: Section 15.3 MSC: Applied TOP: 3Iii 16. Why can’t very bright protostars be seen in visible light?

ANS: They are cool, so they emit mostly in the infrared. In addition, they are embedded in dust cocoons that scatter visible radiation.

DIF: Easy REF: Section 15.3 MSC: Conceptual TOP: 3Iiii 17. Why does a protostar continue to collapse as it is forming?

ANS: It continually radiates away thermal energy so collapse continues to occur until the core is hot and dense enough for nuclear reactions to occur.

DIF: Easy REF: Section 15.3 MSC: Conceptual TOP: 3IIi 18. What is the energy source that powers brown dwarf stars?

ANS: Brown dwarfs shine because they are still radiating the energy produced in their gravitational collapse. Some of the most massive brown dwarf stars may shine briefly by burning deuterium or lithium, but that only provides a small amount of their energy for a brief period of time.

DIF: Medium REF: Section 15.3 MSC: Factual TOP: 3IIiv 19. Why does the surface temperature of a low-mass protostar remain nearly constant as its core contracts?

ANS: It remains nearly constant because of the presence of H– ions in its atmosphere, which determine the opacity of the stellar atmosphere and act as a thermal regulator.

DIF: Easy REF: Section 15.3 MSC: Conceptual TOP: 3IIIii 20. How are Herbig-Haro objects related to T Tauri stars?

ANS: Herbig-Haro objects are ionized gas formed in bipolar outflows, which are strong winds that originate from the poles of protostars. They are possibly driven by the very strong magnetic fields of the protostar-disk systems. These winds clear the envelope of gas and dust that enshrouds the protostar, which at this stage in its evolution is referred to as a T Tauri star.

DIF: Difficult REF: Section 15.4 MSC: Applied TOP: 4Iiii | 4Iiv

CHAPTER 16

Evolution of Low-Mass Stars

CONCEPT MAP Sec 16.1 1. The Life of a Main-Sequence (MS) Star I. Characteristics of the Main Sequence i. Mass and chemical composition determine a star’s evolution; low-mass (3M) and high-mass stars evolve differently (TF: 1, MC: 1–2) ii. Higher core temperatures and pressures increase the rate of nuclear burning; massive stars have higher core temperatures (TF: 2, MC: 3–4, SA: 1–2) iii. MS Lifetime

M/L M 2.5  amount of fuel divided by the rate of fuel burning; lower-mass stars

have longer MS lifetimes (TF: 3, MC: 4–10, SA: 3–4) iv. The structure of a star continually changes as it uses up its fuel (TF: 4, MC: 11, 30) v. All MS stars burn H to He in their cores; the cores fill with He ash and the MS stars must evolve (MC: 12–14) Sec 16.2 2. A Star Runs out of Hydrogen and Leaves the MS I. Leaving the MS and Forming a Degenerate Helium Core i. When a star exhausts its core H, it leaves the MS; for the Sun, this will happen in about 5 billion years (TF: 5–6, MC: 15–19) ii. He core is supported by degenerate electron pressure (TF: 7, MC: 20) iii. Degenerate electron pressure: electrons cannot be squashed into an arbitrarily high density (MC: 21, SA: 5) iv. The degenerate core grows in mass but shrinks in size, and H burning takes place in a shell around

the core (TF: 8, MC: 22) v. The nuclear reaction rate in the shell increases; the core temperature increases, and the star becomes more luminous (TF: 9, MC: 23) II. Tracking the Evolution of the Star on the H-R Diagram i. As the star becomes more luminous, its outer layers grow hotter, causing the star to expand into a subgiant then a red giant (MC: 18, 24–27, SA: 6–7) ii. H ions act as a temperature regulator in red giant stars, controlling how much radiation can escape the star (MC: 24, SA: 8) iii. In a red giant, H burning occurs in a shell around the He core (MC: 13, SA: 9) Sec 16.3 3. Helium Begins to Burn in the Degenerate Core I. After the Red Giant Phase i. In the degenerate core of a red giant, electrons are packed densely but the nuclei are still free to move around (SA: 10) ii. He burning via the triple-alpha process begins at the top of the red giant branch when the core reaches T  108 K (TF: 10, MC: 28–31, SA: 11) iii. Helium flash: He burning begins violently because the pressure of degenerate matter does not respond greatly to changes in temperature iv. Horizontal branch (HB): when He burning in the core and H shell burning is stabilized (TF: 10, MC: 13, 32) v. The luminosity and temperature of a HB star will depend on its chemical composition and mass-loss history (MC: 33) Sec 16.4 4. The Low-Mass Star Enters the Last Stages of Its Evolution I. Asymptotic Giant Branch (AGB) i. When the star’s core fills with C ash, He burning will begin in a shell surrounding the core while H continues to burn in an outer shell ii. The star forms a degenerate carbon core and leaves the HB for the AGB (MC: 34) iii. The surface gravity of an AGB star is very low because its radius is large; g  GM/R2 (MC: 35–37, SA: 6, 7, 12) iv. The temperature in the AGB atmosphere is very low, and dust easily forms

v. AGB and red giant stars have a high rate of mass loss (MC: 36) vi. The Sun will lose 30 percent to 50 percent of its mass before becoming a white dwarf (TF: 11, MC: 39) II. Formation of a Planetary Nebula and a White Dwarf i. Planetary nebula: ionized gas that surrounds a dying low-mass star—a white dwarf—or its remnant (MC: 40, 41, SA: 13) ii. The outer layers of the AGB star are ejected at 20–30 km/s, but after  50,000 years, the nebula fades from sight (MC: 42–44) iii. White dwarf: the hot, small, low-luminosity former core of a low-mass star composed of H, He, or C/O and supported by degenerate electron pressure (TF: 12, MC: 45, SA: 14–15) iv. White dwarfs shine only due to their internal heat; they slowly radiate away their energy, getting cooler and less luminous (MC: 19, 46, 47) Sec 16.5 5. Binary Star Evolution I. Mass Transfer from an Evolving Star onto Its Companion i. An evolving binary star can expand to fill its Roche lobe, resulting in the mass transfer to its companion (TF: 13, MC: 48 SA: 16–17) ii. The most massive star in the binary evolves first to become a white dwarf (MC: 49) iii. An accretion disk around the white dwarf forms when the secondary star transfers matter to it (SA: 17) iv. Matter falls into a deep gravitational well when it accretes onto the white dwarf, releasing a lot of energy in UV, X-rays, etc. (MC: 50, 51) v. If matter falls quickly onto the white dwarf, runaway burning of H will produce a nova; if matter falls slowly then it can build up the white dwarf’s mass (TF: 14, SA: 18) vi. New heavy elements are synthesized in novae II. Type I Supernovae i. The Chandrasekhar limit (1.4 M) is the maximum mass for a white dwarf; if the mass of the white dwarf exceeds this, it will explode as a Type I (hydrogen deficient) supernova (MC: 52, 53, SA: 19) ii. Some supernovae may also be made in the mergers of two white dwarfs (MC: 10, 48, SA: 19) iii. The luminosity of a Type I supernova is  10 billion L (TF: 15, MC: 54, SA: 17) iv. Type I supernovae synthesize lots of iron-peak nuclei and enrich the interstellar medium with heavy

elements Sec 16.6 6. Origins: Stellar Lifetimes and Biological Evolution I. Stellar Lifetimes and Life-Bearing Planets i. Multicellular life took a long time to evolve on Earth ii. Life-bearing planets are expected to be more likely around cool, low-mass, MS stars (MC: 55, SA: 20)

TRUE/FALSE 1. A 10M star will evolve through the same phases as a 1M star.

ANS: F DIF: Easy REF: Section 16.1 MSC: Factual TOP: 1Ii 2. If a main-sequence star’s core temperature increased, fusion reaction rates would decrease because the protons would be moving faster.

ANS: F DIF: Medium REF: Section 16.1 MSC: Applied TOP: 1Iii 3. The more massive a star is, the more hydrogen it has to burn, and the longer its main-sequence lifetime lasts.

ANS: F DIF: Medium REF: Section 16.1 MSC: Applied TOP: 1Iiii 4. Stars evolve primarily because they use up the fuel in their cores.

ANS: T DIF: Easy REF: Section 16.1 MSC: Conceptual TOP: 1Iiv 5. The percentage of hydrogen in the Sun’s core today is roughly half of what it was originally.

ANS: T DIF: Difficult REF: Section 16.2 MSC: Factual TOP: 2Ii 6. The Sun will become a red giant star in about 5 billion years.

ANS: T DIF: Easy REF: Section 16.2 MSC: Applied TOP: 2Ii 7. Pressure from degenerate electrons keeps the core of a red giant star from collapsing.

ANS: T DIF: Medium REF: Section 16.2 MSC: Factual TOP: 2Iii 8. Once the core of a low-mass main-sequence star runs out of hydrogen, fusion in the star stops until the core temperature is high enough for helium fusion to begin.

ANS: F DIF: Medium REF: Section 16.2 MSC: Factual TOP: 2Iiv 9. When a star burns hydrogen in a shell, it will never produce as much energy (per unit time) as when it burns hydrogen in the core because the core has a higher temperature.

ANS: F DIF: Difficult REF: Section 16.2 MSC: Applied TOP: 2Iv

10. A low-mass star that burns helium in its core and hydrogen in a shell surrounding the core is more luminous than a similar star that burns only hydrogen in a shell around a dead core.

ANS: F DIF: Difficult REF: Section 16.3 MSC: Applied TOP: 3Iii | 3Iiv 11. Stars with masses similar to the Sun will lose approximately 30 percent of their mass before they become white dwarfs.

ANS: T DIF: Easy REF: Section 16.4 MSC: Factual TOP: 4Ivi 12. A star like the Sun will eventually become an electron degenerate white dwarf star.

ANS: T DIF: Easy REF: Section 16.4 MSC: Factual TOP: 4IIiii 13. Binary stars can evolve to become novae and supernovae because small differences in the stars’ masses can mean large differences in their main-sequence lifetimes.

ANS: T DIF: Medium REF: Section 16.5 MSC: Conceptual TOP: 5Ii 14. The Sun eventually could become a nova.

ANS: F DIF: Easy REF: Section 16.5 MSC: Factual TOP: 5Iv 15. A Type I supernova can be as luminous as 10 billion L.

ANS: T DIF: Medium REF: Section 16.5 MSC: Factual TOP: 5IIiii

MULTIPLE CHOICE 1. What factor is most important in determining a star’s position on the main sequence and subsequent evolution? a. temperature b. pressure c. mass d. radius e. color ANS: C DIF: Easy REF: Section 16.1 MSC: Factual TOP: 1Ii

2. The evolutionary cutoff between low- and high-mass stars occurs at approximately: a. 1.5 M b. 1 M c. 3 M d. 5 M e. 10 M ANS: C DIF: Medium REF: Section 16.1 MSC: Factual TOP: 1Ii 3. If a main-sequence star were gaining mass by being in an interacting binary system, what would happen to that star’s luminosity and why? a. The luminosity would increase because the star would become a nova. b. The luminosity would increase because the star’s central pressure would rise and the rate of nuclear reactions would increase. c. The luminosity would decrease because the outgoing energy has to pass through more layers in the star. d. The luminosity would decrease because high-mass stars are fainter. e. The luminosity would decrease because the star would quickly turn into a white dwarf.

ANS: B DIF: Medium REF: 16.1 MSC: Conceptual TOP: 1Iii 4. The main-sequence lifetime of a star is given by the equation: a. 

M/L

b. 

L/M

c. 

M2/L

d. 

L2/L

e. 

M/L2

ANS: A DIF: Medium REF: Section 16.1 | Math Tools 16.1 MSC: Applied TOP: 1Iii | 1Iiii

5. Which star spends the longest time as a main-sequence star? a. 0.5 M b. 1 M c. 3 M d. 6 M e. 10 M ANS: A DIF: Easy REF: Section 16.1 | Math Tools 16.1 MSC: Applied TOP: 1Iiii 6. How long will a 2 M star live as a main-sequence star? a. 12 million years b. 180 million years c. 1.8 billion years d. 12 billion years e. 18 billion years ANS: C DIF: Difficult REF: Section 16.1 | Math Tools 16.1 MSC: Applied TOP: 1Iiii 7. If the Milky Way formed stars at approximately a constant rate over the last 14 billion years, what fraction of the M-type stars that ever formed in it can still be found as main-sequence stars today? Note that M-type stars have a mass of approximately 0.5 M. a. 10 percent b. 33 percent c. 50 percent d. 75 percent e. 100 percent ANS: E DIF: Difficult REF: Section 16.1 | Math Tools 16.1 MSC: Applied TOP: 1Iiii

8. For low-mass main-sequence stars in hydrostatic equilibrium, at any interior radius there exists a balance between the downward gravitational force at that radius and: a. the pressure from a degenerate electron core b. the convective force of material rising from the interior c. the energy released from fusion reactions in the core d. the outward gas pressure from the material inside that radius e. the energy released by fusion reactions in a shell surrounding the degenerate core ANS: D DIF: Easy REF: Section 16.1 MSC: Factual TOP: 1Iiii 9. Use the following graph and the relationship 

M/L to estimate the main-sequence lifetime of a star with

a mass equal to 10 times that of the Sun. Note that the Sun’s main-sequence lifetime is about 1010 years.

a. 3 million years b. 30 million years c. 300 million years d. 3 billion years e. 30 billion years ANS: B DIF: Difficult REF: Section 16.1 MSC: Applied TOP: 1Iiii

10. You observe a 0.8 M white dwarf in a binary orbit around a main-sequence star of mass 1.4 M. Which of the following is most likely the original mass of the star that became the white dwarf? a. 0.5 M b. 1 M c. 0.8 M d. 1.4 M e. 3 M ANS: E DIF: Medium REF: 16.1 | 16.5 MSC: Conceptual TOP: 1Iiii | 5IIii

11. Using the data in the table below, identify the spectral type of a star that has a main-sequence lifetime of about 10 billion years.

a. A5 b. F5 c. K0 d. G2 e. M8 ANS: D DIF: Medium REF: Section 16.1 MSC: Applied TOP: 1Iiv 12. As a main-sequence star burns its core supply of hydrogen, what happens? a. Helium begins to fuse throughout the core. b. Helium fuses in a shell surrounding the core. c. Helium fusion takes place only at the very center of the core, where temperature and pressure are highest. d. Helium builds up as ash in the core. e. Helium builds up everywhere in the star’s interior.

ANS: D DIF: Easy REF: Section 16.1 MSC: Factual TOP: 1Iv

13. Place the following evolutionary stages in order from youngest to oldest.

a. 1, 2, 3 b. 2, 3, 1 c. 3, 2, 1 d. 3, 1, 2 e. 2, 1, 3 ANS: D DIF: Medium REF: Section 16.1 | 16.2 | 16.3 MSC: Applied TOP: 1Iv | 2IIiii | 3Iiv

14. A main-sequence star is unique because: a. hydrostatic equilibrium exists at all radii b. energy transport occurs via convection throughout much of its interior c. carbon burning occurs in its core d. it emits strong surface winds e. hydrogen burning occurs in its core ANS: E DIF: Easy REF: Section 16.1 MSC: Conceptual TOP: 1Iv 15. When a star depletes its core supply of hydrogen, _________ causes the core to collapse while increased gas _________ is exerted on the atmosphere. a. pressure; pressure b. radiation; gravity c. gravity; gravity d. gravity; pressure e. gravity, radiation ANS: D DIF: Difficult REF: Section 16.2 MSC: Applied TOP: 2Ii 16. If there were mixing processes in a main-sequence star with a radiative zone (there aren’t) that churned up all the material in the interior, we would expect that the main-sequence lifetime would be _________ because _________. a. shorter, because the star would turn into a giant faster b. shorter, because the star would burn hydrogen faster and have a higher luminosity c. longer, because helium nuclei have a higher mass than hydrogen nuclei d. shorter, because the star would never turn into a red giant e. longer, because more hydrogen would be available to burn ANS: E DIF: Medium REF: Section 16.2 MSC: Applied TOP: 2Ii

17. The Sun will likely stop being a main-sequence star in: a. 5,000 years b. 5 million years c. 50 million years d. 500 million years e. 5 billion years ANS: E DIF: Easy REF: Section 16.2 MSC: Factual TOP: 2Ii 18. The luminosity of a star depends on: a. its mass and age b. its mass c. its age d. its distance e. its mass, age, and distance ANS: A DIF: Medium REF: Section 16.2 MSC: Conceptual TOP: 2Ii | 2IIi

19. During evolutionary phase A in the figure below, the star is _________. In evolutionary phase B, it is _________.

a. expanding; expanding b. expanding; contracting c. contracting; losing mass d. contracting; contracting e. gaining mass; contracting ANS: B DIF: Medium REF: Section 16.2 | 16.4 MSC: Applied TOP: 2Ii | 4IIiv

20. Using this diagram, identify the star with the smallest radius.

a. star A b. star B c. star C d. star D e. star E ANS: E DIF: Medium REF: Section 16.2 MSC: Applied TOP: 2Iii 21. Degenerate refers to a state of matter at: a. low density b. high density c. low luminosity d. high luminosity e. high temperature ANS: B DIF: Medium REF: Section 16.2 MSC: Applied TOP: 2Iiii 22. A low-mass red giant star’s energy comes from: a. hydrogen burning to helium in its core b. helium burning to carbon in its core c. hydrogen burning to helium in a shell surrounding its core d. helium burning to carbon in a shell surrounding its core e. hydrogen burning to carbon in a shell surrounding its core ANS: C DIF: Easy REF: Section 16.2 MSC: Factual TOP: 2Iiv

23. As a red giant star evolves, hydrogen shell burning proceeds increasingly faster due to: a. rotational energy from the star’s rapid rotation b. heat released from the core’s contraction c. pressure from the contracting envelope d. release of energy stored in magnetic fields e. energy from the fusion of heavier elements ANS: B DIF: Difficult REF: Section 16.2 MSC: Factual TOP: 2Iv 24. When a spectral-type G2 star like the Sun leaves the main sequence: a. its luminosity and surface temperature both stay the same b. its luminosity and surface temperature both decrease c. its luminosity increases and its surface temperature decreases d. its luminosity and surface temperature both increase e. its luminosity decreases and its surface temperature increases ANS: C DIF: Medium REF: Section 16.2 MSC: Applied TOP: 2IIi 25. What is the radius of a red giant star that has a luminosity of 300 L and a temperature of 4000 K? (Note that the temperature of the Sun is 5800 K.) a. 8 R b. 13 R c. 25 R d. 36 R e. 65 R ANS: D DIF: Difficult REF: Section 16.2 MSC: Applied TOP: 2IIi

26. As a subgiant star becomes a red giant, its luminosity increases while its temperature remains approximately constant. What does this mean? a. The radius is decreasing. b. The radius is increasing. c. The star is getting hotter. d. The star is losing mass. e. The star is rotating more slowly.

ANS: B DIF: Medium REF: Section 16.2 MSC: Applied TOP: 2IIi 27. As a low-mass main-sequence star runs out of fuel in its core, it grows more luminous. How is this possible? a. It explodes. b. It begins to fuse helium in the core. c. The core expands as it runs out of fuel. d. The core shrinks, bringing more hydrogen fuel into the burning region. e. Convection takes place throughout the interior, bringing more fuel to the core.

ANS: D DIF: Medium REF: Section 16.2 MSC: Applied TOP: 2IIi 28. A low-mass main-sequence star’s climb up the red giant branch is halted by: a. the end of hydrogen shell burning b. the beginning of helium fusion in the core c. electron-degeneracy pressure in the core d. instabilities in the star’s expanding outer layers e. an explosion that destroys the star ANS: B DIF: Easy REF: Section 16.3 MSC: Factual TOP: 3Iii 29. Helium burns in the core of a horizontal branch star via _________ and produces _________. a. the triple-alpha reaction; carbon b. the proton-proton chain; lithium c. the triple-alpha reaction; oxygen d. the proton-proton chain; iron e. the proton-proton chain; calcium ANS: A DIF: Easy REF: Section 16.3 MSC: Factual TOP: 3Iii

30. When helium fusion begins in the core of a red giant star, the situation quickly gets out of control because electron-degeneracy pressure does not respond to changes in: a. luminosity b. density c. gravity d. temperature e. magnetic field strength ANS: D DIF: Difficult REF: Section 16.3 MSC: Applied TOP: 3Iii 31. What is the name of the nuclear reaction illustrated here?

a. the proton-proton chain b. the CNO cycle c. beta decay d. the triple-alpha process e. the alpha-beta reaction ANS: D DIF: Easy REF: 16.3 MSC: Factual TOP: 3Iii 32. During which phase of the evolution of a low-mass star does it have two separate regions of nuclear burning occurring in its interior? a. pre–main sequence b. main sequence c. red giant d. horizontal branch e. white dwarf ANS: D DIF: Medium REF: Section 16.3 MSC: Factual TOP: 3Iiv

33. A star’s surface temperature during the horizontal branch phase is determined primarily by its: a. luminosity b. mass and chemical composition c. magnetic field strength d. rotation rate e. radius ANS: B DIF: Difficult REF: Section 16.3 MSC: Applied TOP: 3Iv 34. A particular asymptotic giant branch star has approximately the same mass as the Sun but 100 times its radius. Compared to the Sun, what is the escape velocity from that star? a. 0.01 times that of the Sun b. 0.1 times that of the Sun c. the same as that of the Sun d. 10 times that of the Sun e. 100 times that of the Sun ANS: B DIF: Difficult REF: Section 16.4 MSC: Applied TOP: 4Iii 35. Asymptotic giant-branch stars have _________ luminosities, _________ radii, and _________ escape velocities. a. large; large; large b. large; small; large c. large; large; small d. small; large; small e. small; small; large ANS: C DIF: Easy REF: Section 16.4 MSC: Factual TOP: 4Iiii 36. Asymptotic giant branch stars have high-mass loss rates because: a. they are rotating quickly b. they have weak magnetic fields c. they have strong magnetic fields d. they have low surface gravity e. they have high surface temperatures ANS: D DIF: Medium REF: Section 16.4 MSC: Conceptual TOP: 4Iiii

37. What is the escape velocity from the surface of a 1 M AGB star that has a radius of 100 R? a. 60 km/s b. 120 km/s c. 240 km/s d. 620 km/s e. 800 km/s ANS: A DIF: Difficult REF: Section 16.4 | Math Tools 16.2 MSC: Applied TOP: 4Iiii 38. When a low-mass star becomes an AGB star and has a temperature of 3300 K, at what wavelength will it shine the brightest? a. 650 nm, red visible b. 880 nm, infrared c. 2.5 m, infrared d. 1 mm, microwave e. 10 m, radio ANS: B DIF: Medium REF: Section 16.4 MSC: Applied TOP: 4Iiv 39. A star like the Sun will lose about _________ of its mass before it evolves to become a white dwarf. a. 3 percent b. 30 percent c. 60 percent d. 75 percent e. 90 percent ANS: B DIF: Easy REF: Section 16.4 MSC: Factual TOP: 4Ivi 40. What is a planetary nebula? a. a planet surrounded by a glowing shell of gas b. the disk of gas and dust surrounding a young star that will soon form a star system c. the ejected envelope of a giant star surrounding the remnant of a star d. a type of young, medium-mass star e. leftover gas from a supernova explosion ANS: C DIF: Easy REF: Section 16.4 MSC: Factual TOP: 4IIi

41. What ionizes the gas in a planetary nebula and makes it visible? a. X-ray photons emitted by a pulsar b. ultraviolet photons emitted by a white dwarf c. the shock wave from a supernova d. hydrogen burning in the nebular gas e. infrared photons from a nova explosion ANS: B DIF: Medium REF: Section 16.4 MSC: Applied TOP: 4IIi 42. What would you need to measure about a planetary nebula to determine how long ago its parent star died? a. the mass of the white dwarf b. the mass and radius of the white dwarf c. the nebula’s temperature and radius d. the nebula’s radius and expansion velocity e. the composition of the gas in the nebula ANS: D DIF: Easy REF: Section 16.4 MSC: Applied TOP: 4IIii 43. The Ring Nebula is a planetary nebula that currently has a radius of 1.2  1013 km and an expansion velocity of 250 km/s. Approximately how long ago did its parent star die and eject its outer layers? a. 1,500 years ago b. 3,200 years ago c. 5,400 years ago d. 8,000 years ago e. 28,000 years ago ANS: A DIF: Medium REF: Section 16.4 MSC: Applied TOP: 4IIii 44. The gas in a planetary nebula is composed of: a. primarily hydrogen from the surrounding interstellar medium b. primarily hydrogen from the post-asymptotic giant branch star c. hydrogen and heavier elements like helium and carbon processed in the core of the post-asymptotic giant branch star d. primarily helium from the post-asymptotic giant branch star e. carbon and helium from the nuclear reactions that took place on the horizontal branch ANS: C DIF: Medium REF: Section 16.4 MSC: Applied TOP: 4IIii

45. A white dwarf with a temperature of 30,000 K would shine brightest at what wavelength? a. 4 nm, X-rays b. 100 nm, ultraviolet c. 400 nm, blue visible d. 1 m, infrared e. 10 m, infrared ANS: B DIF: Medium REF: Section 16.4 MSC: Applied TOP: 4IIiii 46. As a white dwarf star gradually cools, its radius stays approximately constant. What is happening to the white dwarf’s luminosity? a. It stays the same. b. It increases. c. It increases then decreases periodically. d. It decreases. e. You can’t tell from the information given.

ANS: D DIF: Medium REF: Section 16.4 MSC: Conceptual TOP: 4IIiv 47. In a white dwarf, what is the source of pressure that halts its contraction as it cools? a. thermal pressure of the extremely hot gas b. electrons packed so closely that they become incompressible c. neutrons that resist being pressed further together d. carbon nuclei that repel each other strongly because they each contain six protons e. rapid rotation ANS: B DIF: Medium REF: Section 16.4 MSC: Factual TOP: 4IIiv 48. What are two ways that Type I supernovae can be produced? a. mass transfer and stellar mergers b. helium flash and stellar mergers c. mass transfer and helium flash d. helium burning and mass transfer e. carbon burning and mass transfer ANS: A DIF: Easy REF: Section 16.5 MSC: Conceptual TOP: 5 Ii | 5IIii

49. One star in a binary will almost always become a red giant before the other because: a. one star is always larger in radius than the other b. binaries always have one star twice as massive as the other c. small differences in main-sequence masses yield large differences in main-sequence ages d. the more massive binary star always gets more mass from the less massive binary star when both are main-sequence stars e. one star always spins faster than the other ANS: C DIF: Easy REF: Section 16.5 MSC: Conceptual TOP: 5Iii 50. A nova is the result of which explosive situation? a. mass transfer onto a white dwarf b. helium burning in a degenerate stellar core c. a white dwarf which exceeds the Chandrasekhar limit d. the collision of members of a binary system e. runaway nuclear reactions in the core ANS: A DIF: Medium REF: Section 16.5 MSC: Applied TOP: 5Iiv 51. A 1M star in a binary system could create the following chemical element and eject it into the interstellar medium: a. carbon b. helium c. iron d. gold e. all of the above ANS: E DIF: Medium REF: Section 16.5 MSC: Applied TOP: 5Iiv

52. A Type I supernova occurs when a white dwarf exceeds a mass of: a. 0.8 M b. 1.4 M c. 2.3 M d. 5.4 M e. 10 M ANS: B DIF: Easy REF: Section 16.5 MSC: Factual TOP: 5IIi 53. If an 0.8 M white dwarf could accrete matter from a binary companion at a rate of 10 9 M/yr, how long would it take before it exploded as a Type I supernova? a. 600 thousand years b. 20 million years c. 200 million years d. 600 million years e. 1 billion years ANS: D DIF: Medium REF: Section 16.5 MSC: Applied TOP: 5IIi 54. A Type I supernova has a luminosity of approximately: a. 10 thousand L b. 10 million L c. 1 billion L d. 10 billion L e. 10 trillion L ANS: D DIF: Easy REF: Section 16.5 MSC: Factual TOP: 5IIiii

55. Which of the stellar spectral types shown below would be the least likely to have planets with life? a. B0 b. G2 c. K0 d. M0 e. M5 ANS: A DIF: Easy REF: Section 16.6 MSC: Applied TOP: 6Iii

SHORT ANSWER 1. In what two ways does temperature affect the rate of nuclear reactions?

ANS: Higher temperature means atomic nuclei are moving at faster speeds. This results in (1) increased likelihood of collisions, and (2) higher probabilities that they will collide with enough energy to overcome their mutual electric repulsion and fuse.

DIF: Medium REF: Section 16.1 MSC: Applied TOP: 1Iii 2. Why does the core of a main-sequence star have to be hotter to burn helium into carbon than hydrogen into helium?

ANS: A helium nucleus has two protons and a net charge of 2, while the hydrogen nucleus has only one proton and a charge of 1. Because the electrostatic repulsion between a nucleus and proton is proportional to the charges of each, the helium nucleus will repel a proton with two times more force than a hydrogen nucleus. Therefore the helium nuclei must be moving with a higher speed to overcome this repulsion and make the first step in the triple-alpha reactions. Thus, the core of the star must be hotter to burn helium.

DIF: Easy REF: Section 16.1 MSC: Applied TOP: 1Iii 3. Calculate the main-sequence lifetimes of the following stars of different spectral types: B0 (18 M), B5 (6

M), A5 (2 M), F5 (1.3 M), and M0 (0.5 M). What trend do you notice in your results?

ANS: Main-sequence lifetime: MS  1010 yr  (M/M) 2.5 B0: MS  1010 yr  18 2.5  7.3  106 yr  7.3 million yr B5: MS  1010 yr  6 2.5  1.1  108 yr  110 million yr A5: MS  1010 yr  2 2.5  1.8  109 yr  1.8 billion yr F5: MS  1010 yr  1.3 2.5  5.2  109 yr  5.2 billion yr M0: MS  1010 yr  0.5 2.5  5.7  1010 yr  57 billion yr The higher the mass, the shorter the star’s main-sequence lifetime

is.

DIF: Difficult REF: Section 16.1 | Math Tools 16.1 MSC: Applied TOP: 1Iiii 4. How many times longer does a 2 M main-sequence star live compared to a 10 M main-sequence star?

ANS: Main-sequence lifetime: MS  1010 yr  (M/M) 2.5 2M: MS  1010 yr  2 2.5  1.8  109 yr  1.8 billion yr. 10M: MS  1010 yr  10 2.5  3.1  107 yr  0.031 billion yr. Therefore, the 2M star lives 1.8/0.031  58 times longer than the 10M star.

DIF: Difficult REF: Section 16.1 | Math Tools 16.1 MSC: Applied TOP: 1Iiii 5. Explain the two different forms of pressure that support the core of a low-mass main-sequence star and the core of a low-mass red giant star.

ANS: The core of a low-mass main-sequence star is supported by the pressure of the gas via the ideal gas law, and the core of a low-mass red giant star is supported by the pressure of degenerate electrons because quantum mechanics limits the density to which you can squeeze particles.

DIF: Easy REF: Section 16.2 MSC: Factual TOP: 2Iiii 6. Consider a red giant star with a luminosity of 200 L and a radius of 50 R. Using the luminosity– temperature-radius relationship (L

R2T4), calculate how hot this star’s surface temperature will be

compared to the Sun, whose temperature is 5,800 K. ANS: L

R2T4 (L/L)  (R/R)2 (T/T)4 T  (200/502)1⁄4 T  0.53 T  3070 K.

DIF: Difficult REF: Section 16.2 | 16.4 MSC: Applied TOP: 2IIi | 4Iiii 7. Consider a 1 M star’s journey up the red giant branch. Its luminosity will change from 10 L to nearly 1,000 L. How will its temperature and radius change as the star ascends? (Recall that L

R2T4.)

ANS: The red giant star’s temperature will essentially remain constant at a value 1000 K less than it was on the main sequence due to the formation of H ions in the stellar atmosphere. Thus, if its luminosity changes by a factor of 1,000/10  100, then the radius will change by 1001⁄2  10.

DIF: Difficult REF: Section 16.2 | 16.4 MSC: Applied TOP: 2IIi | 4Iiii 8. What stops a red giant from cooling to continuously lower temperatures, and why?

ANS: The H ion: It acts as a thermostat by regulating how much radiation can escape from the star.

When the surface temperature drops by 1000 K compared to its main-sequence value, lots of H ions form in the atmosphere; the increase in H increases the opacity of the star and traps radiation in the stellar atmosphere, preventing further cooling.

DIF: Difficult REF: Section 16.2 MSC: Conceptual TOP: 2IIii 9. Describe the structure of a red giant star just before the helium flash takes place. How does this compare to the structure of a horizontal-branch star?

ANS: A red giant has a nonburning helium core surrounded by a hydrogen-burning envelope. The radius of the star and its luminosity are both very large, while the surface temperature is cool compared to the temperature the star had on the main sequence. After the helium flash, the star settles down to a state that is hotter and smaller. It has a stable, helium-burning core surrounded by a non-burning hydrogen envelope.

DIF: Medium REF: Section 16.2 MSC: Conceptual TOP: 2IIiii 10. How can the core of a star be degenerate with respect to the electrons but nondegenerate with respect to the nuclei?

ANS: (1) Quantum mechanics says electrons take up more space than nuclei when packed together and thus become degenerate sooner than nuclei. (2) Quantum mechanics allows electrons and nuclei to occupy the same physical space so nuclei essentially don’t notice the jam-packed electrons.

DIF: Difficult REF: Section 16.3 MSC: Conceptual TOP: 3Ii 11. Explain what the triple-alpha process is and when it takes place in evolving stars.

ANS: The triple-alpha process is the way that carbon nuclei are formed from the merger of three helium nuclei (helium nuclei are known as “alpha” particles). First, two alpha particles merge to form a nucleus of beryllium (Be). This in turn fuses with a third alpha particle to form a carbon nucleus. This process takes place on the horizontal branch and the asymptotic giant branch.

DIF: Medium REF: Section 16.3 MSC: Conceptual TOP: 3Iii 12. When the Sun becomes an AGB star, its radius will be approximately 100 R. If its mass at this point will be approximately the same as it is now, how will its surface gravity as an AGB star compare to its present surface gravity as a main-sequence star? Note that g  GM/R2.

ANS: Surface gravity: g  GM/R2. gMS / gAGB  (RAGB/RMS)2  (100 R/1 R)2  104  10,000. Thus, when the Sun is an AGB star, its surface gravity will be 10,000 times less than its surface gravity when it

is a main-sequence star.

DIF: Medium REF: Section 16.4 MSC: Applied TOP: 4Iiii 13. What is the shortest phase of evolution for a one solar mass star that we can visibly see?

ANS: The planetary nebula phase is the shortest; the outer layers of the star are ejected in a mere blink of an eye compared to the length of any other stellar phase.

DIF: Medium REF: Section 16.4 MSC: Factual TOP: 4IIi 14. What is “degenerate” in the degenerate core of a white dwarf?

ANS: Only the electrons are degenerate in the core, not the nuclei.

DIF: Medium REF: Section 16.4 MSC: Factual TOP: 4IIiii 15. A white dwarf star has a luminosity equal to 10 4 L and a temperature twice that of the Sun (i.e., T  10,600 K). What is the radius of the white dwarf, expressed as a ratio to the solar radius?

ANS: L

R2T4. (L/L)  (R/R)2(T/T)4.

.

DIF: Difficult REF: Section 16.4 MSC: Applied TOP: 4IIiii 16. Explain the significance of Roche lobes in a binary system.

ANS: The Roche lobe is an imaginary line of equal gravity between two stars in a binary system (that is, a point where both stars exert equal gravitational forces on material). Once one star has swelled to fill its Roche lobe, its outer layers can transfer over to the companion. This is known as mass transfer.

DIF: Easy REF: Section 16.5 MSC: Conceptual TOP: 5Ii 17. What types of chemical elements can low-mass stars contribute to the enrichment of the interstellar medium and how are they produced?

ANS: Low-mass stars can enrich the interstellar medium by ejecting helium and carbon when they become planetary nebulae. Also, their white dwarf remnants can explode as novae or Type I supernovae and thus enrich the interstellar medium in even heavier elements.

DIF: Medium REF: Section 16.5 | Connections 16.1 MSC: Applied TOP: 5Ii | 5IIiii 18. Why are novae thought to reoccur repeatedly?

ANS: The explosive hydrogen burning on the surface of a white dwarf does not destroy the star or signif-

icantly affect the binary system. This means that mass will continue to transfer through the Roche lobe from the giant companion star to the white dwarf after the nova. Continued mass transfer leads to a new build-up of hydrogen-rich material on the surface of the white dwarf and thus, eventually, another nova.

DIF: Medium REF: Section 16.5 MSC: Conceptual TOP: 5Iv 19. What types of stars cause Type I supernovae, and what makes them explode?

ANS: Type I supernovae are formed in one or two ways. A white dwarf may be in a binary system with a red giant or AGB star. Mass shed from the cool, luminous star would form an accretion disk around the white dwarf, which would gradually add to the white dwarf’s mass. If the mass of the white dwarf grows past the Chandrasekhar mass, the white dwarf will collapse and explode from runaway carbon burning.

An alternative way of producing supernovae would be the merger of two white dwarf stars if the total of their masses is greater than 1.4 M.

DIF: Medium REF: Section 16.5 MSC: Conceptual TOP: 5IIi | 5IIii 20. Explain which types of main-sequence stars would be more likely to have planets with complex life.

ANS: Low-mass main-sequence stars have longer lifetimes because, even though they have less fuel, they burn the fuel much more slowly than do higher-mass stars. On Earth, life took a long time to evolve.

Although single-celled life got started relatively quickly, it was only in the last 600 million years that multicellular life was abundant. If life on other planets evolved similarly, then we should not expect to find life on planets around massive main-sequence stars, since they would evolve and die faster than it took multicellular life to appear on Earth. We should then expect to find life-bearing planets around stars with longer lifetimes, which are low-mass main-sequence stars.

DIF: Medium REF: Section 16.6 MSC: Conceptual TOP: 6Iii

CHAPTER 17

Evolution of High-Mass Stars

CONCEPT MAP Sec 17.1 1. High-Mass Stars Follow Their Own Path I. Nuclear Burning in High-Mass Stars i. The evolution of high-mass stars is driven by mass, which sets the core density and the types of nuclear reactions in the core; hydrogen burning takes place via the CNO cycle (TF: 1–3, MC: 1–5, SA: 1) ii. Convection is the dominant was energy is transported in the core (SA: 2) iii. When the core fills with He ash, massive stars respond by compressing the core to T  108 K and igniting the triple-alpha reaction (He burning to C) without the core becoming degenerate, and move to the right on the H-R diagram (TF: 4, MC: 6, SA: 3) iv. More massive elements, all the way up to Si burning to Fe, are fused in the cores of massive stars in a structure like that of an onion (MC: 7–8) II. The Instability Strip i. Stars in the instability strip pulsate in radius and vary in luminosity with a period that depends on the stars’ luminosity (MC: 9–10, SA: 4) ii. Cepheid variables: massive stars in the instability strip with pulsation periods from 1 to 100 days (TF: 5, MC: 11) iii. RR Lyrae stars: low-mass stars in the horizontal branch stage of evolution that pulse with periods of about 1 day (TF: 5, MC: 11, SA: 5) iv. Cepheid and RR Lyrae variable stars’ periods tell us their luminosities, and thus we can derive the stars’ distances (TF: 6, MC: 12, SA: 5–6) III. Stellar Winds and Mass Loss

i. Massive stars have high-velocity winds and large mass loss, with more massive stars losing a higher fraction of their mass (MC: 13, SA: 7, 8) ii. Winds are driven by radiation pressure and can be episodic (MC: 14) Sec 17.2 2. High-Mass Stars Go Out with a Bang I. Nuclear Burning i. Iron is the most massive element that can fuse to produce energy; fusion of more massive elements requires energy (TF: 6–7, MC: 15–17, SA: 9) ii. Binding energy: the amount of energy needed to break up a nucleus (SA: 10) II. The Final Days in the Life of a Massive Star i. Fusing 1 kg of H  He makes more energy than fusing heavier elements, so it’s the most efficient element for burning (MC: 18) ii. Neutrino cooling: when C burning begins, most of the energy is coming out as neutrinos instead of heating the star (MC: 19, SA: 11) iii. Burning of heavier elements supports the star for less and less time (MC: 20–24) iv. The neutrino flux increases as successively heavier elements are ignited, robbing the core of its energy source (TF: 7, MC: 25) III. The Core Collapses and the Star Explodes as a Type II Supernova i. Gravity wins in the end; quantum mechanical pressure from degenerate matter is not enough to slow the collapsing cores of massive stars ii. As the star collapses, gamma rays begin to break up the iron nuclei, robbing the star of more energy iii. The core is so dense that protons and electrons combine to form neutrons, because neutral material can be packed into a tighter volume, which also robs the star of energy (MC: 26) iv. Main-sequence stars with M  8 M explode as a Type II supernovae, ejecting material at speeds of ~30,000 km/s (TF: 8, 9, MC: 27) Sec 17.3 3. The Spectacle and Legacy of Supernovae I. Supernovae Energetics i. Type II supernovae (SNe) can be as bright as 100 billion L, but 100 times that energy comes out as kinetic energy in the ejected gas and 10,000 times that amount is released as neutrinos (MC:

28–29, SA: 12) II. The Energetic and Chemical Legacy of Supernovae i. Gas heated in the supernovae explosions emits X-rays ii. Supernovae shock waves stir up the interstellar medium and may trigger new star formation (MC: 30) iii. Most of the elements heavier than Fe in the universe are made in Type I and Type II supernovae by capture of free neutrons in the explosion (TF: 10, MC: 31, SA: 13) III. Neutron Stars i. Neutron star: remnant left behind after a Type II supernova (MC: 32) ii. Masses: 1.4 M  M  3 M. Radii are about 10 km (TF: 11, MC: 33–37) iii. Pressure is provided by degenerate neutrons; more massive neutron stars are actually smaller in radius (MC: 38, SA: 14) iv. If a neutron star is part of a binary system, the secondary can fill its Roche lobe and dump material onto the neutron star; X-rays will be emitted from the spot in the accretion disk where material hits it (TF: 12, MC: 39) IV. Pulsars i. Pulsars: neutron stars that swing a beam of relativistic jets past your line of sight and pulse on and off in the radio with very short periods (TF: 13, MC: 40, SA: 15) ii. Neutron stars have strong magnetic fields, and charged particles in the jets must spiral along the magnetic field lines and produce synchrotron emission at radio wavelengths (MC: 41) iii. Crab Nebula is a nearby example of a Type II supernovae producing a pulsar (MC: 42–44) Sec 17.4 4. Star Clusters are Snapshots of Stellar Evolution I. Evolution of the H-R Diagram as a Star Cluster Ages i. Star cluster: a group of stars with different masses that all formed from the same gas cloud and thus the stars have the same age, chemical composition, and distance (SA: 16) ii. Massive stars die first and disappear from the H-R diagram; over time the most massive star at the main-sequence turnoff gets lower in mass (TF: 14, 15, MC: 45–50, SA: 17–18) iii. Studies of star-cluster H-R diagrams for clusters with different ages and chemical compositions verify our models of stellar structure and stellar evolution (SA: 19) iv. The number of stars in various stages of the H-R diagram is proportional to how much time the stars spend in that phase (MC: 46–50)

v. From matching the shape of the H-R diagram to a stellar isochrone, one can determine the age, reddening, and distance of a star cluster (MC; 46–50) vi. The color of the integrated light from the cluster is a function of age; bluer clusters are younger because they have bluer, more luminous stars than older clusters (MC: 51, SA: 20) II. Stellar Populations i. Stellar Populations: a group of stars with a common set of ages and chemical compositions inside a galaxy ii. Younger stellar populations are brighter and bluer than older ones (MC: 51) iii. The chemical abundances of massive elements in a stellar population contain information on the cumulative history of its formation (MC: 52) iv. Young stellar populations typically have higher chemical abundances than older stars (MC: 52, 53) Sec 17.5 5. Origins: Seeding the Universe with Heavy Elements I. Understanding the Composition of the Stars i. The composition of stars reflects the composition of the gas out of which they were formed (MC: 52, 53) ii. The elements in stars were mostly formed by previous generations of stars that died as planetary nebula or supernovae (MC: 54, 55)

TRUE/FALSE 1. The evolutionary differences between high- and low-mass stars can be attributed to differences in the amount of mass each star possesses.

ANS: T DIF: Easy REF: Section 17.1 MSC: Conceptual TOP: 1Ii 2. The nuclear reaction that produces most of the energy for massive main-sequence stars is called the CNO cycle.

ANS: T DIF: Easy REF: Section 17.1 MSC: Factual TOP: 1Ii 3. Fusion reactions that create chemical elements heavier than oxygen require energy input; thus, these reactions cannot provide a star with power.

ANS: F DIF: Medium REF: Section 17.1 MSC: Factual TOP: 1Ii

4. High-mass stars differ from low-mass stars in that they burn helium to carbon when on the main sequence.

ANS: F DIF: Easy REF: Section 17.1 MSC: Factual TOP: 1Iiii 5. Pulsating variable stars are more commonly known as pulsars.

ANS: F DIF: Medium REF: Section 17.1 MSC: Factual TOP: 1IIii | 1IIiii 6. Cepheid variable stars are important because we can use them to determine the distance to any stellar group that contains some of these stars.

ANS: T DIF: Medium REF: Section 17.1 MSC: Applied TOP: 1IIiv 7. The production of large numbers of neutrons in nuclear reactions at the core of a massive star helps rob the core of energy and speeds its eventual collapse.

ANS: T DIF: Difficult REF: Section 17.2 MSC: Factual TOP: 2IIiv 8. An 8M star will eventually die as a Type I supernova.

ANS: F DIF: Easy REF: Section 17.2 MSC: Conceptual TOP: 2IIIiv 9. Type I and Type II supernovae are approximately equal in luminosity.

ANS: T DIF: Difficult REF: Section 17.3 MSC: Factual TOP: 2IIIiv 10. Most of the uranium (atomic mass  238) found on the Earth was formed in Type II supernova explosions.

ANS: T DIF: Medium REF: Section 17.3 MSC: Factual TOP: 3IIiii 11. The densest state of matter found in nature occurs inside a white dwarf star.

ANS: F DIF: Easy REF: Section 17.3 MSC: Factual TOP: 3IIIii 12. Neutron stars are sometimes found in binary systems, where matter overflowing from a companion star and accreting onto the neutron star will produce X-rays and other energetic phenomena.

ANS: T DIF: Medium REF: Section 17.3 MSC: Conceptual TOP: 3IIIiv 13. Every pulsar is a neutron star, but not every neutron star is a pulsar.

ANS: T DIF: Medium REF: Section 17.3 MSC: Applied TOP: 3IVi 14. We can determine the age of a star cluster by measuring the color of the reddest red giant stars in the cluster.

ANS: F DIF: Difficult REF: Section 17.4 MSC: Applied TOP: 4Iii

15. We can determine the age of a star cluster because stars of different masses go through their lives at different rates.

ANS: T DIF: Medium REF: Section 17.4 MSC: Applied TOP: 4Iii

MULTIPLE CHOICE 1. The CNO cycle is the dominant mechanism for hydrogen fusion only in high-mass main-sequence stars because of the greater __________ their cores. a. concentration of heavy elements like carbon in b. turbulence in c. abundance of hydrogen in d. temperature of e. rotation speed of ANS: D DIF: Medium REF: Section 17.1 MSC: Factual TOP: 1Ii 2. The principal means by which high-mass stars generate energy on the main sequence is called: a. the proton-proton chain b. the carbon-carbon reaction c. the triple-alpha process d. the CNO cycle e. neutrino cooling ANS: D DIF: Easy REF: Section 17.1 MSC: Factual TOP: 1Ii 3. A main-sequence star of 25 solar masses has about 12.5 times the luminosity of a 10 solar mass star. This is because: a. the more massive star has a hotter core, and therefore nuclear burning proceeds more rapidly b. massive stars have more convection in their cores, which heats up the material there c. the massive star has more hydrogen to burn d. the massive star has more carbon, which speeds up the CNO cycle e. the massive star is probably younger than the 10 solar mass star ANS: A DIF: Medium REF: Section 17.1 MSC: Applied TOP: 1Ii

4. In the CNO cycle, carbon is used a catalyst for the fusion of hydrogen to helium. This means that: a. three helium nuclei fuse to form carbon b. carbon facilitates the reaction but is not consumed in it c. carbon boosts the energy from the reaction, which is why massive stars are luminous d. carbon breaks apart into three helium nuclei e. the reaction produces carbon nuclei in addition to helium ANS: B DIF: Medium REF: Section 17.1 MSC: Factual TOP: 1Ii 5. What is one way that massive stars differ from low-mass stars? a. They are found at cooler temperatures on the main sequence. b. They fuse carbon through silicon without leaving the main sequence. c. Convection is important in their cores, which determines when the stars leave the main sequence. d. They turn into red giants explosively. e. Most of their fusion energy is emitted as neutrinos and not visible light.

ANS: C DIF: Medium REF: Section 17.1 MSC: Factual TOP: 1Ii 6. As a high-mass main-sequence star evolves off the main sequence, it follows a: __________ on the H-R diagram. a. nearly vertical path b. path of constant radius c. roughly horizontal path d. path of declining luminosity e. path of increasing temperature ANS: C DIF: Easy REF: Section 17.1 MSC: Factual TOP: 1Iiii 7. How does nucleosynthesis depend on the mass of the star? a. With increasing mass, heavier and heavier elements are formed throughout their interiors. b. With increasing mass, heavier and heavier elements are formed in their cores. c. With increasing mass, elements between helium and gold are formed in the cores. d. With increasing mass, elements between helium and carbon are formed in the cores. e. All stars more massive than 8 solar masses create elements from helium through uranium in their cores.

ANS: B DIF: Medium REF: Section 17.1 MSC: Factual TOP: 1Iiv

8. Massive stars synthesize chemical elements going from helium up to iron: a. throughout the interior b. primarily at the surface c. only in the core of the star d. along the equator of the star e. in a deep convection zone in the interior of the star ANS: C DIF: Medium REF: Section 17.1 MSC: Applied TOP: 1Iiv 9. The luminosity of a Cepheid star varies in time because: a. the entire star pulsates from its core to its surface b. the outer envelope of the star pulsates in radius c. the star rotates too quickly d. the star is too massive to be stable e. the star undergoes large surface temperature fluctuations ANS: B DIF: Difficult REF: Section 17.1 MSC: Conceptual TOP: 1IIi 10. What mechanism drives the pulsations in Cepheid variables? a. changes in the rate of core nuclear reactions b. the formation and destruction of sunspots c. the ionization and recombination of hydrogen d. the ionization and recombination of helium e. large rates of mass loss ANS: D DIF: Medium REF: Section 17.1 MSC: Factual TOP: 1IIi 11. The main difference between Cepheid stars and RR Lyrae stars is: a. their masses b. that Cepheids form at much greater distances from Earth c. that RR Lyrae were discovered much earlier than Cepheids d. their pulsation mechanisms e. that Cepheids obey a period-luminosity relation, but RR Lyraes do not ANS: A DIF: Medium REF: Section 17.1 MSC: Factual TOP: 1IIii | 1IIiii

12. If you measure the average brightness and pulsation period of a Cepheid variable star, you can also determine its: a. age b. rotation period c. distance d. mass e. composition ANS: C DIF: Easy REF: Section 17.1 MSC: Applied TOP: 1IIiv 13. If a 25M main-sequence star loses mass at a rate of 10 6 M/yr, then how much mass will it lose in its lifetime of 3 million years? a. 3M b. 5M c. 8M d. 10M e. 12M ANS: A DIF: Easy REF: Section 17.1 MSC: Applied TOP: 1IIIi 14. What causes massive stars to expel their outer layers? a. radiation pressure b. high magnetic fields c. rapid rotation d. carbon fusion e. emission of neutrinos ANS: A DIF: Easy REF: Section 17.1 MSC: Factual TOP: 1IIIii 15. An iron core cannot support a massive main-sequence star because: a. iron has low nuclear binding energy b. iron is not present in stellar interiors c. iron supplies too much pressure d. iron fusion only occurs in a degenerate core e. iron cannot fuse to make heavier nuclei and produce energy ANS: E DIF: Easy REF: Section 17.2 MSC: Factual TOP: 2Ii

16. During the main-sequence evolution of a massive star, increasingly heavier elements are fused in the core, giving the core support for: a. longer and longer times b. shorter and shorter times c. an approximately equal amount of time d. approximately 10,000 years e. only a few days ANS: B DIF: Medium REF: Section 17.2 MSC: Factual TOP: 2Ii 17. Each kilogram of hydrogen that fuses into helium releases about 6  1014 Joules of energy. How many tons of hydrogen are fused each second to power a massive main-sequence star with a luminosity of 100 L? Note that 1 L  4  1026 Joule/second and 1 ton  103 kg. a. 2  106 tons b. 7  107 tons c. 2  109 tons d. 7  1010 tons e. 2  1011 tons ANS: D DIF: Difficult REF: Section 17.2 MSC: Applied TOP: 2Ii 18. The nuclear reaction that releases the most energy per kilogram is: a. silicon fusing to iron b. oxygen fusing to silicon c. carbon fusing to magnesium d. helium fusing to carbon e. hydrogen fusing to helium ANS: E DIF: Easy REF: Section 17.2 MSC: Factual TOP: 2IIi

19. Why does the luminosity of a high-mass star remain nearly constant as the star burns heavy elements in its core, even though it is producing millions of times more energy per second than it did on the main sequence? a. Most of the energy is trapped in the core, increasing the core’s temperature. b. All of the extra energy goes into heating the shells of fusion surrounding the core. c. Most of the energy is absorbed by the outer layers of the star, increasing the star’s radius but leaving its luminosity unchanged. d. Most of the energy is carried out of the star by escaping neutrinos. e. All of the energy goes into breaking apart light elements like helium and carbon.

ANS: D DIF: Difficult REF: Section 17.2 MSC: Applied TOP: 2IIii 20. Once silicon burning begins to fuse iron in the core of a high-mass main-sequence star, it only has a few __________ left to live. a. seconds b. days c. months d. years e. million years ANS: B DIF: Difficult REF: Section 17.2 MSC: Factual TOP: 2IIiii 21. Which of these begins first in the core of a massive star? a. silicon fusion to iron b. neon fusion to magnesium c. carbon fusion to neon d. helium fusion to carbon e. hydrogen fusion to helium ANS: E DIF: Medium REF: Section 17.2 MSC: Factual TOP: 2IIiii

22. Massive stars explode soon after fusion to iron begins because: a. iron has the smallest binding energy of all elements b. neutrinos emitted during the fusion to iron are captured by the star’s lighter elements c. fusion of elements heavier than iron requires energy, so the star runs out of fuel and cannot hold itself up against gravity d. stars do not contain elements heavier than iron; these are made in supernova explosions e. iron nuclei are unstable and rapidly break apart into lighter elements ANS: C DIF: Medium REF: Section 17.2 MSC: Applied TOP: 2IIiii 23. The collapse of the core of a high-mass star at the end of its life lasts approximately: a. one second b. one minute c. one hour d. one week e. one year ANS: A DIF: Easy REF: Section 17.2 MSC: Factual TOP: 2IIiii 24. Each stage of nuclear burning in 25 M star is __________ in duration than in a star of 15 M. a. much shorter b. a little shorter c. equally long d. a little longer e. much longer ANS: A DIF: Easy REF: Section 17.2 MSC: Factual TOP: 2IIiii 25. Massive stars explode when they: a. accrete mass from their binary star companion b. generate uranium in their cores c. merge with another massive star d. run out of nuclear fuel in their core, and the cores collapse e. lose a lot of mass in a stellar wind ANS: D DIF: Medium REF: Section 17.2 MSC: Applied TOP: 2IIiv

26. When the core of a massive star collapses, a neutron star forms because: a. all the charged particles are ejected in the resulting explosion b. protons and electrons combine to make neutrons c. iron nuclei disintegrate into neutrons d. neutrinos escaping from the core carry away most of the electromagnetic charge e. the collapse releases a large number of protons, which soon decay into neutrons ANS: B DIF: Easy REF: Section 17.2 MSC: Applied TOP: 2IIIiii 27. What is the minimum mass main-sequence star that becomes a Type II supernova? a. 4M b. 8M c. 10M d. 12M e. 25M ANS: B DIF: Easy REF: Section 17.2 MSC: Factual TOP: 2IIIiv 28. How does the energy in light emitted by a supernova compare to the energy emitted by the Sun during its lifetime? a. The supernova emits far less energy. b. The supernova emits somewhat less energy. c. Both emit about the same energy. d. The supernova emits somewhat more energy. e. The supernova emits far more energy.

ANS: C DIF: Difficult REF: Section 17.3 MSC: Applied TOP: 3Ii 29. Type I and Type II supernovae can be distinguished by what combination of observations? a. light curves and the detection of energetic cosmic rays b. light curves and the detection of neutrons c. light curves and the detection of radio pulses d. spectra and light curves e. spectra and X-ray emission ANS: D DIF: Easy REF: Section 17.3 MSC: Factual TOP: 3Ii

30. Suppose the Milky Way makes 10 new stars per year and only 1 out of 5,000 will explode as a supernova.

What would be the average time between supernova explosions in the Milky Way? a. 50 years b. 500 years c. 5,000 years d. 50,000 years e. 500,000 years ANS: B DIF: Medium REF: Section 17.3 MSC: Applied TOP: 3IIii 31. Essentially all the elements heavier than iron in our Milky Way were formed: a. by supernovae b. during the formation of black holes c. by fusion in the cores of the most massive main-sequence stars d. during the formation of planetary nebulae e. during the initial stages of the Big Bang ANS: A DIF: Easy REF: Section 17.3 MSC: Conceptual TOP: 3IIiii 32. Type I and Type II supernovae are respectively caused by what types of stars? a. white dwarfs, Cepheid variables b. white dwarfs, pulsars c. massive stars, white dwarfs d. massive stars, neutron stars e. white dwarfs, massive stars ANS: E DIF: Easy REF: Section 17.3 MSC: Factual TOP: 3IIIi 33. Neutron stars have masses that range from: a. 3.5 M to 25 M b. 1.2 M to 30 M c. 2.5 M to 10 M d. 1.4 M to 3 M e. 0.1M to 1.4 M ANS: D DIF: Easy REF: Section 17.3 MSC: Factual TOP: 3IIIii

34. A neutron star contains a mass of up to 3M in a sphere with a diameter approximately the size of: a. an atomic nucleus b. an apple c. a school bus d. a city e. the Earth ANS: D DIF: Medium REF: Section 17.3 MSC: Factual TOP: 3IIIii 35. The acceleration from gravity on the surface of a neutron star can be how large compared to the value on the surface of the Earth? For reference, the typical mass of a neutron star is 2 M  and its radius is approximately 10 km. a. equal in size b. 10 times as large c. 104 times as large d. 107 times as large e. 1011 times as large ANS: E DIF: Difficult REF: Section 17.3 MSC: Factual TOP: 3IIIii 36. Using the formula g  GMNS/R2NS, calculate the acceleration of gravity on a neutron star of mass 3 solar masses and radius 10 km, and express this in terms of the acceleration of gravity on the surface of the Earth (g  9.8 m/s2). a. 4  104 b. 4  105 c. 4  108 d. 4  1011 e. 4  1014 ANS: D DIF: Medium REF: Section 17.4 MSC: Applied TOP: 3IIIii

37. Which of the following is NOT a common characteristic of a neutron star? a. extremely high density b. enormous magnetic field c. very short rotation period d. large radius e. source of pulsars ANS: D DIF: Easy REF: Section 17.4 MSC: Factual TOP: 3IIIii 38. What mechanism provides the internal pressure inside a neutron star? a. ordinary pressure from hydrogen and helium gas b. degeneracy pressure from neutrons c. degeneracy pressure from electrons d. rapid rotation e. strong magnetic fields ANS: B DIF: Easy REF: Section 17.3 MSC: Factual TOP: 3IIIiii 39. A neutron star in a mass-transfer binary system is called: a. a quasar b. a double star c. an X-ray binary d. a Cepheid variable e. a white dwarf star ANS: C DIF: Easy REF: Section 17.3 MSC: Factual TOP: 3IIIiv 40. We can identify only a fraction of all the radio pulsars that exist in our galaxy because: a. gas and dust efficiently block radio photons b. few swing their beam of synchrotron emission in our direction c. most have evolved to become black holes, which emit no light d. massive stars are very rare e. neutron stars have tiny radii, and are hard to detect even with large telescopes ANS: B DIF: Medium REF: Section 17.3 MSC: Applied TOP: 3IVi

41. When the first pulsar was discovered, scientists thought it might be a signal from a distant extraterrestrial civilization. However, this idea was quickly discarded because: a. it was realized the signals were interference from cars and trucks passing by the radio observatory b. the government made the scientists hide their original finding c. they realized that Cepheid variables could produce the detected radio signals d. more pulsars were discovered, which meant that these were natural phenomena e. the technology required to create pulsed signals is beyond the power of any civilization ANS: D DIF: Difficult REF: Section 17.3 MSC: Applied TOP: 3IVii 42. The Crab Nebula is an important test of our ideas about supernova explosions because: a. people saw the supernova and later astronomers found a pulsar inside the nebula b. the system contains an X-ray binary c. the nebula is expanding slowly, as expected from mass loss rates in massive stars d. the original star must have been like the Sun before it exploded e. astronomers observed the merger of the two stars ANS: A DIF: Medium REF: Section 17.3 MSC: Conceptual TOP: 3IViii 43. One reason why we think neutron stars were formed in supernova explosions is that: a. all supernova remnants contain pulsars b. pulsars are made of heavy elements, such as those produced in supernova explosions c. pulsars are often found near Cepheids and Wolf-Rayet stars, which are also signs of massive star formation d. pulsars spin very rapidly, as did the massive star just before it exploded e. pulsars sometimes have material around them that looks like the ejecta from supernovae ANS: E DIF: Medium REF: Section 17.3 MSC: Conceptual TOP: 3IViii

44. The Type II supernova that created the Crab Nebula was seen by Chinese and Arab astronomers in the year 1054 CE. Because the star is 6,500 light-years away from us, we know the star exploded in the year: a. 554 CE b. 1054 CE c. 1054 BCE d. 5447 BCE e. 7555 BCE ANS: D DIF: Medium REF: Section 17.3 MSC: Applied TOP: 3IViii 45. What characteristic of a star cluster is used to determine its age? a. the chemical composition of stars in the cluster b. the luminosity of the faintest stars in the cluster c. the color of the main sequence turnoff in the cluster d. the total number of stars in the cluster e. the apparent diameter of the cluster ANS: C DIF: Medium REF: Section 17.4 MSC: Applied TOP: 4Iii 46. Of all the main-sequence stars ever formed with a mass equal to 25 percent of the Sun’s mass, how many are still on the main sequence today? a. none b. 1 percent c. 10 percent d. 50 percent e. 100 percent ANS: E DIF: Difficult REF: Section 17.4 MSC: Applied TOP: 4Iii | 4Iv

47. Which of the clusters in this figure are open clusters?

a. Westerlund 2 and M53 b. NGC 290 and M9 c. NGC 290 and Westerlund 2 d. M9 and Westerlund 2 e. M53 and NGC 290 ANS: C DIF: Medium REF: Section 17.4 MSC: Conceptual TOP: 4Iii | 4Iv 48. You observed three different star clusters and found that the main-sequence turnoff stars in Cluster 1 had spectral type F, the main-sequence turnoff stars in Cluster 2 had spectral type A, and the main-sequence turnoff stars in Cluster 3 had spectral type G. Which star cluster is the youngest and which one is the oldest? a. Cluster 1 is the youngest and cluster 2 is the oldest. b. Cluster 2 is the youngest and cluster 1 is the oldest. c. Cluster 2 is the youngest and cluster 3 is the oldest. d. Cluster 3 is the youngest and cluster 1 is the oldest. e. Cluster 3 is the youngest and cluster 2 is the oldest.

ANS: C DIF: Medium REF: Section 17.4 MSC: Applied TOP: 4Iii | 4Iv

49. Suppose you measured H-R diagrams for the two star clusters pictured below. Which of the following statements is true?

a. Cluster A is younger than cluster B, but both are the same distance away. b. Cluster A is older and farther away than cluster B. c. Cluster A and cluster B have the same age, but cluster B is closer. d. Cluster A is older and closer than cluster B. e. Cluster A is younger and farther away than cluster B. ANS: D DIF: Difficult REF: Section 17.4 MSC: Applied TOP: 4Iii | 4Iv

50. List the following H-R diagrams from oldest to youngest.

a. 2, 1, 3, 4 b. 1, 4, 3, 2 c. 4, 3, 1, 2 d. 1, 2, 4, 3 e. 3, 1, 4, 2 ANS: A DIF: Medium REF: Section 17.4 MSC: Applied TOP: 4Iii | 4Iv 51. You observe a distant galaxy, and see that most of the blue light is coming from regions along spiral arms and in the outer regions of the galaxy. This blue light indicates that these regions contain: a. likely sites for planets with life b. neutron stars and white dwarf stars c. K-type supergiants d. only old, low-mass stars e. young, massive stars ANS: E DIF: Difficult REF: Section 17.4 MSC: Applied TOP: 4Ivi | 4IIii

52. Where did all heavy elements in the Sun come from? a. Previous generations of stars seeded the interstellar medium out of which the Sun formed. b. Nearby supernova explosions directly contaminated the Sun’s surface. c. Nucleosynthesis within the Sun generated all the elements we see in the solar spectrum. d. The Sun gobbled up some planets during the early days of our Solar System. e. The solar wind carries away hydrogen and helium, leaving behind the heavy elements.

ANS: A DIF: Medium REF: Section 17.4 | Section 17.5 MSC: Applied TOP: 4IIiv | 5Ii 53. What might be true about the oldest stars in the Milky Way? a. They would have lots of heavy elements, since they have been around for a long time and have undergone a lot of nucleosynthesis in their cores. b. They would be seen as supergiants. c. They would have few heavy elements, since there was not much chance for earlier generations of stars to explode as supernovae before these stars were formed. d. They would be massive, since they were among the first stars formed. e. They would likely be seen as pulsars.

ANS: C DIF: Difficult REF: Section 17.4 | Section 17.5 MSC: Applied TOP: 4IIiv | 5Ii 54. Where did the iron in your blood come from? a. nucleosynthesis on the surfaces of neutron stars b. nucleosynthesis that took place in supernova explosions c. nucleosynthesis in the cores of low-mass stars d. nucleosynthesis in the cores of massive stars e. nucleosynthesis in red giant and horizontal-branch stars ANS: D DIF: Easy REF: Section 17.5 MSC: Factual TOP: 5Iii 55. Iron has 26 protons in its nucleus, and gold has 79 protons. Where did all the gold on the Earth come from? a. nucleosynthesis on the surfaces of neutron stars b. nucleosynthesis that took place in supernova explosions c. nucleosynthesis in the cores of low-mass stars d. nucleosynthesis in the cores of massive stars e. nucleosynthesis in red giant and horizontal-branch stars ANS: B DIF: Medium REF: Section 17.5 MSC: Applied TOP: 5Iii

SHORT ANSWER 1. Why does the CNO cycle happen only in high-mass stars?

ANS: The CNO cycle requires protons to collide and react with nuclei of carbon, nitrogen, and oxygen.

These nuclei have a strong electrical repulsion, since they have six protons (carbon), seven protons (nitrogen), or eight protons (oxygen). High temperatures, which mean high particle velocities, are required in order to overcome this electrical repulsion. These high temperatures are only achieved in the cores of higher-mass stars, which have a greater internal pressure because of their high mass.

DIF: Difficult REF: Section 17.1 MSC: Applied TOP: 1Ii 2. Explain the effect of core convection on the main-sequence lifetime of massive main-sequence stars and contrast this to the situation in low-mass stars.

ANS: In massive stars, convection mixes the helium ash from nuclear fusion throughout the core, so helium builds up throughout the entire core region. The convection also brings in fresh hydrogen, which is then available for burning. As a result, the main-sequence lifetime is longer than it would be in the absence of convection. In a low-mass star, the helium builds up at the very center of the core, and the star begins to leave the main sequence when the central hydrogen is exhausted.

DIF: Medium REF: Section 17.1 MSC: Factual TOP: 1Iii 3. Do large, high-mass main-sequence stars become red giants?

ANS: No, they do not. Red giants have a degenerate core, and when they start burning helium there is a major change in their structures as they change from giants to horizontal-branch stars. When high-mass stars begin burning helium to carbon in their cores, the core is nondegenerate, and the star smoothly transfers to burning helium without radically changing its structure.

DIF: Medium REF: Section 17.1 MSC: Applied TOP: 1Iiii 4. Describe the physical mechanism that causes pulsations in Cepheid variables.

ANS: During the contraction phase of the pulsations, thermal energy is used to ionize helium, which in turn reduces the pressure further. The star contracts past its equilibrium size until all the helium in the outer envelope is ionized. The pressure rises again as the star contracts, which causes the star to “bounce” and expand past its equilibrium radius. During the expansion, the helium recombines with electrons, releasing energy to drive up the temperature and continue the expansion. Eventually the star reaches a maximum radius and begins the pulsation cycle over again.

DIF: Difficult REF: Section 17.1 MSC: Factual TOP: 1IIi 5. How do Cepheid variable stars differ from RR Lyrae variable stars in their masses, luminosities, and periods?

ANS: Cepheids are supergiant, massive stars, whereas RR Lyrae stars are horizontal branch stars with masses more like that of our Sun. Cepheids are also much more luminous and have longer periods (1 to 100 days) than RR Lyrae stars, which have periods of approximately 1 day.

DIF: Easy REF: Section 17.1 MSC: Factual TOP: 1IIiii | 1IIiv 6. With the Hubble Space Telescope, you discover a Cepheid variable star in a nearby galaxy that has a period of 30 days. If nearby Cepheids follow a period-luminosity relationship that says the luminosity of the star is L  335 L  (P/1 day), then what is this Cepheid’s luminosity and absolute magnitude? Recall that the Sun’s absolute magnitude is M  5. If the apparent magnitude of the Cepheid is 25, what is this galaxy’s distance in Mpc? ANS: The Cepheid’s luminosity is L  335 L  (30 days/1 day)  10,650 L, and its absolute magnitude is M  M

2.5 log (L/L)  5

2.5 log (10,650)  5.0.

Absolute magnitude M, apparent magnitude m, and distance d are related by the equations m

M5

log (d/10 pc) and d  10(m M  5)/5 pc.

Thus the distance of the Cepheid is d  10(25  5  5)/5 pc  107 pc  10 Mpc. DIF: Difficult REF: Section 17.1 MSC: Applied TOP: 1IIiv 7. If an 8M star loses mass at an average rate of 10 6 M/yr in a stellar wind, how many years would it take for its mass be reduced to 6M? Would this amount of mass loss be possible in the star’s lifetime?

ANS: If R is the mass loss rate and M is the amount of mass lost in time t, then R  M/t. If M  8M 6M  2M and R  10 6 M/yr, then t  M/R  2M/(10 6 M/yr)  2  106 yr  2 million years.

In comparison, the main-sequence lifetime of the star is MS  1010 yr  (M/M) 25  1010 yr  (8) 25  5.5  107 yr  55 million years.

Therefore, the star could lose this much mass because its main-sequence lifetime is much longer than the time it would take to do so.

DIF: Difficult REF: Section 17.1 MSC: Applied TOP: 1IIIi

8. Why do main-sequence high-mass stars lose so much mass compared to low-mass stars?

ANS: They have very strong winds because their luminosities are so high that the radiation pressure in their outermost layers is stronger than their surface gravity.

DIF: Medium REF: Section 17.1 MSC: Applied TOP: 1IIIi 9. Why do massive stars explode once they generate an iron core?

ANS: Burning of light elements, up to iron in the periodic table, generate energy. This happens because the binding energy per particle in an atomic nucleus increases as the number of particles in the nucleus increases. The fusion of heavier elements than iron, however, requires energy to be input. As a result, the star cannot generate energy to hold itself up against its self-gravity, and will experience core collapse soon after iron fusion begins.

DIF: Medium REF: Section 17.2 MSC: Factual TOP: 2Ii 10. What is the meaning of nuclear binding energy?

ANS: The binding energy of a nucleus is the energy required to tear it apart and separate the protons and neutrons. Consequently, it is also equal to the energy released during the nuclear reactions that produced the nucleus.

DIF: Easy REF: Section 17.2 MSC: Factual TOP: 2Iii 11. Name three processes that speed the collapse of the core of a dying high-mass star.

ANS: (1) Photodisintegration uses up the thermal energy of the core and speeds the collapse.

(2) Electrons and protons combine to form neutrons. This process uses thermal energy and speeds the collapse.

(3) Neutrinos continue to escape, carrying away energy and speeding the collapse.

DIF: Medium REF: Section 17.2 MSC: Applied TOP: 2IIii 12. Although a Type II supernova shines with a luminosity of 100 billion L in light, most of the energy in the explosion is emitted in another way. What is it, and how much more energy does it carry compared to the light?

ANS: The majority of the energy released in a Type II supernova explosion is carried away by the neutrinos that are released, and the amount of energy the neutrinos carry is 10,000 times that of the light produced. A lot of energy also goes into the kinetic energy of the expanding outer envelope of the supernova.

DIF: Medium REF: Section 17.3 MSC: Factual TOP: 3Ii 13. Why do free neutrons released in supernova reactions so easily create new heavy elements?

ANS: Neutrons have no electric charge, so there is no electric repulsion between neutrons and atomic nuclei that would hinder collisions between neutrons and nuclei. Free neutrons can easily penetrate atomic nuclei, regardless of how many protons the nucleus contains.

DIF: Medium REF: Section 17.3 MSC: Applied TOP: 3IIiii 14. Which has a smaller radius, a 2M neutron star or a 3M neutron star? What supports each of these stars from collapsing to form a black hole?

ANS: The pressure from degenerate neutrons supports a neutron star from collapsing to form a black hole. The strange fact about a star composed of degenerate neutrons is that the more massive ones are physically smaller in radius. Therefore, a 3M neutron star is smaller than a 2M star.

DIF: Easy REF: Section 17.3 MSC: Factual TOP: 3IIIiii 15. Explain why the pulsars we see might only be a fraction of the neutron stars in the Milky Way. ANS: Pulsars are neutron stars that are emitting a beam of radio waves. As the star rotates, that beam sweeps out from the neutron star the way a lighthouse beam does. If that beam happens to pass over the Earth, we could detect the pulsar with a radio telescope. It is likely that most neutron star beams are aimed in random directions, so their beams do not strike the Earth and so go undetected.

DIF: Difficult REF: Section 17.3 MSC: Applied TOP: 3IVi 16. About 2% of the mass of the Sun is in elements heavier than helium. Where did these elements come from?

ANS: The sun formed out of a gas cloud in the interstellar medium. The gas in that cloud was seeded with elements that had been synthesized in earlier generations of stars, and then released into interstellar space in supernova explosions.

DIF: Easy REF: Section 17.4 MSC: Applied TOP: 4Ii 17. Suppose you observe three star clusters. Cluster 1 has a main-sequence turnoff point at spectral type G, Cluster 2 has a turnoff point at spectral type A, and Cluster 3 has a turnoff point at spectral type B. Which cluster is youngest and which is oldest? Explain why. What is the approximate age of the oldest cluster?

ANS: Of the three types of main-sequence turnoff stars, B, A, and G are the most massive to least mas-

sive stars. Since more massive stars have shorter main-sequence lifetimes, Cluster 3 is the youngest and Cluster 1 is the oldest. Cluster 1 is about 10 billion years old because the Sun is a G star and has a main-sequence lifetime of about 10 billion years.

DIF: Medium REF: Section 17.4 MSC: Applied TOP: 4Iii 18. Suppose you measured H-R diagrams for the two star clusters pictured below. Which cluster is older?

Which cluster is farther away? Explain why.

ANS: The spectral type of Cluster A’s main-sequence turnoff is G5, and the spectral type of Cluster B’s main-sequence turnoff is B5. Since G5 main-sequence stars are less massive than B5 stars and the G5 stars have longer lifetimes, Cluster A is older than Cluster B. To judge the distances, we must examine the apparent magnitudes of similar type main-sequence stars in each cluster. In Cluster A, the K0 main-

sequence stars have absolute magnitudes of 14, while in Cluster B the K0 main-sequence stars have absolute magnitudes of 18, which is fainter than in Cluster A. Therefore, Cluster B is farther away than Cluster A. DIF: Difficult REF: Section 17.4 MSC: Applied TOP: 4Iii 19. Why are star clusters helpful for testing our ideas of stellar structure and evolution?

ANS: All the stars in an individual cluster were born at nearly the same time, and therefore have the same age and chemical composition. This allows us to explore the effect of stellar mass on the evolution of individual stars, in particular to verify that high-mass stars evolve more quickly than low-mass stars.

DIF: Medium REF: Section 17.4 MSC: Applied TOP: 4Iiii 20. Explain why a blue star cluster is likely younger than a red star cluster.

ANS: A blue star cluster appears blue because the cluster’s light is dominated by the most massive stars. Because these are so short-lived, the cluster must be young. A red star cluster appears red because its light is dominated by evolved low-mass giant stars and low-mass main-sequence stars. The light from these low-mass stars would easily be outshone by the presence of high-mass stars in the cluster. The fact that we see mostly the light from the faint, low-mass stars means the high-mass stars must have already “died,” which means the red cluster is older than the blue cluster.

DIF: Easy REF: Section 17.4 MSC: Applied TOP: 4Ivi

CHAPTER 18

Relativity and Black Holes

CONCEPT MAP Sec 18.1 1. Beyond Newtonian Physics I. Black Holes i. The upper limit for the mass of a neutron star is ~3 M; a more massive stellar core becomes a black hole (TF: 1–2, MC: 1 SA: 1) Sec 18.2 2. Special Relativity I. A Different View of Space and Time i. The laws of physics are the same in any inertial reference frame (TF: 3, MC: 2–3) ii. In the special theory of relativity, the speed of light in a vacuum is the same for all observers, regardless of the speed at which the emitter is moving (TF: 4–5, MC: 4, SA: 2) iii. The passage of time depends on an observer’s frame of reference; moving clocks run slower if the speed of light is the same for all observers, and “simultaneous” depends on the observers (TF: 6, MC: 5–6, SA: 2) iv. 3-D space and time together form 4-D spacetime (MC: 7, SA: 3) v. Special relativity reverts to basic Newtonian physics when the speed of the observer is slow relative to that of light (MC: 8–10, SA: 4) vi. Objects with v approaching c encounter odd relativistic effects. vii. Many experiments have confirmed the theory of relativity (MC: 11) II. Important Consequences of Special Relativity i. Mass and energy are manifestations of the same thing; E  mc2 (TF: 7, MC: 12, SA: 5)

ii. Nothing can move faster than the speed of light, c; an object with any nonzero rest mass cannot reach the speed of light because it would take an infinite amount of energy (MC: 13–14, SA: 6) iii. Lorentz factor, (MC: 15) iv. Time passes more slowly in a moving reference frame; t’  t (MC: 16–19, SA: 7–8) v. An object is shorter in motion than at rest; L’  L/  (MC: 20, SA: 8) vi. The twin paradox illustrates many aspects of relativity (MC: 21) Sec 18.3 3. Gravity Is a Distortion of Spacetime I. Einstein’s General Theory of Relativity i. The presence of a mass warps the fabric of 4-D spacetime (MC: 22) ii. The inertial mass of an object equals its gravitational mass (TF: 8, MC: 23) iii. Gravity is the result of the shape of the spacetime terrain through which an object moves (TF: 9, MC: 24–25, SA: 9) iv. Equivalence principle: free fall is the same as free float; being stationary in a gravitational field is the same as being in an accelerated reference frame (TF: 10, MC: 26–29, SA: 10) v. Falling objects follow curved paths through spacetime called geodesics (MC: 29, 30, SA: 9, 11) II. Analogy for Spacetime i. Spacetime is like a rubber sheet, and putting down a mass is like making a dip in the fabric of spacetime; objects respond by moving as if there is a “gravitational force” that attracts other objects to the mass (MC: 31, SA: 12) ii. Mass distorts the geometry of spacetime (TF: 10) III. Observable Consequences of General Relativity i. The consequences of curved spacetime include the precession of Mercury’s orbit, gravitational lensing (MC: 32, SA: 13) ii. Gravitational lensing: a massive object distorts spacetime, bending the path of light that travels near it (TF: 11, MC: 33–34, SA: 14) iii. Eddington detected gravitational lensing during a solar eclipse (1919); stars appeared to be deflected outward as the Sun passed in front of them. iv. General relativistic time dilation: time runs more slowly near a massive object (MC: 35–36) v. Gravitational redshift: light trying to escape a massive object will be redshifted depending on how massive the object is (TF: 12, MC: 36–38, SA: 15) vi. Accelerating a massive object gives rise to gravitational waves that ripple through spacetime like

ripples on a pond (MC: 39, SA: 16) vii. There are ongoing attempts to detect gravitational waves, such as with the LIGO experiment (SA: 17) Sec 18.4 4. Black Holes I. Black Holes Warp Spacetime i. Singularity: black holes cause an infinitely deep pit in spacetime (SA: 18) ii. Schwarzschild radius/event horizon: radius where vesc  c; RS  2GMBH/c2 (TF: 13, MC: 40–47, SA: 15, 18) iii. The event horizon is a boundary of no return; infinite gravitational redshift (MC: 48) iv. Hawking radiation: black holes should radiate as a virtual pair of particles are created at the event horizon and one falls in and one streams outward (MC: 49–50) v. Black holes are found through the effects of their gravity (MC: 51–53, SA: 19) vi. Examples of X-ray emission detections of black holes (Cygnus X-1 and LMC X-3): flickering X-ray emission from material falling onto an accretion disk (TF: 14, MC: 54–56, SA: 20) Sec 18.5 5. Origins: Gamma-Ray Bursts I. Black Holes and Energetic Explosions i. X-ray satellites have discovered numerous bursts of high-energy gamma rays, followed by a longer afterglow (TF: 15) ii. We now understand that these are produced by especially energetic supernovae. We see the gamma-ray bursts when a relativistic jet, with ejected material moving at a significant fraction of the speed of light, is aimed in our direction (MC: 57)

TRUE/FALSE 1. The Sun could turn into a black hole at the end of its life.

ANS: F DIF: Easy REF: Section 18.1 MSC: Applied TOP: 1Ii 2. A black hole can be produced if the stellar core left over after a supernova is larger than 3 M.

ANS: T DIF: Easy REF: Section 18.1 MSC: Factual TOP: 1Ii

3. You have a 1 kg mass at rest sitting on a lab bench in front of you. You see somebody fly by in a spaceship traveling at 0.2c. That astronaut flying by also has a 1 kg mass sitting on a bench in front of her.

She would measure that mass to be 1kg.

ANS: T DIF: Medium REF: Section 18.2 MSC: Conceptual TOP: 2Ii 4. Special relativity says that all observers moving at constant speed will measure the same value of the speed of light.

ANS: T DIF: Easy REF: Section 18.2 MSC: Factual TOP: 2Iii 5. Einstein’s special theory of relativity implies that if a person standing at the front of a train traveling at 0.1 km/s shines a flashlight out in front of the train, the emitted photons will travel at a speed of 300,000.1 km/s.

ANS: F DIF: Easy REF: Section 18.2 MSC: Conceptual TOP: 2Iii 6. Special relativity says that moving clocks run slower.

ANS: T DIF: Easy REF: Section 18.2 MSC: Factual TOP: 2Iiii 7. The mass of an atomic nucleus is a measure of how much energy is contained therein.

ANS: T DIF: Easy REF: Section 18.2 MSC: Factual TOP: 2IIi 8. The Earth orbits the Sun because the Sun changes space around it but not time.

ANS: F DIF: Medium REF: Section 18.3 MSC: Conceptual TOP: 3Iii 9. Gravity is nothing more than a curvature in the fabric of spacetime.

ANS: T DIF: Medium REF: Section 18.3 MSC: Conceptual TOP: 3Iiii 10. One result of relativity is that an astronaut in a windowless spacecraft could tell the difference between falling freely and the presence of a nearby gravitational source.

ANS: F DIF: Easy REF: Section 18.3 MSC: Factual TOP: 3Iiv 11. Gravitational lensing allows us to see distant objects brightened and distorted because of massive objects in the path that the light from the object travels.

ANS: T DIF: Difficult REF: Section 18.3 MSC: Conceptual TOP: 3IIIii

12. We can’t detect the gravitational redshift coming from the Sun because the Sun has a very small amount of gravity.

ANS: F DIF: Medium REF: Section 18.3 MSC: Applied TOP: 3IIIv 13. The singularity of a black hole has an infinitely small radius, but its event horizon is finite.

ANS: T DIF: Easy REF: Section 18.4 MSC: Conceptual TOP: 4Iii 14. Black holes emit no light whatsoever.

ANS: F DIF: Difficult REF: Section 18.4 MSC: Applied TOP: 4Ivi 15. Gamma-ray bursts are probably associated with extremely energetic supernova explosions.

ANS: T DIF: Easy REF: Section 18.5 MSC: Factual TOP: 5Iii

MULTIPLE CHOICE 1. What would happen if mass were continually added to a 2M neutron star? a. The star’s radius would increase. b. The star could eventually become a black hole. c. The star could erupt as a nova. d. The star could exceed the Chandrasekhar limit and become a Type II supernova. e. The star could eventually return to the top of the main sequence.

ANS: B DIF: Medium REF: Section 18.1 MSC: Applied TOP: 1Ii 2. What is the meaning of the phrase inertial frame of reference? a. a reference frame that is not accelerating b. a reference frame that is stationary with respect to the Earth c. a reference frame that is in motion at constant speed d. a reference frame that is accelerating at a constant rate e. a reference frame in which there are strong gravitational forces ANS: A DIF: Medium REF: Section 18.2 MSC: Factual TOP: 2Ii

3. What will observers in different inertial frames of reference always agree on? a. how the speed of light varies with the motion of an observer b. the length of the meter, but not the duration of the second c. the rate each frame is accelerating d. the results of physics experiments performed in each frame e. whether events are simultaneous or not ANS: D DIF: Easy REF: Section 18.2 MSC: Factual TOP: 2Ii 4. You observe a distant galaxy apparently moving away at 1/3 the speed of light (v  c/3). If you could measure the speed that this galaxy’s light is passing the Earth, you would get: a. c/3 b. 2c/3 c. c d. 4c/3 e. 5c/3 ANS: C DIF: Medium REF: Section 18.2 MSC: Conceptual TOP: 2Iii 5. The aberration of starlight is: a. a change in the wavelength of light caused by the motion of the light source b. poor focusing of starlight caused by imperfections in the optics of a telescope c. a change in the direction of starlight caused by its passage through the earth’s atmosphere d. an increase in the wavelength of light as it leaves the surface of a gravitating body e. a change in the direction of starlight caused by the orbital motion of the Earth ANS: E DIF: Medium REF: Section 18.2 MSC: Factual TOP: 2Iiii 6. One consequence of Einstein’s ideas about the speed of light is that: a. if two events take place at the same time, all observers will see that event simultaneously b. whether events are seen as simultaneous or not depends on the motion of an observer c. observers will always disagree on when an event took place d. it is not possible to know when an event happened e. moving observers can always know whether they are in motion ANS: B DIF: Easy REF: Section 18.2 MSC: Conceptual TOP: 2Iiii

7. What is the meaning of the word spacetime? a. It is a mental framework for keeping track of numbers in Newtonian physics. b. Space and time form a two-dimensional region where physics takes place. c. The term has no special meaning; it’s just a fancy way to sound important when talking about physics. d. It is the combined treatment of space and time in the theory of relativity. e. It is the idea that observers will always measure the same locations and times of events.

ANS: D DIF: Easy REF: Section 18.2 MSC: Conceptual TOP: 2Iiv 8. How does relativity compare to Newtonian physics? a. Relativity gives the same result as Newtonian physics when objects are moving slowly. b. Relativity gives results that contradict many predictions of Newtonian physics, so we know the latter is incorrect. c. Relativity must be better because it is a newer theory than Newtonian physics. d. Newtonian physics and relativity make the same predictions, but it’s easier to compute results using relativity. e. Newtonian physics makes predictions that are at odds with certain observations, whereas relativity is consistent with those observations.

ANS: A DIF: Medium REF: Section 18.2 MSC: Conceptual TOP: 2Iv 9. When do the predictions of special relativity match those of Newtonian physics? a. in terrestrial laboratories b. inside our Solar System c. when different observers are at rest with one another d. when objects have a low mass e. when objects are moving slowly ANS: E DIF: Easy REF: Section 18.2 MSC: Factual TOP: 2Iv

10. What is true about muons? a. They are always moving at high speed, so they test relativity. b. Relativity explains why we can see short-lived muons that are produced high in the atmosphere from cosmic rays. c. We can see them being deflected from straight lines by the gravity of black holes. d. They have a mass that does not increase if they are moving fast. e. They are examples of Hawking radiation from black holes.

ANS: B DIF: Medium REF: Section 18.2 MSC: Conceptual TOP: 2Iv 11. Has relatively ever been tested? a. No, because it would require us to set up physics experiments in faraway galaxies. b. Yes, because even ordinary motion in automobiles and airplanes produces easily noticeable effects predicted by relativity. c. No, because no one has been able to think of experiments that are able to measure the small differences between the predictions of Newtonian physics and relativity. d. Yes, because (for example) subatomic particles can be accelerated to speeds approaching that of light. e. No, because the theory of relativity contains paradoxes and contradictions, like the twin paradox.

ANS: D DIF: Medium REF: Section 18.2 MSC: Conceptual TOP: 2Ivii 12. According to Einstein’s relativity, which two quantities are different manifestations of the same thing? a. mass and gravity b. light and energy c. energy and mass d. temperature and energy e. distance and time ANS: C DIF: Easy REF: Section 18.2 MSC: Factual TOP: 2IIi

13. According to relativity, spacecraft that can travel faster than the speed of light are: a. impossible, because nothing can travel that fast b. possible, but not useful since they could not contain living beings c. impossible, since objects that travel that fast would get shorter and squeeze out space for the astronauts to live d. possible, if we are clever enough with new technologies e. impossible, because they would require new energy sources that are not yet invented ANS: A DIF: Easy REF: Section 18.2 MSC: Factual TOP: 2IIii 14. Why can an object with a nonzero mass never travel as fast as the speed of light? a. It would take an infinite amount of energy to accelerate it to a speed of c. b. It would emit so much radiation that its energy would decrease and it would slow down again. c. It would lose all its mass and turn into neutrinos. d. An object can actually travel as fast as light, but if it did it would disappear. e. If it were going at the speed of light, it would be converted to pure energy since E  mc2.

ANS: A DIF: Easy REF: Section 18.2 MSC: Factual TOP: 2IIii 15. What is the Lorentz factor for an object moving at 0.85c? a. 1.00 b. 1.67 c. 1.80 d. 1.90 e. 2.05 ANS: C DIF: Medium REF: Section 18.2 MSC: Applied TOP: 2IIiii 16. How fast must a muon be moving if it looks to an observer at rest that it lived for 2  10 4 seconds before it decayed when, in fact, it lived for only 2  10 6 seconds in its moving reference frame? a. 0.05c b. 0.50c c. 0.95c d. 0.995c e. 0.99995c ANS: E DIF: Difficult REF: Section 18.2 MSC: Applied TOP: 2IIiv

17. Assume that a group of explorers traveled to the Orion Nebula, the nearest star-forming cloud at a distance of 1,300 light-years, using revolutionary technology that allowed them to travel at a speed of 0.99 c.

Observers back on Earth using Earth-bound clocks would say it took them __________ to get there, but the travelers with their moving clocks would say it took them only __________ to get there. a. 1,310 years; 185 years b. 1,440 years; 630 years c. 1,310 years; 390 years d. 1,440 years; 425 years e. 1,310 years; 1,300 years ANS: A DIF: Difficult REF: Section 18.2 MSC: Applied TOP: 2IIiv 18. If you were to design a spacecraft that could travel to the galactic center fast enough that the astronauts aboard only aged by 25 years during the trip, how fast would the spacecraft have to go? The galactic center is 25,000 light years away. a. 0.95c b. 0.995c c. 0.99995c d. 0.9999995c e. 0.999999995c ANS: D DIF: Difficult REF: Section 18.2 MSC: Applied TOP: 2IIiv 19. Suppose you detect a pulsar that gives us 1,000 radio pulses per second, but the pulsar is in a distant galaxy that is apparently moving away from us at 50 percent of the speed of light. An observer at rest with respect to the pulsar in that faraway galaxy would measure a pulse rate of: a. 870 per second b. 1,150 per second c. 1,250 per second d. 1,366 per second e. 1,450 per second ANS: D DIF: Difficult REF: Section 18.2 MSC: Applied TOP: 2IIiv

20. Which of the following is a consequence of Einstein’s special theory of relativity? a. Moving clocks run quicker. b. The velocity of light depends on the speed of the observer. c. Distances are shorter for objects traveling close to the speed of light. d. Gravity arises because mass distorts spacetime. e. Faster moving objects require less force to accelerate them.

ANS: C DIF: Medium REF: Section 18.2 MSC: Factual TOP: 2IIv 21. The twin paradox shows that special relativity: a. explains many things but can’t explain everything b. is accurate but contains some worrisome contradictions c. is incorrect d. only explains things in steady motion, but cannot be used to explain objects that accelerate e. correctly accounts for the results of experiments in different reference frames ANS: E DIF: Medium REF: Section 18.2 MSC: Applied TOP: 2IIvi 22. __________ is the result of mass distorting the fabric of spacetime. a. Energy b. Radiation c. Fusion d. Gravity e. Electric charge ANS: D DIF: Easy REF: Section 18.3 MSC: Factual TOP: 3Ii 23. You measure that an object has a mass of exactly 1 kg by weighing it in the Earth’s gravitational field. The equivalence principle says that the mass you would measure by trying to accelerate it would be: a. 1 kg b. greater than 1 kg c. less than 1 kg d. different from 1kg, depending on what it’s made of e. larger than 1 kg because of the Sun’s gravity ANS: A DIF: Medium REF: Section 18.3 MSC: Factual TOP: 3Iii

24. Photons have no mass, and Einstein’s theory of general relativity says: a. their paths through spacetime are curved in the presence of a massive body b. their apparent speeds depend on the observer’s frame of reference c. they should not be attracted to a massive object d. their wavelengths must remain the same as they travel through spacetime e. their wavelengths would grow longer as they travel through empty space ANS: A DIF: Medium REF: Section 18.3 MSC: Applied TOP: 3Iiii 25. What does gravity mean in relativity? a. It is a result of mass and energy being two forms of the same thing. b. It is a consequence of distances getting shorter as objects move faster. c. It is the result of the mass of falling bodies getting bigger because they are in motion. d. It is the force that objects with mass exert on a body. e. It is the result of the distortion in spacetime around an object with any energy density.

ANS: E DIF: Easy REF: Section 18.3 MSC: Factual TOP: 3Iiii 26. The equivalence principle says that: a. the universe is homogeneous and isotropic b. being stationary in a gravitational field is the same as being in an accelerated reference frame c. at any radius inside a star the outward gas pressure must balance the weight of the material on top d. mass and energy are interchangeable and neither can be destroyed e. gravity does not exist in space ANS: B DIF: Medium REF: Section 18.3 MSC: Conceptual TOP: 3Iiv 27. Why do astronauts in space feel no gravity? a. There is no gravity out in space. b. Gravity only happens when objects are accelerating. c. In space, the gravity from the Moon and the Sun cancels out the Earth’s gravity. d. They and their spaceship are both freely falling at the same rate in the gravitational field. e. The astronauts do not have any mass when they are out in space.

ANS: D DIF: Medium REF: Section 18.3 MSC: Conceptual TOP: 3Iiv

28. The principle of equivalence states that the gravitational mass is equal to: a. the mass when moving nearly the speed of light b. the resistance to acceleration c. the mass when near a black hole d. the object’s weight e. the density divided by the volume ANS: B DIF: Medium REF: Section 18.3 MSC: Conceptual TOP: 3Iiv 29. As shown in this figure, an observer in a windowless room notices that light from a flashlight does not follow a straight path, but instead seems to be bent downwards as it crosses the room. What does the observer conclude is happening?

a. The room is experiencing an acceleration from gravity. b. The room is accelerating upward. c. Both A and B at the same time. d. Either A or B; it is impossible to tell the difference. e. Either A or B, depending on whether massive objects in the room fall downward or float upward when released.

ANS: D DIF: Difficult REF: Section 18.3 MSC: Conceptual TOP: 3Iiv | 3Iv

30. A geodesic is the name for: a. the aberration of starlight b. the gravitational field of the Earth c. the solid crust of a terrestrial planet d. the path followed by a freely-falling object in spacetime e. the shape of a body that has mass ANS: D DIF: Easy REF: Section 18.3 MSC: Factual TOP: 3Iv 31. In the rubber-sheet analogy for spacetime, what would you expect for objects (such as golf balls) rolling around in the presence of a massive object that is stretching the rubber sheet? a. Their paths would be straight if they were moving slowly enough. b. Their paths would curve more the closer they come to the massive object. c. Their paths would curve by the same amount no matter how close they come to the mass. d. Their paths would curve towards the mass if they passed close, but bend away from the mass if they passed far from the mass. e. Their paths would curve less the closer they come to the massive object.

ANS: B DIF: Medium REF: Section 18.3 MSC: Factual TOP: 3IIi 32. Name one verified prediction of general relativity. a. Spacetime is flat except inside the Schwarzschild radius of the Sun. b. Relativity predicts Mercury has an orbital precession, but Newtonian physics does not. c. Starlight passing near the Sun should be bent away from the Sun by its gravity. d. Gravitational lensing should make Mercury brighter than it would otherwise be. e. Mercury’s orbital precession is a little bigger than predicted by Newtonian physics.

ANS: E DIF: Medium REF: Section 18.3 MSC: Factual TOP: 3IIIi 33. Gravitational lensing occurs when __________ distorts the fabric of spacetime. a. a star b. dark matter c. a black hole d. any massive object e. a white dwarf ANS: D DIF: Easy REF: Section 18.3 MSC: Applied TOP: 3IIIii

34. The bending of light passing near a massive object is called: a. time dilation b. the twin paradox c. gravitational lensing d. length contraction e. mass increase ANS: C DIF: Easy REF: Section 18.3 MSC: Factual TOP: 3IIIii 35. Compared to a clock on the surface of the Earth, a clock on the International Space Station runs: a. at approximately the same rate, but slightly slower. b. significantly slower c. significantly faster d. sometimes faster, sometimes slower e. at an equal rate, except during eclipses ANS: A DIF: Medium REF: Section 18.3 MSC: Applied TOP: 3IIIiv 36. Light is increasingly redshifted near a black hole because: a. the photons are moving away from us very quickly as they are sucked into the black hole b. the photons are moving increasingly faster in order to escape the pull of the black hole c. the photons lose energy because climbing out of the black hole’s gravity makes them tired d. the curvature of spacetime is increasingly stretched near the black hole, which in turn stretches the wavelengths of the photons e. time is moving increasingly slower as viewed from the observer’s frame of reference ANS: E DIF: Difficult REF: Section 18.3 MSC: Conceptual TOP: 3IIIiv | 3IIIv 37. The gravitational redshift of light should be smallest for light emitted from the surface of: a. a black hole b. the Sun c. a white dwarf d. a planet like the Earth e. a neutron star ANS: D DIF: Medium REF: Section 18.3 MSC: Applied TOP: 3IIIv

38. Two observers are on different floors of a building, as shown in the figure below. The observer on the bottom floor shines a light upward, which is reflected from a mirror in an upper floor and sent back downward. Consider the gravitational redshift of the light due to the Earth. If the light has a wavelength of exactly 600 nm when emitted, the observer on the upper floor will measure a wavelength of __________ while the observer on the lower floor will see the light returning with a wavelength of __________.

a. 600 nm, 600 nm b.  600 nm, 600 nm c.  600 nm,  600 nm d. 600 nm,  600 nm e.  600 nm,  600 nm ANS: B DIF: Difficult REF: Section 18.3 MSC: Conceptual TOP: 3IIIv

39. General relativity predicts that pairs of objects with nonzero masses undergoing accelerations can produce: a. pulses of electromagnetic radiation b. gravitational waves c. high-energy particles d. a slowing of clocks here on the Earth e. redshifted light from the surface of the object ANS: B DIF: Easy REF: Section 18.3 MSC: Factual TOP: 3IIIvi 40. The event horizon of a black hole is defined as: a. the point of maximum gravity b. the radius of the original neutron star before it became a black hole c. the radius from which shock waves course through spacetime due to the strong gravitational distortion of the black hole d. the radius at which the escape speed from the black hole equals the speed of light e. the radius at which the gravitational force is the same as that on the surface of the Sun ANS: D DIF: Easy REF: Section 18.4 MSC: Factual TOP: 4Iii 41. What is the meaning of the Schwarzschild radius around a black hole? a. It is the radius at which an orbiting object would show a precession. b. It is the radius at which gravitational redshift can be detected. c. It is the radius at which the black hole’s spin equals the speed of light. d. It is the radius at which the escape velocity equals the speed of light. e. It is the radius at which a body falling onto the black hole would move at half the speed of light.

ANS: D DIF: Easy REF: Section 18.4 MSC: Factual TOP: 4Iii 42. If the Sun suddenly turned into a black hole, what would be the radius of its event horizon? a. 3 m b. 30 m c. 300 m d. 3 km e. 30 km ANS: D DIF: Medium REF: Section 18.4 MSC: Applied TOP: 4Iii

43. If the Earth were to shrink in size until it became a black hole, its Schwarzschild radius would be: a. 1 cm b. 1 m c. 1 km d. 10 km e. 200 km ANS: A DIF: Medium REF: Section 18.4 MSC: Applied TOP: 4Iii 44. The Schwarzschild radius of a 10M is __________ the size of the Schwarzschild radius of a 5 M black hole. a. 1/5 b. 1/2 c. equal to d. 2 times e. 5 times ANS: D DIF: Medium REF: Section 18.4 MSC: Applied TOP: 4Iii 45. While traveling the galaxy in a spacecraft, you and a colleague set out to investigate the 106M black hole at the center of our galaxy. She hops aboard an escape pod and drops into a circular orbit around the black hole, maintaining a distance of 1 AU, while you remain much farther away in the spacecraft. After doing some experiments to measure the strength of gravity, your colleague signals her results back to you using a green laser. What would you see? Hint: you will need to calculate the location of the event horizon. a. Her signals are shifted only slightly toward the red because she is orbiting well outside the event horizon of the black hole. b. You would see her signals shifted to a much redder wavelength because she is close to the event horizon. c. You would see nothing, because your colleague has crossed the event horizon around the black hole. d. You would see nothing, because no light can escape the gravitational pull of a black hole no matter how far she is from it. e. You would see her signals shifted to a much bluer wavelength because black holes can make highly energetic light.

ANS: A DIF: Difficult REF: Section 18.4 MSC: Applied TOP: 4Iii

46. While traveling the galaxy in a spacecraft, you and a colleague set out to investigate the 106M black hole at the center of our galaxy. He hops aboard an escape pod and drops into a circular orbit around the black hole, maintaining a distance of 10,000 km, while you remain much farther away in the spacecraft. After doing some experiments to measure the strength of gravity, your colleague signals his results back to you using a green laser. What would you see? Hint: you will need to calculate the location of the event horizon. a. You would see his signals unaltered in wavelength because he is orbiting well outside the event horizon of the black hole. b. You would see his signals shifted to a much redder wavelength because he is close to the event horizon. c. You would see nothing, because your colleague has crossed the event horizon around the black hole. d. You would see nothing, because no light can escape the gravitational pull of a black hole no matter how far he is from it. e. You would see his signals shifted to a much bluer wavelength because black holes can make highly energetic light.

ANS: C DIF: Medium REF: Section 18.4 MSC: Applied TOP: 4Iii 47. While traveling the galaxy in a spacecraft, you and a colleague set out to investigate the 106M black hole at the center of our galaxy. She hops aboard an escape pod and drops into a circular orbit around the black hole, maintaining a distance of 4  106 km, while you remain much farther away in the spacecraft.

After doing some experiments to measure the strength of gravity, your colleague signals her results back to you using a green laser. What would you see? Hint: you will need to calculate the location of the event horizon. a. You would see her signals unaltered in wavelength because she is orbiting well outside the event horizon of the black hole. b. You would see her signals shifted to a much redder wavelength because she is close to the event horizon. c. You would see nothing because your colleague has crossed the event horizon around the black hole. d. You would see nothing because no light can escape the gravitational pull of a black hole no matter how far she is from it. e. You would see her signals shifted to a much bluer wavelength because black holes can make highly energetic light.

ANS: B DIF: Difficult REF: Section 18.4 MSC: Applied TOP: 4Iii

48. A person would feel __________ as he approached the event horizon of a black hole. a. extremely strong tidal forces b. intense heating c. strong Hawking radiation d. strong infrared radiation e. nothing ANS: A DIF: Medium REF: Section 18.4 MSC: Conceptual TOP: 4Iiii 49. Hawking radiation from black holes refers to: a. light emitted from matter falling onto a black hole b. the gravitational redshift of light emitted near the event horizon c. the radiation of particles created near the event horizon d. high-energy X-rays and gamma rays from the formation of a black hole e. the optical and infrared light from an energetic supernova explosion ANS: C DIF: Easy REF: Section 18.4 MSC: Factual TOP: 4Iiv 50. Hawking radiation is emitted by a black hole when: a. the black hole rotates quickly b. the black hole accretes material c. a supernova explodes and forms a black hole out of its core d. synchrotron radiation is emitted by infalling charged particles e. a virtual pair of particles is created from the vacuum of space ANS: E DIF: Medium REF: Section 18.4 MSC: Factual TOP: 4Iiv 51. If the Sun were to be instantly replaced by a 1M black hole, the gravitational pull of the black hole on Earth would be: a. much greater than it is now b. the same as it is now c. much smaller than it is now d. larger or smaller, depending on the location of the Moon e. irrelevant because Earth would be quickly fall into the Sun and be destroyed ANS: B DIF: Medium REF: Section 18.4 MSC: Applied TOP: 4Iv

52. Even if a black hole emitted no light, we can still detect it: a. from sound waves produced by material falling onto the black hole b. by tides produced on the Earth’s oceans c. through its Hawking radiation d. through its gravitational effect on surrounding gas or stars e. by looking for dark patches on the sky where the black hole swallows background light ANS: D DIF: Medium REF: Section 18.4 MSC: Conceptual TOP: 4Iv 53. A red giant star is found to be orbiting an unseen object with a short orbital period. By measuring the speed at which it orbits, astronomers deduce that the object has a mass of 10M This object is probably a __________ because __________. a. black hole; the giant star is massive and could only be in orbit about something even more massive b. black hole; its mass is too large to be a neutron star or a white dwarf c. neutron star; any supernova that would have made a black hole would have destroyed the red giant d. M-dwarf star; only such stars would be faint enough to go unseen in this system e. black hole; most red giants orbit neutron stars, and neutron stars can turn into black holes ANS: B DIF: Medium REF: Section 18.4 MSC: Factual TOP: 4Iv 54. Black holes that are stellar remnants can be found by searching for: a. dark regions at the centers of galaxies b. variable X-ray sources c. extremely luminous infrared objects d. objects that emit very faint radio emission e. regular, repeated pulsations at radio wavelengths ANS: B DIF: Medium REF: Section 18.4 MSC: Applied TOP: 4Ivi 55. Most black holes are found: a. by the watching the orbits of nearby stars b. from the bending of the light from background stars c. by finding objects that emit no light d. by the X-rays produced by a surrounding accretion disk e. by the detection of Hawking radiation ANS: D DIF: Medium REF: Section 18.4 MSC: Factual TOP: 4Ivi

56. If you observed that at the centers of some galaxies there were objects emitting lots of X-rays or there was gas in rapid motion, you might conclude that these galaxies: a. are very old b. contain many white dwarfs c. have different physics than on the Earth d. are rotating quickly e. contain black holes ANS: E DIF: Medium REF: Section 18.4 MSC: Applied TOP: 4Ivi 57. Gamma-ray bursts are likely to be: a. the signs of accretion onto black holes b. the product of merging neutron stars c. produced by the most energetic supernova explosions d. the result of neutron stars accreting mass and turning into black holes e. produced by intelligent life in the universe exploding nuclear bombs ANS: C DIF: Easy REF: Section 18.5 MSC: Factual TOP: 5Iii

SHORT ANSWER 1. Describe two possible ways to make a black hole.

ANS: Black holes can be made in supernova explosions if the remnant core is over the maximum mass for a neutron star ( 3 M). They can also be made if enough material is added to a neutron star that is in a mass-transfer binary system until it exceeds 3 M.

DIF: Easy REF: Section 18.1 MSC: Factual TOP: 1Ii 2. What is the central idea in relativity concerning the speed of light? Describe at least two unusual consequences of this idea.

ANS: The central idea is that the speed of light in a vacuum will be the same for all observers, regardless of their relative motion with respect to one another. Some consequences are that moving clocks will run slower, that moving masses will be larger, and that distances will be shorter in the moving frame.

DIF: Medium REF: Section 18.2 MSC: Factual TOP: 2Iii | 2Iiii

3. Explain what four-dimensional spacetime means.

ANS: In relativity, space and time have to be considered together when explaining the motions of objects. The theory blends both of them into a 4-D “spacetime” in which there are three spatial dimensions and one time dimension. The theory explains what happens to lengths and times for objects moving at speeds that are a significant fraction of the speed of light.

DIF: Medium REF: Section 18.2 MSC: Factual TOP: 2Iiv 4. Explain how Newtonian physics is an approximation to relativity.

ANS: Newtonian physics explains many phenomena extremely well, provided that any motions are small compared with the speed of light. The results of relativity are therefore to be viewed as having a broader applicability than Newtonian physics, being valid over a wider range of conditions. The predictions of relativity revert to those of Newtonian physics when gravitational fields are weak or the relative speed of the observer and an object is slow compared to the speed of light.

DIF: Easy REF: Section 18.2 MSC: Conceptual TOP: 2Iv 5. What is the meaning of the equation E  mc2?

ANS: This is the famous equation relating energy (E), mass (m), and the square of the speed of light (c2). It shows that energy and mass are both manifestations of the same thing. Any nuclear or chemical reaction that releases energy is accompanied by a change in the mass of the objects involved in the reaction. Also, adding energy to an object, say increasing its kinetic energy by making accelerate, also increases the inertial mass of the object.

DIF: Medium REF: Section 18.2 MSC: Conceptual TOP: 2IIi 6. Explain why no object that has mass can ever move at a speed equal to the speed of light. At what velocity do massless particles (e.g., photons) travel in vacuum?

ANS: According to relativity, increasing the speed of an object with mass increases the inertial mass of the object. As a result, it would take an infinite amount of energy to bring an object with mass to the velocity of light. Massless particles, such as light, travel at the speed of light in a vacuum.

DIF: Medium REF: Section 18.2 MSC: Conceptual TOP: 2IIii

7. Suppose we discovered radio signals coming from the star Alpha Centauri, whose distance is 4.4 lightyears from us, and we sent a crew in a spacecraft to visit it. If the spacecraft used revolutionary technology allowing it to travel at a speed of 0.5 c, how long would it take the spacecraft to get to Alpha Centauri, and how long a time would the astronauts say passed during the trip? (Ignore the time it would take to accelerate the spacecraft to reach a velocity of 0.5 c.)

ANS: Traveling at a speed of 0.5 c, the time it would take the spacecraft to reach Alpha Centauri would be t  2  4.4 yr  8.8 years. The astronauts experience time dilation, because moving clocks run slower, and they would say that t/ amount of time went by where:   (1

v2/c2) 1⁄2  (1

0.52) 1⁄2 

1.15. Thus, they would think the trip took only (8.8 years)/1.15  7.6 years.

DIF: Difficult REF: Section 18.2 MSC: Applied TOP: 2IIiv 8. Muons are elementary particles that decay into other particles in about 2 microseconds. They are formed in the upper atmosphere of the Earth from high energy cosmic rays, and can be detected on the ground even though they could only travel a few hundred meters before decaying according to Newtonian physics. How does relativity explain how we can detect them on the ground? Explain both in our reference frame and in the frame of the muon.

ANS: From our reference frame, we see the muon created with a downward velocity (approaching us) that is a significant fraction of the speed of light. Since clocks run slower in moving reference frames, we see that the lifetime of the muon is prolonged well beyond its normal lifetime. From the muon’s point of view, it pops into existence with the Earth approaching it at a very high speed. Since distances are shorter in moving frames, the muon sees that the distance to the ground is greatly reduced from what it would be if the Earth were not moving, and so it “thinks” it can cover that distance within its decay time.

DIF: Difficult REF: Section 18.2 MSC: Applied TOP: 2IIiv | 2IIv 9. How would the Newtonian theory explain the orbit of the Earth around the Sun? What would the explanation be in general relativity?

ANS: In Newton’s theory of gravity, the Sun and the Earth exert a mutual force on one another; this causes the Earth to accelerate (change its direction of motion) constantly, giving it an elliptical path around the Sun. General relativity says that the mass of the Sun changes the curvature of spacetime in its vicinity; the Earth then follows this curved path around the Sun. DIF: Medium REF: Section 18.3 MSC: Conceptual TOP: 3Iiii | 3Iv

10. What is the equivalence principle? Describe a consequence of the equivalence principle for astronauts orbiting in the space station.

ANS: The equivalence principle states that action of gravity is indistinguishable from an acceleration in an observer’s reference frame. One consequence is that an object in free fall will experience the same effects as if there were no gravity present. This is why astronauts in the International Space Station don’t feel the effects of gravity, even though the strength of the Earth’s gravity at the Space Station is only a little smaller than it is on the surface of the Earth.

DIF: Medium REF: Section 18.3 MSC: Conceptual TOP: 3Iiv 11. Galileo supposedly experimented with gravity by dropping two objects of different masses from the leaning tower of Pisa at the same instant and observing that they hit the ground at the same time. If Albert Einstein had done the experiment, how would his conclusion have differed from Galileo’s?

ANS: Galileo would say that the acceleration due to gravity was the same for both objects and that, therefore, they fell at the same rate, hitting the ground at the same time even though they had different masses. Einstein would say that the curvature of spacetime at that location dictated a single path that both objects followed, regardless of mass.

DIF: Difficult REF: Section 18.3 MSC: Conceptual TOP: 3Iv 12. What is the “rubber sheet” analogy for spacetime, and why is it useful for explaining gravity and gravitational waves?

ANS: Spacetime can be thought of conceptually like a stretched rubber sheet. If there are no masses present, the sheet is flat, and a very small object rolling on the sheet (say, a small ball bearing) will move in a straight line. Adding masses to spacetime is like putting objects on the rubber sheet; the sheet will dip in the vicinity of the object, and a ball bearing will follow a curved path instead of a straight one.

DIF: Medium REF: Section 18.3 MSC: Conceptual TOP: 3IIi 13. Describe two early verifications of the prediction of relativity.

ANS: In 1919, Eddington measured the gravitational deflection of starlight passing near the Sun. Even earlier, Einstein himself showed that relativity completely accounts for the rate of change in direction of the perihelion of Mercury’s orbit; Newtonian physics explained most of the rate, but could not account for all the observed change in the perihelion.

DIF: Easy REF: Section 18.3 MSC: Factual TOP: 3IIIi

14. Explain why Einstein’s theory of general relativity predicts the existence of gravitational lensing.

ANS: Einstein’s theory of general relativity says that gravity is simply a distortion of the geometry of spacetime. A massive object like a star or galaxy warps the fabric of spacetime like a heavy ball on a rubber sheet distorts the rubber sheet. As light passes by the object it responds to the distortion of spacetime even though the photon has no mass, and the photon’s path is bent around the object creating the effect known as gravitational lensing.

DIF: Medium REF: Section 18.3 MSC: Conceptual TOP: 3IIIii 15. While traveling the galaxy in a spacecraft, you and a colleague set out to investigate a 10M black hole.

Your colleague hops aboard an escape pod and drops into a circular orbit around the black hole maintaining a distance of 50 km from it, while you remain much farther away inside the spacecraft. After doing some experiments to measure the strength of gravity, your colleague signals the results back to you using a green laser. What would you see, and why?

ANS: What you see depends on where your colleague is with respect to the event horizon of the black hole. The Schwarzschild radius marks the event horizon, and it is equal to

Rs  2GMBH/c2  2  6.7  10 11 m3/(kg s2)  2  1030 kg/(3  108 m/s)2  30,000 m  30 km.

Since your colleague is orbiting close to the event horizon, but not inside it, you would still see his or her signals, but instead of seeing a green light you would see a light that is redder due to the gravitational redshift.

DIF: Difficult REF: Section 18.3 | 18.4 MSC: Applied TOP: 3IIIv | 4Iii 16. What are gravitational waves? Have they been detected?

ANS: Gravitational waves are ripples in spacetime that occur when a massive object undergoes acceleration. They have not yet been detected, but there are ongoing experiments such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) that are attempting to find them.

DIF: Easy REF: Section 18.3 MSC: Factual TOP: 3IIIvi 17. How might we be able to detect events like colliding neutron stars even if we don’t detect any light from them?

ANS: One way to detect such an event would be through the gravitational radiation emitted during the collision. Accelerations of massive objects would produce ripples in spacetime, which could be seen with a gravitational wave detector.

DIF: Medium REF: Section 18.3 MSC: Applied TOP: 3IIIvii 18. What is the difference between the singularity and the event horizon of a black hole?

ANS: The singularity is the very center of the black hole, which is infinitely small in size. The event horizon is defined as the finite radius from the center of the black hole, inside of which the escape velocity is greater than the speed of light so nothing can escape the black hole.

DIF: Easy REF: Section 18.4 MSC: Conceptual TOP: 4Ii | 4Iii 19. You repeatedly measure the radial velocity of what seems to be a single main-sequence star like the Sun, but find that it is in fact orbiting in a tight orbit around another object that remains unseen. From measurement of the velocity and orbital period, you calculate that the unseen object has a mass 20 times that of the Sun-like star. Why might this object be a black hole?

ANS: If the other member of this binary system can’t be detected, it must not be emitting much light, at least in comparison to the Sun-like star. This rules out the possibility that the object is a massive mainsequence star or a red supergiant, since it would be easily visible; indeed, it would be far brighter than the star you see. Since it is in a tight orbit, it cannot be a diffuse dark cloud, so it must therefore be some sort of stellar remnant, such as a black hole, a neutron star, or a white dwarf. It can’t be a white dwarf, since the maximum mass of a white dwarf is set by the Chandrasekhar limit (1.4 solar masses). Similarly, it can’t be a neutron star, since neutron stars also have a maximum mass, which is somewhere around three solar masses. Therefore, it could be a black hole.

DIF: Difficult REF: Section 18.4 MSC: Applied TOP: 4Iv 20. Explain one way we can detect black holes.

ANS: Material falling onto a black hole would form an accretion disk around it. Because the black hole has strong gravity, the gas in the disk would orbit the black hole rapidly and would be hot. In addition, the material falling onto the disk would also be moving fast and have a large Doppler shift, and create a hot spot in the disk where it hit. Astronomers know of a number of systems where a black hole is gathering in X-ray-emitting material from a nearby giant companion that is losing mass. These are called X-ray binaries. This is one example that shows black holes can be detected through the effects of their gravity.

DIF: Easy REF: Section 18.4 MSC: Factual TOP: 4Ivi

CHAPTER 19

The Expanding Universe

CONCEPT MAP Sec 19.1 1. Twentieth-Century Astronomers Discovered the Universe of Galaxies I. Discovery of Galaxies i. Charles Messier published the first catalog of 106 nonstellar objects, mostly nearby galaxies, star clusters, and nebulae (TF: 1) ii. The visible universe contains billions and billions of galaxies iii. Early astronomers underestimated the size of the Milky Way because they did not know about extinction from dust and gas (MC: 1) iv. Shapley later estimated the Milky Way’s size as 300,000 light-years, 50 times larger than people had thought v. In 1920s Great Debate, Curtis and Shapley debated the size of the Milky Way and the distances of “nebulae”; Curtis believed nebulae were at large distances external to our Milky Way, and Shapley argued the reverse; Shapley was correct about the size of the Milky Way, but Curtis was correct that nebulae were galaxies outside the Milky Way (TF: 2, MC: 2, SA: 1) vi. In 1925, Hubble used Cepheid variable stars to measure the distance to Andromeda and proved that the nebulae were at great distances and thus were galaxies like our own Milky Way (MC: 3–4, SA: 2) Sec 19.2 2. The Cosmological Principle I. A Key Assumption i. Cosmological principle: the physical laws that apply to one part of the universe apply to all parts of

the universe (TF: 3) ii. The universe is homogeneous (different observers see the same things) and isotropic (the universe is similar in all directions) (MC: 5–8, SA: 3) Sec 19.3 3. The Universe Is Expanding I. Galaxy Redshifts i. In 1912, Slipher discovered that galaxy spectra are redshifted with respect to laboratory wavelengths (MC: 9) ii. Redshift: z  (obs

rest)/ rest  vr/c (MC: 10–11, SA: 4–5)

iii. Hubble’s law: vr  H0d; current data shows H0  22 km/s/Mly (10%) (MC: 12–20, SA: 5) iv. Hubble’s law says that the universe is uniformly expanding; every point in space expands at the same rate (MC: 21–22, SA: 6) II. The Distance Ladder i. Standard candles: objects whose luminosity is constant and can be used to determine distances (MC: 23–24, SA: 7) ii. Distance ladder: a series of standard candles that extend to larger and larger distances (TF: 4–5, MC: 25–26, SA: 8–9) iii. Type I supernovae are the most luminous standard candles and can be seen the farthest away (TF: 5, MC: 26–28, SA: 9) iv. Peculiar velocities: departures from the Hubble flow due to local gravitational forces (MC: 29, TF: 6) III. Hubble’s Law Maps the Universe in Space and Time i. The redshift of a galaxy tells us its distance for a given value of H0. ii. Look-back time: the time it takes light to reach us from a distant object (MC: 30) Sec 19.4 4. The Universe Began in the Big Bang I. Big Bang i. The universe began, and began expanding, at the Big Bang ii. Hubble time: 1/H0, is the approximate age of the universe; 13.7 billion years (MC: 31–32, SA: 10) II. Galaxies Are Not Flying Apart through Space i. The Big Bang happened everywhere; there is no special center to the expansion; galaxies are moving with the expanding space (TF: 7, MC: 33, SA: 11)

ii. Dark energy: an energy that currently dominates the Hubble constant and causes the universe’s expansion rate to accelerate (MC: 34–35) iii. Scale factor: RU(z)  1/(1  z) RU denotes universe’s expansion factor since light at redshift z was emitted (TF: 8–9, MC: 36–39, SA: 12–14) iv. The expansion is only big enough to be noticeable after summing up its effect over very large spatial scales; the expansion of the universe does not affect the physics of stars, atoms, human scales, and so on (TF: 10, MC: 40–41, SA: 15) Sec 19.5 5. Astronomers Observe Radiation Left Over from the Big Bang I. Cosmic Microwave Background i. Alpher and Gamow predicted leftover photons from the hot, early universe would be redshifted to T  5 to 10 K; in 1964, Penzias and Wilson discovered the cosmic background radiation (CBR) (MC: 42) ii. The CBR is the thermal radiation left over from when the universe was hot and ionized just after the Big Bang (TF: 11, MC: 43–45, SA: 16) iii. Recombination: protons and electrons combined to form neutral hydrogen, and the universe became transparent for photons (MC: 39, 46) iv. Since recombination, the CBR has cooled by 1,000 times (MC: 47–48) v. The CBR has a Planck blackbody spectrum at T  2.7 K (TF: 12, MC: 38, 49, 50, SA: 17) II. CBR Measures the Earth’s Motion Relative to the Universe Itself i. Earth’s motion makes the CBR slightly hotter in the direction we are moving and slightly cooler in the opposite direction (MC: 50) ii. Tiny variations in the temperature of the CBR result from structures in place at the time of recombination (SA: 18) Sec 19.6 6. Origins: Big Bang Nucleosynthesis I. The Big Bang Theory Correctly Predicts the Abundance of the Least Massive Elements i. Big Bang nucleosynthesis: out of the Big Bang, H, D, He, and small amounts of Li and Be form as the universe cools (MC: 52, TF: 13) ii. Big Bang theory predicts the universe contains 24 percent He by mass, which observations confirm (TF: 14, MC: 39, 53, SA: 19)

iii. D, 7Li, and 3He abundance also agree with predictions (SA: 19) iv. If more baryons were around during the Big Bang, then the predicted abundances of the elements would change (TF: 15, MC: 54–55, SA: 20)

TRUE/FALSE 1. Charles Messier published the first galaxy catalog containing over 2,000 nearby galaxies.

ANS: F DIF: Easy REF: Section 19.1 MSC: Factual TOP: 1Ii 2. In the Great Debate of 1920, Curtis argued that spiral nebulae were individual galaxies, while Shapley argued spiral nebulae must be part of the Milky Way because the Milky Way was very large in size.

ANS: T DIF: Medium REF: Section 19.1 MSC: Factual TOP: 1Iv 3. The cosmological principle says physical laws that the universe is homogeneous and isotropic.

ANS: T DIF: Easy REF: Section 19.2 MSC: Factual TOP: 2Ii 4. We can measure the value of H0 directly with no intermediate steps.

ANS: F DIF: Easy REF: Section 19.3 MSC: Factual TOP: 3IIii 5. When we measure distances to very faint galaxies with Type I supernovae, the accuracy of the measurement is only as good as that of the calibration of Cepheid stars.

ANS: T DIF: Medium REF: Section 19.3 MSC: Applied TOP: 3IIiii 6. The apparent recessional velocities of galaxies at large distances are due mainly to the actual motions of the galaxies through space.

ANS: F DIF: Medium REF: Section 19.3 MSC: Factual TOP: 3IIiv 7. There is no special center to the expansion of the universe.

ANS: T DIF: Easy REF: Section 19.4 MSC: Factual TOP: 4IIi 8. The value of the scale factor RU depends inversely on the value of redshift z.

ANS: T DIF: Medium REF: Section 19.4 MSC: Factual TOP: 4IIiii 9. It is possible for a galaxy to have an apparent recessional velocity greater than the speed of light.

ANS: T DIF: Difficult REF: Section 19.4 MSC: Applied TOP: 4IIiii

10. As the universe expands, the sizes of galaxies expand a measurable amount as well.

ANS: F DIF: Easy REF: Section 19.4 MSC: Factual TOP: 4IIiv 11. The CMB contains photons left over from the epoch in the universe’s history when electrons recombined with nuclei.

ANS: T DIF: Medium REF: Section 19.5 MSC: Applied TOP: 5Iii 12. The current temperature of the cosmic microwave background radiation is 17 K. ANS: F DIF: Easy REF: Section 19.5 MSC: Factual TOP: 5Iv 13. The only chemical elements that formed as the universe cooled down after the Big Bang were hydrogen, deuterium, and helium.

ANS: F DIF: Medium REF: Section 19.6 MSC: Factual TOP: 6Ii 14. The standard Big Bang theory successfully predicts that the fraction of helium by mass in the primordial universe was 2 percent, similar to what it is in the Sun. ANS: F DIF: Medium REF: Section 19.6 MSC: Factual TOP: 6Iii 15. The abundance of various elements formed from Big Bang nucleosynthesis depended only on the temperature of normal matter in the early universe.

ANS: F DIF: Difficult REF: Section 19.6 MSC: Applied TOP: 6Iiv

MULTIPLE CHOICE 1. What caused early astronomers to believe that our galaxy is only about 6,000 light-years across? a. Telescopes were not powerful enough to observe stars farther away. b. Interstellar dust blocked visible light from stars farther away. c. Stars farther away could not be resolved as individual objects. d. Astronomers miscalculated the distances to stars, believing that the stars were 50 times closer than they actually were. e. Astronomers assumed all red stars were faint main-sequence stars, and they confused more luminous red giants with them.

ANS: B DIF: Easy REF: Section 19.1 MSC: Factual TOP: 1Iiii

2. In the Great Debate of 1920, Curtis and Shapley argued over whether or not: a. the Big Bang occurred b. the age of the universe was 14 billion years c. the universe was contracting d. life existed outside of Earth e. the spiral nebulae were located outside the Milky Way ANS: E DIF: Easy REF: Section 19.1 MSC: Factual TOP: 1Iv 3. Astronomers have known that galaxies are separate entities outside of our own for roughly the last: a. 35 years b. 60 years c. 90 years d. 150 years e. 210 years ANS: C DIF: Easy REF: Section 19.1 MSC: Factual TOP: 1Ivi 4. What did Edwin Hubble study in the Andromeda Galaxy that proved it was an individual galaxy and not part of our own Milky Way? a. Cepheid stars b. Type I supernovae c. globular clusters d. red giant stars e. RR Lyrae variables ANS: A DIF: Medium REF: Section 19.1 MSC: Factual TOP: 1Ivi 5. What do astronomers mean when they say that the universe is homogeneous? a. All stars in all galaxies have planetary systems just like ours. b. The universe looks exactly the same no matter what place you look. c. Galaxies are generally distributed similarly throughout the universe. d. Generally speaking, there is little difference between conditions on Earth, in the Sun, or in outer space. e. The universe looks the same at all times in its history.

ANS: C DIF: Medium REF: Section 19.2 MSC: Factual TOP: 2Iii

6. What do astronomers mean when they say that the universe is isotropic? a. Far away parts of the universe look just like nearby parts. b. All galaxies are spiral galaxies like our own. c. Intergalactic gas has the same density everywhere in the universe. d. The laws of physics apply everywhere in the universe. e. The universe looks the same no matter what direction you look.

ANS: E DIF: Medium REF: Section 19.2 MSC: Factual TOP: 2Iii 7. If we lived in a galaxy one billion light-years from our own, what would we see? a. a universe 1 billion years younger than ours b. a universe 1 billion years older than ours c. much the same universe we see here d. the universe is expanding at a slower rate than we see from Earth e. the universe is expanding at a faster rate than we see from Earth ANS: C DIF: Medium REF: Section 19.2 MSC: Applied TOP: 2Iii 8. In an imaginary universe, astronomers find that there are thousands of galaxies within a few million light years, but beyond those galaxies there is nothing but empty space. Such a universe would be: a. isotropic and homogeneous b. isotropic but not homogenous c. homogeneous but not isotropic d. neither homogeneous nor isotropic e. lacking any evidence for the Big Bang ANS: B DIF: Difficult REF: Section 19.2 MSC: Conceptual TOP: 2Iii 9. The spectra of most galaxies tell us that: a. most galaxies appear to be moving away from us b. their light comes predominantly from objects other than stars c. most galaxies contain clouds of gas that are absorbing their favorite wavelengths d. galaxies in the past rotated at a faster rate than they do today e. galaxies are rushing through space at high speeds ANS: A DIF: Medium REF: Section 19.3 MSC: Factual TOP: 3Ii

10. If you found a galaxy with an H emission line that had a wavelength of 756.3 nm, what would be the galaxy’s redshift? Note that the rest wavelength of the H emission line is 656.3 nm. a. 0.05 b. 0.07 c. 0.10 d. 0.13 e. 0.15 ANS: E DIF: Medium REF: Section 19.3 MSC: Applied TOP: 3Iii 11. If a galaxy has a recessional velocity of 50,000 km/s, at what wavelength will you observe the H

emission line? Note that the rest wavelength of the H emission line is 656.3 nm.

a. 695.7 nm b. 719.4 nm c. 742.3 nm d. 766.0 nm e. 1750 nm ANS: C DIF: Difficult REF: Section 19.3 MSC: Applied TOP: 3Iii 12. According to Hubble’s law, as the distance of a galaxy __________ its __________ increases. a. increases; luminosity b. increases; recessional velocity c. decreases; luminosity d. decreases; recessional velocity e. decreases; peculiar velocity ANS: B DIF: Easy REF: Section 19.3 MSC: Factual TOP: 3Iiii

13. If a galaxy has an apparent velocity of 700 km/s, what is its distance if the Hubble constant is 70 km/s/Mpc? a. 10 Mpc b. 70 Mpc c. 100 Mpc d. 700 Mpc e. 1,000 Mpc ANS: A DIF: Medium REF: Section 19.3 MSC: Applied TOP: 3Iiii 14. You see a galaxy in which the H line (rest wavelength  656.3 nm) is observed at a wavelength of 756.3 nm. What would be the observed wavelength of a particular helium line, which has a rest wavelength of 1,083 nm? a. 1,083 nm b. 1,183 nm c. 1,248 nm d. 1,440 nm e. 3,142 nm ANS: C DIF: Difficult REF: Section 19.3 MSC: Applied TOP: 3Iiii 15. Astronomers use galactic redshift as a measure of: a. gravity b. luminosity c. velocity d. mass e. distance ANS: E DIF: Easy REF: Section 19.3 MSC: Conceptual TOP: 3Iiii

16. If you measured the distances and recessional velocities for a sample of galaxies and plotted the data to get the figure below, what value would you derive for the Hubble constant?

a. 10 km/s/Mpc b. 50 km/s/Mpc c. 70 km/s/Mpc d. 100 km/s/Mpc e. 500 km/s/Mpc ANS: B DIF: Difficult REF: Section 19.3 MSC: Applied TOP: 3Iiii 17. If the distance of a galaxy is 10 Mpc, what is its recessional velocity if the Hubble constant is 70 km/s/Mpc? a. 700 km/s b. 1,000 km/s c. 3,500 km/s d. 5,000 km/s e. 7,000 km/s ANS: A DIF: Medium REF: Section 19.3 MSC: Applied TOP: 3Iiii 18. If you found a galaxy with an H emission line that had a wavelength of 756.3 nm, what would be the galaxy’s distance if the Hubble constant is 70 km/s/Mpc? Note that the rest wavelength of the H emission line is 656.3 nm. a. 650 Mpc b. 760 Mpc c. 3,200 Mpc d. 6,400 Mpc e. 7,600 Mpc ANS: A DIF: Difficult REF: Section 19.3 MSC: Applied TOP: 3Iiii

19. If the spectrum of a distant galaxy is observed to have a calcium K absorption line that occurs at a wavelength of 500.4 nm, what is this galaxy’s distance if the rest wavelength of this absorption line is 393.4 nm? Assume the Hubble constant is 70 km/s/Mpc. a. 720 Mpc b. 950 Mpc c. 1,200 Mpc d. 2,500 Mpc e. 3,700 Mpc ANS: C DIF: Difficult REF: Section 19.3 MSC: Applied TOP: 3Iiii 20. In the figure shown below, the upper spectrum is from hydrogen at rest in a laboratory, and the lower spectrum is from a galaxy. How far away is this galaxy?

a. 110 Mpc b. 170 Mpc c. 230 Mpc d. 280 Mpc e. 340 Mpc ANS: A DIF: Difficult REF: Section 19.3 MSC: Applied TOP: 3Iiii 21. Galaxies move away from us in all directions because: a. the force of gravity increases with distance b. the force of gravity weakens with distance c. space is expanding d. our galaxy has expelled all other galaxies e. we are at the center of the expansion of the universe ANS: C DIF: Easy REF: Section 19.3 MSC: Conceptual TOP: 3Iiv

22. Hubble’s constant, H0, represents: a. the rate of expansion of the universe b. the speed at which galaxies are moving away from us c. the time it takes a galaxy to move twice as far away from us d. the size of the universe e. the amount of time since the Solar System formed ANS: A DIF: Medium REF: Section 19.3 MSC: Factual TOP: 3Iiv 23. To be a standard candle, an object must have a constant: a. lifetime b. brightness c. luminosity d. distance e. mass ANS: C DIF: Easy REF: Section 19.3 MSC: Conceptual TOP: 3IIi 24. If suddenly we find that the distances between the Sun and nearby stars are now 10 percent larger than we thought it was before, what measured properties will we not have to adjust? a. the distances of stars b. the size of our galaxy c. the value of H0 d. the fact that the universe is expanding e. the luminosity of Type II supernovae ANS: D DIF: Difficult REF: Section 19.3 MSC: Applied TOP: 3IIi 25. Which of the following lists distance indicators that are used to measure distances ranging from the very nearest to the very farthest? a. Cepheids, parallax, main-sequence fitting, Type I supernovae b. parallax, main-sequence fitting, Cepheids, Type I supernovae c. parallax, main-sequence fitting, Type I supernovae, Cepheids d. main-sequence fitting, parallax, Cepheids, Type I supernovae e. Cepheids, Type I supernovae, main-sequence fitting, parallax ANS: B DIF: Medium REF: Section 19.3 MSC: Applied TOP: 3IIii

26. Why can Type I supernovae be used to determine a galaxy’s distance? a. Type I supernovae occur only in very luminous galaxies. b. Type I supernovae have approximately the same luminosity. c. Type I supernovae have approximately the same size. d. A Type I supernova occurs in a typical galaxy about once every 100 years. e. Type I supernovae occur even in very small galaxies.

ANS: B DIF: Easy REF: Section 19.3 MSC: Conceptual TOP: 3IIii | 3IIiii 27. Which distance indicator can be used to measure the most distant objects? a. Cepheids b. parallax c. Type I supernovae d. main-sequence fitting e. RR Lyrae stars ANS: C DIF: Easy REF: Section 19.3 MSC: Applied TOP: 3IIiii 28. Type I supernovae have an absolute magnitude of 20. If you discover a Type I supernovae in a distant galaxy that has an apparent magnitude of 22, then how far away is this galaxy? a. 0.3 Mpc b. 40 Mpc c. 1,200 Mpc d. 2,500 Mpc e. 3,500 Mpc ANS: D DIF: Difficult REF: Section 19.3 MSC: Applied TOP: 3IIiii 29. Why are some galaxies’ spectra blueshifted rather than redshifted? a. Some distant galaxies are gravitationally lensed. b. Some distant galaxies show the universe was contracting before the Big Bang started. c. Some nearby galaxies have vigorous star formation and are much bluer than others. d. Some distant galaxies have AGN at their centers. e. Some nearby galaxies have small peculiar velocities in our direction.

ANS: E DIF: Medium REF: Section 19.3 MSC: Conceptual TOP: 3IIiv

30. If the distance of a galaxy at a redshift z  0.5 is 1,800 Mpc, how many years back into the past are we looking when we observe this galaxy? a. 500 million years b. 2 billion years c. 6 billion years d. 9 billion years e. 10 billion years ANS: C DIF: Medium REF: Section 19.3 MSC: Applied TOP: 3IIIii 31. The inverse of the value of H0 is a: a. time b. mass c. density d. size e. luminosity ANS: A DIF: Easy REF: Section 19.4 MSC: Factual TOP: 4Iii 32. If the Hubble constant had a value that was half of its current measured value of 70 km/s/Mpc, the age of the Universe would be about a. 7 billion years b. 14 billion years c. 22 billion years d. 27 billion years e. 33 billion years ANS: D DIF: Medium REF: Section 19.4 MSC: Applied TOP: 4Iii 33. Where in the universe did the Big Bang take place? a. near the Milky Way Galaxy b. near the Virgo cluster c. near some unknown location on the other side of the universe d. everywhere in the universe e. at the center of the universe, not too far from the center of the cosmic background radiation ANS: D DIF: Medium REF: Section 19.4 MSC: Conceptual TOP: 4IIi

34. Is dark energy responsible for the expansion of the universe? a. No. The expansion would happen without dark energy, but dark energy is causing the expansion rate to decrease. b. Yes. Dark energy is currently the driving force in the universe’s expansion. c. No. Dark energy is only responsible for the observed rapid motions of stars within galaxies. d. No. The expansion would happen without dark energy, but dark energy is causing the expansion rate to increase. e. No. The dark energy has no effect on the expansion.

ANS: D DIF: Medium REF: Section 19.4 MSC: Applied TOP: 4IIii 35. What is the interpretation of a redshift larger than 1? a. The object is moving faster than the speed of light. b. The universe has more than doubled in size since the light from that object was emitted. c. The object has an extremely large peculiar velocity. d. The light was shifted to longer wavelengths from gravitational radiation. e. The rate of expansion of the universe is increasing.

ANS: B DIF: Medium REF: Section 19.4 MSC: Applied TOP: 4IIii 36. When we look at galaxies in the universe and measure their star formation rates, we find that galaxies at redshifts z  1 have higher star formation rates than they do now. At that time, the universe was __________ times the size it is today. a. 0.1 b. 0.2 c. 0.5 d. 0.7 e. 0.90 ANS: C DIF: Difficult REF: Section 19.4 MSC: Applied TOP: 4IIiii

37. Distant galaxies we can see today with a redshift of z  6 emitted their light when the universe was: a. five times smaller than it is today b. six times smaller than it is today c. seven times smaller than it is today d. eight times smaller than it is today e. the same size as it is today ANS: C DIF: Difficult REF: Section 19.4 MSC: Applied TOP: 4IIiii 38. The cosmic microwave background radiation was emitted when the universe had a size about 1/1000 of today’s value. What was the temperature of the microwave background radiation when it was emitted? a. 30 K b. 300 K c. 3000 K d. 30,000 K e. 300,000 K ANS: C DIF: Difficult REF: Section 19.4 | 19.5 MSC: Applied TOP: 4IIiii | 5Iv 39. Which of the following is NOT a prediction of the standard Big Bang theory that has been successfully verified by observations? a. The universe is expanding. b. The most distant galaxies are redder because they are older. c. Helium and lithium were made as the universe cooled after the initial Big Bang. d. The early universe was very hot and dense. e. The most distant galaxies are redder because their light has been stretched during the time it took for the light to reach Earth.

ANS: B DIF: Easy REF: Section 19.4 | 19.5 |19.6 MSC: Applied TOP: 4IIiii | 5Iiii | 6Iii

40. Galaxy peculiar velocities are typically about 300 km/s. How far away do you have to look in order to see galaxies recessional velocities that are 10 times this peculiar velocity? a. 12 Mpc b. 25 Mly c. 37 Mpc d. 43 Mpc e. 52 Mpc ANS: D DIF: Difficult REF: Section 19.4 MSC: Applied TOP: 4IIiv 41. Will the Sun get larger over many billions of years? a. Yes, because of changes taking place in its interior. b. Yes, because the rate of expansion of the universe is increasing. c. No, because the Earth is at center of the universe’s expansion. d. Yes, because of the expansion of the universe is pulling the Sun apart. e. No. It has always been the same size.

ANS: A DIF: Medium REF: Section 19.4 MSC: Factual TOP: 4IIiv 42. In the early 1960s, physicists named Penzias and Wilson detected a persistent noise at a wavelength of 1 mm in their radio telescope that came from all directions in the sky due to: a. synchrotron emission from the Crab Nebula b. emission from newly formed stars in the Orion Nebula c. cellphone usage d. photons left over from the Big Bang e. television and radio broadcasting ANS: D DIF: Easy REF: Section 19.5 MSC: Factual TOP: 5Ii 43. The CMB is a snapshot of the radiation in the universe at a redshift of z  1,000 when the universe was: a. 1,000 times smaller than it is today b. 10 times smaller than it is today c. two times smaller than it is today d. the same size as it is today e. 10 times larger than it is today ANS: A DIF: Medium REF: Section 19.4 MSC: Applied TOP: 5Iii

44. The existence of the cosmic background radiation tells us that the early universe was: a. much hotter than it is today b. much colder than it is today c. composed entirely of radiation at early times d. composed entirely of stars at early times e. about the same temperature as today, but was much more dense ANS: A DIF: Medium REF: Section 19.5 MSC: Conceptual TOP: 5Iii 45. Suppose the expansion of the whole universe began only about 2 billion years ago, and before that the universe was static and not expanding. What should we expect in such a situation? a. We would see redshifts increasing with distance until a distance of approximately 2 billion light years. b. We would see a universe that was isotropic but not homogenous. c. Distant galaxies would be much smaller than our own, since they have not been expanding for 13 billion years. d. The cosmic microwave background would be perfectly uniform in temperature. e. We would not expect evidence for a Big Bang.

ANS: A DIF: Difficult REF: Section 19.5 MSC: Conceptual TOP: 5Iii 46. Why is it not possible to look all the way back to the Big Bang itself? a. Photons are not produced until the stars begin to shine at a redshift of z  20. b. From redshifts of z  0 to 100, photons are gravitationally lensed by the dark matter in the universe. c. At redshifts of z  1,000, most of the photons are blocked by large amounts of cold gas and dust. d. For redshifts of z  1,000, photons are heavily altered because they easily react with individual protons and electrons in the universe. e. Photons from the Big Bang would be so strongly redshifted that we could never detect them.

ANS: D DIF: Difficult REF: Section 19.5 MSC: Conceptual TOP: 5Iiii

47. If the wavelength of the background radiation peaked at 1 m at the time of recombination, how old was the universe then compared to its age today? a. 1/10th as old b. 1/100th as old c. 1/1,000th as old d. 1/10,000th as old e. 1/100,000 as old ANS: C DIF: Difficult REF: Section 19.5 MSC: Applied TOP: 5Iiv 48. After the Big Bang, as the universe cooled and protons and electrons combined so that the universe became neutral, what important consequence happened? a. Protons and neutrons combined to form nuclei such as deuterium and helium. b. Neutrinos ceased to interact with normal matter. c. Dark matter ceased to interact with normal matter. d. Photons began to travel freely through the universe. e. Lithium and other light elements were formed by the fusion of hydrogen and helium.

ANS: D DIF: Medium REF: Section 19.5 MSC: Applied TOP: 5Iiv 49. The current temperature of the cosmic background radiation of 2.73 K means that the peak of its spectrum occurs at a wavelength of: a. 0.1 m b. 1 m c. 10 m d. 100 m e. 1,000 m ANS: E DIF: Difficult REF: Section 19.5 MSC: Applied TOP: 5Iv

50. Does the temperature of the cosmic background radiation tell us about the age of the universe? a. Yes, since it was produced at a redshift of approximately 1,000. b. No, we still need to know the Hubble constant to know the universe’s age. c. No, the temperature does not but the size of temperature variations in the CMB does tell us the age. d. Yes, in combination with information on cosmic nucleosynthesis. e. No, the CMB formed at a wide variety of times and gives no information on the age of the whole universe.

ANS: B DIF: Medium REF: Section 19.5 MSC: Conceptual TOP: 5Iv 51. The temperature of the CMB is hotter on one side of the sky than on the other by approximately 3 mK. What does this tell us? a. The Earth has a peculiar velocity of about 8,000 km/s with respect to the CMB. b. The Earth has a peculiar velocity of about 400 km/s with respect to the CMB. c. The Earth is near the center of the universe’s expansion, but not exactly at the middle. d. The universe is expanding faster on one side of us than the other. e. The universe is homogeneous but not isotropic.

ANS: B DIF: Easy REF: Section 19.5 MSC: Factual TOP: 5IIi 52. Which of the following was NOT created as a result of Big Bang nucleosynthesis? a. helium b. lithium c. hydrogen d. deuterium e. carbon ANS: E DIF: Medium REF: Section 19.6 MSC: Factual TOP: 6Ii 53. The Sun is 27 percent helium by mass. Where was the majority of this helium manufactured? a. in Type II supernovae explosions that happened before the Sun formed b. in low-mass stars that lived and died before the Sun formed c. in nuclear burning that happened in the Sun’s core d. in the Big Bang e. by accreting Jupiter-mass planets in the early era of the Solar System ANS: D DIF: Easy REF: Section 19.6 MSC: Applied TOP: 6Iii

54. If there were more baryons in the universe immediately after the Big Bang, then the abundance of __________ in stars and galaxies would be higher. a. hydrogen b. carbon c. uranium d. helium e. oxygen ANS: D DIF: Difficult REF: Section 19.6 MSC: Applied TOP: 6Iiv 55. The figure below shows the current measurements of the abundances of various light elements in the universe (vertical yellow bar), along with theoretical predictions. If you redetermined the abundance of ordinary helium (4He) and it was 90% of the current value, what would you conclude about the density of normal matter in today’s universe?

a. It would be higher, and all the various density indicators would agree. b. It would be higher, but the various density indicators would disagree. c. It would be lower, but the various density indicators would disagree about its value. d. It would be lower, but the various density indicators would agree. e. It would be about the same as current observations suggest.

ANS: C DIF: Difficult REF: Section 19.6 MSC: Applied TOP: 6Iiv

SHORT ANSWER 1. What were the positions taken by Heber Curtis and Harlow Shapley in their “Great Debate,” and how were both of them partially correct?

ANS: Curtis believed in a smaller galaxy (roughly 6,000 light-years across), and he thought the “spiral nebulae” were galaxies outside our own. Shapley believed that our galaxy was larger (roughly 300,000 light-years across) and encompassed everything. Curtis was correct about other galaxies existing outside our own; Shapley was correct about the size of our galaxy.

DIF: Medium REF: Section 19.1 MSC: Factual TOP: 1Iv 2. How did Edwin Hubble definitively prove that “spiral nebulae” were individual galaxies that were separate from the Milky Way? ANS: Hubble identified Cepheid variable stars in the Andromeda Galaxy. Measuring their apparent magnitudes and periods, Hubble was able to calculate Andromeda’s distance and prove it was much larger than the size of the Milky Way. Therefore, the spiral nebulae must be individual galaxies and not part of our own Milky Way, and the sizes of the galaxies were similar to our own Milky Way. DIF: Medium REF: Section 19.1 MSC: Applied TOP: 1Ivi 3. Describe the two assumptions regarding the universe that the cosmological principle makes.

ANS: (1) The universe is homogeneous, meaning it is the same viewed from any location.

(2) The universe is isotropic, meaning that it appears the same no matter what direction you look.

DIF: Easy REF: Section 19.2 MSC: Factual TOP: 2Ii 4. The spectrum of a galaxy is observed to have an H emission line at a wavelength of 928.7 nm. What is its redshift? Note that the rest wavelength of the H emission line is 656.3 nm.

ANS: z  (obs

rest)/rest  (928.7 nm 656.3 nm)/ 656.3 nm  272.4 nm/656.3 nm  0.42.

DIF: Medium REF: Section 19.3 MSC: Applied TOP: 3Iii 5. The spectrum of a galaxy is observed to have an H emission line at a wavelength of 856.3 nm. What is its distance if the Hubble constant is 70 km/s/Mpc? Note that the rest wavelength of the H emission line is 656.3 nm.

ANS: z  (obs

rest)/rest  (856.7 nm 656.3 nm)/ 656.3 nm  200 nm/656.3 nm  0.30.

d  vr/H0  zc/H0  0.3  (3  105 km/s) / (70 km/s/Mpc)  1,290 Mpc.

DIF: Medium REF: Section 19.3 MSC: Applied TOP: 3Iii | 3Iiii 6. We see the universe around us expanding, which gives distant galaxies an apparent velocity of 70 km/s/Mpc. If you were an astronomer living today in a galaxy that was located 1 billion light years away from us, at what rate would you see the galaxies moving away from you?

ANS: Because the universe appears to be homogeneous, all observers would measure the same rate of the universe’s expansion. You would therefore also see that galaxies are moving away at a rate of 70 km/s/Mpc. DIF: Easy REF: Section 19.3 MSC: Conceptual TOP: 3Iiv 7. What is a standard candle, and why is it useful for measuring distances?

ANS: A standard candle is any object that has a known luminosity. Standard candles are used to determine distances by observations of their brightness from Earth: since we know the luminosity, we can figure out how far away they are located in order to be at the brightness we observe.

DIF: Medium REF: Section 19.3 MSC: Conceptual TOP: 3IIi 8. Name three “rungs” in the distance ladder that let us estimate the value of the Hubble constant.

ANS: Trigonometric parallax, period-luminosity relation for Cepheid stars, and luminosities of Type I supernovae.

DIF: Medium REF: Section 19.3 MSC: Factual TOP: 3IIii 9. Order the following objects by the maximum distance they can be detected: Cepheid variables, RR Lyrae stars, Type I supernovae, and low-mass main-sequence stars. Explain your reasoning.

ANS: Objects can be seen to farther distances if they are more luminous. So placing these objects in order of their luminosities will also place them in order of the maximum detectable distance. Thus: lowmass main-sequence stars, RR Lyrae stars, Cepheid variables, and Type I supernovae.

DIF: Difficult REF: Section 19.3 MSC: Applied TOP: 3IIii | 3IIiii 10. If the Hubble constant were equal to 50 km/s/Mpc, what would the approximate age of the universe (the Hubble time) be, assuming that the expansion rate has stayed approximately constant over time? Note that 1 Mpc  3.1  1019 km and 1 year  3.17  107 s.

ANS: The age of the universe would be: 1/H0  1 / (50 km/s/Mpc)  (1 s Mpc)/(50 km)  (1 s  3.1  1019 km  (1yr / 3.1  107 s)) / (50 km) 

2  1010 yr  20 billion yr.

DIF: Difficult REF: Section 19.4 MSC: Applied TOP: 4Iii 11. Does Hubble’s law imply that our galaxy is sitting at the center of the universe? Explain.

ANS: Our galaxy is not at the center of the universe. According to Hubble’s law, all other galaxies appear to be moving away from us, with their speed directly related to their distances from us. However, when viewed from another galaxy, the results would be the same. All Hubble’s law tells us is that the universe is expanding uniformly in all directions.

DIF: Medium REF: Section 19.4 MSC: Conceptual TOP: 4IIi 12. What does the value of RU, the scale factor of the universe, tell us?

ANS: The scale factor tells us how far away objects were at various times relative to the present day.

DIF: Easy REF: Section 19.4 MSC: Factual TOP: 4IIiii 13. You observe a distant quasar in which the Lyman alpha line of hydrogen (rest wavelength  121.6 nm) is found at a wavelength of 547.2 nm. When the light from this quasar was emitted, how large was the universe compared to its current size?

ANS: Using the formula for the redshift, this quasar has a redshift of z  (547.2 nm

121.6 nm) / 121.6

nm, or z  3.5. The expansion factor of the universe is given by 1  z, so at the time the universe was 1/(1  z)  1/(1  3.5)  0.22 its current size.

DIF: Difficult REF: Section 19.4 MSC: Applied TOP: 4IIiii 14. The Hubble Space Telescope can be used to study galaxies at a redshift equal to 2. How much has the universe expanded since that the light was emitted from these galaxies?

ANS: The expansion factor at z  2 is RU(z)  1/(1  z)  1/(1  2)  1/3. Thus, the universe has expanded by a factor of 3 since a redshift of 2.

DIF: Medium REF: Section 19.4 MSC: Applied TOP: 4IIiii 15. Does the expansion of the Universe make the Sun bigger? What about the Milky Way? Why or why not?

ANS: The expansion of the universe is only detectable on very large distance scales, much larger than the size of the Sun or the Milky Way. The expansion of the universe does not affect things that are bound together by gravity, such as stars or galaxies.

DIF: Easy REF: Section 19.4 MSC: Factual TOP: 4IIiv

16. What important event in the universe’s history marked the creation of the cosmic background radiation?

ANS: The CMB is a relic of the time in the universe’s history when free electrons combined with free nuclei in order to make the universe neutral and transparent to photons. The photons in the CMB have been traveling through the universe essentially uninterrupted since that time.

DIF: Medium REF: Section 19.5 MSC: Factual TOP: 5Iii 17. The cosmic microwave background (CMB) has a temperature of approximately 2.7 K and a Planck blackbody spectrum. Calculate the wavelength where the CMB spectrum peaks.

ANS:

peak  2,900 m K/T  2,900 m. K/2.7 K  1,000 m  1 m.

DIF: Medium REF: Section 19.5 MSC: Applied TOP: 5Iv 18. The COBE and WMAP satellites detected fluctuations in the CMB. On average, how big were these fluctuations, and what does that tell us about the universe at a redshift of z  1,000?

ANS: After correcting for the dipole on the sky caused by the Milky Way’s motion, the COBE and WMAP satellites detected tiny fluctuations in the CMB at the level of 1 part in 105, which tells us that structure had formed in the early universe (it was not totally smooth) even before the matter and radiation stopped interacting strongly with one another.

DIF: Difficult REF: Section 19.5 MSC: Factual TOP: 5IIii 19. Name two predictions of the standard Big Bang theory that have been verified by observations.

ANS: The existence of cosmic microwave background (CMB) and primordial abundances of H, D, 3He, 4He, and 7Li.

DIF: Easy REF: Section 19.6 MSC: Factual TOP: 6Iii | 6Iiii 20. If the density of normal matter, such as protons and electrons, were higher in the early universe, would there have been more or less 4He made in the Big Bang?

ANS: If there were more protons and electrons around in the early universe, then more 4He would have been made because there would have been more particles around to form it.

DIF: Easy REF: Section 19.6 MSC: Applied TOP: 6Iiv

CHAPTER 20

Galaxies

CONCEPT MAP Sec 20.1 1. Galaxies Come in Many Types I. Galaxy Shapes i. The apparent 2-D shapes of galaxies tell us their 3-D shapes (TF: 1, MC: 1, SA: 1) ii. The Hubble sequence classifies galaxies by their appearance; it is not an evolutionary sequence (TF: 2–3, MC: 2–4, SA: 2) iii. Elliptical (E) galaxies: round or elliptical in shape, E0  round; E7  highly flattened (SA: 3) iv. Spiral (S) galaxies: flattened disks with spiral arms; Sa  large bulge to disk ratio with tightly wound, smooth spiral arms; Sd  little or no bulge with loosely wound, knotty spiral arms; barred spiral galaxies (SB) form a parallel classification from SBa to SBd (MC: 5–6, SA: 3) v. Irregular (Irr) galaxies: galaxies that are irregular in structure II. Stellar Motions Give Galaxies Their Shapes i. E galaxies have randomly oriented stellar orbits; the longest dimension represents the direction with the fastest stellar orbits (TF: 4–5, MC: 7–8) ii. Stars move in aligned circular orbits in spiral disks (MC: 9) iii. The stars in spiral bulges have more random orbits than in the disk (MC: 9) III. Other Differences among Galaxies i. S galaxies have cold gas, dust, and current star formation in their disk (MC: 10, SA: 4) ii. E galaxies have hot, diffuse, X-ray emitting gas and no star formation; the gas is heated by Type I supernovae (TF: 6, MC: 10–13, SA: 4) iii. Young, massive stars make spiral arms appear blue; their disks also contain older stars (MC: 14– 15)

iv. Irr galaxies have lots of gas and high star formation rates. v. Galaxies span a very wide range in luminosity: 104 to 1012 L vi. The most luminous galaxies are Es at the centers of galaxy clusters. vii. Motions of stars and gas or smoothness of light distribution help determine distances (MC: 16) viii. Only Es and Irrs come in both dwarf (dE, dIrr) and giant types (TF: 7) Sec 20.2 2. In Spiral Galaxies, Stars Form in the Spiral Arms I. Spiral Arms i. UV and 21-cm light can be used to trace star formation in a spiral’s disk ii. Spiral arms have more cold gas and dust and higher star formation rates, but stars and new star formation are spread throughout the disk (TF: 8, MC: 17–18) iii. Dust absorption is a prominent feature of spiral arms iv. Gravitational disturbances in the disk will induce spiral structure (TF: 9, MC: 19, SA: 5) v. Energy from UV radiation and supernovae can induce new star formation. vi. Barred spirals often have two-armed spiral structure and elongated (nonspherical) bulge vii. Spiral density waves do not rotate at the same speed the disk material rotates (MC: 20–21) Sec 20.3 3. Galaxies Are Mostly Dark Matter I. Forms of Matter i. Luminous or “normal” matter: stars, gas, and dust ii. Measuring the rotation curve (velocity of stars or gas as a function of radius) in a galaxy tells us how the mass is distributed (MC: 22–28, SA: 6–7) iii. The Doppler shift is used to measure the velocities of stars or gas; velocities should decrease as v r 1⁄2 outside an object (TF: 10, MC: 24, 29, SA: 8) iv. Rotation curves of spiral galaxies are flat even well beyond the optical extent of the galaxy, implying galaxies are made mostly of dark matter (MC: 24, 30) v. Dark matter: material having mass but not emitting or absorbing radiation (TF: 11) vi. Dark matter is probably exotic elementary particles that have not yet appeared in laboratory experiments; less-favored candidates are MACHOs (Jupiter-like objects, black holes, or white dwarfs) (MC: 31–33, SA: 9) II. Shape of the Mass Distribution

i. In the inner regions of galaxies, luminous matter dominates the mass distribution (MC: 34) ii. X-ray emitting gas can be used to study the dark matter in E galaxies (MC: 35–36, SA: 10) iii. The dark matter in galaxies is more extended and less centrally concentrated than the luminous matter and contains approximately 90 percent of the galaxy’s total mass (TF: 12–13, MC: 37–38) Sec 20.4 4. Most Galaxies Have a Supermassive Black Hole at the Center I. Active Galaxies i. Quasars: quasi-stellar radio sources that usually come from a faint, blue, starlike source with 1012 L  L  1015 L (MC: 39–40, SA: 11) ii. Quasars are quite rare in today’s universe (MC: 41–42) iii. Quasars lie at the heart of very luminous, and often violently interacting, galaxies (MC: 43) iv. Active galactic nuclei (AGNs): point-like with luminosities of 10 to 100 billion L; they are the lowluminosity analogs to quasars and arise from accreting black holes (MC: 44) v. Approximately 3 percent of galaxies in the nearby universe contain AGNs (TF: 14, MC: 45) vi. Seyfert galaxies: S galaxies with AGNs with bright emission lines, which can be broad or narrow in velocity width, that can be used to map the gas rotating around the central black hole (TF: 15, SA: 11–12) vii. Radio galaxies: E galaxies with powerful lobes of synchrotron emission and/or a jet emitted by a central black hole (SA: 11, 13) viii. From the timescale on which an AGN’s light varies, we know it emanates from a region the size of the Solar System (TF: 16, MC: 46–49, SA: 14–15) II. Supermassive Black Holes and Accretion Disks Power AGNs i. AGNs are powered by the liberation of gravitational energy as matter accretes onto a supermassive black hole (MC: 49) ii. Quasars, Seyferts, and radio galaxies can all be explained with a model of a supermassive black hole with an accretion disk surrounding it and a dusty torus outside the accretion disk (MC: 50, SA: 16) iii. Clouds in the accretion disk or surrounding it give off emission lines iv. The interaction of the accretion disk with the black hole powers a jet of material III. Which Type of AGN We See Depends on Our Viewing Angle i. Viewed edge on, an AGN’s accretion disk is blocked by the torus; viewed pole on, we can see ac-

cretion disks and jets of AGNs (TF: 17, SA: 17) ii. Particles in the jet are moving at nearly the speed of light and exhibit relativistic effects such as relativistic beaming (MC: 51) iii. Sometimes we see only one side of a two-sided jet because only one side is beamed into our line of sight (MC: 53) iv. Sometimes the velocities in the relativistic jet appear faster than the speed of light (superluminal motion), but in reality they are not IV. Normal Galaxies and AGNs i. Every giant galaxy may contain a supermassive black hole at its center (TF: 18) ii. AGNs are seen only when material is accreting onto the black hole; most times its is not accreting and is invisible (TF: 19, SA: 18) iii. There were more AGNs in the past than today, probably because galaxy mergers were more common in the past (MC: 54, SA: 19–20) iv. Supermassive black holes with masses of 104 to 5  109 M have been discovered at the centers of many nearby galaxies (TF: 20) v. More massive galaxies have more massive black holes (MC: 55)

TRUE/FALSE 1. We can determine the three-dimensional shape of any individual elliptical galaxy by photographing it.

ANS: F DIF: Easy REF: Section 20.1 MSC: Applied TOP: 1Ii 2. The Hubble tuning fork diagram represents an evolutionary diagram of galaxies from elliptical to spiral.

ANS: F DIF: Easy REF: Section 20.1 MSC: Factual TOP: 1Iii 3. The Hubble diagram is a way of sorting galaxies by their visual appearance.

ANS: T DIF: Easy REF: Section 20.1 MSC: Factual TOP: 1Iii 4. The 3-D shape of an elliptical galaxy depends on the average velocity of its stars along each of the three axes.

ANS: T DIF: Medium REF: Section 20.1 MSC: Factual TOP: 1IIi

5. Elliptical galaxies have the shapes they do because all the stars are orbiting in the same direction in the galaxy.

ANS: F DIF: Easy REF: Section 20.1 MSC: Factual TOP: 1IIi 6. Elliptical galaxies contain hot gas that is heated by supernova explosions.

ANS: T DIF: Easy REF: Section 20.1 MSC: Factual TOP: 1IIIii 7. Although elliptical and irregular galaxies have been observed in both dwarf and giant varieties, we find only giant spiral galaxies.

ANS: T DIF: Easy REF: Section 20.1 MSC: Factual TOP: 1IIIviii 8. Star formation in spiral galaxies is enhanced in the spiral arms because more hot gas is concentrated there.

ANS: F DIF: Difficult REF: Section 20.2 MSC: Factual TOP: 2Iii 9. Any asymmetry in the gravity field of a spiral galaxy can create spiral arms.

ANS: T DIF: Medium REF: Section 20.2 MSC: Factual TOP: 2Iiv 10. We know that spiral galaxies contain dark matter because the gas in the disk rotates with a velocity that decreases in a way that is proportional to r-1/2.

ANS: F DIF: Medium REF: Section 20.3 MSC: Applied TOP: 3Iiii 11. Dark matter consists of material that emits radiation at wavelengths other than visible light.

ANS: F DIF: Medium REF: Section 20.3 MSC: Factual TOP: 3Iv 12. Dark matter is distributed inside galaxies in exactly the same way as luminous matter, although the total mass of dark matter is much larger than that of luminous matter.

ANS: F DIF: Medium REF: Section 20.3 MSC: Factual TOP: 3IIiii 13. In most galaxies, the amount of dark matter and the amount of luminous matter is about equal.

ANS: F DIF: Medium REF: Section 20.3 MSC: Factual TOP: 3IIiii 14. Today in our local universe, approximately 30 percent of galaxies are AGNs. ANS: F DIF: Medium REF: Section 20.4 MSC: Factual TOP: 4Iiv

15. Seyfert galaxies are unique because they have large lobes of radio emission that extend well beyond the visible part of the galaxy.

ANS: F DIF: Medium REF: Section 20.4 MSC: Factual TOP: 4Ivi | 4Ivii 16. We know AGN are about the size of our Solar System because of how fast they change in brightness over time.

ANS: T DIF: Medium REF: Section 20.4 MSC: Factual TOP: 4Iviii 17. The type of AGN we see depends mostly on the angle at which we view the galaxy.

ANS: T DIF: Easy REF: Section 20.4 MSC: Conceptual TOP: 4IIIi 18. Astronomers have evidence that all giant galaxies probably contain supermassive black holes.

ANS: T DIF: Easy REF: Section 20.4 MSC: Factual TOP: 4IVi 19. A galaxy can contain a supermassive black hole that remains invisible because no matter is falling onto the black hole.

ANS: T DIF: Medium REF: Section 20.4 MSC: Conceptual TOP: 4IVii 20. Galaxies that are classified as quasars were much more numerous in the past.

ANS: T DIF: Easy REF: Section 20.4 MSC: Factual TOP: 4IViv

MULTIPLE CHOICE 1. We classify galaxies by: a. their true shapes b. how they appear on the sky c. the amount of gas and dust in them d. whether they are near or far from us e. the amount of dark matter they contain ANS: B DIF: Easy REF: Section 20.1 MSC: Conceptual TOP: 1Ii

2. The Hubble classification scheme for galaxies sorts them by: a. their evolutionary state b. their mass c. the amount of dust d. the amount of dark matter e. their visual appearance ANS: E DIF: Easy REF: Section 20.1 MSC: Factual TOP: 1Iii 3. What type of galaxy is this?

a. a giant elliptical b. an ordinary spiral galaxy c. an irregular galaxy d. a barred spiral e. a dwarf elliptical ANS: D DIF: Easy REF: Section 20.1 MSC: Applied TOP: 1Iii

4. What type of galaxy is this?

a. a giant elliptical b. a regular spiral galaxy c. an irregular galaxy d. a barred spiral e. a dwarf elliptical ANS: A DIF: Easy REF: Section 20.1 MSC: Applied TOP: 1Iii 5. In spiral galaxies, the size of the central bulge is correlated with the: a. tightness of the spiral arms b. luminosity of the galaxy c. age of the galaxy d. thickness of the disk e. presence of an active nucleus ANS: A DIF: Easy REF: Section 20.1 MSC: Factual TOP: 1Iiv

6. Approximately __________ of spiral galaxies are barred. a. 10 percent b. 25 percent c. 35 percent d. 50 percent e. 75 percent ANS: D DIF: Medium REF: Section 20.1 MSC: Factual TOP: 1Iiv 7. Stars in elliptical galaxies have velocities that: a. change constantly b. are very large in size c. are equal in all directions d. are constant in time e. are orientated in random in directions ANS: E DIF: Medium REF: Section 20.1 MSC: Factual TOP: 1IIi 8. Stars in the disks of spiral galaxies have orbits that: a. are randomly oriented b. constantly are getting larger c. mostly aligned in the same plane d. spiral shaped e. unaffected by dark matter ANS: C DIF: Medium REF: Section 20.1 MSC: Factual TOP: 1IIi 9. The three-dimensional shape of __________ is set by the average velocities of the stars along each of the three axes. a. spiral galaxies b. only elliptical galaxies c. spiral bulges d. elliptical galaxies and spiral bulges e. quasars ANS: D DIF: Medium REF: Section 20.1 MSC: Factual TOP: 1IIii | 1IIiii

10. Which of the following is NOT a difference between spiral and elliptical galaxies? a. Spiral galaxies still have ongoing star formation whereas elliptical galaxies do not. b. Spiral galaxies are sometimes very thin in appearance whereas elliptical galaxies are always round or elliptical. c. Spiral galaxy disks contain lots of cold gas and dust whereas elliptical galaxies do not. d. Spiral galaxies tend to have bluish areas whereas elliptical galaxies tend to be all reddish. e. Spiral galaxies have dark matter halos whereas elliptical galaxies do not.

ANS: E DIF: Medium REF: Section 20.1 MSC: Applied TOP: 1IIIi | 1IIIii 11. Active star formation does not occur in elliptical galaxies because they: a. rotate too fast b. contain little molecular hydrogen c. are too massive d. are too far away e. usually contain active nuclei ANS: B DIF: Easy REF: Section 20.1 MSC: Applied TOP: 1IIIii 12. Elliptical galaxies appear red because: a. they are moving away from us b. they contain mostly ionized hydrogen gas c. they contain mostly old stars d. they contain lots of dust e. they contain a mix of old and young stars ANS: C DIF: Easy REF: Section 20.1 MSC: Applied TOP: 1IIIii 13. Which of the following would NOT be found in an elliptical galaxy? a. novae b. Type I supernovae c. Type II supernovae d. planetary nebulae e. hot gas ANS: C DIF: Difficult REF: Section 20.1 MSC: Applied TOP: 1IIIii

14. The disks of spiral galaxies appear blue because: a. they are moving toward us b. they contain a relatively high concentration of low-mass stars c. they contain active regions of star formation d. they contain more metals that, when ionized, emit blue light e. stars collide with each other frequently in these dense regions and explode as Type I supernovae ANS: C DIF: Medium REF: Section 20.1 MSC: Applied TOP: 1IIIiii 15. The image below was probably taken in ____________ wavelengths because _______________________.

a. radio; the disk probably contains many planets with intelligent life, who are beaming out radio and television signals into space b. infrared; the central regions are bright, and cool stars in the bulges emit a lot of infrared light c. visible; there is a lot dust in this galaxy that blocks light in a ring about halfway along the disk d. ultraviolet; the light from hot young stars is mainly found along spiral arms e. X-ray; the central nucleus is easily seen, and AGN emit a lot of X-rays ANS: D DIF: Medium REF: Section 20.1 MSC: Applied TOP: 1IIIiii

16. Two elliptical galaxies have the same apparent size and brightness. Galaxy A appears lumpy, and it is possible to observe individual stars and star clusters. The other one, galaxy B, appears very smooth, with no fine detail. Which one is likely to be more luminous? a. Galaxy A. It has to contain more luminous objects if we can see them individually, and so the galaxy as a whole has to be more luminous. b. Galaxy A. The lumpiness means that there is a lot of dust in the galaxy, which absorbs some of the starlight. Therefore the total amount actually emitted must be larger, if it is to have the same brightness as the other galaxy. c. Neither. If they have the same apparent size and brightness, they must be at equal distances. d. Galaxy B. This galaxy appears smooth because it is farther away and the light from its stars all blends together. Because it is farther away, it has to be more luminous. e. Galaxy B. The smooth light distribution means that the stars are moving around very quickly. Therefore the galaxy has more mass and is more luminous.

ANS: D DIF: Difficult REF: Section 20.1 MSC: Applied TOP: 1IIIvii 17. Which of the following are NOT found mainly in the spiral arms of galaxies? a. molecular clouds b. H II regions c. K- and M-type stars d. Type II supernovae e. O-type stars ANS: C DIF: Medium REF: Section 20.2 MSC: Applied TOP: 2Iii 18. Why is 21-cm radiation a good tracer of spiral arm structure? a. Emitted by neutral hydrogen, it traces the location of high concentrations of gas. b. Emitted by O and B stars, it traces the location of newly formed stars. c. Emitted by ionized hydrogen, it traces the location of H II regions. d. Emitted by molecular hydrogen, it traces the location of clouds in the process of collapsing into stars. e. Emitted by red supergiants, it traces the presence of massive stars that are experiencing high rates of mass loss.

ANS: A DIF: Medium REF: Section 20.2 MSC: Applied TOP: 2Iii

19. Spiral structure can be triggered in a disk galaxy by: a. the merger of close binary stars b. a supernova explosion c. the presence of an active nucleus d. the gravity of distant galaxies e. any disturbance to the disk ANS: E DIF: Easy REF: Section 20.2 MSC: Conceptual TOP: 2Iiv 20. Spiral structure: a. rotates at the same speed as the gas in the disk b. rotates at a speed that is different than the gas in the disk c. stays constant in time as the gas in disk rotates through it d. rotates backward compared to the stellar orbits e. must only last a short time, since few galaxies with disks show spiral patterns ANS: B DIF: Difficult REF: Section 20.2 MSC: Conceptual TOP: 2Ivii 21. Long-lasting spiral density waves occur in the disk of a spiral galaxy when: a. the spiral merges with another galaxy b. the galaxy is young c. the galaxy’s disk is regularly disturbed d. the galaxy absorbs a satellite galaxy e. a supernova explosion takes place ANS: C DIF: Medium REF: Section 20.2 MSC: Conceptual TOP: 2Ivii 22. Which of these would NOT be useful for measuring the mass of the dark matter halo in a spiral galaxy? a. H emission from H II regions in the spiral disk b. the velocities of satellite dwarf galaxies surrounding a spiral galaxy c. absorption lines in the spectra of its globular clusters d. the velocities of star clusters in the outer disk e. all of these can be used to measure the mass of the dark matter ANS: E DIF: Difficult REF: Section 20.3 MSC: Applied TOP: 3Iii

23. A galaxy rotation curve is a plot of the rotation speed as a function of the: a. galaxy’s luminosity b. mass of the dark matter halo c. brightness of the luminous matter in the galaxy d. radius from the center e. ages of star clusters ANS: D DIF: Easy REF: Section 20.3 MSC: Factual TOP: 3Iii 24. Why do rotation curves of galaxies imply that dark matter exists? a. The velocity of stars rises dramatically at large distances from the centers of galaxies, implying large amounts of dark matter cause these stars to move much faster than stars at smaller distances. b. The velocity of stars stays approximately constant at distances well beyond the visible edges of galaxies, implying that galaxies contain much more matter than what we observe in stars and gas. c. The velocity of stars is low in the inner regions of galaxies, suggesting that dark matter is impeding their motion. d. The velocity of stars rises quickly in the inner regions of galaxies, suggesting that unseen matter is gravitationally moving this material at faster rates than expected. e. The velocity of stars falls slowly in the outer regions of galaxies, showing that while there is a lot of dark matter it is distributed the same way as is the visible light.

ANS: B DIF: Medium REF: Section 20.3 MSC: Conceptual TOP: 3Iii | 3Iiii | 3Iiv 25. If you measure the velocity of a cloud of gas that is rotating around the center of a galaxy in a circular orbit with radius R, you can determine: a. the total mass of the galaxy b. the mass of the stars and gas in the galaxy c. the mass of the galaxy enclosed within the radius R d. the mass of the galaxy located outside the radius R e. the mass of luminous matter within the radius R ANS: C DIF: Easy REF: Section 20.3 MSC: Conceptual TOP: 3Iii

26. If you observed a gas cloud orbiting in a disk of a spiral galaxy that was located 10,000 pc from the center and it rotated with a speed of 300 km/s, what is the mass of the galaxy interior to gas cloud’s orbit? a) 1  1011 M b) 2  1011 M c) 8  1011 M d) 1  1012 M e) 4  1012 M ANS: B DIF: Difficult REF: Section 20.3 MSC: Applied TOP: 3Iii 27. If the rotational speed of the gas in the disk of a spiral galaxy is 200 km/s at a distance of 25,000 pc from its center, then what is the mass enclosed within this radius? a. 7.5 billion M b. 18 billion M c. 70 billion M d. 230 billion M e. 400 billion M ANS: D DIF: Difficult REF: Section 20.3 MSC: Applied TOP: 3Iii 28. If you measure the spectrum emitted by a cloud of gas that orbits a nearby spiral galaxy and the observed wavelength of a hydrogen emission line is 658.4 nm while the rest wavelength of that line is 656.3 nm, what is the velocity of this gas cloud along your line of sight (i.e., the radial velocity)? Note that the speed of light is 3  105 km/s. a. 960 km/s b. 1,200 km/s c. 2,300 km/s d. 3,800 km/s e. 4,400 km/s ANS: A DIF: Difficult REF: Section 20.3 MSC: Applied TOP: 3Iii

29. If a galaxy had no dark matter, how would the velocity (v) of a star that orbited far outside the visible extent of the galaxy depend on its distance (r) from the center of the galaxy? a. 

r2

b. 

r1

c.  d. 

r

e. 

r2

ANS: C DIF: Difficult REF: Section 20.3 MSC: Applied TOP: 3Iiii 30. What makes up the majority of the mass of an individual spiral galaxy? a. a central supermassive black hole b. dark matter c. massive O- and B-type stars d. cold molecular gas clouds e. low-mass G-, K-, and M-type stars ANS: B DIF: Easy REF: Section 20.3 MSC: Factual TOP: 3Iiv 31. Dust in galaxies does not count as dark matter because: a. it only absorbs some wavelengths of light but not all wavelengths the way dark matter does b. it interacts with light c. it is only found in spiral disks, whereas dark matter is more widely distributed d. dust is easily destroyed by dark matter e. there is never enough to make all the gravity ANS: B DIF: Easy REF: Section 20.3 MSC: Factual TOP: 3Ivi 32. Dark matter is most likely made up of: a. elementary particles that have mass but do not interact much with normal matter b. supermassive black holes c. faint stellar remnants such as white dwarfs and neutron stars d. free-floating Jupiter-mass planets e. cold concentrations of dust ANS: A DIF: Easy REF: Section 20.3 MSC: Factual TOP: 3Ivi

33. How have astronomers searched for evidence of MACHOs in the halo of the Milky Way? a. They search for stars that are binary companions of MACHOs. b. They search for rapidly moving, massive gas clouds. c. They search for disturbances in the background of gravitational waves. d. They search for stars whose light is briefly amplified by gravitational lensing. e. They search for close binary stars undergoing mass transfer.

ANS: D DIF: Easy REF: Section 20.3 MSC: Applied TOP: 3Ivi 34. The density of ordinary luminous matter can exceed the density of dark matter in which parts of galaxies? a. everywhere inside galaxies b. only in the outer regions of galaxies c. only in star clusters d. near the centers of galaxies e. only around an AGN ANS: D DIF: Easy REF: Section 20.3 MSC: Factual TOP: 3IIi 35. How did astronomers determine that elliptical galaxies are composed mostly of dark matter? a. They measured the velocities of stars in the inner regions of the galaxies. b. They measured the rotation rates of gas as a function of distance from the centers of the galaxies. c. They measured the amount of gravitational pull elliptical galaxies have on companion galaxies. d. They assumed that the proportion of dark matter was roughly the same as in spiral galaxies. e. They measured X-ray emission from hot gas gravitationally bound to the galaxies.

ANS: E DIF: Medium REF: Section 20.3 MSC: Factual TOP: 3IIii 36. If elliptical galaxies do not have cold gas clouds for which we can measure the Doppler shift, then how can we measure the mass of an elliptical galaxy? a. by measuring the speeds of stars in the nucleus b. by observing the velocity of the hot, X-ray emitting gas that surrounds the galaxy c. by measuring the number of H II regions in the galaxy d. by measuring how many supernovae go off each century e. by measuring the amount of blue light in the galaxy ANS: B DIF: Medium REF: Section 20.3 MSC: Conceptual TOP: 3IIii

37. The dark matter in galaxies is __________ the luminous material. a. distributed in roughly the same way as b. much more extended in radius than c. much more centrally concentrated than d. much less uniform in density than e. much clumpier than ANS: B DIF: Easy REF: Section 20.3 MSC: Factual TOP: 3IIiii 38. Roughly what percentage of the total mass of a galaxy is made up of luminous, or normal, matter? a. 5 percent b. 25 percent c. 33 percent d. 50 percent e. 90 percent ANS: A DIF: Easy REF: Section 20.3 MSC: Factual TOP: 3IIiii 39. Quasars were first discovered in which region of the electromagnetic spectrum? a. X-ray b. gamma-ray c. radio d. infrared e. visible ANS: C DIF: Medium REF: Section 20.4 MSC: Factual TOP: 4Ii 40. If the average quasar has a luminosity of 1014 L and an absolute magnitude of 30, and you discover one that has an apparent magnitude of 10, then what is its distance? a. 10,000 Mpc b. 5,000 Mpc c. 1,000 Mpc d. 500 Mpc e. 0.1 Mpc ANS: C DIF: Difficult REF: Section 20.4 MSC: Applied TOP: 4Ii

41. Which of these statements about quasars is FALSE? a. Quasars are extremely luminous. b. Quasars are rare in our local vicinity. c. Quasars have high brightnesses. d. Quasars are extremely massive. e. Quasars are located at the centers of some active galaxies.

ANS: C DIF: Difficult REF: Section 20.4 MSC: Applied TOP: 4Iii | 4Iiii 42. Assume that when a supermassive black hole accretes gas, approximately 50 percent of its mass is converted directly into energy, which is radiated away. If a quasar has a luminosity of 1014 L then what must its mass accretion rate be? a. 1 M per year b. 10 Mper year c. 1 M per century d. 10 M per century e. 1 M per million years ANS: B DIF: Difficult REF: Section 20.4 MSC: Applied TOP: 4Iii 43. What do the broad emission lines in quasar spectra tell astronomers about these objects? a. They are very far away. b. They are moving away from us. c. They have rapid internal motions. d. They are very dense. e. They have few heavy elements.

ANS: C DIF: Medium REF: Section 20.4 MSC: Conceptual TOP: 4Iiv

44. AGNs appear to be powered by: a. extremely dense star clusters b. supermassive black holes c. ultradense molecular clouds d. decaying dark matter e. supernova explosion ANS: B DIF: Easy REF: Section 20.4 MSC: Conceptual TOP: 4Iiv 45. What is the approximate percentage of nearby galaxies that contain AGNs? a. 3 percent b. 15 percent c. 20 percent d. 50 percent e. 80 percent ANS: A DIF: Medium REF: Section 20.4 MSC: Factual TOP: 4Iv 46. How do we know that AGNs have sizes on the order of our Solar System? a. Quasars and Seyfert galaxies are stellar-like sources. b. Their brightness varies by factors of a few. c. The emission lines in their spectra show gas rotating at speeds of thousands of km/s. d. Their brightness varies on timescales ranging from hours to a day. e. We can measure their angular size, which gives us the physical size when we know the distance.

ANS: D DIF: Medium REF: Section 20.4 MSC: Applied TOP: 4Iviii 47. If a Seyfert galaxy’s nucleus varies in brightness on the timescale of 10 hours, then approximately what is the size of the emitting region? a. 20 AU b. 30 AU c. 50 AU d. 70 AU e. 120 AU ANS: D DIF: Difficult REF: Section 20.4 MSC: Applied TOP: 4Iviii

48. You observe a quasar that varies in brightness over a period of 50 days as shown in the figure below.

Using these data, estimate the size of the AGN region in AU.

a. 20 AU b. 100 AU c. 500 AU d. 3,500 AU e. 10,000 AU ANS: D DIF: Difficult REF: Section 20.4 MSC: Applied TOP: 4Iviii 49. The unified model of AGN suggests that quasars, Seyfert galaxies, and radio galaxies: a. are unrelated, although they are all very luminous galaxies at radio wavelengths b. are powered by similar mass accretion rates onto supermassive black holes c. are driven by violent mergers of two gas-rich spiral galaxies d. are similar phenomena but viewed from different orientation angles e. are all powered by high rates of star formation and supernova explosions ANS: D DIF: Easy REF: Section 20.4 MSC: Conceptual TOP: 4IIi

50. What do the formation of the Solar System and the existence of AGNs have in common? a. They both demonstrate the importance of how luminosity varies with time. b. They both demonstrate the high efficiency of nuclear fusion. c. They both demonstrate the importance of accretion disks. d. They both demonstrate the importance of living in the center of a galaxy. e. They both demonstrate the existence of dark matter.

ANS: C DIF: Medium REF: Section 20.4 MSC: Conceptual TOP: 4IIii 51. How do we measure directly that AGNs have black holes at their centers? a. We know that massive stars make black holes, and most galaxies contain a lot of massive stars. b. We know black holes are made of dark matter, and galaxies are mostly made up of dark matter. c. We know that black holes emit radio waves, and many AGNs are bright at radio waves. d. We measure that the mass of the black hole is increasing as material falls into it. e. In some systems, we can see both redshifted and blueshifted emission lines from material orbiting around the black hole.

ANS: E DIF: Medium REF: Section 20.4 MSC: Applied TOP: 4IIIii 52. In some radio galaxies, we only see one side of the jet because: a. the black hole only produces a jet in one direction b. of relativistic beaming c. the other side is inside the black hole’s event horizon d. of dust obscuration e. of dark matter that blocks the light from one side of the jet ANS: B DIF: Medium REF: Section 20.4 MSC: Conceptual TOP: 4IIIiii 53. What happens to active galaxies when their AGNs run out of fuel? a. The black holes powering them collapse even further. b. They simply become normal galaxies. c. They slowly dissipate into clouds of gas and dust. d. They eventually pull in material from farther out, starting the AGN process all over again. e. They explode as gamma-ray bursts.

ANS: B DIF: Easy REF: Section 20.4 MSC: Applied TOP: 4IVii

54. We interpret the fact that we saw more AGNs in the past than we do today to indicate that: a. there were fewer galaxy-galaxy interactions in the past b. there were more galaxy-galaxy interactions in the past c. supermassive black holes were larger in the past d. today’s AGNs are primarily hidden by large amounts of cold gas and dust e. AGNs had bigger central black holes in the past ANS: B DIF: Medium REF: Section 20.4 MSC: Conceptual TOP: 4IViii 55. More luminous giant galaxies tend to have __________ supermassive black holes at their centers. a. more massive b. radio quiet c. more luminous d. slower rotating e. less massive ANS: A DIF: Easy REF: Section 20.4 MSC: Conceptual TOP: 4IVv

SHORT ANSWER 1. What determines the three-dimensional shapes of galaxies?

ANS: The velocities of stars and the orientation of the stellar orbits determine their shapes.

DIF: Easy REF: Section 20.1 MSC: Factual TOP: 1Ii 2. How did Hubble use his tuning fork diagram to classify galaxies? Is there any significance to this organization?

ANS: Elliptical and spiral galaxies are organized along different portions of the diagram. Along the handle, elliptical galaxies are organized based on their how round they appear, with galaxies arranged by increasing flatness. Along the tines, spirals (and barred spirals) are organized based on the prominence of their central bulge as well as the tightness of their spiral arms. Although Hubble originally organized galaxies in this manner in an effort to determine if there was some sort of evolutionary sequence from one type of galaxy to another, today we know that this is not so.

DIF: Medium REF: Section 20.1 MSC: Conceptual TOP: 1Iii

3. How do we know that the stellar disks in spiral galaxies are flat? How do we know that elliptical galaxies are not flat?

ANS: Individual galaxies are oriented at a variety of angles to our line of sight. Sometimes we see that spirals look round on the sky; this means we are looking at the disk face-on. Sometimes spirals look quite thin, which means we are seeing these edge-on. The fraction of galaxies that look thin are what we expect to see if all spiral galaxies have thin disks. Elliptical galaxies, on the other hand, never look thin, so they must be roundish (elongated but not flat) in three dimensions.

DIF: Medium REF: Section 20.1 MSC: Applied TOP: 1Iiii | 1Iiv 4. How do the current star formation rates of spiral and elliptical galaxies compare?

ANS: Spirals contain lots of cold gas in their disks and their current star formation rate is high, but elliptical galaxies have very little or no cold gas (only hot, X-ray-emitting gas) and they have not formed many stars in the last few billion years.

DIF: Easy REF: Section 20.1 MSC: Factual TOP: 1IIIi | 1IIIii 5. Describe two ways by which spiral structure may be induced in a spiral galaxy’s disk.

ANS: Possibilities are: (1) gravitational interactions with other galaxies; (2) gravitational disturbances from a nonspherical central bulge; and (3) star formation triggered by the explosive deaths of previous generations of stars.

DIF: Medium REF: Section 20.2 MSC: Conceptual TOP: 2Iiv 6. What is plotted on a rotation curve, and what can it tell us about a galaxy?

ANS: A rotation curve is a plot of the velocity of stars or gas around the center of the galaxy as a function of the distance from the galaxy’s center. This gives us an indication of the mass of a galaxy, and how the mass is distributed as a function of radius.

DIF: Medium REF: Section 20.3 MSC: Conceptual TOP: 3Iii 7. If the rotational speed of the gas in the disk of a spiral galaxy is 100 km/s at a distance of 26,000 lightyears from its center, then what is the mass enclosed within this radius? If the mass of luminous matter inside this radius is 5 billion M then what fraction of the galaxy’s mass is made of dark matter?

ANS: The radius of the gas is:

r  26,000 ly  26,000  9.5  1015 m  2.5  1020 m and the mass enclosed within this radius is equal to

M  v2 r/G  (105 m/s)2  2.5  1010 m/(6.7  10 11 m3/kg s2)  3.7  1040 kg  (1M/2  1030 kg).

M  1.9  1010 M.

The fraction of mass in dark matter is (M

Mluminous)/M  (19 5)/19  0.74, and thus 74 percent of the

mass inside this radius is made of dark matter.

DIF: Difficult REF: Section 20.3 MSC: Applied TOP: 3Iii 8. Outside the dark matter halo of a galaxy, how should the speed of an orbiting body such as a satellite dwarf galaxy depend on its distance from the giant galaxy’s center?

ANS: When you are outside the giant galaxy’s dark matter halo, the velocity should drop off in a Keplerian way so that  , where r is the distance of the satellite galaxy from the center of the giant galaxy.

DIF: Difficult REF: Section 20.3 MSC: Applied TOP: 3Iiii 9. What is the most likely candidate for making up the dark matter in galaxies?

ANS: The most likely source of the dark matter in galaxies is an elementary particle that does not interact with normal matter and does not radiate much at any wavelength.

DIF: Easy REF: Section 20.3 MSC: Factual TOP: 3Ivi 10. How do we know that many E galaxies contain significant amounts of dark matter?

ANS: Many E galaxies are observed to contain hot, X-ray-emitting gas. Because it is so hot, this gas would quickly evaporate away from the galaxy if the stars were the only objects in the galaxy that contributed to the total amount of gravity. E galaxies therefore contain much more mass than is contained in the stars, and since this mass does not emit or absorb light we attribute it to dark matter.

DIF: Medium REF: Section 20.3 MSC: Factual TOP: 3IIii 11. Give three examples of types of active galactic nuclei and give one reason why each is unique.

ANS: Three types of AGNs: (1) quasars—extremely luminous, stellar-like objects that are really the centers of galaxies at high redshift; (2) Seyfert galaxies—spiral galaxies with very luminous, stellar-like nuclei; and (3) radio galaxies—elliptical galaxies with supermassive black holes at their centers that eject jets of charged particles into space. The particles produce synchrotron emission.

DIF: Medium REF: Section 20.4 MSC: Applied TOP: 4Ii | 4Ivi | 4Ivii

12. What is the difference between a Seyfert galaxy and a normal spiral galaxy?

ANS: A Seyfert galaxy contains a luminous, star-like central nucleus, which is usually brighter than the entire galaxy itself. Normal galaxies do not contain such a nucleus.

DIF: Medium REF: Section 20.4 MSC: Factual TOP: 4Ivi 13. If the radio lobes of a radio galaxy extend out to 10 Mpc from the center of the galaxy and the particles in them are traveling at approximately 0.99 c, then how long must the black hole at the center have been ejecting this material into space? Note that 1 Mpc  3.1  1022 m and 1 year  3  107 s.

ANS: Because   d/t, the length of time the black hole has been emitting particles is given by

t  d/  10 Mpc  (3.1  1022 m/1 Mpc)/ (0.99  3  108 m/s)  (1 yr/3  107 s)  35 million years.

DIF: Difficult REF: Section 20.4 MSC: Applied TOP: 4Ivii 14. How do astronomers know that AGNs are as small as our Solar System?

ANS: AGNs have been observed to vary dramatically over less than a day. As AGNs give off light, any changes in emission will spread out over space depending on the size of the AGN. If variations are observed to vary over time scales of less than a day, then AGNs must be less than a light-day in size, which is about the size of our Solar System.

DIF: Medium REF: Section 20.4 MSC: Applied TOP: 4Iviii 15. If an AGN varies its brightness on a timescale of 4 hours, then what is the size of the AGN measured in AU? ANS: The size of the AGN is the distance that light can travel in the amount of time over which it varies.

Since c  d/t, we have d  ct  3  108 m/s  4 hr  (60 min/1 hr)  (60 s/1 hr)  (1 AU/1.5  1011 m)  29 AU. DIF: Difficult REF: Section 20.4 MSC: Applied TOP: 4Iviii 16. What are the main components of an AGN? ANS: All AGN of any luminosity consist of a supermassive black hole, an accretion disk, surrounding clouds of gas, and often jets or winds emanating from this region.

DIF: Easy REF: Section 20.4 MSC: Factual TOP: 4IIii 17. How does the appearance of an AGN depend on the viewing angle to the nucleus?

ANS: An AGN of any type (e.g., Seyferts or quasars) consist of an accretion disk around a black hole

with a surrounding torus of dusty material). There are also gas clouds in the vicinity of the accretion disk and jets of material leaving the region. If we view the AGN edge-on, the torus blocks the light of the accretion disk, so we only see the jets and emission from gas clouds away from the torus. If we view the system pole-on, we see the accretion disk, jets, and gas clouds that are within the torus.

DIF: Medium REF: Section 20.4 MSC: Factual TOP: 4IIIi 18. If there is a 2 million M black hole at the center of our galaxy, why is our own galaxy not an AGN? ANS: There is currently no infall of matter onto the central supermassive black hole.

DIF: Easy REF: Section 20.4 MSC: Conceptual TOP: 4IVii 19. What is the connection between active galaxies and galactic mergers?

ANS: Astronomers believe that galactic mergers can cause large amounts of material to be funneled down into the central regions of galaxies; when material accretes onto the supermassive black hole then it triggers AGN activity.

DIF: Medium REF: Section 20.4 MSC: Conceptual TOP: 4IViii 20. Why are a higher percentage of distant galaxies classified as AGNs compared to the percentage of galaxies around us locally?

ANS: More distant galaxies are seen at an earlier time due to the finite speed of light. Active galactic nuclei are associated with mergers. Infalling galaxies, often gas-rich dwarfs, will bring in gas to the nuclear regions of a giant galaxy. This gas can fuel the AGN activity as material accretes onto the central black hole. Evidently, such mergers were more common in the past than they are today.

DIF: Difficult REF: Section 20.4 MSC: Applied TOP: 4IViii

CHAPTER 21

The Milky Way—A Normal Spiral Galaxy

CONCEPT MAP Sec 21.1 1. Measuring the Shape and Size of the Milky Way I. Spiral Structure in the Milky Way i. The Sun is inside the galaxy’s flattened disk, so we see the Milky Way as a swath of faint stars and gas clouds (TF: 1, MC: 1–2, SA: 1) ii. 21-cm radiation from hydrogen helps map the spiral pattern (MC: 3, SA: 1) iii. The Milky Way disk has two major spiral arms that emanate from the ends of its bar (TF: 1–2, MC: 4–6) II. Globular Clusters and the Size of the Milky Way Galaxy i. Globular clusters: 150 in the Milky Way; 75 percent are in the halo, 25 percent are associated with the inner disk; 12 to 13 Gyr old, and have low abundances of heavy elements, 10 to 1,000 times less the Sun’s abundance (TF: 4–6, MC: 7, SA: 2–3) ii. Globular clusters contain roughly 500,000 stars in a sphere of radius of 5 pc; total luminosities range from 400 to 106 L (TF: 7, SA: 4) iii. RR Lyrae variable stars used to derive distances (MC: 8–12, SA: 5–6) iv. The Sun is 8 kpc from the center of the Milky Way, about halfway to the edge of the disk (TF: 8, MC: 13–14) Sec 21.2 2. Dark Matter in the Milky Way I. Motions Trace the Mass Distribution i. The Milky Way has a flat rotation curve at 200–250 km/s out to the distance of Large Magellanic

Cloud’s orbit (MC: 15–16, SA: 7–9) ii. Total mass of the Milky Way is 1 to 1.5  1012 M and dominated by dark matter (TF: 9, MC: 17–20) Sec 21.3 3. Stars in the Milky Way I. Stars Have Different Ages and Chemical Compositions i. The abundance of heavy elements in a star records the cumulative history of star formation and gas accretion at the time the star was born (TF: 10, MC: 21–22, SA: 10–11) ii. Stars can be divided into disk and halo based on their ages, chemical compositions, and velocities (MC: 23–30) iii. Open clusters: loosely bound groups of disk stars, with ages from very young to somewhat older than the Sun and with a high fraction of heavy elements, on average, than the Sun (TF: 5–6, MC: 31–32, SA: 2) iv. Younger stars typically have higher abundances of heavy elements than older stars (MC: 33, SA: 12) v. The inner regions of galaxy disks are more enriched because they have had more star formation than the outer regions (MC: 34, SA: 13) vi. Globular clusters have some heavy elements, so we know there must have been a generation of stars that formed before they did (SA: 14) vii. There are no disk stars with very low abundances, so there must have been some enrichment taking place before the disk began to form II. Looking at a Cross-Section through the Disk i. Thin disk: youngest stars; 300 pc in thickness, 30,000 pc in diameter (TF: 11, MC: 25–26, SA: 15) ii. Thick disk: older and intermediate in chemical abundance between the thin disk and halo; 3,700 pc in thickness, about the same diameter as a thin disk (SA: 15) iii. At larger distances from the plane, disk stars become older and lower in chemical abundance (MC: 36) iv. Two hypotheses of the thick disk’s origin: (1) disk stars could have been scattered upward over time by gravitationally interacting with molecular clouds in the disk, or (2) the original thin disk could have been puffed up in mergers of small dwarf galaxy satellites (TF: 12, MC: 37) v. Supernovae can blow disk gas out from the plane in galactic “fountains” (MC: 38) III. The Halo Is More than Globular Clusters

i. The stellar halo contains about 1 percent of the Milky Way’s mass and consists of very old stars ii. In the solar neighborhood, halo stars can be identified by their low chemical abundances and high velocities (TF: 13, MC: 39) iii. Stars in the outer halo (beyond 15 kpc) have lower abundances and often orbit in the retrograde direction (MC: 40, SA: 16) IV. Magnetic Fields and Cosmic Rays Fill the Galaxy i. Cosmic rays: charged particles (mostly protons) moving through space (MC: 41) ii. Cosmic rays come in a very wide range of energies; the highest energy protons can be moving at nearly the speed of light (SA: 17) iii. High-energy cosmic rays enter the Earth’s atmosphere and create a shower of elementary particles iv. Cosmic rays are trapped within our galaxy by its magnetic fields and make synchrotron (radio) emission (MC: 42) v. Magnetic fields are in the disk because they are tied to the charged particles located in the dense molecular clouds (TF: 14, MC: 43) vi. The amount of energy in the magnetic fields and in the kinetic energy of the interstellar gas are all comparable (MC: 44) Sec 21.4 4. The Milky Way Hosts a Supermassive Black Hole I. The Milky Way Hosts a Supermassive Black Hole at Its Center i. X-rays, radio, and far infrared light can be used see to the center of the Milky Way. The radio source at the galactic center is called Sagittarius A* (MC: 45) ii. By tracking the positions of stars orbiting Sagittarius A*, we have discovered a black hole with a mass of 4  106 M (TF: 15–16, MC: 46–47, SA: 18) iii. The center of the Milky Way rotates rapidly and has low-level AGN-like emission at radio and X-ray wavelengths, including bubbles of gas above and below the galactic plane (TF: 17) iv. The black hole is not accreting material and is not very luminous (MC: 48, SA: 19) Sec 21.5 5. The Milky Way Offers Clues about How Galaxies Form I. The Milky Way Formed Hierarchically i. Some generations of stars must have lived and died before globular clusters and halo stars and the disk began to form (TF: 18, MC: 49–50)

ii. Milky Way has more than 20 dwarf galaxy satellites (MC: 51–52) iii. Milky Way formation scenario: inside a large clump of dark matter, small protogalaxies first collapsed and formed stars, some merged together to form the Milky Way, and others stayed as dwarf satellites (TF: 19, MC: 52, SA: 20) iv. The Sagittarius dwarf spheroidal galaxy orbiting the Milky Way is being ripped apart and long tidal tails surround the Milky Way (TF: 20, MC: 53–54) v. The central black hole formed and grew by accretion (MC: 55–56)

TRUE/FALSE 1. Our galaxy is a typical barred spiral galaxy.

ANS: T DIF: Easy REF: Section 21.1 MSC: Factual TOP: 1Ii 2. The Milky Way has two major spiral arms that emanate from the ends of its bar.

ANS: T DIF: Medium REF: Section 21.1 MSC: Factual TOP: 1Iiii 3. Our galaxy is classified as an Sd galaxy in the Hubble sequence.

ANS: F DIF: Medium REF: Section 21.1 MSC: Factual TOP: 1Iiii 4. There are many more individual stars residing in the Milky Way stellar halo than inside globular clusters.

ANS: T DIF: Medium REF: Section 21.1 MSC: Factual TOP: 1IIi 5. Open clusters are only found in the disk of the Milky Way; globular clusters are only found in the halo of the Milky Way. ANS: F DIF: Medium REF: Section 21.1 | Section 21.3 MSC: Factual TOP: 1IIi | 3Iiii 6. Stars in the Milky Way that are 12 to 13 billion years old are only found in globular clusters.

ANS: F DIF: Medium REF: Section 21.1 | Section 21.3 MSC: Conceptual TOP: 1IIi | 3IIIi 7. Globular clusters have much higher stellar densities than the local solar neighborhood.

ANS: T DIF: Easy REF: Section 21.1 MSC: Factual TOP: 1IIii 8. The locations of globular clusters on the sky and their distances tell us that the Sun is located near the center of the Milky Way. ANS: F DIF: Easy REF: Section 21.1 MSC: Factual TOP: 1IIiv

9. The Milky Way is like most other galaxies in that its mass is dominated by dark matter.

ANS: T DIF: Difficult REF: Section 21.2 MSC: Factual TOP: 2Iii 10. The chemical abundance of a main-sequence star usually reflects how much nuclear burning has gone on inside it.

ANS: F DIF: Medium REF: Section 21.3 MSC: Conceptual TOP: 3Ii 11. The youngest disk stars are found in a very thin plane in the disk.

ANS: T DIF: Medium REF: Section 21.3 MSC: Factual TOP: 3IIi 12. There are no known processes that can change the orbits of disk stars over time.

ANS: F DIF: Difficult REF: Section 21.3 MSC: Conceptual TOP: 3IIiv 13. Stars in the halo are most easily identified passing by the Sun because of their distinctively redder colors.

ANS: F DIF: Difficult REF: Section 21.3 MSC: Conceptual TOP: 3IIIii 14. Cosmic rays come mostly from extragalactic sources and contribute little to the overall energy balance in the Milky Way disk.

ANS: F DIF: Medium REF: Section 21.3 MSC: Factual TOP: 3IVv 15. There is a 109 M black hole at the center of the Milky Way that is rapidly accreting stars and gas.

ANS: F DIF: Easy REF: Section 21.4 MSC: Factual TOP: 4Iii 16. Most of the gravity that the Sun feels in the galaxy is caused by the black hole at the center of the Milky Way. ANS: F DIF: Difficult REF: Section 21.4 MSC: Applied TOP: 4Iii 17. We can estimate the mass of the central black hole in the Milky Way by measuring the orbital speeds of stars near it.

ANS: T DIF: Easy REF: Section 21.4 MSC: Factual TOP: 4Iiii 18. Halo stars formed before the Milky Way developed a thin stellar disk.

ANS: T DIF: Medium REF: Section 21.5 MSC: Factual TOP: 5Ii

19. The Milky Way has lived out its life mainly in isolation and has not experienced many mergers with other galaxies.

ANS: F DIF: Medium REF: Section 21.5 MSC: Applied TOP: 5Iiii 20. A dwarf galaxy is currently being ripped apart and accreted onto the Milky Way. ANS: T DIF: Easy REF: Section 21.5 MSC: Factual TOP: 5Iiv

MULTIPLE CHOICE 1. The Milky Way appears as __________ in the night sky because __________. a. randomly distributed stars; the Sun lies near the center of the Milky Way b. a faint band of light sprinkled with dark clouds; the Sun lies in the disk c. a faint band of light; the Sun lies in the halo d. an ellipse of light in the Southern sky; the Sun lies midway along the disk e. a circular disk with spiral arms; the Sun lies far above the galaxy’s plane ANS: B DIF: Easy REF: Section 21.1 MSC: Applied TOP: 1Ii

2. Below are two images of the Milky Way at different wavelengths. Which one was taken at infrared wavelengths, and why do you think so?

a. A; there are many cool stars in the image, which emit a lot of infrared light. b. A; the dust in the galaxy mainly blocks infrared light and lets visible light through. c. B; we see mainly warm dust emitting thermal radiation. d. B; infrared light passes through the dust so we can see the galaxy’s structure more clearly. e. B; stars in the central bulge shine only at infrared wavelengths.

ANS: D DIF: Easy REF: Section 21.1 MSC: Applied TOP: 1Ii

3. Why is 21-cm radiation the best way to map the spiral arms in the Milky Way? a. The molecular hydrogen gas that produces this emission is concentrated in the spiral arms. b. These photons, which are produced by neutral hydrogen, pass through the dense clouds of gas and dust in the disk. c. The emission is produced by supernovae, which are concentrated in the spiral arms. d. Radio telescopes are easier to operate than optical telescopes, and observations can be made even during the daytime. e. Radio photons do not have Doppler shifts, so we can detect clouds of gas without having to worry about the orbital velocities.

ANS: B DIF: Medium REF: Section 21.1 MSC: Conceptual TOP: 1Iii 4. The Milky Way Galaxy is a(n) __________ galaxy. a. irregular b. elliptical c. nonbarred spiral d. barred spiral e. lenticular ANS: D DIF: Easy REF: Section 21.1 MSC: Factual TOP: 1Iiii 5. The Milky Way is classified as an __________ galaxy. a. Sa b. SBbc c. Sd d. SBc e. SBd ANS: B DIF: Difficult REF: Section 21.1 MSC: Factual TOP: 1Iiii

6. Which of these galaxies is most like our own, and what is the name for the type of the galaxy we live in?

a. Galaxy A; giant elliptical b. Galaxy A: nonbarred spiral c. Galaxy B: barred spiral d. Galaxy B: irregular e. Galaxy B: lenticular ANS: C DIF: Medium REF: Section 21.1 MSC: Conceptual TOP: 1Iiii 7. Globular clusters are found in which stellar subgroups of the galaxy? a. the disk b. the halo c. the bulge d. both the halo and the inner disk e. the outer halo ANS: D DIF: Easy REF: Section 21.1 MSC: Factual TOP: 1IIi 8. What type of standard candle is used to determine distances to globular clusters? a. O-type main-sequence stars b. Cepheid variable stars c. T Tauri stars d. Type I supernovae e. RR Lyrae stars ANS: E DIF: Easy REF: Section 21.1 MSC: Factual TOP: 1IIiii

9. By comparing globular clusters, you find that Cluster A’s RR Lyrae stars are 100 times fainter than Cluster B’s RR Lyrae stars. You know that both clusters have approximately the same chemical composition and age, and thus their RR Lyrae stars should have the same luminosities. Which is true about the clusters’ distances? a. Cluster A is 10 times farther away than Cluster B. b. Cluster A is 10 times closer to us than Cluster B. c. Clusters A and B are approximately the same distances from us. d. Cluster A is 100 times farther away than Cluster B. e. Custer A is 100 times closer to us than Cluster B. ANS: A DIF: Medium REF: Section 21.1 MSC: Applied TOP: 1IIiii 10. By comparing globular clusters, you find that Cluster A’s RR Lyrae stars are 100 times fainter than Cluster B’s RR Lyrae stars. You know that Cluster A’s stars are 1.5 times more luminous than Cluster B’s stars, because Cluster A is more metal-rich and somewhat younger. How do the clusters’ distances compare? a. Cluster A is 65 times farther away than Cluster B. b. Cluster A is 12 times farther away than Cluster B. c. Cluster A is 10 times farther away than Cluster B. d. Cluster A is seven times farther away than Cluster B. e. Cluster A and Cluster B are at approximately the same distance.

ANS: B DIF: Difficult REF: Section 21.1 MSC: Applied TOP: 1IIiii 11. If a globular cluster were 1,000 pc away from us and an RR Lyrae star in it had an absolute magnitude of 0.5, what would its apparent magnitude be? a. 4.0 b. 6.5 c. 8.0 d. 10.5 e. 15.5 ANS: D DIF: Medium REF: Section 21.1 MSC: Applied TOP: 1IIiii

12. If RR Lyrae stars in a globular cluster had an apparent magnitude of 11.5 and an absolute magnitude of 0.5, what would the globular cluster’s distance be? a. 1,600 pc b. 1,900 pc c. 2,500 pc d. 3,200 pc e. 8,600 pc ANS: A DIF: Difficult REF: Section 21.1 MSC: Applied TOP: 1IIiii 13. Studying standard candles in globular clusters offered the first conclusive proof that our galaxy was much __________ than originally believed. a. smaller b. rounder c. older d. flatter e. larger ANS: E DIF: Easy REF: Section 21.1 MSC: Factual TOP: 1IIiv 14. The Sun is located approximately: a. halfway out in the disk b. one-third of the way out in the halo c. one-quarter of the way out in the bulge d. three-quarters of the way out in the disk e. near the galactic center ANS: A DIF: Easy REF: Section 21.1 MSC: Factual TOP: 1IIiv

15. How many years does it take the Sun to complete one orbit around the Milky Way? Note that the Sun is traveling at approximately 220 km/s and is 8 kpc from the center of the Milky Way. a. 180 thousand years b. 7 million years c. 35 million years d. 230 million years e. 620 million years ANS: D DIF: Medium REF: Section 21.2 MSC: Applied TOP: 2Ii 16. If we found a star cluster at the edge of the disk of the Milky Way at a distance of 10 kpc from the center, which orbited with a velocity of 300 km/s, what would be the total mass enclosed within this radius? a. 3  1011 M b. 7  1011 M c. 2  1011 M d. 4  1012 M e. 8  1012 M ANS: C DIF: Difficult REF: Section 21.2 MSC: Applied TOP: 2Ii 17. What observed property of the Milky Way suggests that it contains a large amount of matter not in the form of stars? a. the rotation curve b. the velocities of the open star clusters c. the number and shape of the spiral arms d. the thickness of the disk e. the presence of a black hole at the galactic center ANS: A DIF: Easy REF: Section 21.2 MSC: Applied TOP: 2Iii

18. Our galaxy is __________ other galaxies because the dark matter in the Milky Way __________. a. unlike; makes up a small fraction of the total mass than in other galaxies b. unlike; makes up a larger fraction of the total mass than in other galaxies c. like; contains most of the mass in the galaxy d. like; is made up of black holes and other stellar remnants e. like; is made up of dust and faint objects of planetary mass ANS: C DIF: Medium REF: Section 21.2 MSC: Applied TOP: 2Iii 19. Imagine you discovered a barred spiral galaxy that was a “Milky Way twin” because the size of the bulge and disk, the arrangement of spiral arms, and other characteristics were just like those in our galaxy. Then you measured its rotation curve and plotted the figure shown below. What would you conclude about this galaxy?

a. This galaxy has much more dark matter than does the Milky Way. b. This galaxy has about the same amount of dark matter as does the Milky Way. c. This galaxy inexplicably has much less dark matter than does the Milky Way. d. This galaxy probably has no stellar halo. e. This galaxy probably has no central black hole.

ANS: C DIF: Medium REF: Section 21.2 MSC: Conceptual TOP: 2Iii

20. Most of the mass in our galaxy is in the form of: a. stars b. gas c. dust d. dark matter e. globular clusters ANS: D DIF: Easy REF: Section 21.2 MSC: Factual TOP: 2Iii 21. The chemical composition of a star’s atmosphere tells us: a. how much nuclear burning has gone on in the star b. the star’s evolutionary stage c. how many planets have fallen onto the star in its lifetime d. the chemical composition of the cloud from which the star formed e. the amount of heavy elements the entire galaxy had when the star was formed ANS: D DIF: Medium REF: Section 21.3 MSC: Conceptual TOP: 3Ii 22. The oldest stars in the galaxy are usually __________ in heavy elements because __________. a. low; they have had time to accrete unprocessed gas from the interstellar medium b. low; they were formed before much chemical enrichment had taken place c. low; old stars must have low mass, and low-mass stars do not generate many heavy elements d. high; they have turned a lot of their initial hydrogen and helium into heavier elements e. high; stars with higher fractions of heavy elements have shorter lifetimes ANS: B DIF: Medium REF: Section 21.3 MSC: Conceptual TOP: 3Ii

23. You measure the chemical abundances and space motions of stars near the Sun, and plot your data in the figure shown below. The stars separate into two distinct groups, which you label A and B. These groups are, respectively, in which two stellar components of the Milky Way?

a. disk and halo b. bulge and disk c. halo and disk d. halo and bulge e. globular clusters and halo ANS: A DIF: Difficult REF: Section 21.3 MSC: Applied TOP: 3Iii 24. In which part of the Milky Way would you find little or no neutral hydrogen, no current star formation, and stars that are all older than 10 billion years? a. the inner disk b. the outer disk c. the galactic center d. the solar neighborhood e. the halo ANS: E DIF: Easy REF: Section 21.3 MSC: Factual TOP: 3Iii

25. If you found a star cluster that was 13 billion years old and whose stars had fractions of heavy elements that were 100 times less than that in the Sun, what type of star cluster would this be and where would it most likely be located? a. an open cluster that is likely to be in the disk b. an open cluster that is likely to be in the stellar halo c. a globular cluster that is likely to be in the thin disk d. a globular cluster that is likely to be in the stellar halo e. a globular cluster that is likely to be in the central bar ANS: B DIF: Medium REF: Section 21.3 MSC: Conceptual TOP: 3Iii 26. If you found a halo star that was 13 billion years old and had the same amount of heavy elements as the Sun, would you be surprised? a. No; many halo stars have high fractions of heavy elements. b. No; this star probably escaped from a globular cluster. c. No; this star has been around a long time, and has created a lot of heavy elements through nuclear fusion. d. Yes, because most halo or globular cluster stars have few heavy elements. e. Yes; halo stars mostly have ages less than 10 billion years.

ANS: D DIF: Medium REF: Section 21.3 MSC: Conceptual TOP: 3Iii 27. The stars in the disk of the Milky Way near the Sun have: a. chemical abundances similar to the Sun b. much higher velocities than the Sun c. chemical abundances that are on average 10 times higher than the Sun d. much lower velocities than the Sun e. chemical abundances that are on average 10 times lower than the Sun ANS: A DIF: Medium REF: Section 21.3 MSC: Factual TOP: 3Iii

28. Which objects in our Milky Way galaxy have orbits that are NOT similar to stars in elliptical galaxies? a. disk stars b. halo stars c. bulge stars d. globular clusters e. satellite dwarf galaxies ANS: A DIF: Easy REF: Section 21.3 MSC: Applied TOP: 3Iii 29. Which of the following are generally NOT found in our galaxy’s halo? a. globular clusters b. planetary nebulae c. RR Lyrae stars d. stars with high percentages of heavy elements e. low-mass main-sequence stars ANS: D DIF: Difficult REF: Section 21.3 MSC: Applied TOP: 3Iii 30. In a star that has a chemical abundance similar to the Sun, what percentage of its mass is made of heavy elements? a. 0.1 percent b. 0.2 percent c. 2 percent d. 10 percent e. 20 percent ANS: C DIF: Medium REF: Section 21.3 MSC: Factual TOP: 3Iii 31. Open star clusters primarily inhabit which part of spiral galaxies? a. disk b. halo c. bulge d. nucleus e. satellite galaxies ANS: A DIF: Easy REF: Section 21.3 MSC: Factual TOP: 3Iiii

32. Globular clusters, when compared to open clusters, generally: a. are located closer to the center of the Milky Way b. are younger in age c. have fewer amounts of heavy elements d. are less massive e. contain more dark matter ANS: C DIF: Medium REF: Section 21.3 MSC: Factual TOP: 3Iiii 33. On average, we expect that __________ stars have the __________ percentage of heavy elements. a. older; highest b. disk; lowest c. bulge; lowest d. halo; highest e. younger; highest ANS: E DIF: Easy REF: Section 21.3 MSC: Factual TOP: 3Iiv 34. Where are the most metal-rich stars found in the Milky Way? a. in the disk near the Sun b. near the center c. in the halo d. in globular clusters e. in old open clusters ANS: B DIF: Easy REF: Section 21.3 MSC: Factual TOP: 3Iv 35. The ratio of the diameter of the galaxy’s disk to the thickness of its disk is about: a. 10 to 1 b. 30 to 1 c. 100 to 1 d. 1,000 to 1 e. 10,000 to 1 ANS: C DIF: Medium REF: Section 21.3 MSC: Factual TOP: 3IIi

36. We find that interstellar gas and the youngest disk stars are found in a very narrow distribution along the galactic plane, but that older disk stars can be found in a thicker disk. What might this mean? a. Gravitational scattering off molecular clouds changes the orbits of disk stars as they get older. b. The disk is still settling down from a thicker to a thinner state. c. The galaxy has not absorbed any dwarf galaxies in the last several billion years. d. Cosmic rays are responsible for causing star formation to happen in a narrow layer over time. e. Most of the dark matter in the galaxy is in a thin layer along the disk.

ANS: A DIF: Difficult REF: Section 21.3 MSC: Conceptual TOP: 3IIi | 3IIiii 37. The Milky Way has both a thin and thick disk of stars. Which one of the following statements about them is FALSE? a. The thin disk stars on average are younger than the thick disk stars. b. Molecular clouds are distributed more like the thin disk stars rather than the thick disk stars. c. The thin disk stars on average have lower abundances of heavy elements than the thick disk stars. d. The thick disk stars may be older thin disk stars that have higher upward velocities because they have gravitationally interacted more with molecular clouds in the spiral arms. e. The thick disk may have formed when a small dwarf galaxy merged with the Milky Way. ANS: C DIF: Medium REF: Section 21.3 MSC: Factual TOP: 3IIiv 38. The location and motions of interstellar gas can be strongly affected by what? a. Dark matter b. Low-mass stars c. Pulsars d. Supernovae e. Halo stars ANS: D DIF: Easy REF: Section 21.3 MSC: Factual TOP: 3IIv

39. Halo stars near the Sun are distinguished from disk stars by what characteristics? a. low chemical abundances and low relative velocities b. high chemical abundances and high relative velocities c. low chemical abundances and high relative velocities d. high chemical abundances and low relative velocities e. equal chemical abundances and high relative velocities ANS: C DIF: Easy REF: Section 21.3 MSC: Factual TOP: 3IIIii 40. In the Milky Way’s outermost halo, the stars orbit: a. mostly in the prograde direction b. mostly in the retrograde direction c. both in the prograde and retrograde directions d. the same way the disk stars do e. neither in the prograde nor the retrograde directions ANS: C DIF: Easy REF: Section 21.3 MSC: Factual TOP: 3IIIiii 41. Cosmic rays are: a. photons with even higher energy than gamma rays b. high-velocity particles produced in novae c. elementary particles with very high energies d. synchrotron emission from strong magnetic fields e. dark-matter particles falling into the galaxy ANS: C DIF: Medium REF: Section 21.3 MSC: Factual TOP: 3IVi 42. Cosmic rays in our galaxy can produce synchrotron radiation at what wavelengths? a. gamma-ray b. X-ray c. ultraviolet d. infrared e. radio ANS: E DIF: Easy REF: Section 21.3 MSC: Factual TOP: 3IViv

43. Magnetic fields in the Milky Way are concentrated in the disk because: a. halo stars are incapable of producing strong magnetic fields b. the fields are tied to the charged particles in dense molecular clouds c. gravity forces them to sink to the center of the disk d. supernovae explosions continually force them toward the middle of the disk e. supernovae explosions eliminate magnetic fields away from the disk ANS: B DIF: Difficult REF: Section 21.3 MSC: Conceptual TOP: 3IVv 44. Cosmic rays in the Milky Way are important because: a. these energetic photons easily penetrate the Earth’s atmosphere b. they can be collected and used to generate electricity c. they have about the same energy as that contained in magnetic fields and in the kinetic energy of gas clouds d. they carry information about AGN in external galaxies e. they can influence the motion of stars in the halo ANS: C DIF: Medium REF: Section 21.4 MSC: Conceptual TOP: 3IVvi 45. Sagittarius A*, the radio source located at the center of our galaxy, is believed to be: a. a massive star cluster b. a supernova remnant c. a quasar d. a Seyfert nucleus e. a supermassive black hole ANS: E DIF: Easy REF: Section 21.4 MSC: Factual TOP: 4Ii 46. How have astronomers measured the mass of the black hole at the center of our galaxy? a. using the rotation curve derived from 21-cm emission b. by observing the motions of stars near the center of the galaxy c. by measuring the brightness of the quasar d. by measuring the Doppler shift of Sagittarius A* e. by counting the number of supernova explosions near the black hole during the last century ANS: B DIF: Easy REF: Section 21.4 MSC: Applied TOP: 4Iii

47. What is the radius of the event horizon for the 4  106 M black hole at the center of the Milky Way? a. 0.02 AU b. 0.08 AU c. 0.17 AU d. 0.35 AU e. 0.60 AU ANS: B DIF: Medium REF: Section 21.4 MSC: Applied TOP: 4Iii 48. If the central black hole started accreting a lot of gas, it would appear as what kind of an object? a. a red supergiant b. a pulsar c. a planetary nebula d. an AGN e. a supernova remnant ANS: D DIF: Medium REF: Section 21.4 MSC: Applied TOP: 4Iiv 49. Why do we know that at least one generation of stars formed and died before the Milky Way’s globular clusters formed? a. All globular cluster stars have some amount of heavy elements. b. They are so old that nuclear fusion in globular cluster stars has altered their chemical abundances. c. Globular clusters are 9 to 10 billion years old. d. No globular cluster is older than 12 billion years. e. All globular clusters reside in the disk of the Milky Way. ANS: A DIF: Easy REF: Section 21.5 MSC: Factual TOP: 5Ii 50. The oldest disk stars are both __________ and __________ compared to halo stars. a. younger; higher in chemical elements b. older; higher in chemical elements c. younger; lower in chemical elements d. older; lower in chemical elements e. younger; equal in chemical element abundances ANS: A DIF: Easy REF: Section 21.5 MSC: Factual TOP: 5Ii

51. If the Large Magellanic Cloud is orbiting the Milky Way in a circular orbit with a speed of 175 km/s and a distance of 50 kpc from the center, how long would it take for the Large Magellanic Cloud to complete one orbit around the Milky Way? a. 500 million years b. 2 billion years c. 5 billion years d. 9 billion years e. 12 billion years ANS: B DIF: Medium REF: Section 21.5 MSC: Applied TOP: 5Iii 52. The presence of dwarf galaxies around the Milky Way supports what picture of our galaxy’s formation? a. The galaxy formed by the merger of two large galaxies, which scattered their dwarf companions to large distances. b. The galaxy originally formed with no dark matter but gained its dark matter by the absorption of smaller dwarf systems. c. The galaxy formed in a giant explosion caused by the violent collisions of clouds of dark matter. d. The galaxy formed a long time ago and has been passively evolving since. e. The galaxy formed by the merger of smaller systems.

ANS: E DIF: Medium REF: Section 21.5 MSC: Conceptual TOP: 5Iii | 5Iiii 53. Because of the ages of globular clusters in the Milky Way, we think our galaxy’s early formation history was characterized by: a. one single cloud of gas gently collapsing and star formation proceeding slowly within it b. one single cloud of gas that rapidly collapsed and turned most of its gas into stars c. violent merging of protogalactic fragments that stimulated a high rate of star formation d. slow merging of protogalactic fragments after they had already turned most of their gas into stars e. merging of two galaxies, each about half as massive as the Milky Way is today ANS: C DIF: Medium REF: Section 21.5 MSC: Factual TOP: 5Iiv

54. Based on the chemical abundances and velocities of stars in the Milky Way’s disk and halo, we can conclude that the protogalactic fragments that came together to form the galaxy merged when the fragments had: a. not yet formed stars b. converted most of their gas into stars c. begun forming stars d. each formed a supermassive black hole at their center e. gathered a lot of dark matter from surrounding space ANS: C DIF: Medium REF: Section 21.5 MSC: Applied TOP: 5Iiv 55. What is ripping apart the Sagittarius dwarf spheroidal galaxy? a. a supermassive black hole at the center of the dwarf galaxy b. pressure from its passage through the dark matter in the Milky Way c. a violent episode of star formation d. the gravitational tidal force of the Milky Way e. supernova explosions occurring in the dwarf galaxy ANS: D DIF: Easy REF: Section 21.5 MSC: Factual TOP: 5Iv 56. How do we think the central black hole in the galaxy primarily grew to its current mass? a. ordinary stellar evolution of massive stars b. merger of close binary stars c. shock waves from supernova explosions d. dark matter clumping together e. accretion of nearby gas ANS: E DIF: Medium REF: Section 21.5 MSC: Factual TOP: 5Iv

SHORT ANSWER 1. What are the main observational difficulties in observing the shape and spiral arm pattern of the Milky Way? ANS: We are seeing it from within, so we lack perspective. Furthermore, dust obstructs much of the material in the galaxy’s plane.

DIF: Easy REF: Section 21.1 MSC: Conceptual TOP: 1Ii | 1Iii

2. Describe the differences between the ages and chemical abundances of globular and open star clusters.

ANS: Globular clusters are very old, 12–13 billion years, and poor in chemical abundance, 10 to 1,000 times less than the Sun. Open star clusters lie in the disk, and they are 0–10 billion years old and have chemical abundances that are like the Sun or even more metal-rich.

DIF: Medium REF: Section 21.1 | Section 21.3 MSC: Factual TOP: 1IIi | 3Iiii 3. Describe the population of globular star clusters in the Milky Way: how many are there, where do they reside in the Milky Way, and how do their ages and chemical abundances compare to the Sun? ANS: The Milky Way contains over 150 globular clusters (some are hidden by dust and gas). Globular clusters orbit the Milky Way on randomly oriented, elliptical orbits; 75 percent reside in the halo of the Milky Way, and 25 percent in the inner disk. Globular clusters are 12–13 billion years in age and have chemical abundances that are much less (10 to 1,000 times less) than the Sun. DIF: Medium REF: Section 21.1 MSC: Factual TOP: 1IIi 4. A typical globular cluster is composed of 500,000 stars in a sphere whose radius is approximately 5 pc.

The distance between stars in a globular cluster is given approximately by d  r/N1⁄3, where r is the radius and N is the number of stars in the cluster. Calculate this distance, and determine how many times smaller it is than the typical distance between stars in the solar neighborhood, which is 3 light-years.

ANS: The typical distance between stars in a globular cluster is (5 pc)/(500,000)1⁄3  0.06 pc, which is 16 times smaller than the typical distance of 1 pc between stars in the solar neighborhood.

DIF: Medium REF: Section 21.1 MSC: Applied TOP: 1IIii 5. Explain how a standard candle allows you to determine the distance to an object. What is it that you have to measure or know about the standard candle to derive its distance?

ANS: A standard candle is a star that can be seen at great distances and whose luminosity we know simply by identifying the star (that is, using its period or some other property to recognize it). Its distance is then easily calculated using , where L is the known luminosity and b is the measured apparent brightness.

DIF: Medium REF: Section 21.1 MSC: Conceptual TOP: 1IIiii

6. By comparing globular clusters, you find that Cluster A’s RR Lyrae stars are 225 times fainter than Cluster B’s RR Lyrae stars. You know that both clusters have approximately the same chemical composition and age, and thus their RR Lyrae stars should have the same luminosities. Which cluster has the larger distance, and what is the ratio of the clusters’ distances?

ANS: Distance is given by , where L is the known luminosity and b is the apparent brightness. Taking the ratio of the distances, we get Therefore, Cluster A is 15 times farther away than Cluster B. DIF: Difficult REF: Section 21.1 MSC: Applied TOP: 1IIiii 7. The Sun is rotating about the center of the galaxy at roughly 220 km/s at a distance of about 8 kpc. How long will it take to complete one revolution? How many times has the Sun orbited the galaxy in its lifetime?

ANS: Disk stars rotate in approximately circular orbits. The circumference of a circle C  2r, and thus the orbital period is P  C/v  2r/v. For the Sun, P  2  (8  103  3.1  1016 m) / (220  103 m/s)  (1 yr / 3.1  107 s)  230 million years. The age of the sun is 4.6  109 yr, and thus it has orbited the galaxy about 20 times in its lifetime.

DIF: Medium REF: Section 21.2 MSC: Applied TOP: 2Ii 8. You find a star orbiting about the center of the galaxy at a speed of 220 km/s at a distance of 15 kpc. How much mass is within the orbit of that star? You can assume the star is on a circular orbit around the center of the Milky Way. ANS: The mass is derived from the equation M  rv / G, where v is the velocity, r the radius of the orbit, and G is Newton’s gravitational constant. The first step is to convert r to meters, by noting that r  15,000 pc  (3.09  1016 m/ 1 pc)  4.64  1020 m. Thus M  (4.64  1020 m)  (2.2  105 m/s)2 / (6.67  10−11 m3/kg s2)  3.37  1041 kg  (1 M / 2  1030 kg)  1.7  1011 M.

DIF: Difficult REF: Section 21.2 MSC: Applied TOP: 2Ii 9. What objects are used to determine that the galaxy is mostly dark matter?

ANS: The presence of dark matter is indicated by the orbital speeds of stars in the disk, by the velocities of globular cluster and halo stars near the Sun, and by the motions of satellite galaxies such as the Large Magellanic cloud.

DIF: Medium REF: Section 21.2 MSC: Factual TOP: 2Ii 10. Why can the observed chemical abundance of a star tell you something about its age?

ANS: As a galaxy evolves, gas is converted to stars, the stars die and eject newly made chemical ele-

ments, the gas cools back down and forms new stars, with the newly formed stars being more enriched in heavy elements than the prior generation of stars. Therefore, the chemical abundance of stars in galaxies generally grows with time.

DIF: Medium REF: Section 21.3 MSC: Applied TOP: 3Ii 11. Suppose it was discovered that the galaxy has constantly been absorbing large amounts of gas that never had any stars form in it. Would this complicate our interpretation of ages and chemical enrichment?

ANS: Such a discovery might indeed complicate matters. The gas would be expected to consist only of hydrogen and helium, because no stars ever formed in it and therefore no massive stars would have seeded the gas with new heavy elements. As a result, the gas in our galaxy’s interstellar medium would be diluted, and its heavy element fraction reduced, by the absorption of this unprocessed gas. Consequently, star formation in our galaxy would not enrich the gas with new heavy elements as fast as we would expect, and new generations of stars would not form with ever higher fractions of heavy elements.

DIF: Difficult REF: Section 21.3 MSC: Applied TOP: 3Ii 12. The Sun has 2 percent of its mass in heavy elements (those elements that are not hydrogen and helium).

What does this tell us about chemical enrichment in the galaxy?

ANS: The chemical composition of stars is determined by the chemical composition of the gas from which the stars were made. The Sun is about 4.5 billion years old. That it has 2 percent of its mass in heavy elements tell us that previous generations of massive stars were able to enrich the gas in the region the Sun formed to a level of 2 percent heavy elements.

DIF: Medium REF: Section 21.3 MSC: Applied TOP: 3Iiv 13. Where would we find the stars with the richest abundance of massive elements? Where would we find those with the smallest amounts of massive elements? Why? ANS: Stars with the highest abundance of massive elements are the young stars in the innermost regions of the disk. Stars with the fewest massive elements are the halo stars (such as globular cluster stars).

The reason is that the halo stars formed first when the interstellar medium of the galaxy was the least enriched; the disk stars, particularly in the inner disk where gas is more concentrated, have formed from material that has been (and continues to be) enriched by many generations of star formation.

DIF: Medium REF: Section 21.3 MSC: Conceptual TOP: 3Iv

14. How do we know that globular cluster stars were not the first stars that formed in our galaxy? Would this still be true if some or all globular clusters came into the galaxy by accretion of small satellite galaxies?

ANS: They contain some massive elements and therefore must have been enriched by at least one earlier generation of stars. This would still be true even if a globular cluster was formed elsewhere and brought into the galaxy by accretion: since the fraction of massive elements is not zero, there must have been some chemical evolution before the globular cluster was formed.

DIF: Medium REF: Section 21.3 MSC: Conceptual TOP: 3Ivi 15. Describe a scale model of the disk of the Milky Way using a scale in which 1 cm  1000 light-years. Note that the diameter of the disk is approximately 100,000 light-years, the thickness of young stars is 500 lightyears, and the thickness of the older stars is 4,000 light-years.

ANS: In this model, the disk would be 100 cm (1 m) in diameter. The young stars would be found in a disk 0.5 cm ( 5 mm) thick by 100 cm wide, and the older stars would be in a disk 4 cm thick by 1 meter wide. The disk is thin indeed!

DIF: Medium REF: Section 21.3 MSC: Applied TOP: 3IIi | 3IIii 16. If you wanted to study the properties of nearby stars that were some of the first stars to form in the Milky Way, but were NOT members of a globular cluster, how could you go about finding them?

ANS: You would look for stars that were members of the Milky Way halo. The stars would have highly elliptical orbits, not rotating like other disk stars, and you could identify them by the very high velocities they have as they pass through the solar neighborhood. Also, you could identify halo stars by their low chemical abundance, and main-sequence halo stars by their relatively bluer colors than disk stars.

DIF: Difficult REF: Section 21.3 MSC: Applied TOP: 3IIIiii 17. The energy of a cosmic ray is given by mc2, where m is the mass of the particle, c is the speed of light, and  is the Lorentz factor and . If a cosmic ray proton is traveling at a speed of   0.999999c, what is its energy? How does that compare to the kinetic energy of a grape thrown up in the air and caught in your mouth? Assume the grape has a mass of about 5 grams and a speed of 0.5 m/s when you catch it. The mass of a proton is 1.7  10−27 kg.

ANS: The cosmic ray proton has a Lorentz factor of and an energy equal to E  mc2  702  1.7  10−27 kg  (3  108 m/s)2  1.1  10−7 N. The grape has a kinetic energy equal to E  1⁄2 mv2  1⁄2  5  10−3 kg  (0.5 m/s)2  6.4  10−4 N. Therefore, the grape has 5,800 times more energy than the cosmic ray proton.

DIF: Difficult REF: Section 21.3 MSC: Applied TOP: 3IVii 18. Calculate the size of the event horizon of the supermassive black hole at the center of the Milky Way, the mass of which is 4  106 M. Give your answer in units of AU. ANS: Rs  2GM/c2  2  6.7  10−11 m3/kg s2  4  106 M  (2  1030 kg/1 M)/(3  108 m/s)2.

Rs  1.2  1010 m  (1 AU/1.5  1011 m)  0.08 AU. DIF: Difficult REF: Section 21.4 MSC: Applied TOP: 4Iii 19. Why is the supermassive black hole in the center of our galaxy not an AGN right now? Could it be active in the future? Why or why not?

ANS: It is not an AGN (active galactic nucleus) right now because it currently is not accreting much gas or dust. It is sitting dormant in an inactive stage. It could become active in the future if gas in the inner regions is disturbed, say by gravitational instabilities in the disk or from an orbiting satellite, and it flows down into the center and accretes onto the black hole.

DIF: Easy REF: Section 21.4 MSC: Conceptual TOP: 4Iiv 20. Describe the basic scenario that describes the formation and evolution of the Milky Way, including a description of how the halo and disk formed.

ANS: Several protogalactic fragments began forming stars but merged quickly, forming the stellar halo including the globular clusters, while the gas left in the fragments settled down and formed the disk of the Milky Way. A supermassive black hole formed at the center of the galaxy. Star formation stopped in the halo as all the gas settled down into a disk, but star formation proceeded in the gas-rich disk and continues on to today. Over time, dwarf galaxy satellites of the Milky Way may collide and merge with it, adding to the mass of the Milky Way and stimulating new bursts of star formation.

DIF: Medium REF: Section 21.5 MSC: Applied TOP: 5Iiii

CHAPTER 22

Modern Cosmology

CONCEPT MAP Sec 22.1 1. The Universe Has a Destiny and a Shape I. Modern Cosmology i. The universe has a fate depending on the density of matter and energy in it (MC: 1) ii. Gravity acts to slow the expansion of the universe (MC: 2, 3) iii. Critical density: the density of the universe for which there is exactly enough matter (assuming no dark energy) to cause the universe to recollapse; 9.5  10 −27 kg/m3  1 proton/m3 for H0  70 km/s/Mpc (MC: 4, SA: 1) iv. mass: the mass density of the universe divided by the critical density (SA: 2) v. In a universe with only gravity, mass dictates the evolution of the universe (MC: 5) vi. Assuming no dark energy, if mass  1, the universe recollapses in a Big Crunch; if mass  1, the universe expands forever (TF: 1, MC: 6, 7, SA: 3) vii. For luminous matter, mass  0.02; including dark matter, mass  0.2

0.3 (TF: 2–3, 7)

Sec 22.2 2. The Accelerating Universe I. Dark Energy and the Cosmological Constant i. If gravity were slowing the universe’s expansion, we would see distant galaxies experiencing a larger Hubble constant (larger recessional velocities) (MC: 8, SA: 4) ii. Measurements of Type I supernovae and their host galaxies found the expansion rate was actually speeding up over time; confirmed by WMAP (TF: 4, MC: 9–11, SA: 5) iii. For the expansion rate to increase, there must be a force opposing gravity (MC: 12)

iv. : the density of dark energy relative to the critical density (MC: 13–16) v. Observations of the CMB, Type I supernovae and galaxy clusters tell us that   0.7 and the total   mass  1 (MC: 17) vi. Dark energy has been accelerating the universe’s expansion for the last 5–6 billion years (TF: 5, MC: 3) vii. The cosmological constant might not be constant in time; the Big Rip (MC: 2, 3, 18) viii. Current observations suggest that  is constant with time and that the universe will expand forever (MC: 19, 20, SA: 6) II. Age of the Universe i.  and mass determine the expansion rate and the age of the universe (MC: 21) ii. If the Hubble constant is constant in time, then the age of the universe is 1/H0 (13.6 billion years); if the expansion has been increasing with time, then the universe is actually older than the Hubble time (MC: 22, SA: 7) iii. The oldest globular clusters are about 13 billion years old; WMAP tells us that the universe is about 13.7 billion years old (MC: 3, 23, SA: 8) III. The Universe Has a Shape i. Three basic shapes are possible: flat with   mass  1, open (saddle) geometry with   mass  1, or closed (spherical geometry) with   mass  1 (SA: 9) ii. Shape of the universe determines the geometry of objects in space (MC: 24) iii. In a flat and open universe, spacetime can be infinite in size; in a closed universe, spacetime is finite in size and closes back on itself (MC: 25) Sec 22.3 3. Inflation I. The Universe Is Much Too Flat i. A universe that starts out perfectly flat remains so over time (TF: 6, MC: 26, 27) ii. Flatness problem (MC: 28, 29, SA: 10) II. The Cosmic Background Radiation Is Much Too Smooth i. The CMB is remarkably constant and smooth in temperature across the sky (SA: 11) ii. Uncertainty principle (SA: 11) iii. Horizon problem (MC: 30, SA: 11) III. Inflation Solves These Problems i. In the early 1980s, Alan Guth proposed inflation to solve the flatness and horizon problems (TF: 7–

9, MC: 31, 32, SA: 12) ii. Small fluctuations in the universe are essentially erased by inflation (SA: 12) iii. Inflation makes the universe extremely flat (  mass  1) (SA: 12) iv. Inflation takes pieces of the universe that were in causal connection and moves them so far apart they are no longer connected (SA: 12) Sec 22.4 4. The Earliest Moments I. The Forces of Nature i. The four fundamental forces of nature (TF: 10–11, MC: 33, SA: 13, 14) ii. Strong force binds neutrons and protons into nuclei (SA: 14) iii. Weak force mediates beta decay (TF: 12, SA: 14) iv. Quantum electrodynamics (QED): describes weak and electromagnetic forces (SA: 15) v. QED predicted three particles later discovered in accelerator experiments (MC: 34) vi. Quantum chromodynamics: describes the strong nuclear force (MC: 35) vii. Quarks are bound together inside protons and neutrons (TF: 13) viii. QED and QCD together form the standard model II. A Universe of Particles and Antiparticles i. Every particle has a corresponding antiparticle (MC: 36) ii. Annihilation and pair production (TF: 14, MC: 37–39, SA: 16) III. Frontiers of Physics i. As the universe expanded and cooled, the average energy of the photons was too low to create a particle and antiparticle (MC: 40) ii. Asymmetry ratio: 1 extra particle per 10 billion matter-antimatter pair (MC: 41, 42) iii. Grand unified theory (GUT): explains all four forces of nature (TF: 15, MC: 43–45) IV. Toward a Theory of Everything i. The Planck Era (MC: 46) ii. Theory of everything (TOE) combines GUT with gravity (MC: 47, SA: 17, 18) iii. Superstring theory: an 11-dimensional universe is required to unite gravity and quantum mechanics (TF: 16, MC: 48, 49, SA: 19) V. The Forces Separated in the Cooling Universe i. Gravity separated first, followed by strong force, then electroweak split (MC: 50–52) ii. Nucleosynthesis: as the universe cools, nuclei form first, followed by atoms (MC: 53)

Sec 22.5 5. Multiple Multiverses I. Multiple Universes Are Possible i. Can only observe up to 13.7 billion light years away (MC: 54) ii. Four known possible types of multiverses (MC: 55, SA: 20) Sec 22.6 6. Origins: Our Own Universe Must Support Life I. Anthropic Principle i. Values of physical constants must allow for our existence (MC: 56)

TRUE/FALSE 1. If the sum of the mass and energy density in the universe yields   1, then the universe will recollapse in a Big Crunch.

ANS: T DIF: Easy REF: Section 22.1 MSC: Applied TOP: 1Ivi 2. The ratio of the average density in all forms of matter (luminous and dark) in our universe to the critical density is approximately 0.3.

ANS: T DIF: Easy REF: Section 22.1 MSC: Factual TOP: 1Ivii 3. The ratio of the density of luminous matter in our universe to the critical density is 0.3.

ANS: F DIF: Medium REF: Section 22.1 MSC: Factual TOP: 1Ivii 4. Observations of Type I supernovae in distant galaxies confirm that the expansion of the universe is decreasing over time.

ANS: F DIF: Easy REF: Section 22.2 MSC: Factual TOP: 2Iii 5. Dark energy is a force that continually decreases the expansion rate of the universe.

ANS: F DIF: Easy REF: Section 22.2 MSC: Conceptual TOP: 2Ivi 6. The fact that the currently observed mass   is very close to 1 means that its value has always been very close to 1.

ANS: T DIF: Medium REF: Section 22.3 MSC: Conceptual TOP: 3Ii 7. The concept of inflation helps solve both the problems of the flatness of the universe and smoothness of the cosmic background radiation.

ANS: T DIF: Easy REF: Section 22.3 MSC: Conceptual TOP: 3IIIi 8. The theory of inflation assumes that at some point in the future dark energy will decay and the universe will expand rapidly by a factor of 1030.

ANS: F DIF: Medium REF: Section 22.3 MSC: Factual TOP: 3IIIi 9. According to the theory of inflation. when the universe was approximately 1 second old, its size grew by a factor of 1030.

ANS: F DIF: Difficult REF: Section 22.3 MSC: Factual TOP: 3IIIi

10. The three fundamental forces of nature are gravity, electromagnetism, and the strong force.

ANS: F DIF: Easy REF: Section 22.4 MSC: Factual TOP: 4Ii 11. The four fundamental forces of nature are gravity, electromagnetism, the weak force, and the strong force.

ANS: T DIF: Easy REF: Section 22.4 MSC: Factual TOP: 4Ii 12. The weak force mediates the beta decay of neutrons into protons, electrons, and antineutrinos.

ANS: T DIF: Medium REF: Section 22.4 MSC: Factual TOP: 4Iiii 13. The force that binds three quarks together to make up a proton or neutron is called the weak force.

ANS: F DIF: Medium REF: Section 22.4 MSC: Factual TOP: 4Ivii 14. Directly after the Big Bang, the universe was filled with a variety of charged particles, antiparticles, and photons.

ANS: T DIF: Easy REF: Section 22.4 MSC: Factual TOP: 4IIii 15. Astronomers have confirmed the Grand Unified Theory by observing that protons decay in laboratory experiments.

ANS: F DIF: Easy REF: Section 22.4 MSC: Factual TOP: 4IIIiii 16. In the superstring theory that successfully unites gravity and quantum mechanics, the universe must have 11 dimensions (10 spatial and 1 temporal).

ANS: T DIF: Medium REF: Section 22.4 MSC: Factual TOP: 4IViii

MULTIPLE CHOICE 1. Why does the overall mass and energy density of the universe decide its ultimate fate? a. The greater the density of the universe, the more black holes will form, which will devour surrounding material more quickly. b. The greater the density of the universe, the more gravitationally bound the universe will be. c. The greater the density of the universe, the more massive stars it will contain, and the faster it will evolve. d. The greater the density of the universe, the more stars it will form over time and the longer the universe will last. e. The greater the density of the universe, the more supernova will explode, forcing the universe to expand forever.

ANS: B DIF: Medium REF: Section 21.1 MSC: Conceptual TOP: 1Ii 2. Gravity acts to __________ the expansion of the universe, and dark energy acts to __________ the expansion. a. slow; slow b. slow; speed up c. speed up; slow d. speed up; speed up ANS: B DIF: Easy REF: Section 22.1, 22.2 MSC: Applied TOP: 1Iii | 2Ivii 3. Which of the following is FALSE? a. When the universe was young, matter worked to slow the expansion rate of the universe. b. The Big Bang occurred approximately 13.7 billion years ago. c. Only in the last 5 to 6 billion years has dark energy caused the universe’s expansion rate to increase over time. d. Throughout the age of the universe, dark energy has caused its expansion rate to increase over time. e. Astrophysicists are unsure whether or not dark energy always will continue to increase the expansion rate of the universe.

ANS: D DIF: Medium REF: Section 22.1 | Section 22.2 MSC: Factual TOP: 1Iii | 2Ivi | 2Ivii | 2IIiii

4. The critical density of the universe is closest to: a. 1 proton/m3 b. 1 g/m3 c. 1 kg/m3 d. 1 MEarth/m3 e. 1 M/m3 ANS: A DIF: Medium REF: Section 22.1 MSC: Factual TOP: 1Iiii 5. If there were no dark energy in the universe, the value of __________ would determine the evolution and fate of the universe. a. H0 b. 1/H0 c. luminous d. mass e.  ANS: D DIF: Easy REF: Section 22.1 MSC: Applied TOP: 1Iv 6. If mass  0.5 today and there were no dark energy, the universe would: a. expand forever b. slow its expansion but never reverse it c. stop expanding and eventually collapse d. oscillate between expansion and collapse e. expand so quickly that the universe is ripped apart ANS: A DIF: Medium REF: Section 22.1 MSC: Applied TOP: 1Ivi

7. If the fate of the universe were determined SOLELY by what we currently know to be the total mass of the universe in luminous and dark matter (excluding dark energy), astronomers would predict that we live in a universe that will: a. expand forever b. slow its expansion but never reverse it c. stop expanding and eventually collapse d. oscillate between expansion and collapse e. expand so quickly that the universe is ripped apart ANS: A DIF: Medium REF: Section 22.1 MSC: Applied TOP: 1Ivi | 1Ivii 8. If gravity were slowing down the expansion of the universe, how would very distant galaxies differ from nearby galaxies? a. Distant galaxies would have a larger value for H0 than nearby galaxies. b. Distant galaxies would have a smaller value for H0 than nearby galaxies. c. Distant galaxies would contain more dark energy than nearby galaxies. d. Distant galaxies would contain less dark energy than nearby galaxies. e. Distant and nearby galaxies would be the same.

ANS: A DIF: Easy REF: Section 22.2 MSC: Applied TOP: 2Ii 9. To determine how the expansion rate of the universe has changed over time, astronomers directly measure the __________ for a sample of Type I supernovae in distant galaxies. a. redshift and luminosity b. redshift and brightness c. redshift and distance d. distance and luminosity e. distance and brightness ANS: B DIF: Easy REF: Section 22.2 MSC: Applied TOP: 2Iii

10. In the 1990s, astronomers found that distant Type I supernova were __________ than they expected, leading them to conclude that __________. a. brighter; the universe’s expansion rate was decreasing with time b. brighter; the universe was finite in size c. fainter; the universe’s expansion rate has been increasing with time d. fainter; the universe was infinite in size e. fainter; the universe was finite in size ANS: C DIF: Medium REF: Section 22.2 MSC: Applied TOP: 2Iii 11. Observations of Type I supernovae in distant galaxies have shown that: a. the star formation rate in galaxies decreases with increasing redshift b. the expansion rate of the universe is increasing c. the cosmological constant is zero d. dark energy is negligible at the present time e. there were more stars in the past than at the present time ANS: B DIF: Medium REF: Section 22.2 MSC: Applied TOP: 2Iii 12. The observations that show that the expansion of the universe is speeding up tell us the universe must contain: a. gravity b. dark matter c. a force opposing gravity d. black holes e. cosmic microwave background radiation ANS: C DIF: Easy REF: Section 22.2 MSC: Applied TOP: 2Iiii 13. The Big Rip could occur in a universe where the effect of __________ increases with time. a. quantum mechanics b. luminous matter c. dark energy d. gravity e. dark matter ANS: C DIF: Medium REF: Section 22.2 MSC: Applied TOP: 2Iiv

Figure 1

14. Figure 1 shows five graphs of the scale factor of the universe as a function of time.

Which of these graphs would occur for a universe with mass  1 and   0? a. A b. B c. C d. D e. E ANS: E DIF: Difficult REF: Section 22.2 MSC: Applied TOP: 2Iiv 15. Figure 1 shows five graphs of the scale factor of the universe as a function of time.

Which of these graphs would occur for a universe with mass  1 and   0? a. A b. B c. C d. D e. E ANS: C DIF: Difficult REF: Section 22.2 MSC: Applied TOP: 2Iiv

16. Figure 1 shows five possible graphs of the scale factor of the universe as a function of time.

Which of these graphs represent our universe where mass  0.3 and   0.7? a. A b. B c. C d. D e. E ANS: C DIF: Medium REF: Section 22.2 MSC: Applied TOP: 2Iiv 17. Current observations suggest that the density of luminous matter, all matter, and dark energy are: a. luminous  0.1, mass  0.3,   0.7 b. luminous  0.02, mass  0.5,   0.5 c. luminous  0, mass  0.1,   0.9 d. luminous  0.02, mass  0.3,   0.7 e. luminous  0.1, mass  0.2,   0.7 ANS: D DIF: Medium REF: Section 22.1 MSC: Factual TOP: 2Iv 18. What do astronomers believe is currently having the largest effect on the change in the expansion rate of the present-day universe? a. dark matter b. baryonic (normal) matter c. dark energy d. radiation pressure e. gravity ANS: C DIF: Medium REF: Section 22.2 MSC: Applied TOP: 2Ivii 19. If mass    1 today and dark energy were a cosmological constant, the universe would: a. expand forever b. slow its expansion but never reverse it c. stop expanding and eventually collapse d. oscillate between expansion and collapse e. expand so quickly that the universe is ripped apart ANS: A DIF: Medium REF: Section 22.2 MSC: Applied TOP: 2Iviii

20. How does the existence of dark energy affect the expansion of the universe? a. It is possible for the mass density of the universe to be below the critical density and still collapse. b. It is possible for the mass density of the universe to be below the critical density and still expand forever. c. It is impossible to have a collapsing universe, regardless of its density. d. It is impossible to have an expanding universe, regardless of its density. e. It is impossible for the mass density of the universe to be above the critical density and still expand forever.

ANS: B DIF: Medium REF: Section 22.2 MSC: Applied TOP: 2Iviii 21. What two properties of the universe are determined by the values of mass and ? a. size and temperature b. expansion rate and size c. size and age d. age and expansion rate e. age and temperature ANS: D DIF: Easy REF: Section 22.2 MSC: Factual TOP: 2IIi 22. In a universe with an accelerating expansion rate, the actual age of the universe is __________ the Hubble time. a. larger than b. smaller than c. negligible compared to d. the same as e. independent of ANS: A DIF: Medium REF: Section 22.2 MSC: Applied TOP: 2IIii

23. From the analysis of distant Type I supernovae and the Cosmic Microwave Background, we know that the Universe is approximately a. 4.6 million years old b. 4.6 billion years old c. 6.5 thousand years old d. 13.7 million years old e. 13.7 billion years old ANS: E DIF: Easy REF: Section 22.2 MSC: Applied TOP: 2IIiii 24. In a closed (spherical) universe, the sum of the angles in a triangle is: a. 180 degrees b.  180 degrees c. 120 degrees d.  180 degrees e. 240 degrees ANS: D DIF: Easy REF: Section 22.2 MSC: Factual TOP: 2IIIii 25. The universe can be infinite in size for which shapes of the universe? a. open b. closed c. flat d. closed and flat e. open and flat ANS: E DIF: Medium REF: Section 22.2 MSC: Factual TOP: 2IIIiii

Figure 2 26. Figure 2 shows a graph of the value of mass as a funtion of time in a universe with no dark energy. The five different curves correspond to universes with slightly different values for mass one second after the Big Bang.

Which line corresponds to a universe with the largest value of mass one second after the Big Bang? a. A b. B c. C d. D e. E ANS: A DIF: Medium REF: Section 22.3 MSC: Applied TOP: 3Ii 27. Figure 2 shows a graph of the value of mass as a funtion of time in a universe with no dark energy. The five different curves correspond to universes with slightly different values for mass one second after the Big Bang.

Which line corresponds to a universe with the smallest value of mass one second after the Big Bang? a. A b. B c. C d. D e. E ANS: E DIF: Medium REF: Section 22.3 MSC: Applied TOP: 3Ii 28. Why would it be very improbable for our universe to have mass    0.9? a. It would quickly evolve to have a much different value of mass  . b. In order to exist, every universe must have mass    1. c. We know mass  1, based on the average density of galaxies. d. The fluctuation in the CMB tell us that mass    0.7. e. Dark energy studies tell us   0.9.

ANS: A DIF: Difficult REF: Section 22.3 MSC: Applied TOP: 3Iii

29. The flatness problem arises because only a universe with __________ can have that value forever. a. mass    0 b. mass    1 c. mass  luminous  1 d. luminous    1 e. luminous    0 ANS: B DIF: Easy REF: Section 22.3 MSC: Applied TOP: 3Iii 30. Why is the fact that the cosmic microwave background radiation (CMB) is very smooth considered a problem? a. A universe as smooth as predicted by the CMB should not have formed as many galaxies as have been observed. b. A universe as smooth as predicted by the CMB should have collapsed by now. c. A universe as smooth as predicted by the CMB should be expanding much faster than we are now. d. A universe as smooth as predicted by the CMB should never occur because quantum mechanical fluctuations would have been imprinted on it. e. A universe as smooth as predicted by the CMB should never have formed any stars or galaxies.

ANS: D DIF: Difficult REF: Section 22.3 MSC: Applied TOP: 3IIiii 31. What can simultaneously solve both the flatness and horizon problems in cosmology? a. GUT b. quantum mechanics c. TOE d. inflation e. dark energy ANS: D DIF: Easy REF: Section 22.3 MSC: Factual TOP: 3IIIi

32. Which of the following is a FALSE statement about inflation? a. Inflation occurred when the universe was  10−33 seconds old. b. Inflation occurred when the universe expanded by a factor of approximately 1030. c. Inflation solves the horizon problem. d. Inflation is currently driving the expansion of the universe. e. Inflation solves the flatness problem.

ANS: D DIF: Medium REF: Section 22.3 MSC: Factual TOP: 3IIIi 33. Which of the following are NOT considered carrier particles used to mediate forces between other particles? a. photons b. gluons c. gravitons d. quarks e. W particles ANS: D DIF: Medium REF: Section 22.4 MSC: Applied TOP: 4Ii 34. Which was a triumph of quantum electrodynamics (QED)? a. QED predicted the existence of three carrier particles before they were discovered in laboratory experiments. b. QED unites the strong and weak nuclear forces. c. QED is an example of a theory of everything. d. QED successfully explains the origin of quantum mechanical fluctuations in the CBR. e. QED combined the electric and magnetic forces.

ANS: A DIF: Difficult REF: Section 22.4 MSC: Factual TOP: 4Iv 35. Quantum chromodynamics describes how __________ works. a. gravity b. the strong nuclear force c. electricity d. magnetism e. light ANS: B DIF: Easy REF: Section 22.4 MSC: Factual TOP: 4Ivi

36. How would astronomers describe the antiparticle of an electron? a. It would have the same charge but opposite spin. b. It would have the same mass but opposite charge. c. It would have the opposite charge and higher mass. d. It would have the opposite spin and higher mass. e. It would have the opposite charge but same spin.

ANS: B DIF: Medium REF: Section 22.4 MSC: Conceptual TOP: 4IIi 37. The figure below illustrates pair production in the early universe, with one particle labeled with a question mark.

What type of particle must this be? a. proton b. electron c. antiproton d. positron e. neutron ANS: D DIF: Easy REF: Section 22.4 MSC: Applied TOP: 4IIii 38. In order to produce an electron and a positron, two photons must have a combined energy of: a. E  2mec2 b. E  mec2 c. E  1⁄2mec2 d. E  mec2 e. This cannot happen.

ANS: A DIF: Easy REF: Section 22.4 MSC: Applied TOP: 4IIii

39. What was the temperature of the universe when a typical CMB photon was no longer able to spontaneously create an electron and positron pair? Note that a photon has an energy equal to E  hc/, the CMB at any point in time has a blackbody spectrum that peaks at a wavelength peak  (2900 m K)/T, and the mass of a single electron is 9  10−31 kg. a. 2000 K b. 400,000 K c. 2 million K d. 40 million K e. 2 billion K ANS: E DIF: Difficult REF: Section 22.4 MSC: Applied TOP: 4IIii 40. In the early universe, which type of particle and anti-particle first stopped being spontaneously formed out of photons in the CMB? a. Protons and neutrons, because their formation requires a larger number of photons b. Electrons and positrons, because their formation requires a smaller number of photons c. Protons and antiprotons, because their formation requires higher energy photons d. Electrons and neutrinos, because their formation requires lower energy photons e. Protons, antiprotons, electrons, and neutrinos stopped forming at the same time.

ANS: C DIF: Medium REF: Section 22.4 MSC: Applied TOP: 4IIIi 41. What would the universe be like if there were complete symmetry between matter and antimatter? a. It would look similar to our universe, but half of it would be composed of antimatter. b. We would observe two universes, one an antimatter reflection of the other. c. There would be no universe, because all of the matter and antimatter would have been annihilated. d. There would be a universe, but it would be completely composed of photons from the CMB. e. It would look similar to our universe, but the charges of all of the particles would be reversed.

ANS: D DIF: Medium REF: Section 22.4 MSC: Applied TOP: 4IIIii

42. At what stage in the universe’s history do we think the asymmetry between matter and antimatter was created? a. at the very moment the Big Bang occurred b. around the time gravity separated out of the single superforce c. around the time the strong force separated out of the grand unified theory d. around the time the weak force and electromagnetism separated e. around the time the electric and magnetic forces separated ANS: C DIF: Difficult REF: Section 22.4 MSC: Factual TOP: 4IIIii 43. A grand unified theory unites which forces? a. only electromagnetism, weak nuclear, and strong nuclear forces b. only gravity and strong nuclear forces c. only electromagnetism, gravity, and weak nuclear forces d. only gravity and electromagnetism forces e. all four known forces ANS: A DIF: Easy REF: Section 22.4 MSC: Factual TOP: 4IIIiii 44. To verify whether or not some grand unified theories are correct, physicists are searching for: a. the Big Rip b. mini–black holes c. antimatter d. protons that decay e. dark matter that decays ANS: D DIF: Medium REF: Section 22.4 MSC: Factual TOP: 4IIIiii 45. Grand unified theories are very attractive because they can explain: a. why we have five fundamental forces in the universe today b. why the Big Bang never made any antimatter c. why the universe consists mostly of matter d. why the CMB is very smooth e. what happens inside a black hole ANS: C DIF: Medium REF: Section 22.4 MSC: Factual TOP: 4IIIiii

46. What is the Planck era? a. the earliest moments after the Big Bang b. a period when quantum mechanics is needed to describe spacetime in addition to particles c. the time period when a theory of everything (TOE) is needed to understand the universe d. When spacetime can be described as a quantum mechanical “foam” rather than a smooth sheet e. all of the above ANS: E DIF: Medium REF: Section 22.4 MSC: Factual TOP: 4IVi 47. The standard model of particle physics is incomplete because it leaves which question(s) unanswered? a. Why do neutrinos have mass? b. Why is the strong nuclear force much stronger than the weak nuclear force? c. How is gravity related to the other three fundamental forces? d. How does gravity work over such long distances? e. all of the above ANS: C DIF: Difficult REF: Section 22.4 MSC: Conceptual TOP: 4IVii 48. In some particle physics theories, the universe must have more than three spatial dimensions, but we experience only three. Why would we not see the other spatial dimensions? a. The other nine spatial dimensions are too small to be noticeable. b. The other seven spatial dimensions are tightly wrapped around each other and have not expanded. c. The other seven spatial dimensions undergo inflation and flatten themselves out. d. The other nine spatial dimensions wrap around each another and form the temporal dimension. e. The other seven spatial dimensions are completely full of dark matter.

ANS: B DIF: Difficult REF: Section 22.4 MSC: Conceptual TOP: 4IViii 49. In the superstring theory that successfully unites gravity and quantum mechanics, the universe must have: a. four dimensions (three spatial and one temporal) b. six dimensions (three spatial and three temporal) c. seven dimensions (six spatial and one temporal) d. nine dimensions (eight spatial and one temporal) e. 11 dimensions (10 spatial and 1 temporal) ANS: E DIF: Medium REF: Section 22.4 MSC: Factual TOP: 4IViii

50. As the universe cooled shortly after the Big Bang, which was the first fundamental force to separate itself out from the others? a. electromagnetism b. gravity c. the nuclear force d. the strong force e. the weak force ANS: B DIF: Medium REF: Section 22.4 MSC: Factual TOP: 4Vi 51. Soon after the Big Bang, we believe the four fundamental forces of nature were united into one superforce, and __________ was the first to split off from the others. a. the strong force b. electromagnetism c. the weak force d. gravity e. nucleosynthesis ANS: D DIF: Easy REF: Section 22.4 MSC: Factual TOP: 4Vi 52. Why does a unified force split to become two separate forces? a. The universe expands so much that carrier particles become too dense. b. The average energy of photons in the universe becomes less than the mass of the unified force’s carrier particles. c. Dark energy becomes significant at later times and forces the two to split. d. The spatial dimensions split and so must the forces. e. Too many particles are created for the unified force to manage.

ANS: B DIF: Difficult REF: Section 22.4 MSC: Conceptual TOP: 4Vi

53. Put the following types of objects in order, starting with the first one to form after the Big Bang and ending with the last one to form: a. neutral atoms, protons, nuclei b. protons, nuclei, neutral atoms c. nuclei, neutral atoms, protons d. protons, neutral atoms, nuclei e. nuclei, protons, neutral atoms ANS: B DIF: Easy REF: Section 22.4 MSC: Applied TOP: 4Vii 54. Even with infinitely powerful telescopes, we can look back in time only until the time when a. galaxies first formed b. hydrogen and helium formed c. stars first formed d. gravity split off from a superforce e. recombination happened ANS: E DIF: Difficult REF: Section 225 MSC: Factual TOP: 5Ii 55. Which of these are possible types of multiple universes that could exist? a. parallel universes with different physics and different mathematical explanations b. other parts of an infinite universe which are so far away that we cannot observe them c. quantum mechanical parallel universes, which are created each time something happens that has a probability of occurring in a different way, such as a dice roll d. a multiverse with constant inflation, where each universe forms due to quantum fluctuations stopping that inflation e. all of the above ANS: E DIF: Difficult REF: Section 22.5 MSC: Conceptual TOP: 5Iii

56. Which of these is a restatement of the anthropic principle? a. Life evolves to create more complex species. b. Any galaxy must have living things. c. The values of physical constants must allow for our existence. d. There must be other civilizations in the universe. e. Dark energy will pull the universe apart.

ANS: C DIF: Easy REF: Section 22.6 MSC: Applied TOP: 6Ii

SHORT ANSWER 1. Why does the critical mass density of the universe depend on the value of Hubble’s constant, H0?

ANS: H0 is an indication of how fast the universe is expanding; and the faster the rate of expansion, the greater the amount of mass required to keep it gravitationally bound.

DIF: Medium REF: Section 22.1 MSC: Conceptual TOP: 1Iiii 2. The critical mass density of the universe today is 9.5  10−27 kg/m3. Using the current observed value of luminous, what is the average density of protons in the universe? If they were spread evenly throughout the universe, what would be their typical separation? Note that the typical separation between protons would be roughly equal to the cube root of the average volume occupied by one proton, and the mass of a proton is 1.7  10−27 kg.

ANS: The luminous  0.02 there the average density of protons in the universe is 0.02  9.5  10−27 kg/m3  1.9  10−28 kg/m3. Therefore, the volume a typical proton occupies, V, is equal to the mass of a proton divided by the average density of luminous matter. Thus, V  1.7  10−27 kg / (1.9  10−28 kg/m3)  8.9 m3, and the typical distance between protons is V1⁄3  2.1 m.

DIF: Difficult REF: Section 22.1 MSC: Applied TOP: 1Iiv 3. In a universe with no dark energy, what will happen to the expansion of the universe in the future if: (a) mass  1, (b) mass  1, and (c) mass  1?

ANS: For each case: (a) The universe’s expansion will slow until it reverses, and the universe will recollapse in a Big Crunch. (b) The universe’s expansion will continue forever. (c) The universe’s expansion will continue forever.

DIF: Easy REF: Section 22.1 ` MSC: Applied TOP: 1Ivi

4. If there were enough mass to slow down the expansion rate of the universe, how would the Hubble constant measured using very distant galaxies be different from what is observed in galaxies that are closer to us? Explain your answer. Is this what we observe in our universe?

ANS: If the universe were slowing down, it must have been expanding faster in the past. Looking farther away means looking farther back in the past, when the expansion rate would have been larger. Therefore, more distant galaxies would be experiencing a larger expansion rate than more nearby galaxies, meaning we would measure a larger Hubble constant for the more distant galaxies. This is the opposite of what we observe in our universe, because dark energy is making the expansion rate increase as time goes on.

DIF: Difficult REF: Section 22.2 MSC: Applied TOP: 2Ii 5. What are two observations that, when combined, tell us that the universe has mass    1 and is flat?

ANS: Correct answers are any two pairs of (1) fluctuations in the CMB, (2) redshifts and distances of Type I supernovae, and (3) galaxy clustering.

DIF: Easy REF: Section 22.1 MSC: Applied TOP: 2Iii 6. If dark energy is currently causing the expansion rate of the universe to increase with time, does this mean that you should worry that the Sun, the Earth, and your body itself are expanding? Why or why not?

ANS: Dark energy is indeed causing space to expand, but it is at such a small rate that it is only noticeable if you add up the expansion of very large distances, like the distances to galaxies. It does not make an appreciable difference on the scale of the Solar System, Sun, Earth, or your body, so you have nothing to worry about.

DIF: Easy REF: Section 22.2 MSC: Applied TOP: 2Iviii 7. If the Hubble constant were larger than it is, how would that change the measured age of the universe?

Explain your answer.

ANS: If the Hubble constant were larger, then the universe would be expanding faster. This means it would have taken less time for the universe to grow to its current scale factor, implying a younger universe. So, we would measure a younger age for the universe.

DIF: Medium REF: Section 22.2 MSC: Applied TOP: 2IIii

8. If you found a globular cluster that had an age of 20 billion years, what cosmological observations would conflict with this observation?

ANS: The WMAP results on the CMB say the universe’s age is 13.7 billion years; therefore, this globular cluster would be older than the age of the universe.

DIF: Easy REF: Section 22.2 MSC: Applied TOP: 2IIiii 9. Why does the sum of mass and  determine the shape of our universe?

ANS: The mass and energy density affects the curvature of spacetime. Therefore, a measurement of the density of the universe is an indication of its overall shape (flat, open, or closed).

DIF: Easy REF: Section 22.2 MSC: Conceptual TOP: 2IVi 10. What is the “flatness problem” in cosmology?

ANS: If the mass and energy density of the universe does not make   1 exactly, then it would be very unlikely to observe a value of  that is very close, but not equal, to 1 because the universe would very quickly diverge from   1. Therefore, if you observe that   1 then it probably is true that   1.

DIF: Medium REF: Section 22.3 MSC: Conceptual TOP: 3Iii 11. Explain what the uncertainty principle means, and how it relates to the horizon problem.

ANS: The uncertainty principle says that, as you look at smaller size scales, there are bigger fluctuations in particles. This results in ripples in the universe at very early times after the Big Bang. Observations show that the cosmic microwave background radiation is very smooth and constant, meaning these initial fluctuations have been smoothed out, even over the largest distances where energy could not have been exchanged between those areas of the universe. This is the horizon problem.

DIF: Difficult REF: Section 22.3 MSC: Conceptual TOP: 3IIi | 3IIii | 3IIiii 12. What can solve the flatness and horizon problem in cosmology, and why?

ANS: The concept of inflation in the very early universe neatly solves the flatness and horizon problem because when the universe expands greatly it becomes flat everywhere and it washes out quantum mechanical fluctuations by carrying them over the horizon and making the CMB look very smooth.

DIF: Medium REF: Section 22.3 MSC: Conceptual TOP: 3IIIi | 3IIIii | 3IIIiii | 3IIIiv

13. What are the names of the two particles that mediate the electromagnetic force in quantum electrodynamics (QED) and the strong force in quantum chromodynamics (QCD)?

ANS: The photon is the particle that mediates electromagnetic force in QED, and the gluon is the particle that mediates the strong force in QCD. DIF: Easy REF: Section 22.4 MSC: Factual TOP: 4Ii 14. Describe the four fundamental forces of nature by listing their names and describing what effect each force has on objects in the universe.

ANS: The four fundamental forces of nature are: (1) Gravity—is really the effect of mass and energy distorting spacetime, but which appears as an attractive force between two objects having mass.

(2) Strong force—this force holds the quarks together inside an individual proton or neutron, and it also holds the nuclei of atoms together.

(3) Weak force—mediates beta decay, which is when a neutron decays into a proton, electron, and antineutrinos.

(4) Electromagnetism—generates the repulsive or attractive electrical force and magnetism.

DIF: Medium REF: Section 22.4 MSC: Applied TOP: 4Ii | 4Iii | 4Iiii 15. In quantum electrodynamics (QED), what mediates the electromagnetic force between particles?

ANS: In QED, the electromagnetic force between two objects is determined by all the possible ways in which they can exchange virtual photons.

DIF: Difficult REF: Section 22.4 MSC: Conceptual TOP: 4Iiv 16. What was the temperature of the universe when a typical CMB photon was no longer able to spontaneously create a proton and antiproton pair? Note that a photon has an energy equal to E  hc/, the CBR at any point in time has a blackbody spectrum that peaks at peak  (2,900 m K)/T, and the mass of a single proton is 1.7  10−27 kg.

ANS: In order to create a proton and antiproton, the photon must have an energy E  2mc2. Therefore, the wavelength of the photon must be   hc/2mc2  h/2mc, which would mean that

T  2,900 m K  2mc/h.

T  (2,9.00  10−6 m K  2  1.7  10−27 kg  3  108 m/s)/(6.6  10−34 kg m2/s).

T  4  1012 K. T  400 trillion K.

DIF: Difficult REF: Section 22.4 MSC: Applied TOP: 4IIii 17. What is the difference between a grand unified theory and a theory of everything?

ANS: A TOE can link all four fundamental forces together, whereas a GUT can link electromagnetism and weak and strong forces together, but not gravity.

DIF: Easy REF: Section 22.4 MSC: Applied TOP: 4IVii 18. Explain why astronomers cannot accurately model the exact history of the universe in the first few fractions of a second after the Big Bang.

ANS: At the very moment of its creation, the universe was so incredibly small and dense that quantum mechanics played a significant role in dictating the evolution of the universe. According to quantum mechanics, it is impossible to talk about the early state of the universe as a certainty. Instead we can talk about it only in terms of probabilities.

DIF: Difficult REF: Section 22.4 MSC: Conceptual TOP: 4IVii 19. According to superstring theory, how are different varieties of elementary particles similar? How do they differ?

ANS: In superstring theory, all elementary particles are thought to be composed of “strings.” Differences between the particles arise because the strings vibrate in different ways.

DIF: Easy REF: Section 22.4 MSC: Applied TOP: 4IViii 20. Explain why inflation could be lead to an infinite number of multiverses inside our own universe. Are they likely to be similar or different than our own universe?

ANS: Because inflation made the universe tremendously large, large enough that different areas are no longer in causal contact, there are multiple regions that can be considered their own observable universe because they are disjointed from one another. They are likely to be very similar to our universe, because the universe is homogeneous and isotropic on that large a physical scale.

DIF: Medium REF: Section 22.5 MSC: Applied TOP: 5Iii

CHAPTER 23

Large-Scale Structure in the Universe

CONCEPT MAP Sec 23.1 1. Galaxies Form Groups, Clusters, and Larger Structures I. Galaxies Are the Fundamental Building Blocks of Structure in Our Universe i. Galaxy groups (MC: 1) ii. The Local Group (TF: 1) iii. Galaxy clusters (MC: 1–2) iv. Approximately 25 percent of the clusters are dominated by elliptical galaxies rather than spiral galaxies; however, by number, the most common type of galaxy is a dwarf galaxy (MC: 3) v. Superclusters (MC: 1) II. Mapping the Universe i. Hubble’s law (v  H0/D) is used to map the structure in the universe (SA: 1–2) ii. Voids and large-scale structure (MC: 4) iii. Peculiar velocities (TF: 2–3, MC: 5, SA: 3) III. Dark Matter Dominates the Mass of Galaxy Groups and Clusters i. Detected by looking at the motions of galaxies (TF: 4, MC: 6, SA: 4) ii. Detected using hot X-ray-emitting gas in a galaxy cluster (MC: 6–7, SA: 4) iii. Detected by gravitational lensing of background galaxies (MC: 6, SA: 4) Sec 23.2 2. The Origin of Structure I. Large-Scale Structure Is Caused by Gravity i. Hierarchical clustering (TF: 5–6, MC: 8–13)

ii. Quantum fluctuations in early universe were seeds for future galaxies (MC: 14) iii. Lambda-CDM (MC: 15) II. Galaxies Formed Because of Dark Matter i. CMB fluctuations are too small to explain structure observed now (MC: 16) ii. The underlying dark matter was clumped to a higher degree because dark matter does not interact with the photons (TF: 7, MC: 17–18, SA: 5) iii. Pressure from radiation (photons) smoothes out lumpiness in the distribution of normal matter, which interacts strongly with photons (MC: 18) iv. Dark matter provides the seeds for the observed structure of the universe (TF: 8, MC: 19–20) III. Hot and Cold Dark Matter i. Cold dark matter; candidates include axion and photino (MC: 21, SA: 6) ii. Hot dark matter, e.g., neutrinos (MC: 22, SA: 6) iii. Hot dark matter may contribute to 5 percent of the mass of the universe, but cold dark matter is its main constituent iv. The two types of dark matter form structure in opposite ways: cold dark matter  hierarchical formation, warm dark matter  top-down formation (MC: 23, SA: 7) v. Cold dark matter forms today’s structures Sec 23.3 3. First Light I. Before Stars and Galaxies i. Dark Ages: lasted from 200 to 600 million years after the Big Bang; when stars began to form from cold gas (MC: 24) ii. Re-ionization: when star formation and radiation from accreting black holes ionized the hydrogen in the universe (MC: 20, 25) II. The First Stars i. Formed from only H, He, and Li, no dust (MC: 26) ii. Formed in dark matter minihalos (TF: 9) iii. Probably 10 to 100 M, much more massive than the average star today (MC: 26, 27) iv. Resulting black holes may have merged together emitting large amounts of gravitational waves and creating supermassive black holes (MC: 27) v. Supernova created and scattered first heavy elements into galaxies (MC: 28)

vi. Second generation of stars had small amounts of heavy elements (MC: 28) vii. First stars not visible; second stars might be observable inside galaxies today III. The First Galaxies i. Formed from the merger of smaller dark matter minihalos (MC: 20) ii. Shaped by the radiation and energy input from the first stars and black holes iii. Their light is redshifted to infrared wavelengths (MC: 29) iv. Formed when universe was 400–500 million years old v. High redshift galaxies are ~20 times smaller than Milky Way vi. Ultrafaint dwarf galaxies (MC: 30) Sec 23.4 4. Galaxy Evolution I. Early Universe i. Galaxies smaller and closer together in early universe ii. Computer simulations and stellar archaeology used to study galaxy evolution II. Galaxy Evolution i. Younger galaxies are much messier (more disturbed) than those seen today; more peculiar, less spiral galaxies in the past (MC: 31–34) ii. Supermassive black hole growth likely connected with galaxy mergers (MC: 27) iii. Accretion onto supermassive black holes effects the star formation rates of galaxies; the growth of bulges and supermassive black hole masses are correlated (MC: 27) iv. Mergers of spiral galaxies can create elliptical galaxies (MC: 35–36) v. The average star formation rate in the universe peaked at z  3, about 2.5 to 3 Gyr ago (MC: 37) vi. Structures form hierarchically on all scales, including galaxy clusters (MC: 38–40, SA: 8) III. Simulating the Evolution of Structure i. Computers combine simulated dark matter and real data to model formation and evolution of universe, e.g., Bolshoi simulation (SA: 9) ii. Only sophisticated high-spatial resolution simulations can contain enough physics to produce structure that is comparable to observations (MC: 12–13, SA: 9) Sec 23.5 5. The Deep Future I. Great Eras: Past, Present, and Future

i. Primordial era (MC: 41–42, SA: 10) ii. Stelliferous era (TF: 10, MC: 42–43, SA: 10) iii. Degenerate era (MC: 42, SA: 10) iv. Black hole era (MC: 42, SA: 10) v. The dark era (MC: 42, 44–45, SA: 10) Sec 23.6 6. Origins: We Are the 4 or 5 Percent I. What Is the Future of the Universe? i. Contents of the universe: dark matter is 22 percent, dark energy is 74 percent, and normal matter is 4.6 percent

TRUE/FALSE 1. The Local Group is a part of the Virgo Supercluster.

ANS: T DIF: Medium REF: Section 23.1 MSC: Factual TOP: 1Iii 2. The peculiar velocities of galaxies allow us to determine how matter is distributed in the universe.

ANS: T DIF: Medium REF: Section 23.1 MSC: Conceptual TOP: 1IIiii 3. Astronomers have determined that the Milky Way is being pulled toward a massive structure called the “Great Attractor” due to the mass of the Great Attractor alone.

ANS: F DIF: Difficult REF: Section 23.1 MSC: Factual TOP: 1IIiii 4. In clusters of galaxies, the total mass of dark matter exceeds the luminous matter by about a factor of 1,000.

ANS: F DIF: Medium REF: Section 23.1 MSC: Factual TOP: 1IIIi 5. Even at its earliest moments, the universe had about as much structure in it as it has at the current time.

ANS: F DIF: Easy REF: Section 23.2 MSC: Factual TOP: 2Ii 6. As structure formed in the universe, galaxy-sized objects formed before cluster-sized objects.

ANS: T DIF: Easy REF: Section 23.2 MSC: Factual TOP: 2Ii 7. There are fluctuations in the CMB due to the fact that clumps of dark matter had already begun to form in the universe when the light from the CMB was given off.

ANS: T DIF: Easy REF: Section 23.2 MSC: Applied TOP: 2IIii 8. Current theoretical models suggest that most of the universe is composed of hot dark matter.

ANS: F DIF: Easy REF: Section 23.2 MSC: Applied TOP: 2IIiv 9. The first stars formed in the universe did not require the effects of dark matter to form.

ANS: F DIF: Medium REF: Section 23.3 MSC: Factual TOP: 3IIii 10. We currently find ourselves in the stelliferous era in the history of our universe.

ANS: T DIF: Medium REF: Section 23.5 MSC: Factual TOP: 5Iii

MULTIPLE CHOICE 1. Which of these shows the correct order of collections of galaxies, starting with the smallest and ending with the largest? a. group, supercluster, cluster b. cluster, supercluster, group c. group, cluster, supercluster d. cluster, group, supercluster e. supercluster, group, cluster ANS: C DIF: Easy REF: Section 23.1 MSC: Factual TOP: 1Ii | 1Iiii | 1Iv 2. If a galaxy is a member of a large cluster of galaxies, like the Coma cluster, the galaxy would have a typical velocity of 1,000 km/s. If the cluster is 10 Mpc in diameter, how long would it take the galaxy to cross from one side of the cluster to another? a. 10,000 years b. 1 million years c. 10 million years d. 100 million years e. 10 billion years ANS: E DIF: Difficult REF: Section 23.1 MSC: Applied TOP: 1Iiii 3. The most common type of galaxy found in a galaxy cluster is a ________ galaxy. a. spiral b. giant elliptical c. giant irregular d. dwarf e. barred spiral ANS: D DIF: Easy REF: Section 23.1 MSC: Factual TOP: 1Iiv

4. What does the large-scale structure of the universe look most like? a. a sponge with many large holes b. a loaf of wheat bread with many tiny holes c. a plate of flat noodles d. a jar of marbles e. a pizza with evenly spread out pepperoni ANS: A DIF: Easy REF: Section 23.3 MSC: Applied TOP: 1IIii 5. Peculiar velocities are produced by: a. erroneous redshifts b. gravity c. supernovae d. eclipsing binary stars e. interstellar winds ANS: B DIF: Easy REF: Section 23.1 MSC: Factual TOP: 1IIiii 6. Which of the following is NOT a way in which astronomers detect dark matter in clusters of galaxies? a. by determining the amount of mass necessary to gravitationally collapse clouds of gas to form the number of stars present b. by determining the amount of mass necessary to gravitationally hold onto the hot gas surrounding galaxy clusters c. by determining the amount of mass necessary to gravitationally hold a cluster of galaxies together d. by determining the amount of mass necessary to gravitationally lens images of distant objects e. None of the above; all of these are ways that astronomers detect dark matter in galaxy clusters.

ANS: A DIF: Medium REF: Section 23.1 MSC: Applied TOP: 1IIIi | 1IIIii | 1IIIiii 7. Which of these components makes up the largest amount of mass in a typical large cluster of galaxies? a. supermassive black holes b. stars c. cold gas d. hot gas e. neutron and white dwarf stars ANS: D DIF: Difficult REF: Section 23.1 MSC: Factual TOP: 1IIIii

8. What process is most responsible for shaping the large-scale structure of the universe? a. supernovae from the first generation of stars b. gravity c. matter/antimatter annihilation d. magnetic fields e. electric force ANS: B DIF: Easy REF: Section 23.2 MSC: Applied TOP: 2Ii 9. Which is NOT an ingredient needed to build a cosmological simulation of large-scale structure formation? a. the spectrum of density fluctuations that quantum mechanics imprints on the early universe b. the density of dark matter and luminous matter in the universe c. the grand unified theory d. the density of dark energy in the universe e. None of the above; all of the above are necessary.

ANS: C DIF: Medium REF: Section 23.2 MSC: Applied TOP: 2Ii 10. Structure formation in our universe: a. occurs for the largest structures first b. occurs for the smallest structures first c. begins on all spatial scales at the same time d. begins after clusters form e. begins with planets ANS: B DIF: Easy REF: Section 23.2 MSC: Applied TOP: 2Ii 11. Structure formation in the universe proceeds hierarchically, meaning that: a. large objects collapse then fragment to form smaller objects b. large objects form at the same times as smaller objects c. small objects collapse then merge to form larger objects d. only small objects form and are stable over time e. normal matter collapses first and dark matter collapses later ANS: C DIF: Easy REF: Section 23.2 MSC: Applied TOP: 2Ii

Figure 1

12. Figure 1 shows images taken from the Bolshoi simulation of the formation of the large-scale structure in the universe. Which one of these images represents the state of the universe at the highest redshift? a. A b. B c. C d. D e. All of these occur at the same redshift, just in different regions of the universe.

ANS: C DIF: Difficult REF: Section 23.2 | 23.4 MSC: Applied TOP: 2Ii | 4IIIii 13. Figure 1 shows images taken from the Bolshoi simulation of the formation of the large-scale structure in the universe. Which one of these images shows the current structure of the universe? a. A b. B c. C d. D e. All of these occur at the same redshift, just in different regions of the universe.

ANS: B DIF: Medium REF: Section 23.2 | 23.4 MSC: Applied TOP: 2Ii | 4IIIii 14. Quantum fluctuations in the early universe: a. were the seeds that grew into today’s galaxies b. are the reason dark matter exists c. were made of small black holes d. had no effect on the current structure of the universe e. can be observed with radio telescopes ANS: A DIF: Easy REF: Section 23.2 MSC: Factual TOP: 2Iii

15. The “Lambda-CDM” model combines the properties of __________ to explain the formation of structure in the universe. a. black holes and neutron stars b. dark energy and cold dark matter c. star formation and angular momentum d. nucleosynthesis and hot dark matter e. gravity and nuclear forces ANS: B DIF: Easy REF: Section 23.2 MSC: Factual TOP: 2Iiii 16. If the quantum fluctuations imprinted on the dark matter halos at the time of the formation of the CMB were ten times larger and the Hubble Constant was the same as it is now, galaxies would likely be: a. smaller than they are now b. larger than they are now c. more numerous than they are now d. nonexistent e. exactly the same as they are now ANS: B DIF: Medium REF: Section 23.2 MSC: Applied TOP: 2IIi 17. Why can’t dark matter halos collapse to be the same size as the visible parts of galaxies? a. Dark matter can’t dissipate its energy through collisions. b. Dark matter is mostly made of mini–black holes. c. Dark matter has much more angular momentum. d. Dark matter annihilates when it begins to get that dense. e. Dark matter particles are too large to collapse that much.

ANS: A DIF: Medium REF: Section 23.2 MSC: Applied TOP: 2IIii

18. In the early universe, why were inhomogeneities in the distribution of normal matter much smaller than the inhomogeneities in the dark matter? a. Normal matter is pushed away by supernova explosions. b. Magnetic fields smoothed the distribution of charged particles in the normal matter but not in dark matter. c. Dark matter particles were more massive and cooled off before normal matter, thus dark matter fluctuations had a longer time over which to grow. d. Dark matter was 10 times more massive than normal matter. e. Radiation pressure affected normal matter but not dark matter.

ANS: E DIF: Difficult REF: Section 23.2 MSC: Applied TOP: 2IIii | 2IIiii 19. Our current ideas on galaxy formation suggest that the visible parts of galaxies: a. form first and are incorporated into dark matter halos later b. form only in the densest parts of dark matter halos c. can tell you the total size of the dark matter halo d. can tell you everything about the formation history of that galaxy e. spread out over larger distances than dark matter halos ANS: B DIF: Easy REF: Section 23.2 MSC: Conceptual TOP: 2IIiv 20. Which of these lists shows the correct chronological order of the events listed, starting with the earliest and ending with the most recent? a. reionization, dark matter halos collapse, recombination, first galaxies are formed b. dark matter halos collapse, reionization, first galaxies are formed, recombination c. reinonization, first galaxies are formed, dark matter halos collapse, reionization d. recombination, dark matter halos collapse, first galaxies are formed, reionization e. first galaxies are formed, dark matter halos collapse, reionization, recombination ANS: D DIF: Difficult REF: Section 23.2 | 23.3 MSC: Applied TOP: 2IIiv | 3Iii | 3IIIi

21. Which of the following is NOT a possible candidate for dark matter? a. axions b. positrons c. photinos d. neutrinos e. None of the above; all of these could be dark matter.

ANS: B DIF: Medium REF: Section 23.2 MSC: Factual TOP: 2IIIi 22. How much of the total mass of the universe is made up of hot dark matter? a. less than 1 percent b. 5 percent c. 25 percent d. 45 percent e. 90 percent ANS: B DIF: Difficult REF: Section 23.3 MSC: Factual TOP: 2IIIii 23. If neutrinos have mass but do not interact much with normal matter, why can’t they be the dominant form of dark matter in the universe? a. Structure formation would have started with large objects fragmenting into smaller objects. b. Structure formation would have started with small objects merging to form larger objects. c. Neutrinos would decay over time and disappear, causing galaxies to fall apart. d. Neutrinos would not gravitationally lens background galaxies. e. Neutrinos are charged particles.

ANS: A DIF: Medium REF: Section 23.2 MSC: Applied TOP: 2IIIiv 24. Which of the following statements about the Dark Ages of the universe is FALSE? a. The first stars began forming during the Dark Ages. b. The end of the Dark Ages coincided with reionization. c. The Dark Ages lasted from 200 to 600 million years after the Big Bang. d. During the Dark Ages, photons could travel freely through the universe. e. Recombination occurred before the Dark Ages.

ANS: D DIF: Difficult REF: Section 23.3 MSC: Conceptual TOP: 3Ii

25. Reionization of the neutral gas in the universe occurred due to the: a. decay of dark matter particles b. emission of neutrinos by the first stars that formed c. release of jets of charged particles from supermassive black holes d. radiation from the first stars, supernovae, and black holes that formed e. positron and electron annihilations ANS: D DIF: Easy REF: Section 23.3 MSC: Factual TOP: 3Iii 26. The first stars formed in the universe had __________ compared to the stars formed today. a. higher metallicity and higher mass b. higher metallicity and lower mass c. lower metallicity and higher mass d. lower metallicity and lower mass e. higher mass and longer lifetimes ANS: C DIF: Easy REF: Section 23.3 MSC: Factual TOP: 3IIi | 3IIiii 27. Which of these statements about black holes in the early universe is NOT true? a. Supermassive black holes affected the star formation rates in early galaxies. b. The growth of supermassive black holes is likely linked with galaxy mergers. c. The first generation of stars had high enough masses to leave black holes behind after exploding as supernova. d. The black holes in the early universe should have been larger than those seen today. e. Supermassive black holes may have formed from mergers of smaller black holes.

ANS: D DIF: Difficult REF: Section 23.3 | Section 23.4 MSC: Applied TOP: 3IIiii | 3IIiv | 4IIii | 4IIiii

28. Which of the following best explain the difference between the heavy-element abundances seen in the first stars and those seen in stars that we observe today? a. Stars today have more heavy elements, because modern stars have higher masses, allowing them to create more heavy elements through nuclear fusion. b. Stars today have more heavy elements, because the gas that formed the current stars was enriched by the higher mass elements that were formed in the first stars. c. Stars today have a fewer heavy elements, because they have been around long enough to use up the larger mass atoms. d. Stars today have a smaller abundance of heavy elements because they haven’t been around long enough to make as many of the larger atoms. e. The stars that astronomers observe now are the first generation of stars.

ANS: B DIF: Medium REF: Section 23.3 MSC: Conceptual TOP: 3IIv | 3IIvi 29. The most distant galaxies visible to us at z  10 are best observed with __________ wavelengths of light. a. infrared b. visible c. X-ray d. gamma ray e. radio ANS: A DIF: Medium REF: Section 23.3 MSC: Applied TOP: 3IIIiii 30. If astronomers discover a new ultrafaint galaxy, where would it most likely be found? a. on its own, away from other galaxies b. a few billion light years away from Earth c. at high redshift d. in a large galaxy cluster e. orbiting the Milky Way ANS: E DIF: Easy REF: Section 23.3 MSC: Applied TOP: 3IIIvi

31. We expect the kinds of galaxies that we see at a redshift of z  4 to be: a. much like what we see today b. smaller and much more irregular looking than today c. smaller versions of what we see today d. far more numerous but with fewer spiral galaxies e. larger versions of what we see today ANS: B DIF: Easy REF: Section 23.4 MSC: Applied TOP: 4IIi 32. Ignoring the effect of redshift, we expect the galaxies that we see at a redshift of z  3 will be __________ than galaxies today. a. more irregular and redder b. larger and redder c. smaller and bluer d. smaller and redder e. larger and bluer ANS: C DIF: Medium REF: Section 23.4 MSC: Applied TOP: 4IIi 33. Compared to what we see today, galaxies in the past are: a. more ordered and more likely to have spiral structure b. more ordered and less likely to have spiral structure c. messier and more likely to have spiral structure d. messier and less likely to have spiral structure e. exactly the same as they are today ANS: D DIF: Easy REF: Section 23.4 MSC: Applied TOP: 4IIi

34. The figure shown below shows two different versions of the Hubble tuning fork diagram. One was made using what galaxies are like now, while the other was made using what galaxies were like 6 billion years ago.

Which one was made using observations of what galaxies were like 6 billion years ago? a. Version A is from 6 billion years ago because the galaxy mergers that have occurred since that time created more peculiar galaxies. b. Version A is from 6 billion years ago because there were more galaxies in the past than there are today. c. Version B is from 6 billion years ago because it has a higher percentage of elliptical galaxies. d. Version B is from 6 billion years ago because the higher merger rate in the past leads to a larger percentage of peculiar galaxies. e. There is no way to know since galaxies don’t change much as they get older.

ANS: D DIF: Difficult REF: Section 23.4 MSC: Applied TOP: 4IIi

35. In the future, we expect our universe will go through the ___________ era, when most of the normal matter in the universe is locked up in brown dwarfs, white dwarfs, neutron stars, and black holes. a. remnant b. neutral c. degenerate d. retirement e. quiet ANS: C DIF: Easy REF: Section 23.3 MSC: Factual TOP: 4IIiv 36. Which of the following is NOT likely to happen when two spiral galaxies collide? a. A more massive elliptical galaxy might form out of the merger. b. The two supermassive black holes at their centers could form a binary black hole system. c. Individual stars collide to create many supernovae. d. A burst of star formation will occur in the merged galaxy. e. The cold gas in the merged galaxy might be blown away by supernovae.

ANS: C DIF: Medium REF: Section 23.4 MSC: Conceptual TOP: 4IIiv 37. By measuring the star formation rate in galaxies as a function of their redshift, we have learned that the average star formation rate in galaxies peaked approximately _________ years ago. a. 1 billion b. 3 billion c. 5 billion d. 7 billion e. 12 billion ANS: B DIF: Medium REF: Section 23.3 MSC: Factual TOP: 4IIv 38. Which of these statements about galaxy clusters is true? a. Galaxy clusters do not require dark matter in order to form. b. Galaxy clusters are the largest structures in the universe. c. Small galaxy clusters form first, then merge together to form larger galaxy clusters. d. Galaxy clusters are evenly distributed throughout the universe. e. There are such large distances between galaxy clusters that they never actually run into each other.

ANS: C DIF: Easy REF: Section 23.4 MSC: Factual TOP: 4IIvi

39. Each of these statements describes the steps that occur during star formation. Which of these is NOT also true for galaxy formation? a. Angular momentum leads to the formation of a disk. b. For both stars and galaxies, the largest objects form first, with smaller objects coming together later. c. A gas cloud radiates energy, allowing it to collapse further than when it was hotter. d. Gravitational instability leads to collapse. e. The original large gas cloud splits into smaller fragments because areas with higher density also have greater gravitational pull.

ANS: B DIF: Difficult REF: Section 23.4 MSC: Conceptual TOP: 4IIvi 40. Which probably formed last in the course of the evolution of the universe? a. a typical proton inside a water molecule on the Earth b. a helium atom on the surface of the Sun c. a typical star that is a member of a globular cluster star in our Milky Way d. the Milky Way e. the Virgo Supercluster ANS: E DIF: Easy REF: Section 23.3 MSC: Conceptual TOP: 4IIvi

41. The figure below shows a diagram separating out the different eras of the universe based on the events that occurred during that era. Each era is labeled by a different letter.

Which of the sections of the image represents the primordial era? a. A b. B c. C d. D e. E ANS: E DIF: Medium REF: Section 23.5 MSC: Applied TOP: 5Ii 42. In order from first to last, the specific eras in the universe’s history are known as the: a. primordial, stelliferous, degenerate, and dark eras b. stelliferous, black hole, and entropy eras c. primordial, stelliferous, black hole, and entropy eras d. primodial, degenerate, and black hole eras e. primordial, stelliferous, degenerate, black hole, and dark eras ANS: E DIF: Medium REF: Section 23.5 MSC: Factual TOP: 5Ii | 5Iii | 5Iiii | 5Iiv | 5Iv

43. We currently live in the universe during the: a. era of stars b. era of degeneracy c. era of black holes d. era of darkness e. era of particle-antiparticle formation ANS: A DIF: Easy REF: Section 23.5 MSC: Factual TOP: 5Iii 44. What do astronomers believe will be the final state of our universe? a. a Big Crunch in which everything collapses back in on itself b. an ever-expanding universe filled with nothing but hydrogen and helium gas c. a universe that stops expanding and is filled with nothing but white dwarfs, neutron stars, and black holes d. an ever-expanding universe filled with photons and elementary particles e. a universe that stops expanding once enough stars become black holes ANS: D DIF: Easy REF: Section 23.5 MSC: Factual TOP: 5Iv

45. The figure below shows a diagram separating out the different eras of the universe based on the events that occurred during that era. Each era is labeled by a different letter.

Which of the sections of the image represents the dark era? a. A b. B c. C d. D e. E ANS: A DIF: Medium REF: Section 23.5 MSC: Applied TOP: 5Iv

SHORT ANSWER 1. Describe the large-scale structure of the universe.

ANS: The structure of the universe is somewhat like a sponge where galaxies and clusters of galaxies lie along walls and filaments that surround voids in space that contain much fewer galaxies.

DIF: Easy REF: Section 23.1 MSC: Applied TOP: 1IIi

2. Use Hubble’s Law to explain how measurements of redshift help astronomers map out the large-scale structure of the universe.

ANS: Hubble’s law (v  H0 / D) states that, due to the expansion of the universe, the farther a galaxy is away from us, the faster it will be receding away from us. Astronomers can therefore measure a galaxy’s redshift and use that calculated velocity to determine the distance to that galaxy.

DIF: Medium REF: Section 23.1 MSC: Conceptual TOP: 1IIi 3. How do we know how fast the Milky Way is moving relative to the local universe? What are we moving toward, and what do we think is mostly responsible for this motion?

ANS: The Doppler shift of the CMB tells us how fast we are moving relative to the CMB, which is the local standard of rest relative to the universe at large. We are moving toward a mass overdensity that is named the Great Attractor, although the recently discovered Shapley Supercluster is likely the dominant cause for this motion.

DIF: Difficult REF: Section 23.1 MSC: Conceptual TOP: 1IIiii 4. Describe two ways in which you could measure the mass of a galaxy cluster.

ANS: Here are three possible answers: (1) Determine the mass needed to bind the hot X-ray-emitting gas to the cluster.

(2) Determine the velocities of galaxies in the cluster and use their kinetic energy to estimate the total mass of the cluster.

(3) Determine the mass using the gravitational lensing of background galaxies.

DIF: Medium REF: Section 23.1 MSC: Applied TOP: 1IIIi | 1IIIii | 1IIIiii 5. Explain why a galaxy can collapse to a much smaller size than its dark matter halo.

ANS: Dark matter collapses down until it is supported by the random motions of its particles. However, normal matter can collapse further as gas clouds collide with one another and dissipate their energy and angular momentum. Dark matter cannot get rid of its energy and angular momentum.

DIF: Difficult REF: Section 23.2 MSC: Applied TOP: 2IIii 6. What is the fundamental difference between the two basic types of dark matter? List an example of each type.

ANS: Cold dark matter particles are heavy and move relatively slowly, whereas hot dark matter particles are lighter and move very rapidly. Two candidates for cold dark matter are elementary particles called

axions or photinos, while hot dark matter is made of neutrinos.

DIF: Medium REF: Section 23.2 MSC: Conceptual TOP: 2IIIi | 2IIIii 7. Why do astronomers think that cold dark matter (as opposed to hot dark matter) is the primary component of dark matter in galaxies?

ANS: Because cold dark matter can condense around galaxies far better than hot dark matter can. Also, galaxy observations indicate that galaxies formed hierarchically, which points to cold dark matter, since hot dark matter would lead to top-down structure formation.

DIF: Medium REF: Section 23.2 MSC: Conceptual TOP: 2IIIiv 8. How do astronomers explain the formation of elliptical galaxies?

ANS: Elliptical galaxies most likely formed as a result of mergers between smaller galaxies. As a result of such a merger, any structure that was present in the galaxies would have disappeared, leaving behind an elliptical shape. In the burst of star formation that the merger induces, the gas is expelled by supernovae explosions and leads to the cessation of star formation.

DIF: Easy REF: Section 23.4 MSC: Applied TOP: 4IIvi 9. Explain how computer simulations of structure formation and observations of the structure in the universe today can help astronomers determine the nature of dark matter.

ANS: Only simulations with specific types and amounts of dark matter result in the formation of the types of structures that actually exist in the universe. If the dark matter parameters that were used in the simulation cannot result in the structures we see today, then the universe could not actually be dominated by those “failed” types of dark matter DIF: Medium REF: Section 23.4 MSC: Conceptual TOP: 4IIIi | 4IIIii 10. Put the following eras in the history of the evolution of the universe in their proper order from the earliest to the latest: black hole era, dark era, degenerate era, primordial era, and stelliferous era.

ANS: From first to last are the primordial, stelliferous, degenerate, black hole, and dark eras.

DIF: Easy REF: Section 23.5 MSC: Factual TOP: 5Ii | 5Iii | 5Iiii | 5Iiv | 5Iv

CHAPTER 24

Life

CONCEPT MAP Sec 24.1 1. Life’s Beginnings on Earth I. Definition of Life i. Life: when complex biochemical processes allow organisms to grow and sustain themselves by drawing energy from their environment ii. Organisms reproduce and evolve using RNA and DNA iii. All terrestrial life involves carbon-based chemistry and liquid water iv. Primordial Earth (MC: 1) v. Urey-Miller experiment (MC: 2–3, SA: 1) vi. Terrestrial life probably began in the primordial Earth’s oceans or tide pools (MC: 4) II. First Life i. The young Earth was bombarded by Solar System debris for the first 100 million years (MC: 5) ii. Stromatolites: simple microbes (MC: 6–7, SA: 2) iii. Extremophiles: extreme forms of life (MC: 8) iv. Cyanobacteria: single-celled organisms; blue green algae (MC: 9) v. Oxygenation of the Earth’s atmosphere happened gradually (TF: 1, MC: 5, SA: 2) vi. Terrestrial life is divided into three categories based on DNA (MC: 10) vii. Bacteria and archaeans are prokaryotes viii. Eukaryotes are more complex than prokaryotes with their DNA enclosed in nuclear membranes (MC: 11–12) ix. 80 percent of all life on Earth has been in the form of microbes (TF: 2) x. The first eukaryotes came along ~2 billion years ago (MC: 12)

xi. The first multicellular eukaryotes came along only 1 billion years ago (MC: 5) III. Life Becomes More Complex i. Prokaryotic and eukaryotic organisms share a similar genetic code (MC: 12) ii. Evolutionary Tree of Life (MC: 13) iii. Animals are actually distant descendants of the fungi branch (MC: 14) iv. Single-celled and primitive organisms ruled the Earth’s oceans for 3 billion years v. Cambrian explosion: 540 to 500 million years ago, explosion of diversity (MC: 15) vi. 475 million years ago—plants evolved on land (MC: 16) vii. 230 million years ago—the age of dinosaurs occurred (MC: 5) viii. 65 million years ago—impact of a comet or asteroid wiped out over 50 percent of all plant and animal species, giving mammals their big evolutionary break (MC: 17) ix. Few million years ago—first humans appear (SA: 2) x. 10,000 years ago—first civilizations evolved (MC: 18, 19) xi. History of the universe condensed to the scale of 1 day (MC: 6, 18, 20) IV. Evolution Is a Means of Change and Advancement i. To create life, only one self-replicating molecule needs to form by chance ii. Mutation (TF: 3, MC: 21, SA: 3) iii. Heredity (MC: 22) iv. Natural selection (TF: 4, SA: 3) v. Genetic evolution is inevitable (TF: 4) Sec 24.2 2. The Chemistry of Life I. The Chemistry of Life i. Most of life on Earth is made of four to six chemical elements (TF: 5, MC: 23, SA: 4) ii. DNA is composed of combinations of a few 1010 atoms of five elements (MC: 24) iii. Proteins iv. Terrestrial life employs 20 specific amino acids (TF: 6) v. Terrestrial life is carbon based (organic) (TF: 7) vi. Long, diverse chains of atoms can be made with carbon as the backbone (MC: 25) vii. Other life forms could use an element other than carbon as its base (TF: 7) II. Life, the Universe, and Everything i. Entropy (MC: 26)

ii. The Second Law of Thermodynamics (MC: 27, SA: 5) Sec 24.3 3. Life beyond Earth I. Exploration of the Solar System i. Astrobiology (TF: 8, MC: 28) ii. Mars (MC: 29) iii. Jupiter’s moon Europa (MC: 29) iv. Saturn’s moon Titan (MC: 29) v. Saturn’s moon Enceladus (MC: 29) vi. Finding life on two bodies increases the odds of life being everywhere (MC: 30) II. Habitable Zones i. The habitable zone: 0.7 to 1.4 AU in our Solar System (MC: 31–32) ii. It is useful to search for stars whose lifetimes are  4.3 billion years (MC: 32, SA: 6) iii. A planet’s mass is also a factor as to its ability to hold an atmosphere (MC: 32) iv. Galactic habitable zone (MC: 33) Sec 23.4 4. The Search for Signs of Intelligent Life I. How to Search for Intelligent Life i. In the 1970s, Pioneer and Voyager spacecraft (MC: 34) ii. In 1974, Arecibo message (MC: 35–36) iii. Most searches for intelligent life monitor radio wavelengths (MC: 37, SA: 7) II. The Drake Equation i. Frank Drake and Drake equation (TF: 9, MC: 38, SA: 8–9) ii. The most pessimistic estimate says we are the only one in our galaxy (MC: 38) iii. The most optimistic estimate says the nearest could be only 40–50 light-years away (MC: 38) iv. Any civilization we discover will almost certainly be advanced III. Technologically Advanced Civilizations i. Fermi paradox (MC: 39, SA: 10) ii. Search for Extraterrestrial Intelligence (SETI) iii. SETI’s Allen Telescope Array (ATA) (TF: 10, MC: 40) iv. Finding another advanced civilization suggests the galaxy is teeming with life

v. We cannot go to these stars in person to investigate Sec 24.5 5. Origins: The Fate of Life on Earth I. Eventually, the Sun Must Die i. In 5 billion years, the Sun will become a red giant (SA: 11) ii. Terrestrial planets are likely to burn, unless they move outward (SA: 11) iii. It is unlikely that a comet or asteroid collision will end all life on Earth (SA: 11) iv. To ensure survival, humans must learn to manage the threat of impacts (SA: 11) v. Humanity is the worst threat to its own survival

TRUE/FALSE 1. Scientists have shown that ammonia and methane, mixed together with liquid water and either hydrogen, carbon dioxide, or nitrogen, and exposed to electrical sparks, can produce some of the amino acids essential for life.

ANS: T DIF: Medium REF: Section 24.1 MSC: Factual TOP: 1IIv 2. Approximately 10 percent of all life on Earth has been in the form of microorganisms.

ANS: F DIF: Medium REF: Section 24.1 MSC: Factual TOP: 1IIix 3. Mutations always lead to improvements in an organism’s ability to survive and reproduce.

ANS: F DIF: Easy REF: Section 24.1 MSC: Applied TOP: 1IVii 4. According to the theory of evolution, chemical reactions resulting in the mutation of a molecule are a natural and inevitable occurrence.

ANS: T DIF: Easy REF: Section 24.1 MSC: Factual TOP: 1IViv | 1IVv 5. The four main chemicals of which most life forms on Earth are composed are hydrogen, carbon, oxygen, and calcium.

ANS: F DIF: Easy REF: Section 24.2 MSC: Factual TOP: 2Ii 6. Terrestrial life utilizes 20 different acids that are composed of only four different chemical elements.

ANS: T DIF: Difficult REF: Section 24.2 MSC: Factual TOP: 2Iiv 7. Some forms of life on Earth are silicon-based.

ANS: F DIF: Easy REF: Section 24.2 MSC: Factual TOP: 2Iv | 2Ivii 8. The habitable zone for a 1M star ranges from 0.7 to 1.4 AU from the star.

ANS: T DIF: Difficult REF: Section 24.3 MSC: Applied TOP: 3IIi 9. The Drake equation estimates the number of solar systems in our galaxy today that may harbor life.

ANS: F DIF: Easy REF: Section 24.4 MSC: Factual TOP: 4IIi 10. The main goal of the Allen Telescope Array is to search for high redshift galaxies.

ANS: F DIF: Easy REF: Section 24.4 MSC: Factual TOP: 4IIIiii

MULTIPLE CHOICE 1. Which of the following is a likely source of the energy needed to create the first life on Earth? a. land volcanoes b. asteroid impacts c. lightning d. fire e. earthquakes ANS: C DIF: Easy REF: Section 24.1 MSC: Applied TOP: 1Iiv 2. In 1952, chemists Harold Urey and Stanley Miller mixed ammonia, methane, and hydrogen; in a closed container; zapped it with electrical sparks and found that: a. they could induce cold fusion to occur b. they could not induce any amino acids to form c. single-celled microorganisms had been spontaneously created d. they had created many of the amino acids contained in DNA e. they created life in a test tube ANS: D DIF: Easy REF: Section 24.1 MSC: Factual TOP: 1Iv

3. What was the goal of the experiment depicted below?

a. to create a microorganism b. to simulate the formation of Earth c. to simulate the Big Bang d. to simulate the early universe e. to simulate the formation of the building blocks of life ANS: E DIF: Medium REF: Section 24.1 MSC: Applied TOP: 1Iv 4. Where do we think life first formed on the Earth? a. in the air b. in the oceans c. on land d. on meteoroids that carried microorganisms to Earth e. deep in the interior of Earth ANS: B DIF: Easy REF: Section 24.1 MSC: Factual TOP: 1Ivi

5. Imagine if you built a time machine and successfully traveled 2 billion years backward in time. What would happen? a. You would see dinosaurs roaming Earth. b. You would see numerous multicellular species on Earth. c. You probably would be killed by the ongoing heavy bombardment of Earth. d. You would die due to lack of oxygen. e. You would be floating in space because Earth hadn’t formed yet.

ANS: D DIF: Medium REF: Section 24.1 MSC: Applied TOP: 1IIi | 1IIv | 1IIxi | 1IIIvii 6. If we model the history of the universe as a single day, at what time would the first primitive forms of singlecelled microorganisms appear? a. 5:00 A.M. b. 9:00 A.M. c. 12:00 noon d. 3:00 P.M. e. 5:20 P.M. ANS: E DIF: Medium REF: Section 24.1 MSC: Applied TOP: 1IIii | 1IIIxi

7. The image below is of an Australian shoreline. Which of the following forms of early life on Earth does it depict?

a. stromatolites b. cyanobacteria c. eukaryotes d. meteorites e. fungi ANS: A DIF: Medium REF: Section 24.1 MSC: Medium TOP: 1IIii 8. In which of the following locations has life NOT been found on Earth? a. near deep-oceans hydrothermal vents b. in extremely dry deserts c. in Arctic ice d. in hot sulfur springs e. None of the above. Life has been found in all of these locations.

ANS: E DIF: Easy REF: Section 24.1 MSC: Factual TOP: 1IIiii 9. Why did the presence of cyanobacteria on Earth in the past allow humans to exist? a. It was the first form of life. b. It oxygenated the atmosphere. c. It fertilized the soil to let plants grow. d. It increased the carbon dioxide in the atmosphere, causing the greenhouse effect. e. It was the first life form based on DNA. ANS: B DIF: Easy REF: Section 24.1 MSC: Conceptual TOP: 1IIiv

10. Which one of the following is NOT one of the three categories of terrestrial life based on their DNA characteristics? a. bacteria b. eukarya c. archaea d. fungi ANS: D DIF: Difficult REF: Section 24.1 MSC: Factual TOP: 1IIvi 11. Complex microorganisms that have complex DNA enclosed in a cell nucleus are called: a. algae b. bacteria c. fungi d. eukaryotes e. prokaryotes ANS: D DIF: Easy REF: Section 24.1 MSC: Factual TOP: 1IIviii 12. What is the major difference between prokaryotes and eukaryotes? a. Eukaryotes have DNA, and prokaryotes have RNA. b. Eukaryotes have a nuclear membrane surrounding their DNA, and prokaryotes do not. c. Eukaryotes are planets, and prokaryotes are animals. d. Eukaryotes appeared on Earth before prokaryotes. e. Eukaryotes are single-celled, and prokaryotes are multicellular.

ANS: B DIF: Medium REF: Section 24.1 MSC: Factual TOP: 1IIviii | 1IIx | 1IIIi

13. Which of the following sketches could describe the evolutionary tree of life on Earth? In each diagram, time increases as you move upward, and each branch represents a different species.

a. A b. B c. C d. D e. E ANS: A DIF: Medium REF: Section 24.1 MSC: Applied TOP: 1IIIii 14. Animals are most closely related to which of the following branches? a. bacteria b. archaea c. flagellates d. fungi e. cyanobacteria ANS: D DIF: Applied REF: Section 24.1 MSC: Applied TOP: 1IIIiii 15. Which of these occurred approximately 500 million years ago? a. the extinction of the dinosaurs b. the Cambrian explosion c. The Moon was formed. d. the rise of mammals e. the birth of the first humans ANS: B DIF: Difficult REF: Section 24.1 MSC: Factual TOP: 1IIIv

16. Why could life not have existed on land any earlier than 475 million years ago? a. The temperature was too high. b. There was too little sunlight. c. There was too little oxygen in the atmosphere. d. Earth was covered in erupting volcanoes. e. There was no land because Earth was covered in water.

ANS: C DIF: Medium REF: Section 24.1 MSC: Applied TOP: 1IIIvi 17. Why was a comet impact 65 million years ago a chance happening that benefited the evolution of humans? a. It deposited a significant amount of nitrogen into the Earth’s atmosphere. b. It led to an increase in global UV radiation, which killed off most of the forests and jungles. c. Mammals got an evolutionary boost. d. Plant life began to decline. e. It brought human DNA to Earth.

ANS: C DIF: Easy REF: Section 24.1 MSC: Conceptual TOP: 1IIIviii 18. If we model the history of the universe as a single day, at what time would the first Homo sapiens appear? a. 11:59:58.5 P.M. b. 11:59:20 P.M. c. 11:35:00 P.M. d. 10:00 P.M. e. 6:00 P.M. ANS: A DIF: Medium REF: Section 24.1 MSC: Applied TOP: 1IIIx | 1IIIxi 19. Based on what you know of the Earth’s evolution, what percent of Earth-like planets in a star cluster with an age of 2 billion years likely has intelligent life? a. 0 b. 10 percent c. 30 percent d. 50 percent e. 100 percent ANS: A DIF: Difficult REF: Section 24.2 MSC: Applied TOP: 1IIIx

20. If we model the history of the universe as a single day, at what time would the dinosaurs be wiped out by a large asteroid? a. 5:20:00 P.M. b. 11:00:00 P.M. c. 11:35:00 P.M. d. 11:53:10 P.M. e. 11:59:25 P.M. ANS: D DIF: Medium REF: Section 24.1 MSC: Applied TOP: 1IIIxi 21. If a mutation occurred and spawned a population B that had a 25 percent higher chance of reproducing than the original population A, in how many generations would B make up 90 percent of the total population? a. 10 b. 16 c. 20 d. 24 e. 32 ANS: A DIF: Difficult REF: Section 24.1 MSC: Applied TOP: 1IVii 22. The ability for one generation to pass on its characteristics to future generations is known as: a. natural selection b. mutations c. heredity d. self-replication e. duplication ANS: C DIF: Easy REF: Section 24.1 MSC: Factual TOP: 1IViii

23. Which one of the following is NOT one of the six chemical elements commonly found in living organisms? a. helium b. hydrogen c. oxygen d. phosphorus e. nitrogen ANS: A DIF: Easy REF: Section 24.2 MSC: Factual TOP: 2Ii 24. Which of the following elements are NOT found in DNA molecules? a. carbon b. nitrogen c. hydrogen d. helium e. oxygen ANS: D DIF: Easy REF: Section 24.2 MSC: Factual TOP: 2Iii 25. Carbon forms the backbone of our DNA primarily because: a. it is the most abundant element in the universe after hydrogen and helium b. it reacts easily with oxygen c. it remains solid even at high temperatures d. a carbon atom can bond with up to four atoms at a time e. it can form multiple types of crystal structures ANS: D DIF: Difficult REF: Section 24.2 MSC: Conceptual TOP: 2Ivi

26. Which of these pictures depicts the situation with the lowest entropy?

a. A b. B c. C d. D e. E ANS: D DIF: Medium REF: Section 24.2 MSC: Applied TOP: 2IIi 27. The second law of thermodynamics states that: a. all systems must become more disordered with time b. all systems must become more ordered with time c. some systems can become more ordered if others become less ordered by the same amount or more d. Earth must revolve around the Sun e. energy must always be conserved ANS: C DIF: Easy REF: Section 24.2 MSC: Conceptual TOP: 2IIii 28. The field of astrobiology uses our knowledge of ___________ to study life in the universe. a. biology b. chemistry c. physics d. astronomy e. all of the above ANS: E DIF: Easy REF: Section 24.3 MSC: Conceptual TOP: 3Ii

29. Which of the following Solar System objects is NOT a good candidate for future searches for life? a. Mars, because it once had liquid water on the surface b. Jupiter’s moon Europa, because it appears to have liquid water under its frozen surface c. Saturn’s moon Titan, because it has an atmosphere containing many organic molecules d. Pluto, because a large portion of it is made of water ice e. Saturn’s moon Enceladus, because its cryovolcanoes indicate that it has liquid water under the surface ANS: D DIF: Easy REF: Section 24.3 MSC: Applied TOP: 3Iii | 3Iiii | 3Iiv | 3Iv 30. If future exploration of the moons of Jupiter and Saturn discovered primitive life forms, it would ___________ the probability of finding an advanced civilization somewhere else in the Milky Way because ___________. a. increase; it shows that life can exist in environments which are very different from those found on Earth b. increase; it would show that life can survive travel through space after leaving the Earth c. not affect; primitive life forms have nothing to do with the existence of advanced civilizations d. decrease; it would show that primitive life forms outside the Earth cannot evolve into more complex species e. decrease; it would show that primitive life forms are far more common than advanced ones ANS: A DIF: Medium REF: Section 24.3 MSC: Conceptual TOP: 3Ivi 31. The habitable zone around a star depends most on its: a. mass and age b. radius and distance c. age and radius d. color and distance e. luminosity and velocity ANS: A DIF: Medium REF: Section 24.3 MSC: Factual TOP: 3IIi

32. Based on what we know about the evolution of life on Earth, which of the planets below would be most likely to contain an advanced civilization?

a. Planet A b. Planet B c. Planet C d. Planet D e. Planet E ANS: C DIF: Medium REF: Section 24.3 MSC: Applied TOP: 3IIi | 3IIii | 3IIiii 33. If we wanted to search the Milky Way for other civilizations, where should we look? a. near high-mass stars, because they live longer b. near low-mass stars, because their habitable zones are farther from their stars c. near the galactic center, because the higher temperature makes life more likely d. in the disk of the Milky Way, because planets there are more protected from harmful gamma rays and Xrays coming from the galactic center e. in the halo of the Milky Way, because stars in the halo have more heavy elements than stars in the disk ANS: D DIF: Easy REF: Section 24.3 MSC: Applied TOP: 3IIiv

34. When the Pioneer and Voyager spacecraft were launched into space in the 1970s, they were outfitted with messages describing where they came from. Why is it fairly unlikely that an alien civilization will use them to find us? a. The extremely large distances between stars means it will take a very long time before they reach another solar system. b. They will probably rust and fall apart as they get older. c. They will burn up as the Sun’s gravity pulls them in. d. They will likely run into Kuiper Belt objects before they leave the Solar System. e. They are moving so fast through space that they would be very difficult to catch.

ANS: A DIF: Medium REF: Section 24.4 MSC: Applied TOP: 4Ii 35. In 1974, astronomers sent a message toward globular cluster M13. If life exists there, and it returns our signal, we won’t receive it for at least another 44,000 years. Why? a. It will take that long for the space probe carrying our signal to reach the life forms there. b. Based on the age of the stars in M13, we anticipate it would take that long for a civilization to evolve enough to interpret and respond to our signal. c. M13 is far enough away that even light takes a very long time to reach it. d. It will take that long before our Solar System and M13 are properly aligned again. e. The universe will have expanded substantially after M13 receives our message, therefore taking much longer for their response to make it back to Earth.

ANS: C DIF: Medium REF: Section 24.4 MSC: Applied TOP: 4Iii 36. A value of N  0.1 for the Drake equation signifies that: a. one out of every 10 solar systems in our galaxy harbors intelligent life b. one out of every 10 solar systems in our galaxy harbors life of some kind c. one out of every 10 galaxies in our universe harbors intelligent life d. one out of every 10 galaxies in our universe harbors life of some kind e. approximately one intelligent civilization in the universe is created every 10 billion years.

ANS: C DIF: Medium REF: Section 24.4 MSC: Applied TOP: 4Iii

37. We search for intelligent life in the universe most effectively by: a. sending out spacecraft with messages on them b. using radio telescopes to search for radio signals c. monitoring ultraviolet radiation emitted by stars d. sending spacecraft to explore other worlds e. all of the above ANS: B DIF: Easy REF: Section 24.4 MSC: Factual TOP: 4Iiii 38. If the most pessimistic assumptions in the Drake equation were true, we would: a. have to wait for millions of years to get a message back from the nearest intelligent life b. have to wait for approximately 40 years to get a message back from the nearest intelligent life c. be the only intelligent life in the universe d. need to concentrate on the Andromeda Galaxy when searching for intelligent life e. need to concentrate on the most distant galaxies to find signs of intelligent life ANS: A DIF: Difficult REF: Section 24.4 MSC: Applied TOP: 4IIi | 4IIii | 4IIiii 39. People argue against the possibility of time travel by saying, “If humans will eventually be able to travel back in time, then where are all the tourists from the future?” This idea is similar to the ___________, only applied to time travel instead of aliens. a. Drake equation b. cosmological principle c. Fermi paradox d. Urey-Miller experiment e. evolutionary tree of life ANS: C DIF: Medium REF: Section 24.4 MSC: Conceptual TOP: 4IIIi 40. SETI’s Allen Telescope Array is designed to search ___________ for signs of intelligent life. a. over a thousand stars b. over a million stars c. over a billion stars d. galaxies in the Local Group e. the 100 closest spiral galaxies ANS: B DIF: Medium REF: Section 24.4 MSC: Factual TOP: 4IIIiii

SHORT ANSWER 1. Describe the experiment depicted below. What was the goal of the experiment? Was the experiment a success?

ANS: This image shows the Urey-Miller experiment. The goal of the experiment was to simulate the conditions in the primitive atmosphere of a young Earth. The experiment successfully created many of the proteins necessary for life on Earth.

DIF: Medium REF: Section 24.1 MSC: Applied TOP: 1Iv 2. If the entire history of the universe were scaled to fit into one day, what time of day would microorganisms first form, oxygen start becoming a significant component of the atmosphere, and humans split off from their genetic ancestors?

ANS: The Earth is 4.5 billion years old. Microbes have existed since at least 3.6 billion years ago, so in our scaled day that occurs at (4.5

3.6)/4.5  24 hr  4.8 hr or 4:48 A.M. Oxygen began to fill the at-

mosphere 2 billion years ago, so in our scaled day that would occur at (4.5

2)/4.5  24 hrs  13 hr or

1 P.M. Humans evolved approximately 6 million years ago, so in our scaled day that would occur at (4.5 0.006)/4.5  24 hr  23.968 hr or 11:58 P.M. DIF: Difficult REF: Section 24.1 MSC: Applied TOP: 1IIii | 1IIv | 1IIIix 3. What determines whether or not a specific mutation is passed on to future generations?

ANS: Mutations can be either good or bad for a species. When an advantageous mutation occurs and it increases the reproduction rate of the organism then that mutation is carried onto future generations.

DIF: Medium REF: Section 24.1 MSC: Conceptual TOP: 1IVii | 1IViv 4. What are the four main elements that make up all living organisms on the Earth?

ANS: CHON: carbon, hydrogen, oxygen, and nitrogen DIF: Easy REF: Section 24.2 MSC: Applied TOP: 2Ii 5. As living organisms evolved to become more complex during the history of the Earth, did this violate the second law of thermodynamics?

ANS: As a specific organism evolved to become more complex and more ordered, somewhere else in the universe the entropy was increased to compensate for it.

DIF: Easy REF: Section 24.2 MSC: Applied TOP: 2IIii 6. If you wanted to find intelligent life in the universe, what spectral types of stars would you study and why?

ANS: It took approximately 4.5 billion years for life to develop on the Earth; therefore, one should search G- and K-type stars because their main-sequence lifetimes are longer than 4.5 billion years.

DIF: Medium REF: Section 24.3 MSC: Applied TOP: 3IIii 7. Why would the 21-cm line be a good place to search for signals from intelligent life in the universe?

ANS: Other civilizations might decide to send us a signal at that wavelength because most of the universe is composed of hydrogen, and they would assume other intelligent life would be mapping the sky at this wavelength to study the structure of our galaxy.

DIF: Easy REF: Section 24.4 MSC: Applied TOP: 4Iiii 8. Using life on Earth as an example, calculate the maximum likelihood that any technologically advanced civilization that arose still exists on its home planet today.

ANS: The maximum value of L is simply the lifetime of a technically advanced civilization divided by the total lifetime of our Sun. Technologically advanced civilization is a recent development on Earth, and it will likely go on until our Sun evolves to become a red giant. The Sun has roughly 1–2 billion years until its luminosity increases to the point at which temperatures on Earth will be too high to sustain life, and thus L  1–2 billion years/10 billion years  0.1–0.2.

DIF: Difficult REF: Section 24.4 MSC: Applied TOP: 4IIi

9. What would be the number of intelligent civilizations in our galaxy if all stars formed planets, each planet has a 0.1 percent change of supporting life, and every time conditions are right life develops, but only 0.001 percent of the time intelligent life develops, and once it develops it survives indefinitely?

ANS: The Drake equation says the number of intelligent civilizations in the galaxy is equal to N  few  100 billion stars  1  0.001  1  0.00001  1  1,000.

DIF: Difficult REF: Section 24.4 MSC: Applied TOP: 4IIi 10. What is SETI, what is its main objective, and how does it plan to achieve it?

ANS: SETI is the Search for Extraterrestrial Intelligence. Its main objective is to search for intelligent life in the universe using radio telescopes to detect nonnatural emission from intelligent life at radio wavelengths.

DIF: Easy REF: Section 24.4 MSC: Factual TOP: 4IIIi 11. Name three main dangers (one human and two astrophysical) that threaten the continued existence of human life on Earth.

ANS: Some examples include the following: (1) The Sun evolving to become a red giant and burning away the Earth’s atmosphere (2) A large asteroid or comet hitting the Earth, although some life, even if it were in primitive form, might survive even a large collision (3) Human overpopulation and overuse of natural resources (4) Increasing carbon emission leading to global warming (5) Global nuclear war DIF: Easy REF: Section 24.5 MSC: Factual TOP: 5Ii | 5Iii | 5Iiii | 5Iiv

Credits FIGURE CREDITS Chapter 7 Page 96, Graph: “Radial Velocity/Year” from Exoplanets.org. Reprinted by permission of the Department of Terrestrial Magnetism, Carnegie Institution of Washington.

PHOTO CREDITS Chapter 8 Pages 107, 116 (a): Jim Wark/Visuals Unlimited/Corbis; (b): Frank Lukasseck/Corbis; (c): USGS Hawaiian Volcano Observatory; (d): Stuart Wilson/ Photo Researchers, Inc; p. 109–110, 117 (all): NASA/JPL/Caltech; p.

114: ESA/ DLR/ FU Berlin (G. Neukum); p. 115 (both): NASA/JPL/Malin Space Science Systems.

Chapter 9 Pages 125, 132 (from left to right): NASA/NSSDC.

Chapter 10 Page 141: M. Wong and I. de Pater (University of California, Berkeley); p. 145: Courtesy of John Clark, Boston University and NASA/STScI.

Chapter 11 Page 154 (no. 9): NASA/JPL/Caltech; (no. 10): NASA/JPL/Space Science Institute; (no. 11): NASA/JPL/Ted Stryk; (no. 12): NASA/JPL/Space Science Institute; p. 156: NASA/JPL/Caltech; p. 158, 163: NASA/JPL/Space Science Institute.

Chapter 12

Page 169: Dr Robert McNaught; p. 172 (no. 44): Tony Hallas/Science Faction/Corbis; (no. 47): Mike and Carol Hood; p. 173: Courtesy of Joe Orman; p. 176: Courtesy of Terry Acomb.

Chapter 14 Page 198: © 2001 by Fred Espenak, Courtesy of www .MrEclipse.com; p. 199: NASA/SDO/Solar Dynamics Observatory.

Chapter 15 Page 207: ESO; p. 208 (I): ESA/Planck Collaboration; (II)(c): Axel Mellinger; (III): NASA/JPL-Caltech/IRAS /2MASS/COBE; p. 209: ESO. Chapter 17 Page 239 (top left): ESA/Hubble & NASA; (top right): NASA & ESA; (bottom left): European Space Agency & NASA; (bottom right): NASA/CXC/Univ. de Liège/Y. Naze et al.

Chapter 20 Page 278 (no. 3): NASA, ESA, and The Hubble Heritage Team (STScI/AURA); (no. 4): Johannes Schedler / Panther Observatory; p. 280: Credit & Copyright: Robert Gendler (robgendlerastropics.com).

Chapter 21 Page 292 (a), (c) Axel Mellinger; (b): NASA/JPL-Caltech/IRAS/2MASS/COBE; p. 293 (both): European Southern Observatory (ESO).

Chapter 23 Page 323 (all): Courtesy Joel Primack and George Blumenthal; p. 326: NASA, ESA, Sloan Digital Sky Survey, R. Delgado-Serrano and F. Hammer (Observatoire de Paris).

Chapter 24 Page 334: Chris Boydell/Australian Picture Library/Corbis.