Essential Cosmic Perspective, The, 9th edition Jeffrey O. Bennett Test Bank.pdf (22)

For exam and test prep, contact excellentessaywriters@gmail.com 7) This x-ray image shows a loop of hot gas above the surface of the Sun. Suppose we took another photo looking at the same place 1 hour later. What would we see?

A) The loop would look about the same as it does in this photo.

B) The loop would have shot much higher up above the Sun's surface.

C) The loop would have collapsed back down, so we would not see it anymore.

D) The loop would have turned blue as the gas became hotter.

Answer: A 8) What layer of the Sun are we seeing in this photograph, and in what wavelength band was it photographed?

A) corona, photographed in x-rays B) chromosphere, photographed in visible light C) photosphere, photographed in visible light D) convection zone, photographed in infrared light E) radiation zone, photographed in x-rays Answer: A 9) Study this figure and its axis labels. What is this graph showing us?

A) The number of sunspots on the Sun tends to increase and decrease with an approximately 11year cycle.

B) The latitude at which sunspots appear varies between 0 and 0.5 degrees.

C) The intensity of Sunlight varies significantly over time, and reached an unusually high peak around 1960.

D) The Sun pulsates much like a beating heart, but with each beat taking about a decade.

Answer: A

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10) In terms of the global warming that has been occurring on Earth over the past few decades, what is the key message from this figure?

A) Changes in the Sun's energy output (irradiance) cannot explain the observed warming.

B) The Sun's energy output (irradiance) tracks quite well with the observed warming trend.

C) The Sun's energy output (irradiance) and Earth's global average temperature both vary in an 11-year cycle.

D) Scientists have no way to explain the observed global warming trend.

Answer: A

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End-of-Chapter Questions Visual Skills Check Use the following questions to check your understanding of some of the many types of visual information used in astronomy.

Figure 14.23b, repeated above, shows the latitudes at which sunspots appeared on the surface of the Sun during the last century. Answer the following questions, using the information provided in the figure.

1) Which of the following years had the least sunspot activity? A) 1930 B) 1949 C) 1961 D) 1987 Answer: D 2) What is the approximate range in latitude over which sunspots appear?

Answer: Sunspots appear over a range of 40°N latitude to 40°S latitude. 3) According to the figure, how do the positions of sunspots appear to change during one sunspot cycle?

Do they get closer to or farther from the equator with time? Answer: Sunspots get closer to the equator in average position during a sunspot cycle. Individual sunspots are able to appear near the equator at any time, but sunspots spread across a wide range of latitudes at the beginning of the sunspot cycle, and the spread then narrows considerably by the end of the sunspot cycle. Chapter Review Questions 1) Briefly describe how gravitational contraction generates energy. When was it important in the Sun's history? 2) What two forces are balanced in gravitational equilibrium? What does it mean for the Sun to be in energy balance?

3) State the Sun's luminosity, mass, radius, and average surface temperature, and put the numbers into a perspective that makes them meaningful.

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4) Briefly describe the distinguishing features of each of the layers of the Sun shown in Figure 14.4.

5) What is the difference between nuclear fission and nuclear fusion? Which one is used in nuclear power plants? Which one takes place in the Sun? 6) Why does nuclear fusion require high temperatures and pressures?

7) What is the overall nuclear fusion reaction in the Sun? Briefly describe the proton-proton chain.

8) Describe how a natural "solar thermostat" keeps the core fusion rate steady in the Sun. 9) Why has the Sun gradually brightened with time?

10) Describe how energy generated by fusion makes its way to the Sun's surface. How long does it take?

11) How do mathematical models help us learn about the Sun, and what gives us confidence in the models?

12) What are neutrinos? What was the solar neutrino problem, and how was it solved?

13) What do we mean by solar activity? Describe sunspots, prominences, flares, and coronal mass ejections.

14) Describe the appearance and temperature of the Sun's photosphere. Why does the surface look mottled? How are sunspots different from the surrounding photosphere?

15) How do magnetic fields keep sunspots cooler than the surrounding plasma? Explain.

16) Why are the chromosphere and corona best viewed with ultraviolet and x-ray telescopes, respectively? Briefly explain how these regions are heated.

17) What is the sunspot cycle? Why is it sometimes described as an 11-year cycle and sometimes as a 22-year cycle? Are there longer-term changes in solar activity?

18) Describe the leading model for the sunspot cycle. Does the sunspot cycle influence Earth's climate? Explain.

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Does It Make Sense?

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

Example: I walked east from our base camp at the North Pole.

Solution: The statement does not make sense because east has no meaning at the North Pole–all directions are south from the North Pole.

19) Before Einstein, gravitational contraction appeared to be a perfectly plausible mechanism for solar energy generation.

20) A temperature rise in the Sun's core is nothing to worry about, because conditions in the core will soon return to normal.

21) If fusion in the solar core ceased today, worldwide panic would break out tomorrow as the Sun began to grow dimmer.

22) Astronomers have recently photographed magnetic fields churning deep beneath the solar photosphere.

23) I wear a lead vest to protect myself from solar neutrinos.

24) If there are few sunspots this year, we should expect many more in about 5 years.

25) News of a solar flare caused concern among businesses involved in communication and electrical power generation.

26) By observing solar neutrinos, we can learn about nuclear fusion deep in the Sun's core.

27) If the Sun's magnetic field somehow disappeared, there would be no more sunspots on the Sun. 28) Scientists are currently building an infrared telescope designed to observe fusion reactions in the Sun's core.

Quick Quiz 29) Which of these groups of particles has the greatest mass?

A) a helium nucleus with two protons and two neutrons B) four electrons C) four individual protons Answer: No Correct Answer Was Provided.

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30) X-ray images of the Sun generally show the A) photosphere.

B) chromosphere.

C) corona.

Answer: No Correct Answer Was Provided.

31) Which of these layers of the Sun is coolest?

A) core B) radiation zone C) photosphere Answer: No Correct Answer Was Provided.

32) Scientists estimate the central temperature of the Sun using A) probes that measure changes in Earth's atmosphere.

B) mathematical models of the Sun. C) laboratories that create miniature versions of the Sun. Answer: No Correct Answer Was Provided.

33) Why do sunspots appear darker than their surroundings?

A) They are cooler than their surroundings.

B) They block some of the sunlight from the photosphere.

C) They do not emit any light.

Answer: No Correct Answer Was Provided.

34) At the center of the Sun, fusion converts hydrogen into A) plasma.

B) radiation and elements like carbon and nitrogen.

C) helium, energy, and neutrinos.

Answer: No Correct Answer Was Provided.

35) Solar energy leaves the core of the Sun in the form of A) photons.

B) rising hot gas.

C) sound waves.

Answer: No Correct Answer Was Provided.

36) The fact that we observe neutrinos from the Sun provides direct evidence of A) fusion in the Sun's core.

B) convection in the Sun's interior.

C) the existence of the solar wind.

Answer: No Correct Answer Was Provided.

37) What causes the cycle of solar activity?

A) changes in the Sun's fusion rate B) changes in the organization of the Sun's magnetic field C) changes in the speed of the solar wind Answer: No Correct Answer Was Provided.

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38) Which of these things poses the greatest hazard to communication satellites?

A) photons from the Sun B) solar magnetic fields C) particles from the Sun Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

39) Group Discussion: Arthur Walker, Scientist and Mentor.

Arthur B. C. Walker Jr., an African-American scientist who spent much of his career at Stanford University, was a pioneer in developing the technology for taking ultraviolet and x-ray images of the Sun. a) Working independently, learn basic details of Dr. Walker's career, including when and where he earned his Ph.D., the name and occupation of his first graduate student, the purpose of the investigative commission he was appointed to in 1986, and the purpose of the award that now bears his name. b) Gather in small groups to discuss whether Dr. Walker was likely to have faced discrimination in his efforts to become a scientist. Be sure to consider the time period when he was earning his Ph.D. c) Within the group, share what you've learned about Dr.Walker's first graduate student, including what was posthumously revealed about the student's sexual orientation. Discuss why such a person might have considered Dr.Walker a particularly suitable mentor. d) Discuss what you learned about the 1986 commission. In addition to scientific and technical knowledge, what other skills do people on such a commission need to have? e) What is the purpose of the award that bears Dr. Walker's name, and who has won it to date?

Does your group think this award is an appropriate way to honor him? Why or why not?

The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

40) Inside the Scientists claim to know what is going on inside the Sun, even though we Sun. cannot observe the solar interior directly. What is the basis for these claims, and how do they align with the hallmarks of science outlined in Section 3.4?

41) The Solar Neutrino Problem. Early solar neutrino experiments detected only about a third of the number of neutrinos predicted by the theory of fusion in the Sun. Why didn't scientists abandon their fusion models at this point, and what ultimately led them to the investigations that resolved the discrepancies? Explain how their decision-making process exemplified the hallmarks of science from Chapter 3.

42) The Role of the Sun. Briefly discuss how the Sun affects us here on Earth. Be sure to consider not only factors such as its light and warmth but also how the study of the Sun has led us to new understandings in science and to technological developments. Overall, how important has solar research been to our lives?

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43) The Sun and Global Warming. Some people still claim that part or all of the global warming observed over the past century may be due to changes in the Sun, but the data in Figure 14.26 seem to rule that out (as do several other lines of evidence beyond those discussed in this text).

Discuss how the people in your life talk about global warming, what information sources they view as credible, and whether they consult scientific data when making up their minds about the causes of global warming.

44) Group Activity: The Sun's Future. Working in small groups, discuss the following questions in light of what you have learned about how the Sun came to shine steadily with energy from fusion in its core. Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39. a) Will the Sun's core temperature go up or down after the core runs out of hydrogen for fusion?

Why? b) If your group thinks the temperature will go up, decide what would ultimately stop the temperature from rising forever. If you think the temperature will go down, decide what could stop it from falling to absolute zero. c) Propose and describe an experiment or a set of observations that could test whether your answers to parts a and b are correct.

Investigate Further

45) The End of Fusion I. Describe what would happen in the Sun if fusion reactions suddenly ceased. 46) The End of Fusion II. If fusion reactions in the Sun were to suddenly cease, would we be able to tell? If so, how? 47) A Really Strong Force. How would the interior temperature of the Sun be different if the strong force that binds nuclei together were 10 times as strong? 48) Covered with Sunspots. Describe what the Sun would look like from Earth if the entire photosphere were the same temperature as a sunspot. 49) Inside the Sun. Describe how scientists determine what the interior of the Sun is like. Why haven't we sent a probe into the Sun to measure what is happening there? 50) Solar Energy Output. Observations over the past century show that the Sun's visible-light output varies by less than 1%, but its x-ray output can vary by a factor of 10 or more. Explain why changes in x-ray output can be so much larger than those in the output of visible light. 51) An Angry Sun. A Time magazine cover once suggested that an "angry Sun" was becoming more active as human activity changed Earth's climate. It's certainly possible for the Sun to become more active at the same time that humans are affecting Earth, but is it possible that the Sun could be responding to human activity?

Why or why not?

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52) Research: Current Solar Weather. Daily information about solar activity is available at NASA's website spaceweather.com. Where are we in the sunspot cycle right now? When is the next solar maximum or minimum expected? Have there been any major solar storms in the past few months? If so, did they have any significant effects on Earth? Summarize your findings in a one- to two-page report.

53) Research: Solar Observatories in Space. Visit NASA's website for the Sun-Earth connection and explore some of the current and planned space missions designed to observe the Sun. Choose one mission to study in greater depth, and write a one- to two-page report on the status and goals of the mission and what it has taught or will teach us about the Sun. 54) Research: Nuclear Power. There are two basic ways to generate energy from atomic nuclei: through nuclear fission (splitting nuclei) and through nuclear fusion (combining nuclei). All current nuclear reactors are based on fission, but fusion would have many advantages if we could develop the technology. Research some of the advantages of fusion and some of the obstacles to developing fusion power. Do you think fusion power will be a reality in your lifetime? Explain.

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

55) Chemical Burning and the Sun. When an object burns in a fire, the amount of energy released through this chemical burning is typically about 108 joules per kilogram of mass burned. Use this fact to estimate how long the Sun would last if its energy source were a huge fire releasing chemical energy (rather than its actual source, fusion energy).

56) The Lifetime of the Sun. The Sun's chemical composition was about 70% hydrogen when it formed, and about 13% of this hydrogen was available for eventual fusion in the core. (The rest remains in layers of the Sun where the temperature is currently too low for fusion.) a) Use these data and the Sun's mass to calculate the total mass of hydrogen available for fusion over the lifetime of the Sun. b) The Sun fuses about 600 billion kilograms of hydrogen each second. Based on your result from part a, calculate how long the Sun's initial supply of hydrogen can last. Give your answer in both seconds and years. c) Given that our solar system is now about 4.6 billion years old, when will we need to worry about the Sun running out of hydrogen for fusion?

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57) Solar Power Collectors. This problem leads you through the calculation and discussion of how much solar power can in principle be collected by solar cells on Earth. a) Imagine a giant sphere with a radius of 1 AU surrounding the Sun. What is the surface area of this sphere, in square meters? (Hint: The formula for the surface area of a sphere is 4ℼr2.) b) Because this imaginary giant sphere surrounds the Sun, the Sun's entire luminosity of 3.8 × 1026 watts must pass through it. Calculate the power passing through each square meter of this imaginary sphere in watts per square meter. Explain why this number represents the maximum power per square meter that a solar collector in Earth orbit can collect. c) List several reasons why the average power per square meter collected by a solar collector on the ground will always be less than what you found in part b. d) Suppose you want to put a solar collector on your roof. If you want to optimize the amount of power you can collect, how should you orient the collector? (Hint: The optimal orientation depends on both your latitude and the time of year and day.)

58) Solar Power for the United States. Total annual U.S. energy consumption is about 2 × 1020 joules. a) What is the average power requirement for the United States, in watts? (Hint: 1 watt = 1 joule/s.) b) The best available solar cells can generate an average (day and night) power of about 200 watts/m2. What total area would we need to cover with solar cells to supply all the power needed for the United States? Give your answer in square kilometers and compare it with the total surface area of the continental United States.

59) The Color of the Sun. Use Wien's law (see Mathematical Insight 5.2) and the Sun's average surface temperature of about 5800 K to calculate the wavelength of peak thermal emission from the Sun. What color does this wavelength correspond to in the visible-light spectrum? Why do you think the Sun appears white or yellow to our eyes?

60) The Color of a Sunspot. Use Wien's law (see Mathematical Insight 5.2) and a typical sunspot temperature of 4000 K to calculate the wavelength of peak thermal emission from a sunspot.

What color does this wavelength correspond to in the visible-light spectrum? How does this color compare with the overall color of the Sun? 61) Solar Mass Loss. Estimate how much mass the Sun loses through fusion reactions during its 10-billion-year life. You can simplify the problem by assuming the Sun's energy output remains constant. Compare the amount of mass lost with Earth's mass.

62) Pressure of the Photosphere. The gas pressure of the photosphere changes substantially from its upper levels to its lower levels. Near the top of the photosphere, the temperature is about 4500 K and there are about 1.6 × 1016 gas particles per cubic centimeter. In the middle, the temperature is about 5800 K and there are about 1.0 × 1017 gas particles per cubic centimeter. At the bottom of the photosphere, the temperature is about 7000 K and there are about 1.5 × 1017 gas particles per cubic centimeter. Use the ideal gas law (Mathematical Insight 14.2) to compare the pressures of each of these layers; explain the reason for the trend that you find. How do these gas pressures compare with Earth's atmospheric pressure at sea level?

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63) Tire Pressure. Air pressure at sea level is about 15 pounds per square inch. The recommended air pressure in your car tires is about 30 pounds per square inch. Use the ideal gas law to determine how the density of gas particles inside your tires compares with that of gas particles in the outside air. How does this explain what happens when a tire springs a leak? How is the tire's response to a leak similar to the response of the Sun's core as its number of independent particles gradually declines with time? How is the tire's response different?

64) Personal Energy Content. The average power of a human body (for living and metabolism) is about 100 watts. Suppose your body could run on fusion power and could convert 0.7% of its mass into energy. How much energy would be available through fusion? For how long could your body then operate on fusion power?

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter 15 Surveying the Stars Section 15.1 1) What are the standard units for luminosity?

A) watts B) joules C) newtons D) kilograms E) watts per second Answer: A 2) What is a star's luminosity?

A) the apparent brightness of the star in our sky B) the surface temperature of the star C) the lifetime of the star D) the total energy that the star radiates each second Answer: D 3) What are the standard units for apparent brightness?

A) watts B) joules C) newtons D) watts per second E) watts per square meter Answer: E 4) What two things do we need to measure in order to determine a star's luminosity?

A) apparent brightness and distance B) distance and mass C) apparent brightness and temperature D) apparent brightness and mass Answer: A 5) If we have measured a star's apparent brightness and distance, then we can easily calculate its A) luminosity B) mass C) surface temperature D) radius E) lifetime Answer: A

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6) Suppose the distance to a star was doubled but everything else about the star stayed the same.

What would happen to the star's luminosity and apparent brightness?

A) Luminosity is decreased by a factor of four and the apparent brightness is decreased by a factor of four.

B) Luminosity is decreased by a factor of two and the apparent brightness is decreased by a factor of two.

C) Luminosity remains the same, but the apparent brightness is decreased by a factor of two.

D) Luminosity remains the same, but the apparent brightness is decreased by a factor of four.

E) Luminosity is decreased by a factor of four, but the apparent brightness remains the same.

Answer: D 7) Suppose the distance to a star was doubled but everything else about the star stayed the same.

What would happen to the star's luminosity?

A) It would decrease by a factor of 2.

B) It would decrease by a factor of 4.

C) It would increase by a factor of 2.

D) It would increase by a factor of 4.

E) It would remain the same.

Answer: E 8) Suppose the distance to a star was doubled but everything else about the star stayed the same.

What would happen to the star's apparent brightness in our sky?

A) It would decrease by a factor of 2.

B) It would decrease by a factor of 4.

C) It would increase by a factor of 2.

D) It would increase by a factor of 4.

E) It would remain the same.

Answer: B 9) Stars X and Y have the same luminosity, but star Y is 10 times as far from Earth as star X. How much brighter will star X appear in our sky than star Y? A) 10 times B) 100 times C) 1000 times D) 10,000 times E) 100,000 times Answer: B 10) Suppose that the Sun—Earth distance were 10 AU instead of 1 AU. Then the energy of sunlight received per square meter on Earth each second would be ________ it is today.

A) 10 times as great as B) about the same as C) 1/100 as much as D) 100 times as great as E) 1/10 as much as Answer: C

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11) Suppose the star Sirius were moved to a distance 7 times closer to Earth than it is now. How would its apparent brightness change?

A) It would be 7 times as bright. B) It would be 1/7 as bright. C) It would be 1/49 as bright. D) It would be 49 times as bright. Answer: D 12) Suppose the star Sirius were moved to a distance 7 times closer to Earth than it is now. How would its parallax angle change? A) It would stay the same. B) It would get smaller. C) It would get larger. Answer: C 13) You observe two stars that are identical to our Sun, but that are located at different distances from Earth. Which star is the most luminous? A) They are both the same luminosity. B) The one that appears brightest in our sky is more luminous. C) The one that appears faintest in our sky is more luminous. D) There is no way to tell from the information given. Answer: A 14) Which of the following properties of stars is directly observed (i.e., it can be measured by a calibrated instrument on a telescope, from a single observation without a need for additional calculations)? A) luminosity B) radius C) surface temperature D) apparent brightness Answer: D 15) Star X has a smaller parallax angle than star Y. What can you conclude? A) Star X is nearer to Earth than star Y. B) Star X is farther from Earth than star Y. C) Star X is more luminous than star Y. D) 10 Star X is less luminous than star Y. E) Star X is smaller in radius than star Y. Answer: B

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16) Suppose you measure the parallax angle for a particular star to be 0.1 arcseconds. What is the distance to this star?

A) 10 light-years B) 10 parsecs, which is about 32.6 light-years C) 0.1 light-year D) 0.1 parsec, which is about 0.326 light-year E) impossible to determine Answer: B 17) Suppose that you measure the parallax angle for a particular star to be 0.5 arcseconds. What is the distance to this star?

A) 0.5 light-year B) 0.5 parsec, which is about 1.63 light-years C) 5 light-years D) 5 parsecs, which is about 16.3 light-years E) 2 parsecs, which is about 6.52 light-years Answer: E 18) Suppose that you measure the parallax angle for a particular star to be 0.1 arcseconds. What is the distance to this star? (Hint: A star at a distance of 3.26 light-years has a parallax angle of 1.0 arcseconds.)

A) 0.1 light-years B) 0.326 light-years C) 10 light-years D) 32.6 light-years E) 3.26 light-years Answer: D 19) Star X has an apparent magnitude of 3, and star Y has an apparent magnitude of 8. What can we conclude?

A) Star X appears brighter in the night sky than star Y. B) Star Y appears brighter in the night sky than star X. C) Star X is farther away from us than star Y. D) Star Y is farther away from us than star X. Answer: A 20) Star X has an apparent magnitude of 6, and star Y has an apparent magnitude of 2. How do they compare in observed brightness?

A) Star X appears brighter in the night sky than star Y. B) Star Y appears brighter in the night sky than star X. C) Star X is farther away from us than star Y. D) Star Y is farther away from us than star X. Answer: B

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21) A star's spectral type can be used to determine its A) mass. B) surface temperature. C) luminosity. D) core temperature. E) radius. Answer: B 22) How do we estimate the surface temperature of a star? A) by measuring the Doppler shifts of absorption lines in its chromosphere B) by obtaining its spectrum and classifying the spectral type C) by estimating its parallax D) by estimating its luminosity Answer: B 23) Which of the following correctly lists spectral types in order of decreasing temperature (from hottest to coolest)?

A) MKGFABO B) OBAGFKM C) OBAFGKM D) ABFGKMO E) BAGFKMO Answer: C 24) Star X has spectral type B. Star Y has spectral type M. What can we conclude? A) Star X has a higher surface temperature than star Y. B) Star Y has a higher surface temperature than star X. C) Star X is more massive than star Y. D) Star Y is more massive than star X. Answer: A 25) Which stellar spectral type has the strongest molecular absorption lines in its visible light spectrum? A) O B) B C) G D) K E) M Answer: E 26) Which spectral type represents stars with the hottest surface temperatures? A) G B) M C) O D) A E) B Answer: C

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27) Which of the following stars has the hottest surface temperature?

A) the blue star B) the orange star C) the green star D) the red star E) the yellow star Answer: A 28) Which of these stars is the bluest in color?

A) O star B) G star C) M star D) B star Answer: A 29) Which spectral type represents stars with spectra that peak at the shortest ultraviolet wavelengths?

A) O B) B C) G D) K E) M Answer: A 30) Which of the following statements about spectral types of stars is always true?

A) A star with spectral type G has a lower interior temperature than a star with spectral type B. B) A star with spectral type G has a lower mass than a star with spectral type B. C) A star with spectral type G has a cooler surface temperature than a star with spectral type B. D) A star with spectral type G has a smaller radius than a star with spectral type B. E) All of the above are true.

Answer: C 31) Which of the following people reorganized the spectral classification scheme into the one we use today and personally classified over 400,000 stars?

A) Annie Jump Cannon B) Williamina Fleming C) Cecilia Payne-Gaposchkin D) Henry Draper E) Edward Pickering Answer: A

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32) Which of the following people used the ideas of quantum mechanics to describe why the spectral classification scheme is in order of decreasing temperature?

A) Annie Jump Cannon B) Williamina Fleming C) Cecilia Payne-Gaposchkin D) Henry Draper E) Edward Pickering Answer: C 33) Annie Jump Cannon became famous for A) developing our modern system for classifying stars.

B) discovering the planet Uranus.

C) discovering the planet Neptune.

D) building the world's largest telescope.

Answer: A 34) Which of the following terms is given to a pair of stars that appear to change positions in the sky, indicating that they are orbiting one another?

A) visual binary B) eclipsing binary C) spectroscopic binary D) double star E) none of the above Answer: A 35) Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?

A) visual binary B) eclipsing binary C) spectroscopic binary D) double star E) none of the above Answer: C 36) Besides mass, what else can be determined about the stars in an eclipsing binary system by observing only their light curve?

A) their luminosities B) their surface temperatures C) their radii D) their distances from Earth Answer: C

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37) How do we measure stellar masses?

A) We measure their apparent brightness and distance (from parallax).

B) We don't really know: stellar masses are based on an informed guess.

C) We measure the orbital periods and velocities of stars in binary systems.

D) We measure their density and radius.

Answer: C 38) Astronomers generally can measure a star's mass if only it is a member of a binary star system. What characteristics of the stars must we know to measure the masses of the stars in a binary system?

A) their spectral types and their distance from Earth B) their absolute magnitudes and their luminosities C) their orbital period and average orbital distance D) their luminosities and distance from Earth Answer: C Section 15.2 1) Which of the following best describes the axes of a Hertzsprung-Russell (H-R) diagram?

A) surface temperature on the horizontal axis and luminosity on the vertical axis B) mass on the horizontal axis and luminosity on the vertical axis C) surface temperature on the horizontal axis and radius on the vertical axis D) mass on the horizontal axis and stellar age on the vertical axis E) interior temperature on the horizontal axis and mass on the vertical axis Answer: A 2) A star's position along the horizontal axis of an H-R diagram tells us all of the following except the star's A) radius.

B) spectral type.

C) surface temperature.

D) surface color.

Answer: A 3) Which two quantities must you know in order to plot a star on an H-R diagram?

A) mass and lifetime B) surface temperature and luminosity C) mass and radius D) apparent brightness and distance Answer: B

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4) Which of the following stellar properties has the greatest range in values?

A) mass B) radius C) core temperature D) surface temperature E) luminosity Answer: E

5) Based on the H-R diagram shown above, what is the approximate surface temperature of an F star?

A) 5000 K B) 6000 K C) 7000 K D) 12,000 K Answer: C

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6) Based on the H-R diagram shown above, what is the approximate surface temperature of a B star?

A) 6000 K B) 40,000 K C) 8000 K D) 20,000 K Answer: D 7) Based on the H-R diagram shown above, what is the spectral type of a main-sequence star with a luminosity 100 times greater than that of the Sun? A) O B) A C) G D) M Answer: B 8) Based on the H-R diagram shown above, what is the approximate luminosity of a main-sequence M star?

A) 10−6 times that of the Sun B) 0.01 times that of the Sun C) 1 times that of the Sun D) 10 times that of the Sun Answer: B 9) A main-sequence star is observed with a surface temperature of 3500 K and a luminosity 10−2 times that of the Sun. Based on the H-R diagram shown above, what is the approximate mass of this star?

A) 0.2 times that of the Sun B) 0.5 times that of the Sun C) 0.9 times that of the Sun D) 1.5 times that of the Sun Answer: A 10) A star is observed with a surface temperature of 3000 K and a luminosity of 10−3 times that of the Sun. Based on the H-R diagram shown above, what is the approximate radius of this star?

A) 0.1 solar radius B) 0.5 solar radius C) 1 solar radius D) The radius cannot be determined.

Answer: A

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11) A star is observed with a surface temperature of 3000 K and a luminosity of 105 times that of the Sun. Based on the H-R diagram shown above, what is the approximate radius of this star? A) 10 solar radii B) 102 solar radii C) 103 solar radii D) The radius cannot be determined. Answer: C 12) Based on the H-R diagram shown above, about how long does a main-sequence star of spectral type O spend fusing hydrogen into helium in its core?

A) 1000 years B) 10,000 years C) 10 million years D) 100 million years E) 1 billion years Answer: C 13) Based on the H-R diagram shown above, about how long does a main-sequence star of spectral type G spend fusing hydrogen into helium in its core?

A) 1000 years B) 10,000 years C) 1 million years D) 100 million years E) 10 billion years Answer: E 14) You have observed a star and classified it as a red giant. It has a surface temperature of 3000 K and a luminosity of 105 times that of the Sun. Can you determine its mass by plotting its position on an H-R diagram?

A) Yes, its mass is 0.3 MSun. B) Yes, its mass is 3 MSun. C) Yes, its mass is 30 MSun. D) No, a red giant's mass cannot be determined from its position on the H-R diagram.

Answer: D 15) You have observed a star and classified it as a white dwarf. It has a surface temperature of 3000 K and a luminosity of 10−3 times that of the Sun star based on its position in the H-R diagram. Can you determine its mass by plotting its position on an H-R diagram?

A) Yes, its mass is 0.3 MSun. B) Yes, its mass is 3 MSun. C) Yes, its mass is 30 MSun. D) No, a white dwarf's mass cannot be determined from its position on the H-R diagram.

Answer: D

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16) On a Hertzsprung-Russell diagram, where would we find stars that are cool and dim?

A) upper right B) lower right C) upper left D) lower left Answer: B 17) On a HertzsprungRussell diagram, where would we find stars that are cool and luminous? A) upper right B) lower right C) upper left D) lower left Answer: A 18) On a Hertzsprung-Russell diagram, where would we find stars that have the largest radii? A) upper right B) lower right C) upper left D) lower left Answer: A 19) On a Hertzsprung-Russell diagram, where on the main sequence would we find stars that have the greatest mass? A) upper right B) lower right C) upper left D) lower left Answer: C 20) On a Hertzsprung-Russell diagram, where would we find red giant stars? A) upper right B) lower right C) upper left D) lower left Answer: A 21) On a Hertzsprung-Russell diagram, where would we find white dwarfs? A) upper right B) lower right C) upper left D) v left Answer: D

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22) Where do we find our Sun on the H-R diagram?

A) near the middle of the main sequence B) at the far upper left of the main sequence C) with the red giants D) at the far lower right of the main sequence E) with the white dwarfs Answer: A 23) How do stars on the main sequence obtain their energy?

A) from chemical reactions B) from gravitational contraction C) by converting hydrogen to helium in their cores D) by converting helium to carbon, nitrogen, and oxygen E) from nuclear fission Answer: C 24) Which of the following is the most common type of main-sequence star?

A) an O star B) an A star C) an F star D) an M star E) a G star Answer: D 25) Which main sequence stars have the lowest mass?

A) spectral type O B) spectral type M C) spectral type A D) spectral type G E) can't tell from information provided Answer: B 26) Which main sequence stars have the shortest lifetimes?

A) spectral type M B) spectral type G C) spectral type O D) spectral type A E) can't tell from information provided Answer: C

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27) For main-sequence stars, which of the following is true about low-mass stars compared to high-mass stars? A) Low-mass stars are cooler and less luminous than highmass stars. B) Low-mass stars are hotter and more luminous than high-mass stars. C) Low-mass stars are cooler but more luminous than high-mass stars. D) Low-mass stars are hotter but less luminous than high-mass stars. E) Low-mass stars have the same temperature and luminosity as high-mass stars. Answer: A 28) What does the luminosity of a main-sequence star tell us? A) the star's distance from us B) the rate at which it converts hydrogen to helium C) the star's radius D) the star's surface temperature Answer: B 29) Star X is a supergiant of spectral type M. Star Y is a giant but of identical spectral type M. What can you conclude? A) Star X is more luminous and larger in radius than star Y. B) Star Y is more luminous and larger in radius than star X. C) Star X is hotter than star Y. D) Star Y is hotter than star X. Answer: A 30) The most fundamental property of a star is its A) luminosity. B) abundance of elements besides hydrogen and helium. C) mass. D) surface temperature. E) radius. Answer: C 31) In a pulsating variable star, which characteristic of the star changes dramatically with time? A) mass B) core temperature C) luminosity D) energy-generation process E) rotation rate Answer: C

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32) Sometimes the outermost layers of a pulsating variable star are too opaque to allow much energy to escape to space. What happens to the star as a consequence of this?

A) Pressure builds up, causing the outer layers to expand until they are transparent enough for the energy to escape.

B) The star builds up so much energy that it explodes as a supernova.

C) The entire star heats up dramatically, causing the rate of fusion to increase in the core.

D) The star becomes a red giant.

Answer: A Section 15.3 1) Which of the following statements about an open star cluster is true?

A) All stars in the cluster are approximately the same color.

B) All stars in the cluster are approximately the same age.

C) All stars in the cluster have approximately the same mass.

D) There is an approximately equal number of all types of stars in the cluster.

Answer: B 2) Which of the following is a safe assumption about all the stars in a star cluster?

A) They all have approximately the same surface temperature.

B) They are all approximately the same distance away from us.

C) They all have approximately the same mass.

D) They will all have approximately the same lifetimes.

E) They all have approximately the same apparent brightness in our sky.

Answer: B 3) You observe a star cluster with a main-sequence turnoff point at spectral type G (the same spectral type as the Sun). What is the approximate age of this star cluster?

A) 10,000 years B) 4.6 billion years C) 10 billion years D) 100 billion years E) 10 million years Answer: C

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4) The brightest main-sequence star in a cluster has a surface temperature of 30,000 K and a luminosity of about 104 LSun. Referring to the figure above as needed, what is the approximate age of the cluster? A) 107 years B) 109 years C) 1011 years D) 1013 years Answer: A 5) The brightest main-sequence star in a cluster has a surface temperature of 10,000 K and a luminosity approximateof about 10 LSun. Referring to the figure above as needed, what is the age of the cluster?

A) 107 years B) 109 years C) 1011 years D) 1013 years Answer: B

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6) The youngest star clusters are ones in which their brightest main-sequence stars have spectral type A) M. B) B. C) A. D) O. E) G. Answer: D General Chapter Questions 1) Suppose you see two main-sequence stars of the same spectral type. Star 1 is dimmer in apparent brightness than star 2 by a factor of 100. What can you conclude? (Neglect any effects that might be caused by interstellar dust and gas.)

A) Without first knowing the distances to these stars, you cannot draw any conclusions about how their true luminosities compare to each other.

B) The luminosity of star 1 is a factor of 100 less than the luminosity of star 2.

C) Star 1 is 100 times more distant than star 2.

D) Star 1 is 100 times nearer than star 2.

E) Star 1 is 10 times more distant than star 2.

Answer: E 2) The Sun's life time is 10 billion years. About how long is the lifetime of a star with a star has a mass of 10 MSun and a luminosity of 10,000 LSun? A) 1 million years B) 100 billion years C) 100 trillion years D) 10 million years Answer: D 3) The Sun's life time is 10 billion years. About how long is the lifetime of a star with a star has a mass of 0.1 MSun and a luminosity of 0.01 LSun? A) 1 billion years B) 100 million years C) 100 billion years D) 10 million years Answer: C 4) Which two factors determine a star's luminosity?

A) radius and surface temperature B) radius and distance from Earth C) surface temperature and distance from Earth D) mass and radius E) mass and surface temperature Answer: A

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5) Luminosity has units of energy per time. Apparent brightness has units of energy per time per area. Which of the following expressions correctly states the relationship between luminosity, distance, and apparent brightness (known as the "inverse square law for light")?

A) luminosity = B) apparent brightness = C) apparent brightness = luminosity × 4π × (distance)2 D) distance = Answer: B 6) The apparent brightness of an object is calculated as:

. If one knew the

luminosity and apparent brightness of a star that was too distant for a parallax measurement, what would be the best approach to determining its distance?

A) Insert the values for luminosity and apparent brightness into the equation, then guess the value for the distance that satisfies the equation.

B) Solve the equation for distance, then insert the values for luminosity and apparent brightness into the new equation.

C) The distance cannot be determined.

Answer: B Reading Quiz Questions 1) What is the approximate chemical composition (by mass) with which all stars are born?

A) 3/4 hydrogen, 1/4 helium, no more than about 2% heavier elements B) half hydrogen and half helium C) 98% hydrogen, 2% helium D) 95% hydrogen, 4% helium, no more than 1% heavier elements Answer: A 2) The total amount of power (in watts, for example) that a star radiates into space is called its A) apparent brightness.

B) absolute magnitude.

C) luminosity.

D) flux.

Answer: C 3) According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?

A) Its apparent brightness will decrease by a factor of 9.

B) Its apparent brightness will increase by a factor of 9.

C) Its apparent brightness will decrease by a factor of 3.

D) Its apparent brightness will increase by a factor of 3.

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4) Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?

A) Calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity.

B) Determine both the star's distance and luminosity from its apparent brightness.

C) Determine the distance to the star from its apparent brightness.

D) Calculate the star's surface temperature if we know either its luminosity or its distance.

Answer: A 5) If star A is closer to us than star B, then Star A's parallax angle is A) larger than that of Star B. B) smaller than that of Star B. C) hotter than that of Star B. D) fewer parsecs than that of Star B. Answer: A 6) Star A has an apparent magnitude = 3 and star B has an apparent magnitude = 5. Which star is brighter in our sky?

A) Star A B) Star B C) There is not enough information to answer the question.

D) The two stars have the same brightness in our sky, but Star A is closer to us than Star B. Answer: A 7) From hottest to coolest, the order of the spectral types of stars is A) OBAFGKM. B) OBAGFKM. C) ABFGKMO. D) OMKGFBA. E) ABCDEFG. Answer: A 8) Our Sun is a star of spectral type A) G. B) F. C) S. D) M. Answer: A 9) Astronomers can measure a star's mass in only certain cases. Which one of the following cases might allow astronomers to measure a star's mass?

A) The star is a member of a binary star system.

B) The star is of spectral type G. C) The star is of spectral type A. D) We know the star's luminosity and distance.

Answer: A

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10) Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?

A) spectroscopic binary B) eclipsing binary C) visual binary D) double star Answer: A 11) The axes on a Hertzsprung-Russell (H-R) diagram represent A) luminosity and surface temperature.

B) mass and luminosity.

C) luminosity and apparent brightness.

D) mass and radius.

Answer: A 12) What can we infer, at least roughly, from a star's luminosity class?

A) its size (radius) B) its mass C) its age in years D) its surface temperature Answer: A 13) On an H-R diagram, stellar radii A) increase diagonally from the lower left to the upper right.

B) are greatest in the lower left and least in the upper right.

C) decrease from left to right.

D) are impossible to determine.

Answer: A 14) On an H-R diagram, stellar masses A) can be determined for main sequence stars but not for other types of stars.

B) are greatest in the lower left and least in the upper right.

C) decrease from upper left to lower right.

D) are impossible to determine.

Answer: A 15) High-mass stars have ________ lifetimes than low-mass stars.

A) much shorter B) slightly shorter C) much longer D) slightly longer Answer: A

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16) On an H-R diagram, a red supergiant would be located in the A) upper right. B) upper left. C) lower left. D) lower right. Answer: A 17) What is the common trait of all mainsequence stars? A) They generate energy through hydrogen fusion in their core. B) They are in the final stage of their lives. C) They are all spectral type G. D) They all have approximately the same mass. Answer: A 18) Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true? A) Earth would be inside the supergiant. B) The supergiant's surface temperature would be much hotter than the surface temperature of our Sun. C) Earth would fly off into interstellar space. D) The supergiant would appear as large as the full Moon in our sky. Answer: A 19) What is a white dwarf? A) It is the remains of a star that ran out of fuel for nuclear fusion. B) It is a main sequence star of spectral type F, which tends to look white in color. C) It is a type of star that produces energy by gravitational contraction. D) It is a star that follows a periodluminosity relation. Answer: A 20) Which of the following statements comparing open and globular star clusters is true? A) Stars in open clusters are relatively young, while stars in globular clusters are very old. B) Open clusters are found both in the disk and the halo of the galaxy, while globular clusters are found only in the halo. C) Globular clusters typically contain a few hundred stars, while open clusters typically contain tens to hundreds of thousands of stars. D) Open clusters contain only main-sequence stars, while globular clusters contain only giants. Answer: A

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21) What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us? A) It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age. B) It is the point in a star cluster beyond which main sequence stars are not found, and it tells us the cluster's distance. C) It is the luminosity class of the largest star in a star cluster, and it tells us the cluster's age. D) It is the mass of the most massive star in the star cluster, and it tells us the cluster's size. Answer: A Concept Quiz Questions 1) All stars are born with the same basic composition, yet stars can differ greatly in appearance. Which two factors are most important in determining the current appearance of a star? A) mass and stage of life B) luminosity and stage of life C) age and location in the galaxy D) mass and surface temperature E) apparent brightness and luminosity Answer: A 2) Which units are appropriate for measurement of apparent brightness? A) watts per square meter B) watts C) joules D) newtons Answer: A 3) Star A is identical to Star B, but Star A is twice as far from us as Star B. Therefore A) both stars have the same luminosity, but the apparent brightness of Star B is four times that of Star A. B) both stars have the same luminosity, but the apparent brightness of Star B is twice that of Star A. C) both stars have the same apparent brightness, but the luminosity of Star B is four times that of Star A. D) both stars have the same luminosity, but the apparent brightness of Star A is four times that of Star B. Answer: A 4) A star with a parallax angle of 1/20 arcsecond is A) 20 parsecs (≈ 65.2 light-years) away. B) 20 light-years away. C) 1/20 parsec (≈ 0.163 light-year) away. D) 10 parsecs (≈ 32.6 light-years) away. Answer: A

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5) A star's color is related to its surface temperature because A) stars emit thermal radiation. B) the colors of stars depend mainly on the emission lines in the spectra. C) stars have absorption lines in their spectra. D) the color of a star depends on its mass, and mass is related to surface temperature. Answer: A 6) Which of the following statements about spectral types of stars is not generally true? A) The spectral type of a star can be used to determine its distance. B) The spectral type of a star can be used to determine its surface temperature. C) The spectral type of a star can be used to determine its color. D) The spectral type of a star can be determined by identifying lines in the star's spectrum.

Answer: A 7) Sirius is a star with spectral type A star and Rigel is a star with spectral type B star. What can we conclude? A) Rigel has a higher surface temperature than Sirius. B) Rigel has a higher core temperature than Sirius. C) Sirius has a higher core temperature than Rigel. D) Sirius has a higher surface temperature than Rigel. Answer: A 8) To calculate the masses of stars in a binary system, we must measure their A) orbital period and average orbital distance. B) spectral types and distance from Earth. C) absolute magnitudes and luminosities. D) luminosities and distance from Earth. Answer: A 9) Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes significantly dimmer for about 2 hours. The most likely explanation is that A) the star is a member of an eclipsing binary star system. B) the star is a Cepheid variable. C) the star is periodically ejecting gas into space, every 73 hours. D) the star is a white dwarf. Answer: A

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10) The sketch shows groups of stars on the H-R diagram, labeled (a) through (e); note that (a) represents the entire main sequence, while (c) and (d) represent only small parts of the main sequence. Which group represents stars that are cool and dim?

A) a B) b C) c D) d E) e Answer: D 11) The sketch shows groups of stars on the H-R diagram, labeled (a) through (e); note that (a) represents the entire main sequence, while (c) and (d) represent only small parts of the main sequence. Which group represents stars of the largest radii?

A) a B) b C) c D) d E) e Answer: E 12) The sketch shows groups of stars on the H-R diagram, labeled (a) through (e); note that (a) represents the entire main sequence, while (c) and (d) represent only small parts of the main sequence. Which group represents the most common type of stars?

A) b B) c C) d D) e Answer: C

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13) The sketch shows groups of stars on the H-R diagram, labeled (a) through (e); note that (a) represents the entire main sequence, while (c) and (d) represent only small parts of the main sequence. Which group represents stars that are extremely luminous and emit most of their radiation as ultraviolet light?

A) a B) b C) c D) d E) e Answer: C 14) The sketch shows groups of stars on the H-R diagram, labeled (a) through (e); note that (a) represents the entire main sequence, while (c) and (d) represent only small parts of the main sequence. Which group represents stars with the longest main-sequence lifetimes? A) a B) b C) c D) d E) e Answer: D 15) The sketch shows groups of stars on the H-R diagram, labeled (a) through (e); note that (a) represents the entire main sequence, while (c) and (d) represent only small parts of the main sequence. Which group represents stars fusing hydrogen in their cores? A) a B) b C) c D) d E) e Answer: A 16) The sketch shows groups of stars on the H-R diagram, labeled (a) through (e); note that (a) represents the entire main sequence, while (c) and (d) represent only small parts of the main sequence. Which group represents stars that have no ongoing nuclear fusion? A) a B) b C) c D) d E) e Answer: B

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17) You observe a star and you want to plot it on an H-R diagram. You will need to measure all of the following, except the star's A) mass.

B) distance.

C) apparent brightness.

D) spectral type.

Answer: A 18) The approximate main-sequence lifetime of a star of spectral type O is A) 3 million years.

B) 10,000 years.

C) 300 million years.

D) 10 billion years.

Answer: A 19) How did astronomers discover the relationship between spectral type and mass for main sequence stars?

A) by measuring the masses and spectral types of main-sequence stars in binary systems B) by using computer models of hydrogen fusion and stellar structure C) by measuring stellar radii with powerful telescopes D) by comparing stars with the same spectral type but different luminosities Answer: A 20) The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is the youngest?

A) The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.

B) The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.

C) The diagram shows main-sequence stars of all the spectral types except O and B, along with a few giants and supergiants.

D) The diagram shows no main-sequence stars at all, but it has numerous supergiants and white dwarfs.

Answer: A 21) The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is most likely to be located in the halo of our galaxy?

A) The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.

B) The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.

C) The diagram shows main-sequence stars of all the spectral types except O and B, along with a few giants and supergiants.

D) The diagram shows no main-sequence stars at all, but it has numerous supergiants and white dwarfs.

Answer: B

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Visual Quiz Questions 1) This star map shows stars as we see them in our sky from Earth, centered around the constellation Canis Major. Larger dots represent brighter stars, and a few of the brightest stars are identified. From this view alone, what can you conclude about Sirius?

A) It has the greatest apparent brightness of any star in this region of the sky.

B) It is the most luminous star in this region of the sky.

C) It is the nearest star in this region of the sky.

D) It is actually a binary star system in which the second star is a white dwarf.

E) All of the above choices are true.

Answer: A

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2) Why do some of the bright stars (such as the one indicated by the arrow) in this photo have cross-shaped spikes over them?

A) The spikes are an artifact of photography through a telescope.

B) The spikes are flares ejected from the surfaces of bright stars.

C) Very bright stars tend to be slightly nonspherical in shape and therefore look like they have spikes in photos.

D) The stars with spikes are actually binary star systems.

Answer: A 3) All the stars in this photo are at about the same distance from Earth (some 25,000 light-years away). Which stars in this picture are the largest in size (radius)?

A) the bright red stars B) the blue/white stars that look largest on the photo C) the stars that have the spikes D) the dim red stars Answer: A

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4) This graph shows how the apparent brightness of an eclipsing binary system changes with time. Which of the four labeled regions represents the system at a time when one star is eclipsing the other?

A) I B) II C) III D) IV E) both II and IV F) both I and II Answer: E

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5) Study this H-R diagram. What is the spectral type of the star Sirius? (The red arrow helps you locate Sirius on the diagram.)

A) A B) 102 C) main sequence D) blue Answer: A

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6) Study this H-R diagram. Which of the following stars is the most massive? (The red arrows help you locate these stars on the diagram.)

A) Beta Centauri B) Vega C) Bellatrix D) DX Cancri Answer: A

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7) Study this H-R diagram. Which of the following stars is the largest in size (radius)? (The red arrows help you locate these stars on the diagram.)

A) Antares B) Canopus C) Alpha Centauri B D) Aldebaran Answer: A

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8) Study this H-R diagram. Which of the following stars has (or had) the longest hydrogen fusing lifetime? (The red arrows help you locate these stars on the diagram.)

A) Barnard's star B) Alpha Centauri B C) Betelgeuse D) the Sun Answer: A

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9) This photo shows an object located in the halo of our Milky Way galaxy. What kind of object is it?

A) a globular cluster of stars B) an open cluster of stars C) a spiral galaxy D) a supergiant star E) a "beehive" cluster Answer: A 10) This photo shows an object located in the halo of our Milky Way galaxy. Notice the many bright red dots in the photograph. What are they?

A) red giant stars B) cool, red main-sequence stars of spectral type M C) old white dwarf stars that have turned red in color D) massive supergiant stars E) starspots Answer: A

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End-of-Chapter Questions Visual Skills Check Use the following questions to check your understanding of some of the many types of visual information used in astronomy.

The figure to the right, similar to Figure 15.13, uses a zoom-in to compare the sizes of giant and supergiant stars to the sizes of Earth and the Sun. 1) Suppose we wanted to represent allthese of objects using the 1-to-10-billion scale from Chapter 1, on which the Sun is about the size of a grapefruit. Approximately how large in diameter would the star Aldebaran be on this scale?

A) 40 centimeters (the size of a typical beach ball) B) 6 meters (roughly the size of a dorm room) C) 15 meters (roughly the size of a typical house) D) 130 meters (slightly larger than a football field) Answer: B 2) Approximately how large in diameter would the star Betelgeuse be on this same scale?

A) 40 centimeters (the size of a typical beach ball) B) 6 meters (roughly the size of a dorm room) C) 130 meters (slightly larger than a football field) D) 3 kilometers (the size of a small town) Answer: D 3) Approximately how large in diameter would the star Procyon B be on this same scale?

A) 10 centimeters (the size of a large grapefruit) B) 1 centimeter (the size of a grape) C) 1 millimeter (the size of a grape seed) D) 0.1 millimeter (roughly the width of a human hair) Answer: C

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The H-R diagram to the left is identical to Figure 15.11. Answer the following questions based on the information given in the figure.

4) What are the approximate luminosity and lifetime of a star whose mass is 10 times that of the Sun? Answer: luminosity: about 10,000LSun; lifetime: slightly longer than 10 million years 5) What are the approximate luminosity and lifetime of a star whose mass is 3 times that of the Sun? Answer: luminosity: about 100LSun; lifetime: slightly shorter than 1 billion years 6) What are the approximate luminosity and lifetime of a star whose mass is twice that of the Sun? Answer: luminosity: about 10LSun; lifetime: approximately 1 billion years

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Chapter Review Questions 1) Briefly explain how we can learn about the lives of stars even though their lives are far longer than human lives.

2) In what ways are all stars similar? In what ways do they differ?

3) How is a star's apparent brightness related to its luminosity? Explain the inverse square law for light.

4) How do we use stellar parallax to determine a star's distance, and how can we then determine its luminosity?

5) What are apparent and absolute magnitudes, and how are they related to apparent brightness and luminosity?

6) What do we mean by a star's spectral type, and how is spectral type related to surface temperature and color?

7) How was the spectral sequence discovered, and why does it have the order OBAFGKM? Which stars are hottest and coolest in this sequence?

8) What are the three basic types of binary star systems? Why are eclipsing binaries so important to measuring masses of stars?

9) Draw a sketch of a Hertzsprung-Russell (H-R) diagram. Label the main sequence, giants, supergiants, and white dwarfs. Where on this diagram do we find stars that are cool and dim?

Cool and luminous? Hot and dim? Hot and luminous?

10) What do we mean by a star's luminosity class? Briefly explain how we classify stars by spectral type and luminosity class.

11) What is the defining characteristic of a main-sequence star? Briefly explain why massive main-sequence stars are more luminous and have hotter surfaces than less massive mainsequence stars.

12) Which stars have longer lifetimes: massive stars or less massive stars? Explain why.

13) Why is a star's birth mass its most fundamental property?

14) How do giants and supergiants differ from main-sequence stars? What are white dwarfs?

15) How does the luminosity of a pulsating variable star change with time?

16) Describe in general terms how open clusters and globular clusters differ in their numbers of stars, ages, and locations in the galaxy.

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17) Explain why H-R diagrams look different for star clusters of different ages. How does the location of the main-sequence turnoff point tell us the age of the star cluster?

Does It Make Sense?

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

Example: I walked east from our base camp at the North Pole.

Solution: The statement does not make sense because east has no meaning at the North Pole–all directions are south from the North Pole.

18) Two stars that look very different must be made of different kinds of elements.

19) Two stars that have the same apparent brightness in the sky must also have the same luminosity.

20) Sirius looks brighter than Alpha Centauri, but we know that Alpha Centauri is closer because its apparent position in the sky shifts by a larger amount as Earth orbits the Sun. 21) Stars that look red have hotter surfaces than stars that look blue.

22) Some of the stars on the main sequence of the H-R diagram are not converting hydrogen into helium.

23) The smallest, hottest stars are plotted in the lower left-hand portion of the H-R diagram.

24) Stars that begin their lives with the most mass live longer than less massive stars because they have so much more hydrogen fuel.

25) Star clusters with lots of bright, blue stars of spectral type O and B are generally younger than clusters that don't have any such stars.

26) All giants, supergiants, and white dwarfs were once main-sequence stars.

27) Most of the stars in the sky are more massive than the Sun. Quick Quiz

28) If the star Alpha Centauri were moved to a distance 10 times as far from Earth as it is now, its parallax angle would A) get larger.

B) get smaller.

C) stay the same.

Answer: No Correct Answer Was Provided.

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29) What do we need to measure in order to determine a star's luminosity?

A) apparent brightness and mass B) apparent brightness and temperature C) apparent brightness and distance Answer: No Correct Answer Was Provided.

30) What two pieces of information would you need in order to measure the masses of stars in an eclipsing binary system?

A) the time between eclipses and the average distance between the stars B) the period of the binary system and its distance from the Sun C) the velocities of the stars and the Doppler shifts of their absorption lines Answer: No Correct Answer Was Provided.

31) Which of these stars has the coolest surface temperature?

A) an A star B) an F star C) a K star Answer: No Correct Answer Was Provided.

32) Which of these stars is the most massive?

A) a main-sequence A star B) a main-sequence G star C) a main-sequence M star Answer: No Correct Answer Was Provided.

33) Which of these stars has the longest lifetime?

A) a main-sequence A star B) a main-sequence G star C) a main-sequence M star Answer: No Correct Answer Was Provided.

34) Which of these stars has the largest radius?

A) a supergiant A star B) a giant K star C) a supergiant M star Answer: No Correct Answer Was Provided.

35) Which of these stars has the greatest surface temperature?

A) a 30MSun main-sequence star B) a supergiant A star C) a Cepheid variable star Answer: No Correct Answer Was Provided.

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36) Which of these star clusters is youngest? A) a cluster whose brightest main-sequence stars are white B) a cluster whose brightest stars are red C) a cluster containing stars of all colors Answer: No Correct Answer Was Provided. 37) Which of these star clusters is oldest? A) a cluster whose brightest main-sequence stars are white B) a cluster whose brightest mainsequence stars are yellow C) a cluster containing stars of all colors Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

38) Research: Women in Astronomy.

Until fairly recently, men greatly outnumbered women in professional astronomy. Nevertheless, many women made crucial discoveries in astronomy throughout history–including discovering the spectral sequence for stars. Choose a female astronomer from any time period and learn enough about her life and discoveries to write a oneto three-page scientific biography.

39) Discussion: Annie Jump Cannon's Disability. Annie Jump Cannon (discussed in Section 15.1) was one of the greatest astronomers of the 20th century, male or female. She also had a significant physical disability: a childhood disease had made her completely deaf by the time she reached adulthood. a) Gather in small groups to discuss the potential impacts of Annie Jump Cannon's deafness on her career, and begin the discussion by making a list of ways in which you think her deafness might have made a career in astronomy more challenging. b) Cannon's greatest contributions came from the fact that she visually classified the spectra of more than 400,000 stars during her lifetime. Discuss whether her deafness might have provided her with any advantages in this work and, if so, list what they might have been. c) When Cannon was considered for membership in the National Academy of Sciences in 1923, one of its members said he could not vote for Cannon because she was deaf. Why do you think someone would have voiced that objection in 1923? Discuss how and why scientists' views about physically disabled people have changed during the past century.

The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

40) Classification.As discussed in the text, Annie Jump Cannon and her colleagues developed our modern system of stellar classification. Why do you think rapid advances in our understanding of stars followed so quickly on the heels of their efforts? What other areas in science have had huge advances in understanding follow directly from improved systems of classification?

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41) Snapshot of the Heavens. The beginning of the chapter likened the problem of studying the lives of stars to learning about human beings through a 1-minute glance at human life. What could you learn about human life by looking at a single snapshot of a large extended family, including babies, parents, and grandparents? How is the study of such a snapshot similar to what scientists do when they study the lives of stars? How is it different?

42) Life Spans of Stars. Scientists estimate the life spans of stars by dividing the total amount of energy available for fusion by the rate at which the stars radiate energy into space. Those calculations predict that the life spans of high-mass stars are shorter than those of low-mass stars.

Describe a type of observation that can test this prediction and verify that it is correct.

43) Group Activity: Comparing Stellar Properties. The following activity makes use of Tables F.1 and F.2 in Appendix F. Before starting, split into two groups, one of which will gather data from Table F.1 and the other from Table F.2. a) Working in your separate groups, count and record the number of stars of each spectral type (OBAFGKM) and of each luminosity class (I, II, III, IV, V) in your assigned table. Ignore the white dwarfs. b) Working together, compare the counts of spectral types in the two tables. Are there any patterns you can identify? If so, develop a hypothesis that could explain them. c) Are there any patterns to the differences in the counts you found for the luminosity classes in the two tables? If so, develop a hypothesis that could explain them. d) Considering the data from both tables, identify the nearest star of each spectral type. Based on your results, discuss whether it is possible that there is an undiscovered star of spectral type O within 200 light-years of the Sun. Explain your reasoning.

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Investigate Further 44) Stellar Data. The table below gives basic data for several bright stars; Mv is absolute magnitude and mv is apparent magnitude. Use these data to answer the following questions.

Include a brief explanation with each answer. [Hint: Remember that the magnitude scale runs backward, so brighter stars have smaller (or more negative) magnitudes.]

a) Which star appears brightest in our sky? b) Which star appears faintest in our sky? c) Which star has the greatest luminosity? d) Which star has the least luminosity? e) Which star has the highest surface temperature? f) Which star has the lowest surface temperature? g) Which star is most similar to the Sun? h) Which star is a red supergiant? i) Which star has the largest radius? k) Which stars have finished fusing hydrogen in their cores? l) Among the main-sequence stars listed, which one is the most massive? m) Among the main-sequence stars listed, which one has the longest lifetime?

45) Interpreting the H-R Diagram. Using the information in Figure 15.10, describe how Proxima Centauri differs from Sirius.

46) Parallax from Jupiter. Suppose you could observe stellar parallax from the orbit of Jupiter.

How would it differ from the stellar parallax we observe from Earth? Would it be easier or more difficult to measure stellar distances? Explain.

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47) An Expanding Star. Describe what would happen to the surface temperature of a star if its radius doubled but there was no change in luminosity.

48) Colors of Eclipsing Binaries. Figure 15.8 shows an eclipsing binary system consisting of a small blue star and a larger red star. Explain why the decrease in apparent brightness of the combined system is greater when the blue star is eclipsed than when the red star is eclipsed.

49) Visual and Spectroscopic Binaries. Suppose you are observing two binary star systems at the same distance from Earth. Both are spectroscopic binaries consisting of similar types of stars, but only one of these binary systems is a visual binary. Which of these star systems would you expect to have the greater Doppler shifts in its spectra? Explain your reasoning.

50) Life of a Star Cluster. Imagine you could watch a star cluster from the time of its birth to an age of 13 billion years. Describe in one or two paragraphs what you would see happening during that time.

51) Research: GAIA and Parallax. The European Space Agency's GAIA mission, launched in 2013, has measured parallax for more than a billion stars. Learn how this satellite can measure much smaller parallax angles than can be measured from the ground and how those measurements affect our knowledge of the universe. Write a one- to two-page report on your findings.

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

52) The Inverse Square Law for Light.

Use Earth's distance of about 150 million kilometers from the Sun, the Sun's apparent brightness in our sky of about 1300 watts/m2, and the inverse square law for light to calculate the apparent brightness that we would measure for the Sun were if we located at the following positions. a) half Earth's distance from the Sun b) twice Earth's distance from the Sun c) five times Earth's distance from the Sun 53) The Luminosity of Alpha Centauri A. Alpha Centauri A lies at a distance of 4.4 light-years and has an apparent brightness in our night sky of 2.7 × 10−8 watt/m2. Recall that 1 light-year = 9.5 × 1012 km = 9.5 × 1015 m. a) Use the inverse square law for light to calculate the luminosity of Alpha Centauri A. b) Suppose you have a light bulb that emits 10 watts of visible light. How far away would you have to put the light bulb for it to have the same apparent brightness as Alpha Centauri A in our sky? (Hint: Use 10 watts as L in the inverse square law for light, and use the apparent brightness given above for Alpha Centauri A. Then solve for the distance.)

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54) More Practice with the Inverse Square Law for Light. Use the inverse square law for light to answer each of the following questions. a) Suppose a star has the same luminosity as our Sun (3.8 × 1026 watts) but is located at a distance of 10 light-years. What is its apparent brightness? b) Suppose a star has the same apparent brightness as Alpha Centauri A (2.7 × 10−8 watt/m2) but is located at a distance of 200 light-years. What is its luminosity? c) Suppose a star has a luminosity of 8 × 1026 watts and an apparent brightness of 3.5 × 10−12 watt/m2. How far away is it? Give your answer in both kilometers and light-years. d) Suppose a star has a luminosity of 5 × 1029 watts and an apparent brightness of 9 × 1015 watts/m2. How far away is it? Give your answer in both kilometers and light-years.

55) Parallax and Distance. Use the parallax formula to calculate the distance to each of the following stars. Give your answers in both parsecs and light-years. a) Alpha Centauri: parallax angle of 0.7420″ b) Procyon: parallax angle of 0.2860″ 56) The Magnitude System. Use the definitions of the magnitude system to answer each of the following questions. a) Which is brighter in our sky: a star with apparent magnitude 2 or a star with apparent magnitude 7? By how much? b) Which has a greater luminosity: a star with absolute magnitude −4 or a star with absolute magnitude +6? By how much?

57) Measuring Stellar Mass. The spectral lines of two stars in a particular eclipsing binary system shift back and forth with a period of 6 months. The lines of both stars shift by equal amounts, and the amount of the Doppler shift indicates that each star has an orbital speed of 80,000 m/s. What are the masses of the two stars? Assume that each of the two stars traces a circular orbit around their center of mass. (Hint: See Mathematical Insight 15.4.)

58) Calculating Stellar Radii. Sirius A has a luminosity of 26Lsun and a surface temperature of about 9400 K. What is its radius? (Hint: See Mathematical Insight 15.5.)

59) Time Spent as a Red Giant. The H-R diagram in Figure 15.20 shows a star cluster with a large number of red giants in it. a) What is the approximate mass of the most massive stars that remain on the main sequence of this star cluster? b) What is the luminosity of the most luminous stars in the cluster? c) Compute the ratio of the luminosity from part b to the mass from part a. How does that ratio compare with the Sun's ratio of luminosity to mass? d) From your answer to part c, estimate the maximum amount of time these very luminous stars can last as red giants.

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter 16 Star Birth Section 16.1 1) Where do stars form and why?

A) Stars form around giant planets because planets provide the "seeds" of star formation.

B) Stars form in cold gas clouds because gravity has to be stronger than gas pressure.

C) Stars form in warm dust clouds because the dust shields the protostars from radiation.

D) Stars form in hot gas clouds because stars have to be hot to start fusion reactions.

Answer: B 2) What is the approximate composition of the interstellar medium (by mass) in our region of the Milky Way? A) 60% hydrogen, 40% helium B) 70% hydrogen, 28% helium, 2% heavier elements C) 70% hydrogen, 20% helium, 10% heavier elements D) 50% hydrogen, 48% helium, 2% heavier elements Answer: B 3) Which of the following accurately describes the density and temperature of molecular clouds compared to other interstellar clouds?

A) Molecular clouds are relatively low in both density and temperature.

B) Molecular clouds are relatively high in both density and temperature.

C) Molecular clouds are relatively high in density and low in temperature.

D) Molecular clouds are relatively low in density and high in temperature.

Answer: C 4) Suppose there is a cloud containing interstellar dust between a star and us. How will the dust affect the light we see from the star?

A) It makes the star appear dimmer and redder in color.

B) It makes the star appear dimmer and bluer in color.

C) It does not affect the star's brightness but makes it appear redder in color.

D) It makes the star appear dimmer but does not affect the star's color.

Answer: A 5) If you wanted to observe stars behind a molecular cloud, in what wavelength of light should you observe?

A) ultraviolet B) visible C) infrared D) x-ray E) gamma ray Answer: C

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6) The thermal pressure within a gas depends on A) density only. B) temperature only. C) density and temperature. D) composition. E) gravity. Answer: C 7) The strength of gravity within a molecular cloud depends on A) density only. B) temperature only. C) density and temperature. D) composition. E) thermal pressure. Answer: A 8) What condition is necessary in an interstellar gas cloud for it to begin collapsing? A) a low density B) a low temperature C) a high temperature D) a high heavy element content E) a low heavy element content Answer: B 9) Which of the following must be occurring in order for a molecular cloud to collapse and form stars? A) The inward pull of gravity must be stronger than the outward push of thermal pressure. B) The mass of the cloud must continually increase, thereby increasing the strength of gravity. C) The interior of the cloud must continually increase in temperature. D) The gas in the cloud must be supported by electron degeneracy pressure. Answer: A 10) The buildup of thermal energy in a collapsing gas cloud leads to thermal pressure that can halt the gravitational collapse.

What must happen for the gravitational collapse to continue? A) The cloud must radiate away some of its thermal energy. B) The cloud must gain mass. C) The cloud must have a very strong magnetic field. D) The cloud must get a push from the radiation of nearby hot, young stars. Answer: A 11) How do we think the first generation of stars would have been different from stars born today? A) They would have lacked elements heavier than hydrogen and helium. B) They would have generated energy through fission rather than fusion. C) They would have all been very low in mass. D) They would have shined only with ultraviolet light, not visible light. Answer: A

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Section 16.2 1) What key event marks the transition of a protostar into a true (main-sequence) star?

A) the onset of sustained nuclear fusion in its core B) the clearing of the gas from the surrounding gas cloud into interstellar space C) the moment when the luminosity first matches the luminosity of the lowest-mass main sequence stars D) the onset of a strong stellar wind Answer: A 2) What is the primary source of energy for protostars (that have not yet become hot enough for fusion in their cores)?

A) fission from concentrated radioactive elements B) high-energy photons from nearby stars C) gravitational contraction D) the conversion of mass into energy in accord with E = mc2 Answer: C 3) What is the key distinction between protostars and true (main-sequence) stars?

A) Protostars generate most of their energy from gravitational contraction, and stars generate it from nuclear fusion.

B) Protostars are larger and more luminous than stars.

C) Protostars have stronger magnetic fields than stars.

D) Protostars are supported by thermal pressure, and stars are supported by degeneracy pressure.

Answer: A 4) What happens to the rotation of a molecular cloud as it collapses to form a star?

A) The rotation rate remains the same at all times.

B) The rotation slows as the cloud collapses.

C) The rotation rate increases as the cloud collapses.

Answer: C 5) Why do disks form around young stars?

A) The surrounding gas can only approach the star from a single direction.

B) Collisions between gas particles flatten the rotating gas cloud.

C) The rotation of the cloud causes gas to be spun outward from the central star.

D) Intense winds from nearby massive stars flatten the gas cloud.

Answer: B 6) Approximately how long does the protostellar stage last for a starlike our Sun? A) 300 years B) 30,000 years C) 100,000 years D) 30 million years E) 30 billion years Answer: D 3 Copyright © 2020 Pearson Education, Inc.

7) In general, the time it takes from when in interstellar cloud fragment first begins collapsing until it gives birth to a main-sequence star is A) shorter for less massive stars.

B) longer for less massive stars.

C) about the same for all stars.

D) dependent on the rotation rate of the star.

Answer: B 8) Which stars spend the shortest amount of time as protostars?

A) the most massive stars B) the least massive stars C) Mass doesn't matter, all stars spend about the same amount of time as protostars.

Answer: A

9) The diagram above shows the life track of a 1 MSun protostar from the time it first begins to form until it reaches the main sequence, with four stages labeled. What is happening to the protostar's surface temperature and luminosity during Stage 1, when gravity assembles the protostar from a collapsing cloud?

A) Its surface temperature and luminosity increase.

B) Its surface temperature remains the same and its luminosity decreases.

C) Its surface temperature and luminosity decrease.

D) Its surface temperature decreases and its luminosity increases.

E) Its surface temperature and luminosity remain the same.

Answer: A

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10) The diagram above shows the life track of a 1 MSun protostar from the time it first begins to form until it reaches the main sequence, with four stages labeled. What is happening to the protostar's surface temperature and luminosity during Stage 2, when it is undergoing convective contraction, and its life track moves almost straight down on the HR diagram? A) Its surface temperature and luminosity increase. B) Its surface temperature remains the same and its luminosity decreases. C) Its surface temperature and luminosity decrease. D) Its surface temperature decreases and its luminosity increases. E) Its surface temperature and luminosity remain the same. Answer: B 11) The diagram above shows the life track of a 1 MSun protostar from the time it first begins to form the until it reaches the main sequence, with four stages labeled. What is happening to

protostar's surface temperature and luminosity during Stage 3, when it is undergoing radiative contraction and its life track moves up and to the left?

A) Its surface temperature and luminosity increase.

B) Its surface temperature remains the same and its luminosity decreases.

C) Its surface temperature and luminosity decrease.

D) Its surface temperature decreases and its luminosity increases.

E) Its surface temperature and luminosity remain the same.

Answer: A 12) The diagram above shows the life track of a 1 MSun protostar from the time it first begins to form the until it reaches the main sequence, with four stages labeled. What is happening to protostar's surface temperature and luminosity during Stage 4, when its life track moves straight down to join the main sequence?

A) Its surface temperature and luminosity increase.

B) Its surface temperature increases, and its luminosity decreases.

C) Its surface temperature and luminosity decrease.

D) Its surface temperature decreases, and its luminosity increases.

E) Its surface temperature and luminosity remain the same.

Answer: B Section 16.3 1) About what is the smallest mass a newborn star can have?

A) 0.01 times the mass of the Sun B) 0.08 times the mass of the Sun C) 0.8 times the mass of the Sun D) the mass of the Sun E) twice the mass of the Sun Answer: B 5 Copyright © 2020 Pearson Education, Inc.

2) The most massive stars have masses somewhere around A) 5 solar masses. B) 10 solar masses. C) 30 solar masses. D) 150 solar masses. E) 3000 solar masses. Answer: D 3) What eventually halts the gravitational collapse of an interstellar gas cloud that forms a brown dwarf rather than a true (main-sequence) star? A) The central object becoming hot enough to sustain nuclear fusion in its core B) Degeneracy pressure caused by the high density of electrons in the core C) Thermal pressure from the high core temperature D) The onset of convection in the object's outer layers Answer: B 4) What happens to an object supported by degeneracy pressure as the object cools with time? A) The degeneracy pressure weakens and the object collapses. B) The degeneracy pressure strengthens and the object expands. C) The degeneracy pressure is unaffected by the temperature change, so the object remains stable. Answer: C 5) What prevents a brown dwarf from undergoing nuclear fusion? A) Degeneracy pressure halts the contraction of a protostar so the core never becomes hot or dense enough for nuclear fusion. B) There is not enough mass to maintain nuclear reactions in a self-sustaining way. C) The surface temperature never rises high enough for the radiation to be trapped and heat its interior to the temperature required for nuclear fusion. D) Radiation pressure halts the contraction of a protostar so the core never becomes hot or dense enough for nuclear fusion. E) There are too many heavy elements and not enough hydrogen for fusion to occur in a self sustaining way. Answer: A 6) Why doesn't a brown dwarf become a star? A) It does not have enough mass for its core to get hot enough to start fusion reactions before degeneracy pressure prevents further collapse. B) It always has a companion star that is hotter, and therefore, the brown dwarf is more like a planet than a star. C) It does not have enough heavy elements to start fusion reactions. D) Its luminosity is not high enough to qualify as an official star. Answer: A

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7) What is thought to prevent the formation of stars with masses larger than about 300 times that of our Sun? A) Molecular clouds do not have enough material to form such massive stars.

B) Such massive stars would fragment into binary stars because of their rapid rotation.

C) Such massive stars would generate so much light that radiation pressure would blow them apart.

D) The onset of degeneracy pressure would prevent such massive stars from forming.

Answer: C 8) Which statement accurately describes how our Sun fits into the mass range of stars?

A) The Sun is completely average, with half of all stars being more massive than the Sun and half of all stars being less massive than the Sun. B) The Sun is near the bottom of the overall mass range.

C) The Sun is near the top of the overall mass range.

D) The Sun is near the middle of the overall mass range, but most stars are more massive than the Sun. E) The Sun is near the middle of the overall mass range, but most stars are less massive than the Sun. Answer: E 9) Of the mass ranges below, which represents the most common stars?

A) 0.08 to 0.5 MSun B) 0.5 to 2 MSun C) 2 to 10 MSun D) 10 to 150 MSun Answer: A 10) The most common main-sequence stars have spectral type A) G. B) A. C) O. D) M. Answer: D General Chapter Questions 1) What did Carl Sagan mean when he said that we are all "star stuff"?

A) that life would be impossible without energy from the Sun B) that the Earth formed at the same time as the Sun C) that the carbon, oxygen, and other elements essential to life were created by fusion in stellar cores D) that the Sun formed from the interstellar medium: the "stuff" between the stars E) that the universe contains billions of stars Answer: C

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Short Answer Questions 1) Briefly describe how a star forms.

Answer: In cold, dense molecular clouds, gravity brings material together. As gas moves inward, it converts gravitational potential energy to thermal energy and warms up. Once the cloud becomes so dense that the thermal radiation cannot escape, the temperature rises rapidly, nuclear fusion begins, and the dense core becomes a protostar. As the cloud has collapsed from a large size to a small size, it must spin very fast to conserve angular momentum. This results in the formation of a protostellar disk around the protostar. Planets may form in this disk as the star continues to grow. Eventually stellar winds and jets clear away the surrounding gas and a newly formed star emerges.

2) Why does a cloud collapse rapidly at first, and then slow down as it gets denser?

Answer: The self-gravity causes the cloud to collapse. Gravitational potential energy is turned to heat (through friction) and released as infrared light from the cloud. As the cloud continues to collapse and becomes smaller and denser, molecules collide more frequently and do not have time to release their excess energy as photons. The thermal energy in the cloud is trapped and its temperature rises. The rising temperature increases the pressure and slows down the collapse.

Reading Quiz Questions 1) What do we mean by the interstellar medium?

A) the gas and dust that lies in between the stars in the Milky Way galaxy B) the dust that fills the halo of the Milky Way galaxy C) the middle section of the Milky Way galaxy D) the name of an oracle who can channel messages from beings that live near the star called Vega Answer: A 2) The interstellar clouds called molecular clouds are A) the cool clouds in which stars form.

B) the clouds in which elements such as carbon, nitrogen, and oxygen are made.

C) the clouds that are made mostly of complex molecules such as carbon dioxide and sulfur dioxide.

D) the hot clouds of gas expelled by dying stars.

Answer: A 3) Which of the following types of molecule is the most abundant in an interstellar molecular cloud?

A) H2 B) CO C) H2O D) NH3 Answer: A

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4) Interstellar dust consists mostly of A) microscopic particles of carbon and silicon.

B) ozone "smog."

C) hydrogen and helium atoms.

D) tiny grains of water ice.

E) the same tiny particles found in household dust.

Answer: A 5) Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?

A) infrared B) visible light C) ultraviolet D) blue light Answer: A 6) Suppose you look at a star that can be seen through the edge of a dusty interstellar cloud. The star will look ________ than it would if it were outside the cloud.

A) dimmer and redder B) dimmer and bluer C) more redshifted D) brighter and redder Answer: A 7) Most interstellar clouds remain stable in size because the force of gravity is opposed by ________ within the cloud.

A) thermal pressure B) degeneracy pressure C) radiation pressure D) stellar winds Answer: A 8) What kind of gas cloud is most likely to give birth to stars?

A) a cold, dense gas cloud B) a hot, dense gas cloud C) a cold, low-density gas cloud D) a hot, low-density gas cloud Answer: A 9) How are magnetic fields thought to affect star formation in molecular clouds?

A) They can help resist gravity, so that more total mass is needed before the cloud can collapse to form stars.

B) They accelerate the star formation process.

C) They allow small stars to form in isolation within gas clouds.

D) There are no magnetic fields in interstellar space, so there is no effect.

Answer: A

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10) Which of the following statements about the first stars that were born in the universe is expected to be true?

A) They were made only from hydrogen and helium.

B) They were made from pure energy.

C) They were probably orbited only by terrestrial planets but no jovian planets.

D) They were made approximately of 98% hydrogen and helium and 2% of heavier elements.

Answer: A 11) What is a protostar?

A) a star that is still in the process of forming B) a star that has planets C) an intermediate-mass star D) a star in its final stage of life Answer: A 12) Which of the following phenomena is not commonly associated with the star-formation process?

A) intense ultraviolet radiation coming from a protostar B) the formation of a spinning disk of material around a protostar C) powerful "jets" shooting out along the rotation axis of a protostar D) strong winds of particles blowing out into space from a protostar Answer: A 13) What law explains why a collapsing cloud usually forms a protostellar disk around a protostar?

A) conservation of angular momentum B) Kepler's third law C) the universal law of gravitation D) Wein's law Answer: A 14) What can we learn about a star from a life track on an H-R diagram?

A) the surface temperature and luminosity the star will have at each stage of its life B) the star's age C) the star's current stage of life D) how the star's distance from Earth varies at different times in its life Answer: A 15) When does a protostar become a main-sequence star?

A) when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space B) when a piece of a molecular cloud first begins to contract into a star C) when it becomes luminous enough to emit thermal radiation D) at the instant that the first hydrogen fusion reactions occur in the protostar's core Answer: A

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16) Approximately what core temperature is required before hydrogen fusion can begin in a star? A) 10 million K B) 10,000 K C) 10 trillion K D) 10 billion K E) 1 billion K Answer: A 17) Which of the following stars spends the longest time in the protostellar phase of life? A) a 1 solar-mass star B) a 2 solar-mass star C) a 3 solar-mass star D) a 4 solar-mass star E) a 5 solar-mass star Answer: A 18) What is the approximate range of masses that newborn main-sequence stars can have? A) 0.1 to 150 solar masses B) 0.001 to 1500 solar masses C) 0.1 to 1500 solar masses D) 0.001 to 10 solar masses E) 0.1 to 10 solar masses Answer: A 19) The vast majority of stars in a newly formed star cluster are A) less massive than the Sun. B) very high-mass, type O and B stars. C) red giants. D) about the same mass as our Sun. Answer: A 20) Which of the following statements about brown dwarfs is not true? A) Brown dwarfs eventually collapse to become white dwarfs. B) Brown dwarfs are supported against gravity by degeneracy pressure, which does not depend on the object's temperature. C) Brown dwarfs form like ordinary stars but are too small to sustain nuclear fusion in their cores. D) All brown dwarfs have masses less than about 8% that of our Sun. Answer: A

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Concept Quiz Questions 1) Which two processes can generate energy to help a star or gas cloud maintain its internal thermal pressure? A) nuclear fusion and gravitational contraction B) nuclear fusion and nuclear fission C) nuclear fusion and supernova D) nuclear fission and supernova Answer: A 2) About what percentage of the mass of a molecular cloud is in the form of dust? A) 1% B) 10% C) 50% D) 98% Answer: A 3) How do we learn the chemical composition of the interstellar medium? A) by studying spectra of interstellar gas clouds B) We make an educated guess based on the Sun's composition. C) We collect samples of gas and dust from interstellar space. D) We use computer simulations of the interstellar medium. Answer: A 4) What happens to the visible light radiated by stars located within a dusty gas cloud? A) It is absorbed by dust, which heats the dust grains so that they emit the absorbed energy as infrared light. B) It is blocked by dust and its energy is thereby lost. C) It is reflected by dust back to the star from whence it came. D) It passes through the cloud unaffected.

Answer: A 5) Under which circumstances can you be sure that the thermal pressure within a gas cloud is increasing? A) The cloud's temperature and density are both increasing. B) The cloud's temperature is increasing and its density is decreasing. C) The cloud's temperature and density are both decreasing. D) The cloud's temperature is decreasing and its density is increasing. E) impossible to say Answer: A

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6) Which process is required to allow a gravitationally-collapsing gas cloud to continue to collapse?

A) The cloud must radiate away much of its thermal energy.

B) The cloud must trap most of its thermal energy.

C) The cloud must collide with other clouds.

D) New dust particles must continually be made in the cloud.

Answer: A 7) According to current understanding, the first generation of stars in the universe ________ than differ stars born today.

A) were much more massive B) contained much more hydrogen and helium C) were much cooler in temperature D) were much more likely to be members of binary star systems Answer: A 8) Angular momentum plays an important role in star formation. Which of the following characteristics of a protostellar system is probably not strongly affected by the star's angular momentum?

A) the onset of core hydrogen fusion B) the existence of protostellar jets C) the strength of protostellar winds D) the formation of a protostellar disk Answer: A 9) Binary star systems are thought to form when A) gravity pulls two neighboring protostars quite close together, but angular momentum causes them to orbit each other rather than collide.

B) two interstellar gas clouds happen to contract so close together that there's no room for a disk or planets.

C) the protostellar disk around a protostar has enough material to form a second star.

D) a protostar emits two jets, each of which turns into a star.

Answer: A 10) Generally speaking, a main-sequence star is ________ than it was during the time it was a protostar A) hotter and dimmer B) hotter and brighter C) cooler and dimmer D) cooler and brighter Answer: A

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11) Where does a 1 solar-mass protostar appear on an H-R diagram?

A) It is to the right of the main sequence and higher up than the Sun. B) It is to the right of the main sequence and lower down than the Sun. C) It is to the left of the main sequence and higher up than the Sun. D) Nowhere; only stars that have fusion in their cores can be shown on H-R diagrams.

Answer: A 12) Why does the rotation of a protostar slow down over time?

A) Magnetic fields can transfer angular momentum to the protostellar disk and protostellar winds can carry angular momentum away.

B) All rotating objects slow down over time.

C) The onset of fusion causes the rotation rate to slow dramatically.

D) Magnetic fields of other stars interact with the magnetic fields of the protostars, slowing its rotation.

Answer: A 13) During the time before fusion begins, a protostar's core gradually A) shrinks and heats.

B) shrinks and cools.

C) shrinks and maintains a constant temperature.

D) expands and cools.

Answer: A 14) The core of a protostar that will eventually become a brown dwarf shrinks until A) the type of pressure called degeneracy pressure becomes important.

B) its central temperature is high enough to support fusion reactions.

C) it forms a rocky core.

D) it radiates brown light.

Answer: A 15) If a star is extremely massive (more than 100-solar masses), why isn't it likely to survive for long?

A) It may blow itself apart because of radiation pressure.

B) It explodes as a supernova after just a few dozen years.

C) It eventually divides into two lower-mass stars.

D) Its great mass will cause it to collapse immediately to make a black hole.

Answer: A 16) Consider a large molecular cloud that will give birth to a cluster of stars. Which of the following would you expect to be true?

A) A few massive stars will form, live, and die before the majority of the star's clusters even complete their protostar stage.

B) All the stars in the cluster will be of about the same mass.

C) All the stars in the cluster will become main-sequence stars at about the same time.

D) All the stars in the cluster will have approximately the same luminosity and surface temperature.

Answer: A

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17) We do not know for certain whether the general trends we observe in stellar birth masses also apply to brown dwarfs. But if they do, then which of the following would be true?

A) Brown dwarfs would outnumber all ordinary stars.

B) Brown dwarfs would be responsible for most of the overall luminosity of our Milky Way Galaxy.

C) Brown dwarfs would be extremely rare.

D) Most of the brown dwarfs in the Milky Way Galaxy would be quite young in age.

Answer: A 18) Where would a brown dwarf be located on an H-R diagram?

A) below and to the right of the lowest part of the main sequence B) above and to the left of the highest part of the main sequence C) in the upper right corner of the H-R diagram D) in the lower left corner of the H-R diagram Answer: A Visual Quiz Questions 1) The dark area stretching from the center of this picture to the upper right is about 50 lightyears long and lies in the plane of the Milky Way Galaxy. What is it?

A) a star-forming cloud B) a region of the galactic plane in which there are few stars C) the expanding remains of a supernova D) a black hole Answer: A

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2) Photos A and B both show the same field of view containing a star-forming molecular cloud.

One of the photos was taken in visible light and the other in infrared light. Which one is the visible-light photo, and how do you know?

A) Photo A is the visible-light photo, and we can tell because dust in the molecular cloud absorbs visible light, making the cloud appear dark.

B) Photo A is the visible-light photo because its stars are more colorful.

C) Photo B is the visible-light photo because more stars are visible in the molecular cloud.

D) Photo B is the visible-light photo because the molecular cloud has a reddish tint.

Answer: A 3) The main photo and the smaller inset (at the lower left) both show the same field of view containing a star-forming molecular cloud. One of the photos was taken in visible light and the other in infrared light. Which one is the infrared photo, and how do you know?

A) The main (larger) photo is the infrared because molecular clouds emit much more infrared light than visible light.

B) The main (larger) photo is the infrared one because star-forming clouds generally are pink in color.

C) The smaller inset photo is the infrared one because the cloud appears dark and infrared light is invisible.

D) The smaller inset photo is the infrared because edge of the cloud glows red in the image.

Answer: A

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4) This famous image from the Hubble Space Telescope shows what is sometimes called the "pillars of creation." Which of the following best describes what it shows?

A) The pillars are clouds of gas and dust in which many new stars are forming; the edges of the pillars are sculpted by ultraviolet radiation from stars outside the pillars.

B) Each of the pillars is a cloud of gas that will give birth to a single star, which will emerge from the top of its pillar.

C) There is a hot, young star at the top of each pillar that is shooting a jet of gas that forms the pillar below the star.

D) There is a single star outside the frame at the bottom, and the pillars are clouds of accreting particles that will form planets orbiting the star.

Answer: A 5) This figure shows frames from a computer simulation of star formation, starting at the left with a large molecular cloud measuring more than a light-year across. What is happening as time passes (from left to right)?

A) The cloud is fragmenting into smaller pieces that will form stars.

B) The cloud is developing hot spots that shine more brightly within it.

C) Magnetic fields are forcing the gas to align in intricate ways along twisted magnetic field lines.

D) The cloud has been hit by a shock wave from a nearby stellar explosion, which causes the gas to move violently around.

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6) This photo shows gas associated with a protostar. What is this gas doing?

A) It is flowing outward in two opposite directions from the protostar.

B) It is gas that is gradually accreting onto the protostar.

C) It is part of a protostellar disk in which planets may someday form.

D) It is gas blown off the protostar's surface as a protostellar wind.

Answer: A

7) This figure shows the life track of a 1 solar-mass star from its beginnings in a collapsing cloud fragment until it becomes a main-sequence star. At which of the numbered points is the object generating energy primarily from gravitational contraction?

A) Points 1, 2, and 3 B) Point 1 only C) Point 2 only D) Point 3 only E) Point 4 only F) Points 1 and 2 only Answer: A 18 Copyright © 2020 Pearson Education, Inc.

8) This figure shows the life track of a 1 solar-mass star from its beginnings in a collapsing cloud fragment until it becomes a main-sequence star. At which of the numbered points is the object generating energy primarily from hydrogen fusion? A) Point 4 only B) Point 1 only C) Point 2 only D) Point 3 only E) Points 1, 2, and 3 F) Points 1 and 2 only Answer: A 9) This diagram shows the life tracks from protostar to the main sequence for several stars of different masses. Which track represents the star that takes the longest amount of time to reach the main sequence?

A) Track 1 B) Track 2 C) Track 3 D) Track 4 E) Track 5 Answer: A

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10) What is the main idea captured by this graph?

A) Low-mass stars are much more common than higher-mass stars.

B) Low-mass stars are smaller and redder than higher-mass stars.

C) Stars are best understood by dividing them into four categories by mass.

D) Objects with mass below 0.08 MSun are brown dwarfs rather than true stars.

Answer: A

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End-of-Chapter Questions Visual Skills Check Use the following questions to check your understanding of some of the many types of visual information used in astronomy.

This frame from Figure 16.12 is from a supercomputer model of the formation of one of the first stars in the universe. Color indicates gas temperature: Blue and green represent cool gas (100— 300 K), orange and red represent warm gas (500—3000 K), and yellow represents gas at intermediate temperatures (300—500 K). Use the information in the figure to answer the following questions.

1) What is the approximate width of the region inside the black square? Answer: 200— 300 light-years 2) What is the approximate diameter of the roundish region with a temperature of 300— 500 K? Answer: About 1000 light-years 3) The image shows three different streams of gas that are flowing into the central cloud.

Does most edgesof ofthe cool gas in these streams flow down the middle of the stream or along the the stream?

Answer: Cool gas is mostly in the centers of 21 the streams.

Copyright © 2020 Pearson Education, 4) Does the cold streaming gas heat up or cool down when itInc. enters the central cloud?

Answer: The cold gas heats up when it enters the central cloud.

Chapter Review Questions 1) What is the interstellar medium? What is its chemical composition, and how do we measure it?

2) What is a molecular cloud? How does a molecular cloud compare in temperature and density with the rest of the interstellar medium?

3) What is interstellar dust? How does it interact with visible light? What are the consequences for our view of the heavens, and how is that view different in infrared light?

4) What features of molecular clouds make the conditions favorable for star formation?

5) What happens to the thermal energy released into molecular clouds as gravity makes them contract? Why doesn't it build up and stop star formation?

6) Why do stars tend to form in clusters? Describe the process by which a single cloud gives birth to an entire cluster of stars.

7) Why do we think the very first stars were much more massive than the Sun? 8) What happens to a contracting cloud when its thermal energy can no longer escape the cloud's interior in the form of photons? How does the trapped thermal energy affect the process of star formation?

9) What is a protostar? How does it form? Why does its mass increase with time?

10) What is a protostellar disk? Describe how such a disk enables additional matter to accrete onto the protostar.

11) What are jets? Why do we think they are related to a protostar's rotation? How do they affect the cloud surrounding the protostar?

12) Describe the final stages a protostar goes through before fusion begins in its core. How are these stages represented on a life track?

13) What is degeneracy pressure, and how does it differ from thermal pressure? Explain why degeneracy pressure can support a stellar core against gravity even when the core becomes very cold.

14) What is the minimum mass for a star, and why can't objects with lower masses be true stars?

What is a brown dwarf?

15) What is the maximum mass of a star? What kind of pressure limits how massive a star can be?

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16) How do the numbers of low-mass stars compare with those of higher-mass stars in new star clusters?

Does It Make Sense?

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

Example: I walked east from our base camp at the North Pole.

Solution: The statement does not make sense because east has no meaning at the North Pole–all directions are south from the North Pole.

17) If you want to get a more accurate count of the number of stars in our galaxy, use an infrared telescope to observe them instead of a visible-light telescope.

18) A molecular cloud needs to trap all the energy released by gravitational contraction in order for its center to become hot enough for fusion.

19) Low-mass stars form more easily in clouds that are unusually cold and dense.

20) The current mass of any star is the same as the mass it had when it first became a protostar.

21) The rotation of a protostar always speeds up with time because a protostar is surrounded by a spinning disk.

22) Some of the stars in a star cluster live their entire lives and then die off before many of the cluster's stars initiate fusion.

23) Protostars are generally best observed in ultraviolet light because their surfaces have to get very hot before fusion can begin.

24) Degeneracy pressure exists only in objects that are very cold.

25) If Jupiter were 10 times as massive, we would consider it a brown dwarf, and if it were 100 times as massive, it would be a star.

26) Most of the stars that formed from the same cloud as the Sun have already died off.

Quick Quiz

27) Which of these colors of light passes most easily through interstellar clouds?

A) red light B) green light C) blue light Answer: No Correct Answer Was Provided.

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28) Molecular clouds stay cool because their molecules emit photons. Which of these molecules produces the largest number of photons in a molecular cloud?

A) molecular hydrogen (H2) B) carbon monoxide (CO) C) water (H2O) Answer: No Correct Answer Was Provided.

29) What happens to a cloud's thermal pressure if its temperature falls while its density rises?

A) Thermal pressure goes up.

B) Thermal pressure goes down.

C) More information is needed to determine what thermal pressure does.

Answer: No Correct Answer Was Provided.

30) What happens within a contracting cloud in which gravity is stronger than pressure and temperature remains constant?

A) It breaks into smaller fragments.

B) Thermal pressure starts to push back more effectively against gravity.

C) It traps all the energy released by gravitational contraction.

Answer: No Correct Answer Was Provided.

31) Why are the very first stars thought to have been much more massive than the Sun? A) The clouds that made them were much more massive than today's star-forming clouds.

B) The temperatures of the clouds that made them were higher because the clouds consisted entirely of hydrogen and helium.

C) Star-forming clouds were much denser early in time.

Answer: No Correct Answer Was Provided.

32) What slows down the contraction of a star-forming cloud when it makes a protostar?

A) production of fusion energy B) magnetic fields C) trapping of thermal energy inside the protostar Answer: No Correct Answer Was Provided.

33) What effects are jets and magnetic fields thought to have on a protostar?

A) They carry away angular momentum, helping the protostar grow in mass.

B) They carry away thermal energy, helping the protostar cool.

C) They transfer gravitational energy to the core, causing fusion to start.

Answer: No Correct Answer Was Provided.

34) Which kind of pressure prevents the formation of stars of extremely large mass?

A) thermal pressure B) radiation pressure C) degeneracy pressure Answer: No Correct Answer Was Provided.

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35) Which kinds of stars are most common in a newly formed star cluster?

A) O stars B) G stars C) M stars Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

36) Group The world'sDiscussion: premier observatory ALMA and International Collaboration. for studying star birth is the Atacama Large Millimeter/submillimeter Array (ALMA), located on a high plateau in the Atacama Desert of Chile(see the photo on page 165). Science agencies from many nations collaborated to build ALMA and shared the $1) 4 billion cost. a) There is great demand among astronomers for observing time at ALMA, which can typically execute only about 10% of the observations that astronomers propose for a given year. Discuss whether access to ALMA observing time should be granted to participating countries in proportion to the amount of money they have invested in the project or whether proposals from astronomers in every nation should be treated equally, with acceptance based only on the quality of the proposed investigation. Make a list of the pros and cons of these differing approaches. b) Astronomers from Chile are given 10% of all ALMA observing time, thanks to an agreement negotiated before ALMA was built. This agreement in essence treats Chile's abundance of outstanding sites for astronomical research as a natural resource that entitles the country to compensation, which in this case takes the form of efforts to help it broaden its inclusion in astronomical research. Do you think this policy makes sense? Does it benefit the astronomical community as a whole? Why or why not? c) What general obligations do you think astronomical funding agencies have to local people with ancestral ties to the land on which telescopes are built?

The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

37) Judging an Incomplete Model.

The general picture of star formation presented in this chapter is widely accepted, but even the most detailed models for star formation cannot yetall explain of the observations made by astronomers. For example, they do not explain why some molecular clouds are many times larger than the minimum mass for star formation, nor do they explain the distribution of stellar birth masses. Discuss the observational evidence that supports the general picture of star formation. Then express your level of confidence in this basic view. Do you find it convincing? Why or why not?

38) Predicting the Properties of Brown Dwarfs. Models of star formation predict that objects less massive than 0.08MSun become brown dwarfs instead of true hydrogen-fusing stars. Once they have formed, these objects must cool because fusion cannot replace the thermal energy lost from their surfaces. How would you then expect the properties of brown dwarfs in an older star cluster to compare with those of brown dwarfs in a younger one? Propose an observing program that could test your hypothesis.

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39) Life in a Molecular Cloud? As far as we know, molecular clouds are the only places other than planets that contain the kinds of complex molecules needed to support life, including water molecules and many more complex carbon compounds. Do you think it is possible for life to exist in a molecular cloud? What would life have to be like to survive there?

40) A Star Is "Born." Our discussion of star formation in this chapter talks about star "birth," even though stars are not really living things like humans, plants, or animals. In what sense is star birth like the birth of a living being? How is it different? Do you think it is appropriate to use the word birth in connection with star formation?

41) Group Activity: Conservation Laws and Star Formation. Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39. a) The amount of thermal energy inside a protostar increases with time, even though the protostar is losing radiative energy from its surface. How can we tell how much radiative energy the protostar is losing and how much thermal energy remains in the star? Which type of energy can we measure and which type do we infer from the law of conservation of energy? b) A star-forming cloud flattens into a disk that spins faster as the cloud contracts because of conservation of angular momentum. Then material from the disk gradually spirals inward toward the newly forming star at the center, accreting onto its surface and adding to its mass. What would we find if we could measure the orbital speeds of gas particles in the accretion disk?

Suppose there were no friction in the disk orbiting the protostar; would this affect the orbital speeds? Overall, how would a lack of friction change the process?

Investigate Further

42) Interstellar Dust. Describe how our view of the night sky would be different if there were no dust grains in the interstellar medium.

43) Pressure vs. Gravity. Suppose pressure and gravity are perfectly balanced within a certain molecular cloud. Describe what would happen to that balance if the temperature suddenly dropped. What would happen if the temperature suddenly rose? What would happen if the density suddenly increased without a change in temperature? What would happen if the cloud gained a little bit of mass?

44) Protostars. Describe the life story of a protostar from its beginning as part of a molecular cloud to the moment hydrogen fusion begins. Give as many details as possible. How would that life story be different if the protostar formed in a cloud without any angular momentum?

45) Approach to the Main Sequence. As a protostar approaches the main sequence, its radius shrinks because it is converting gravitational potential energy into thermal energy to replace the thermal energy it is radiating away. Figure 16.16 shows the stages that a star like the Sun goes through before it becomes a main-sequence star. How do you think the time the star spends as a 10LSun protostar compares with the time it spends as a 10LSun protostar? Explain your reasoning.

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46) Understanding Life Tracks. Compare the life tracks of pre-main-sequence stars in Figure 16.17. Which star's luminosity changes the least as the star approaches the main sequence?

Which star's luminosity changes the most? Which star's surface temperature changes the most?

Which star's surface temperature changes the least?

47) Degeneracy Pressure. Describe how Jupiter would be different if there were no such thing as degeneracy pressure.

48) Brown Dwarfs. How are brown dwarfs like jovian planets? In what ways are brown dwarfs like stars?

49) A Newborn Star Cluster. Sketch the development of a newborn star cluster on a series of H-R diagrams. Assume that all the stars are initially protostars with a surface temperature that places them on the extreme right-hand side of the H-R diagram in Figure 16.17. What would the H-R diagram of this star cluster look like after 100,000 years? One million years? Ten million years?

One hundred million years?

50) Light from a Newborn Star Cluster. Suppose a new star cluster is born with one O star, 10 A stars, 100 G stars, and 1000 M stars. Which stellar type dominates the light output from the cluster? What would the color of this star cluster appear to be if you observed it from a distance so great that you could not make out the individual stars?

51) Research: Molecules in Space. More than a hundred different kinds of molecules have been identified in molecular clouds, including some of the molecules necessary for life. Find a list of molecules that have been discovered in space. Choose one that is necessary for life and write a short summary of how it is important to life, how it was discovered in space, and where it has been found in space.

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

52) Water in Molecular Clouds.The water molecules now in your body were once part of a molecular cloud. Only about one-millionth of the mass of a molecular cloud is in the form of water molecules, and the mass density of such a cloud is roughly 10-21 g/cm3. Estimate the volume of a piece of molecular cloud that has the same amount of water as your body. How does this volume compare with the volume of the entire Earth?

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53) Interstellar Dust Grains. Dark interstellar gas clouds contain so many dust grains that starlight cannot pass through, even though the dust grains are tiny and the spaces between them are quite large by earthly standards. A typical dust grain has a radius of about 10-7 meter and a mass of about 10-14 gram. a) Estimate how many dust particles there are in a cloud containing 1000MSun of dusty gas if 1% of the cloud's mass is in the form of dust grains. b) Estimate the total surface area these grains would cover if you could put them side by side.

You can assume that the grains are approximately spherical so each grain covers an area πr2, where r is the grain's radius. State your answer in square light-years. c) Estimate the total surface area the cloud covers, assuming that its matter density is like that of a typical molecular cloud, about 10-21 g/cm3. [Hint: First calculate the cloud's volume from its mass and density. Then determine the cloud's radius using the formula for the radius of a sphere, R = (3 × volume / 4π[)1/3.] State your answer in square light-years. d) Based on your answers to parts b and c, what do you think the chances are that a photon passing through the cloud will hit a dust grain?

54) An Isolated Star-Forming Cloud. Isolated molecular clouds can have a temperature as low as 10 K and a particle density as great as 100,000 particles per cubic centimeter. What minimum mass does such a cloud need to form a star?

55) Masses of the First Stars. Models of the first star-forming clouds indicate that they had a temperature of roughly 200 K and a particle density of roughly 300,000 particles per cubic centimeter at the time they started trapping their internal thermal energy. Estimate the mass at which thermal pressure balances gravity for these values of pressure and temperature. How does that mass compare with the Sun's mass? What is the estimated lifetime of a star with that mass?

56) Internal Temperature of the Sun. The Sun is essentially a gas cloud in which the forces of pressure and gravity balance each other. We can therefore use the equation in Mathematical Insight 16.1 to estimate the interior temperature of the Sun from its mass and particle density. a) What is the average number density of particles within the Sun, given that the average mass per particle is about 10-24 gram? (Hint: The volume of a sphere of radius r is equal to 4πr3 / 3.) b) What is the approximate temperature necessary for gas pressure to balance gravity within the Sun, given the average particle density from part a? c) How does your estimate compare with the internal core temperature of the Sun?

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57) Internal Temperature of a Brown Dwarf. The maximum temperature inside a brown dwarf is the temperature at which thermal pressure would balance gravity. In reality, degeneracy pressure halts the contraction of a brown dwarf before it can reach that maximum value. You can use the formula in Mathematical Insight 16.1 to estimate the maximum interior temperature of a brown dwarf with a mass of 0.05MSun and a radius of 0.1RSun. a) What is the average number density of particles inside the brown dwarf, given that the average mass per particle is about 10-24 gram? (Hint: The volume of a sphere of radius r is equal to 4πr3 / 3.) b) What is the approximate temperature necessary for gas pressure to balance gravity within the brown dwarf, given the average particle density from part a? c) How does that temperature compare with the 1 x 107 K needed to sustain hydrogen fusion? d) Explain how conditions inside the brown dwarf would change if you raised its mass to 0.1MSun. 58) Angular Momentum of a Close Binary. Some close binary star systems have orbital periods as short as several days. Here we will estimate the angular momentum of such a system with two 1MSun stars in a circular orbit having an orbital period of 10 days. a) Determine the average orbital separation of this system using Newton's version of Kepler's third law. b) Determine the average speed of each star with respect to the center of mass of the system. c) The center of mass of this system lies at the midpoint between the two stars because their masses are equal. The distance of each star from the center of mass is therefore equal to half the orbital separation. What is the angular momentum of each star with respect to the center of mass? d) Suppose the Sun had the same angular momentum as these two stars combined. Estimate the speed at which the Sun's surface would be moving around the Sun's rotation axis. e) How does the speed from part d compare with the escape velocity from the Sun's surface?

What would happen if the Sun were spinning this fast?

59) Mass of a Brown Dwarf. Suppose you observe a binary system containing a main-sequence star and a brown dwarf. The orbital period of the system is 1 year, and the average separation of the system is 1 AU. You then measure the Doppler shifts of the spectral lines from the mainsequence star and the brown dwarf, finding that the orbital speed of the brown dwarf in the system is 20 times that of the main-sequence star. How massive is the brown dwarf?

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter 17 Star Stuff Section 17.1 1) What do astronomers mean when they say that we are all "star stuff"?

A) Life would be impossible without energy from the Sun. B) Earth formed at the same time as the Sun. C) Many of the elements from which we and Earth were made were created by stars.

D) The Sun formed from the interstellar medium: the "stuff" between the stars.

E) The universe contains billions of stars.

Answer: C 2) In order to remain stable, a star must have an internal balance between A) gravity and pressure.

B) gravity and temperature.

C) temperature and pressure.

D) pressure and nuclear fusion.

E) nuclear fusion and chemical reactions.

Answer: A 3) Which of the following statements correct describes overall energy balance in a star?

A) Gravity balances pressure.

B) Energy generated by fusion in the core balances the energy emitted from the star's surface.

C) Temperature balances light energy emitted at the photosphere.

D) Energy in sunspots balances energy in magnetic fields.

Answer: B 4) Why are main-sequence lifetimes shorter for more massive stars?

A) They aren't; higher mass stars have longer lifetimes.

B) Strong stellar winds cause higher mass stars to lose mass quickly.

C) Higher core temperatures allow fusion to proceed much more rapidly.

D) More massive stars are made of heavier elements that don't fuse as easily.

Answer: C 5) When discussing stellar lives, astronomers divide stars by mass into low-mass, intermediatemass, and high-mass stars. Which category includes our Sun? A) The Sun is a low-mass star.

B) The Sun is an intermediate-mass star.

C) The Sun is a high-mass star.

Answer: A

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6) When discussing stellar lives, astronomers divide stars by mass into low-mass, intermediate- mass, and high-mass stars. Which category has the longest lifetimes? A) low-mass stars B) intermediate-mass stars C) high-mass stars Answer: A 7) When discussing stellar lives, astronomers divide stars by mass into low-mass, intermediatemass, and high-mass stars. Which category includes a 25 MSun star?

A) low-mass stars B) intermediate-mass stars C) high-mass stars Answer: C Section 17.2 1) What can we learn about a star from a life track on an H-R diagram?

A) how long ago it was born B) when it will die C) where it is located D) what surface temperature and luminosity it will have at each stage of its life E) all of the above Answer: D 2) About what percentage of a star's total lifetime is spent on the main sequence?

A) 10% B) 20% C) 50% D) 90% E) 100% Answer: D 3) What change slowly occurs during the main-sequence lifetime of a star?

A) the solar wind blows material into space, the decreasing mass reduces pressure in theAs core,

which in turn reduces the fusion rate and the luminosity.

B) As hydrogen is used up in the core, the fusion rate decreases and reduces the luminosity.

C) It gathers more gas from interstellar space, increasing its mass and hence the

luminosity.

D) Its core temperature slowly increases, increasing the fusion rate and hence the luminosity.

Answer: D

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4) Why does a star grow larger in radius immediately after it exhausts its core hydrogen?

A) The outer layers of the star are no longer gravitationally attracted to the core.

B) Hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.

C) Helium fusion in the core generates enough thermal pressure to push the upper layers outward.

D) Helium fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.

Answer: B 5) What happens to a star after it exhausts its core hydrogen?

A) Its core contracts but its outer layers expand.

B) Its core expands but its outer layers contract.

C) The entire star (both core and outer layers) contracts.

D) The entire star (both core and outer layers) expands.

Answer: A 6) What is happening deep inside a star while it expands into a subgiant?

A) It is fusing hydrogen to produce helium in the core.

B) It is fusing hydrogen to produce helium in a shell around the core.

C) It is fusing helium to produce carbon in the core.

D) It is fusing helium to produce carbon in a shell around the core.

E) It is not fusing any element; it is contracting and heating up.

Answer: B 7) How does a red giant compare to a main-sequence star of the same mass?

A) The red giant has higher surface temperature and higher luminosity.

B) The red giant has higher luminosity but lower surface temperature.

C) The red giant has lower surface temperature and lower luminosity.

D) The red giant lower luminosity but higher surface temperature.

E) Both stars have the same mass and therefore have the same surface temperature and luminosity.

Answer: B 8) Which of the following is required for helium fusion as compared to hydrogen fusion?

A) much higher temperature B) much higher density C) a star of much greater mass D) a star that is much older in age Answer: A

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9) How many helium nuclei fuse to make one carbon nucleus in the overall helium fusion reaction? A) 2 B) 3 C) 4 D) varies depending on the type of carbon being made Answer: B 10) What is created by the fusion of three helium nuclei? A) hydrogen B) oxygen C) carbon D) nitrogen E) iron Answer: C 11) What happens after the helium flash in the core of a star?

A) The core quickly heats up and expands as helium fusion begins all at once throughout the core. B) The star breaks apart in a violent explosion. C) The core suddenly contracts because the helium disappears after being fused. D) The core stops fusing helium. E) The star starts to fuse helium in a shell outside the core. Answer: A 12) What will happen in the Sun immediately after it has exhausted its supply of hydrogen in its core? A) The helium core will shrink, heat, and start helium fusion reactions to produce carbon. B) With the core hydrogen used up, the Sun will no longer be able to generate energy in any way. C) The helium core will shrink and heat while hydrogen fusion begins in a shell around the core. D) The core will collapse, resulting in a supernova. Answer: C 13) The star Arcturus is a red giant that is easily visible in the night sky. Based on models of stellar evolution, what is happening to Arcturus as this stage of its life? A) It is increasing in size, and hydrogen fusion reactions are occurring in a shell around the core. B) It is stable in its size, and hydrogen fusion reactions are occurring in its core. C) It is decreasing in size, and hydrogen fusion reactions are occurring in its core. D) It is increasing in size, and hydrogen fusion reactions are occurring in its core. E) It is decreasing in size, and hydrogen fusion reactions are occurring in a shell around the core. F) It is stable in its size, and hydrogen fusion reactions are occurring in a shell around the core. Answer: A

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14) What happens to the core of a star after a planetary nebula occurs?

A) It contracts from a protostar to a main-sequence star.

B) It breaks apart in a violent explosion.

C) It becomes a white dwarf.

D) It becomes a neutron star.

E) It can become either a neutron star or a black hole.

Answer: C 15) How does a white dwarf compare to a main-sequence star of the same mass?

A) The white dwarf has higher surface temperature and higher luminosity.

B) The white dwarf has higher luminosity but lower surface temperature.

C) The white dwarf has lower surface temperature and lower luminosity.

D) The white dwarf has lower luminosity but higher surface temperature.

E) The white dwarf and main-sequence star have the same mass and therefore have the same surface temperature and luminosity.

Answer: D 16) Most of the carbon in your body was produced in A) the Big Bang.

B) interstellar gas in the space between the stars.

C) red giant stars.

D) supernova explosions.

E) the solar nebula.

Answer: C

17) Refer to the H-R diagram above that shows the life track of a 1-solar-mass star, with various stages labeled with Roman numerals. After all its life stages are completed, the remaining core of this star will be left behind as A) a white dwarf made primarily of carbon and oxygen.

B) a white dwarf made primarily of hydrogen.

C) a neutron star.

D) a black hole.

E) a supernova.

Answer: A 5 Copyright © 2020 Pearson Education, Inc.

18) Refer to the H-R diagram above that shows the life track of a 1-solar-mass star, with various stages labeled with Roman numerals. A star at the point labelled iii is A) a protostar.

B) on the main sequence.

C) a red giant.

D) experiencing shell fusion.

Answer: B 19) How will the Sun end its life?

A) as a white dwarf B) as a white dwarf supernova C) as a massive star supernova D) as a neutron star or black hole Answer: A 20) Which of the following sequences correctly describes the stages of life (from beginning to end) for a low-mass star?

A) red giant, protostar, main-sequence, white dwarf B) protostar, main-sequence, red giant, white dwarf C) white dwarf, main-sequence, red giant, protostar D) protostar, main-sequence, white dwarf, red giant E) protostar, red giant, main-sequence, white dwarf Answer: B Section 17.3 1) High-mass stars fuse hydrogen into helium through the CNO cycle rather than solely by the proton-proton chain. Which property of the CNO cycle explains why high mass stars have much shorter lives than low mass stars?

A) The CNO cycle is only possible when the abundance of CNO elements is very high.

B) The CNO cycle is very fast at converting hydrogen to helium.

C) The CNO cycle is very fast at converting hydrogen to carbon, nitrogen, oxygen (CNO).

D) The CNO cycle is only possible when the star has lived for a very long time.

Answer: B 2) Why don't low-mass stars have the CNO cycle occurring in their cores?

A) The CNO cycle makes elements heavier than carbon, nitrogen, and oxygen.

B) They don't have enough carbon, nitrogen, and oxygen.

C) Their core temperatures are too low.

Answer: C

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3) Which element has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission? A) hydrogen B) oxygen C) silicon D) iron E) uranium Answer: D 4) Which element is the last to be produced by fusion in the cores of the most massive stars (just before their lives end in supernova explosions)? A) iron B) lead C) oxygen D) uranium E) hydrogen Answer: A

5) Refer to the figure above showing the abundance of elements in the galaxy relative to hydrogen. What is the least abundant element with an atomic number less than iron's?

A) hydrogen B) lithium C) beryllium D) nitrogen Answer: C 7 Copyright © 2020 Pearson Education, Inc.

6) Refer to the figure above showing the abundance of elements in the galaxy relative to hydrogen. Based on our understanding of stellar lives, why does iron have a higher abundance than elements similar in atomic number?

A) It is the end product of core fusion in massive stars.

B) It has a higher density than other elements.

C) It does not; this apparent higher abundance is due to iron's many spectral lines making it easier to find.

D) The Big Bang created larger amounts of iron than of other elements with similar atomic number.

Answer: A 7) Refer to the figure above showing the abundance of elements in the galaxy relative to hydrogen. What is the general trend in elemental abundance?

A) Elements with more protons are less abundant.

B) Elements with more protons are more abundant.

C) Elements heavier than iron do not exist.

D) Elements heavier than iron are relatively abundant.

E) There is no apparent trend in the abundance of elements.

Answer: A 8) Refer to the figure above showing the abundance of elements in the galaxy relative to hydrogen. Why are elements with even numbers of protons more abundant, on average, than elements with odd numbers of protons?

A) because elements are mainly made in reactions with helium nuclei B) Because when elements split in a fission reaction, they prefer to split with all the protons paired.

C) There's no explanation, it's something of a mystery to be explained, or it may be a coincidence.

D) because elements are mainly made in reactions with hydrogen nuclei Answer: A 9) What happens to the core of a star if gravity is strong enough to overcome neutron degeneracy pressure?

A) The core contracts and becomes a white dwarf.

B) The core contracts and becomes a ball of neutrons.

C) The core contracts and becomes a black hole.

D) The star explodes violently, leaving nothing behind.

E) Gravity is not able to overcome neutron degeneracy pressure.

Answer: C

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10) What types of stars end their lives with supernovae?

A) all stars that are red in color B) all stars that are yellow in color C) high-mass stars that can continue to have core fusion until they have iron cores D) stars that are similar in mass to the Sun E) stars that have reached an age of 10 billion years Answer: C 11) After a high-mass star explodes in a supernova, what is left behind?

A) always a white dwarf B) always a neutron star C) always a black hole D) almost always either a white dwarf or a neutron star E) almost always either a neutron star or a black hole Answer: E Section 17.4 1) You discover a binary star system in which one member is a 15MSun main-sequence star and the other star is a 10MSun giant. Why should you be surprised, at least at first?

A) It doesn't make sense to find a giant in a binary star system.

B) The odds of ever finding two such massive stars in the same binary system are so small as to make it inconceivable that such a system could be discovered.

C) The two stars in a binary system should both be at the same point in stellar evolution; that is, they should either both be main-sequence stars or both be giants.

D) The two stars should be the same age, so the more massive one should have become a giant first.

E) A star with a mass of 15MSun is too big to be a main-sequence star.

Answer: D 2) How can the stars in a close binary system dramatically affect each other?

A) The close orbits can allow tidal forces to rip one or both stars apart.

B) Transfer of mass from one star to another can change how both stars evolve.

C) Intense radiation from one star can heat the surface of the other star, causing it to undergo a supernova explosion.

Answer: B General Chapter Questions 1) When discussing stellar lives, astronomers divide stars by mass into low-mass stars (masses similar to the Sun or lower) and high-mass stars (masses more than about 8 times that of our Sun). Which category includes stars that die in supernova explosions?

A) high-mass stars only B) low-mass stars only C) They both die in supernova explosions.

D) Neither: Supernovas only occur in binary star systems.

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2) During which of the following phases of life is a star's balance of pressure and gravity out of equilibrium?

A) when it is on the main sequence B) when it is undergoing helium core fusion C) when it is a white dwarf D) when it is expanding as a red giant E) A star is never out of equilibrium.

Answer: D 3) What kind of star is destined to die by supernova explosion?

A) a brown dwarf B) a red giant C) a low mass star D) a high mass star Answer: D

4) Refer to the sketch above of an H-R diagram for a star cluster. What is the age of the cluster?

A) Less than 1 billion years B) About 1 billion years C) About 2 billion years D) About 10 billion years E) More than 15 billion years Answer: D 5) Refer to the sketch above of an H-R diagram for a star cluster. Which statement about this cluster is true?

A) It is probably located in the disk of the galaxy.

B) It contains some stars that will soon undergo supernova explosions.

C) It is probably located in a globular cluster.

D) It is probably embedded in a molecular cloud in which new stars are being born.

Answer: C

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6) Refer to the sketch above of an H-R diagram for a star cluster. Consider the star to which the arrow points. How is it currently generating energy?

A) by gravitational contraction B) by hydrogen shell burning around an inert helium core C) by core hydrogen fusion D) by core helium fusion combined with hydrogen shell burning E) by both hydrogen and helium shell burning around an inert carbon core Answer: B 7) Refer to the sketch above of an H-R diagram for a star cluster. Consider the star to which the arrow points. Which of the following statements about this star is true?

A) It is somewhat more massive than the Sun. B) It is smaller in radius than the Sun. C) It has a lower luminosity than the Sun. D) Its surface temperature is higher than the Sun's.

E) Its core temperature is lower than the Sun's.

Answer: A Short Answer Questions 1) Briefly summarize the stages of life for a low-mass star.

Answer: [Note: For a "brief" summary, students need recognize only the key stages; additional details are included here for reference.] The protostar assembles from the molecular clouds, heats up from gravitational contraction, and begins hydrogen fusion in the core. The star settles onto the main sequence, where it will fuse hydrogen in its core for 10 billion years. When the core hydrogen is used up, the core contracts until it is degenerate, hydrogen fusion continues in a shell outside the core, and the outer layers expand and cool the star becomes a red giant. Helium fusion begins in the core, but since the core is degenerate, a helium flash takes place and rapidly spreads throughout the core. Helium fusion stabilizes, and the star moves left on the H-R diagram. Core helium is used up and helium begins fusing in a shell outside the core, with hydrogen still fusing in a shell above it. The outer layers expand, and the star again becomes a red giant. The star undergoes thermal pulses and loses its outer layers through a stellar wind. The core shrinks and heats up but is not able to fuse any more elements. The star becomes a planetary nebula as heat from the core blows away and heats up the gas left over from the red giant phase.

Only the naked degenerate core is left, a white dwarf.

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2) Briefly summarize the stages of life for a high-mass star.

Answer: [Note: For a "brief" summary, students need recognize only the key stages; additional details are included here for reference.] The first stages are similar to those of a low-mass star, except that they happen over much shorter time periods. While on the main sequence, the star fuses hydrogen by the CNO cycle and remains at this stage only for several million years. In addition to helium fusion, high-mass stars also undergo alpha capture, which creates heavier elements by fusing a helium nucleus with an existing atom. After helium is used up in the core, the core contracts while helium and hydrogen fusion continue in outer shells. The core contracts until carbon ignition occurs, and the star moves left again on the H-R diagram while carbon fusion occurs in the core. The process continues for stars of still higher mass, zigzagging across the H-R diagram as heavier elements are fused in the core and used up as fuel. Each fusion stage requires less time until iron is finally produced in the core. Iron cannot be fused to produce energy, so the core collapses and pressures increase so that electrons and protons are converted to neutrons. A high quantity of neutrinos is released, which may help force the outer layers violently outward in an explosion called a supernova. Elements heavier than iron are created, the outer layers move away from the core at great velocities, and only a neutron star or black hole is left as a remnant.

3) Briefly explain why high-mass stars have shorter lifetimes than low-mass stars.

Answer: [Note: For a "brief" summary, you may consider it sufficient for students to state that high-mass stars have much higher luminosities, which more than make up for their additional fuel.] High-mass stars have much more fuel for fusion, but they use it at a much greater rate, primarily because their hydrogen fusion proceeds through the CNO cycle rather than the protonproton chain.

Reading Quiz Questions 1) Which of the following stars will live longest?

A) 1 solar-mass star B) 2 solar-mass star C) 3 solar-mass star D) 4 solar-mass star Answer: A 2) In the context of understanding stellar lives, "high-mass" stars have masses A) more than about 8 times the mass of our Sun. B) more than about 3 times the mass of our Sun. C) the same as our Sun. D) more than about 100 times the mass of our Sun. Answer: A 3) Which of the following lists the stages of life for a low-mass star in the correct order?

A) protostar, main-sequence star, red giant, planetary nebula, white dwarf B) protostar, main-sequence star, red giant, supernova, neutron star C) protostar, main-sequence star, planetary nebula, red giant D) main-sequence star, white dwarf, red giant, planetary nebula, protostar Answer: A

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4) What happens when a main-sequence star exhausts its core hydrogen fuel supply?

A) The entire star shrinks in size.

B) The core shrinks while the rest of the star expands.

C) The core immediately begins to fuse its helium into carbon.

D) The star becomes a neutron star.

Answer: B 5) The primary source of energy for a star as it grows in size to become a red giant is A) hydrogen fusion in the central core.

B) helium fusion in the central core.

C) hydrogen fusion in a shell surrounding the central core.

D) gravitational contraction.

Answer: C 6) The overall helium fusion reaction is A) three helium nuclei fusing to form one carbon nucleus.

B) two helium nuclei fusing to form one beryllium nucleus.

C) two hydrogen nuclei fusing to form one helium nucleus.

D) four helium nuclei fusing to form one oxygen nucleus.

Answer: A 7) What is a helium flash?

A) It is the sudden onset of helium fusion in the core of a low-mass star.

B) It is the ignition of helium shell fusion in a high-mass star with a carbon core.

C) It is a sudden brightening of a low-mass star, detectable from Earth by observing spectral lines of helium.

D) It is another name for the helium fusion reaction.

Answer: A 8) An H-R diagram for a globular cluster will show a horizontal branch, which is a line of stars above the main-sequence but to the left of the subgiants and red giants. Which of the following statements about these horizontal branch stars is true?

A) They have inert (nonfusing) carbon cores.

B) Their sole source of energy is hydrogen shell fusion.

C) They generate energy through both hydrogen fusion and helium fusion.

D) In a particular star cluster, all horizontal branch stars have the same spectral type.

Answer: C 9) What is a planetary nebula?

A) gas ejected from a low-mass star in the final stage of its life B) gas created from the remains of planets that once orbited a dead star C) interstellar gas from which planets are likely to form in the not-too-distant future D) the remains of a high-mass star that has exploded Answer: A

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10) The ultimate fate of our Sun is to A) become a white dwarf that will slowly cool with time.

B) explode in a supernova.

C) become a rapidly spinning neutron star.

D) become a black hole.

Answer: A 11) Assume the following stars all have the same mass as our Sun. Which one does not have fusion occurring in its central core?

A) a red giant B) a main-sequence star C) a helium core-fusion star D) a main-sequence star in a binary star system Answer: A 12) How are low-mass red giant stars important to our existence?

A) These stars manufactured most of the carbon atoms in our bodies.

B) These stars manufactured virtually all the elements out of which we and our planet are made.

C) These stars generate the energy that makes life on Earth possible.

D) These stars provide most of the light that reaches us from globular clusters.

Answer: A 13) Which of the following pairs of atomic nuclei would feel the strongest repulsive electromagnetic force if you tried to push them together?

A) helium and helium B) hydrogen and hydrogen C) hydrogen and helium D) hydrogen and deuterium Answer: A 14) Which of the following stars will end its life in a supernova?

A) a 10 solar-mass star B) the Sun C) a 1 solar-mass red giant star D) a neutron star Answer: A 15) What is the CNO cycle?

A) a set of steps by which four hydrogen nuclei fuse into one helium nucleus B) the process by which helium is fused into carbon, nitrogen, and oxygen C) the process by which carbon is fused into nitrogen and oxygen D) the set of fusion reactions that have produced all the carbon, nitrogen, and oxygen in the universe Answer: A

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16) To predict whether a star will eventually fuse oxygen into a heavier element, what do you need to know about the star?

A) its mass B) its luminosity C) how much oxygen it now has in its core D) its overall abundance of elements heavier than helium Answer: A 17) Why is iron significant to understanding how a supernova occurs?

A) Iron cannot release energy either by fission or fusion.

B) Iron is the heaviest of all atomic nuclei, and thus no heavier elements can be made.

C) Supernovae often leave behind neutron stars, which are made mostly of iron.

D) The fusion of iron into uranium is the reaction that drives a supernova explosion.

Answer: A 18) After a supernova explosion, the remains of the stellar core can be A) either a neutron star or a black hole.

B) a neutron star only.

C) a black hole only.

D) a white dwarf, neutron star, or black hole.

Answer: A 19) Why is Supernova 1987A particularly important to astronomers?

A) It is the nearest supernova to have occurred at a time when we were capable of studying it carefully with telescopes.

B) It was the first supernova detected in nearly 400 years.

C) It provided the first evidence that supernovae really occur.

D) It occurred only a few light-years from Earth.

Answer: A 20) Algol consist of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant. Why does this seem surprising, at least at first?

A) The two stars should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star.

B) The two stars in a binary system should both be at the same stage of life; that is, they should either both be main-sequence stars or both be subgiants.

C) It doesn't make sense to find a subgiant in a binary star system.

D) A star with a mass of 3.7 MSun is too big to be a main-sequence star.

Answer: A

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Concept Quiz Questions 1) Our Sun is considered to be a A) low-mass star.

B) intermediate-mass star.

C) high-mass star.

D) brown dwarf.

Answer: A 2) Which of the following types of data provide evidence that helps us understand the life tracks of low-mass stars?

A) H-R diagrams of globular clusters B) H-R diagrams of open clusters C) observing a low-mass star over many years D) spacecraft observations of the Sun Answer: A 3) Why is a 1-solar-mass red giant more luminous than a 1-solar-mass main-sequence star?

A) Fusion reactions are producing energy at a greater rate in the red giant.

B) The red giant has a hotter core.

C) The red giant's surface is hotter.

D) The red giant is more massive.

Answer: A 4) Which of the following describes a star with a hydrogen-fusing shell and an inert (nonfusing) helium core?

A) It is a subgiant that grows in luminosity until helium fusion begins in the central core.

B) It is a red giant that grows in luminosity until it dies in a planetary nebula.

C) It is a subgiant that gradually grows dimmer as its hydrogen-fusing shell expands and cools.

D) It is what is known as a helium core-fusion star, which has both helium fusion in its core and hydrogen fusion in a shell.

Answer: A

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5) This diagram represents the life track of a 1 solar-mass star from its pre-main-sequence stages to just before its final death. Refer to the life stages labeled with roman numerals.

During which stage is the star's energy supplied primarily by gravitational contraction?

A) Stage ii B) Stage iii C) Stage v D) Stage vi E) Stage viii Answer: A 6) This diagram represents the life track of a 1 solar-mass star from its pre-main-sequence stages to just before its final death. Refer to the life stages labeled with roman numerals.

During which stage does the star have an inert (nonfusing) helium core?

A) Stage iii B) Stage iv C) Stage vi D) Stage vii E) Stage viii Answer: B 7) This diagram represents the life track of a 1 solar-mass star from its pre-main-sequence stages to just before its final death. Refer to the life stages labeled with roman numerals.

Which stage lasts the longest?

A) Stage i B) Stage iii C) Stage iv D) Stage viii Answer: B

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8) This diagram represents the life track of a 1 solar-mass star from its pre-mainsequence stages to just before its final death. Refer to the life stages labeled with roman numerals. During which stage does the star have an inert (nonfusing) carbon core surrounded by shells of helium and hydrogen fusion? A) Stage ii B) Stage iii C) Stage iv D) Stage vi E) Stage viii Answer: E 9) This diagram represents the life track of a 1 solar-mass star from its pre-main-sequence stages to just before its final death. Refer to the life stages labeled with roman numerals. What will happen to the star after stage viii? A) Its outer layers will be ejected as a planetary nebula and its core will become a white dwarf.

B) It will continue to expand gradually until carbon fusion begins in its core. C) It will explode as a supernova and leave a neutron star or black hole behind. D) It will remain in stage viii for about 10 billion years, after which its outer layers will shrink back and cool.

Answer: A 10) Carbon fusion occurs in high-mass stars but not in low-mass stars because A) the cores of low-mass stars never get hot enough for carbon fusion. B) the cores of lowmass stars never contain significant amounts of carbon. C) only high-mass stars do fusion by the CNO cycle. D) carbon fusion can occur only in the stars known as carbon stars.

Answer: A 11) Which of the following statements about various stages of core nuclear fusion (hydrogen, helium, carbon, and so on) in a high-mass star is not true? A) Each successive stage lasts for approximately the same amount of time. B) As each stage ends, the core shrinks and heats further. C) Each successive stage creates an element with a higher atomic number and atomic mass number. D) As each stage ends, the reactions that occurred in previous stages continue in shells around the core. Answer: A 12) Which event marks the beginning of a supernova? A) the sudden collapse of an iron core into a compact ball of neutrons B) the onset of helium fusion after a helium flash C) the beginning of neon fusion in an extremely massive star D) the sudden initiation of the CNO cycle Answer: A

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13) Suppose that the star Betelgeuse (the upper left shoulder of Orion) were to supernova tomorrow (as seen here on Earth). What would it look like to the naked eye?

A) Betelgeuse would remain a dot of light but would suddenly become so bright that for a few weeks we'd be able to see this dot in the daytime.

B) We'd see a cloud of gas expanding away from the position where Betelgeuse used to be. Over a period of a few weeks, this cloud would fill our entire sky.

C) Because the supernova destroys the star, Betelgeuse would suddenly disappear from view.

D) Betelgeuse would suddenly appear to grow larger in size, soon reaching the size of the full Moon. It would also be about as bright as the full Moon.

Answer: A 14) Suppose that hydrogen, rather than iron, had the lowest mass per nuclear particle. Which of the following would be true?

A) Nuclear fusion could not power stars.

B) Stars would be brighter.

C) Stars would be less massive.

D) All stars would be red giants.

Answer: A 15) Observations show that elements with atomic mass numbers divisible by 4 (such as oxygen16, neon-20, and magnesium-24) tend to be more abundant in the universe than elements with atomic mass numbers in between. Why do we think this is the case?

A) At the end of a high-mass star's life, it produces new elements through a series of helium capture reactions.

B) The apparent pattern is thought to be a random coincidence.

C) Elements with atomic mass numbers divisible by 4 tend to be more stable than elements in between.

D) This pattern in elemental abundances was apparently determined during the first few minutes after the Big Bang.

Answer: A 16) A spinning neutron star has been observed at the center of a A) supernova remnant.

B) planetary nebula.

C) red supergiant.

D) protostar.

Answer: A 17) Which of the following observations would not be likely to provide information about the final, explosive stages of a star's life?

A) decades of continuous monitoring of red giants in a globular cluster B) studying the light rings around Supernova 1987A in the Large Magellanic Cloud C) observing the structures of planetary nebulae D) neutrino detections from nearby supernovae Answer: A

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18) Which is more common: a star blows up as a supernova, or a star ejects a planetary nebula?

A) Planetary nebulas are more common.

B) Supernovae are more common.

C) They both occur in about equal numbers.

D) It is impossible to say.

Answer: A 19) You discover a binary star system in which one star is a 15 MSun main-sequence star and the other is a 10 MSun giant. How do we think that a star system such as this might have come to exist?

A) The giant must once have been the more massive star, but it is now less massive because it transferred some of its mass to its companion.

B) Although both stars probably formed from the same clump of gas, the more massive one must have had its birth slowed so that it became a main-sequence stars millions of years later than its less massive companion.

C) The two stars probably were once separate, but became a binary when a close encounter allowed their mutual gravity to pull them together.

D) The two stars are simply evolving normally and independently, and one has become a giant before the other.

Answer: A 20) Tidal forces are important to the Algol system today but were not important when both stars were still on the main sequence. Why not?

A) Main-sequence stars in a system like the Algol system are small compared to their physical separation.

B) Main-sequence stars are too big to be affected by tidal forces.

C) Main-sequence stars are too massive to be affected by tidal forces.

D) Main-sequence stars are unaffected by tidally induced mass transfer.

Answer: A Visual Quiz Questions 1) In this diagram, red balls represent protons and gray balls represent neutrons. What reaction is being shown?

A) fusion of helium into carbon B) fusion of hydrogen into helium C) fusion of carbon into oxygen D) the "iron catastrophe" that initiates a supernova Answer: A

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2) This is the H-R diagram for stars in a 12-billion-year-old globular cluster. Based on our understanding of stellar lives, which of the four labeled stars is currently generating energy through hydrogen fusion in a shell around an inert (nonfusing) helium core?

A) I B) II C) III D) IV Answer: C

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3) This H-R diagram shows the life track of a 1 MSun star from the time it first becomes a mainsequence star. Which numbered point represents the star when it has both hydrogen-fusing and helium-fusing shells around an inert carbon core?

A) Point 1 B) Point 2 C) Point 3 D) Point 4 Answer: D

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4) This Hubble Space Telescope photo shows a planetary nebula. What is the white dot in the center (indicated by the arrow)?

A) a white dwarf B) a red giant star with a strong stellar wind C) a neutron star D) a protostar that will soon become a main-sequence star Answer: A

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5) Suppose a particular star has a core that is undergoing several stages of fusion simultaneously, as shown in this diagram. Based on our understanding of stellar lives, the mass of this star is probably A) greater than 8 times the mass of the Sun. B) about 2 times the mass of the Sun. C) about the same mass as the Sun. D) less than 0.1 times the mass of the Sun. Answer: A 6) Suppose a particular star has a core that is undergoing several stages of fusion simultaneously, as shown in this diagram. Which of the following statements about this star is most likely to be true?

A) The star will explode as a supernova within a few days.

B) The star was born from a gas cloud with an unusually large proportion of elements heavier than hydrogen and helium.

C) The star is a red giant.

D) A million years from now, all that will remain of this star is a white dwarf.

Answer: A

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7) According to this diagram, how much more abundant is hydrogen in the universe than nitrogen?

A) Hydrogen is about 10,000 times as abundant as nitrogen.

B) Hydrogen is about 30% more abundant than nitrogen.

C) Hydrogen is about 10 times as abundant as nitrogen.

D) Hydrogen is about 4 times as abundant as nitrogen.

Answer: A 8) This photo shows the famous Crab Nebula. What is it?

A) an expanding cloud of remains from a star that died in a supernova B) a classic example of a planetary nebula C) a star that is right now undergoing a supernova explosion D) a star forming cloud that will eventually give birth to hundreds of stars Answer: A

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9) The arrow in the photo on the left points to the location of the star that has exploded as a supernova in the photo on the right. From this photo pair alone, what can we conclude about the star that exploded? (Assume that we know the star was not a member of a binary system.)

A) It was a high-mass star with at least 8 times the mass of the Sun. B) It is now a black hole.

C) It was a red giant star.

D) Before the supernova, the star lived for at least a billion years.

Answer: A 10) The arrow in the photo on the left points to the location of the star that has exploded as a supernova in the photo on the right. Suppose you had looked with your naked eye at the supernova on the night it was photographed in the "during/after" photo. Assuming that it was bright enough to see (it was), what would it have looked like to your naked eye?

A) like a single star in the night sky B) like a huge white blob, about the angular size of the full Moon but not quite as bright C) like a reddish cloud spread out over a region of the sky about half the angular size of the full moon D) like a pulsating bright light, dimming and brightening about every 5 minutes Answer: A

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End-of-Chapter Questions Visual Skills Check Use the following questions to check your understanding of some of the many types of visual information used in astronomy.

This figure, similar to the left side of Figure 17.8, shows the future life stages of the Sun on an H-R diagram. Answer the following questions, using the information provided in the figure.

1) What will the Sun's approximate luminosity be during the subgiant stage?

Answer: Approximately 10LSun 2) When the Sun is a red giant, what will its approximate surface temperature be?

Answer: Approximately 3500 K 3) Just before the Sun produces a planetary nebula, what will its approximate luminosity be?

Answer: Approximately 104 LSun

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4) When the Sun becomes a white dwarf with a surface temperature similar to its current surface temperature, what will its luminosity be?

Answer: Approximately 10-4 LSun Chapter Review Questions 1) Why is mass so important to a star's life? How and why do we divide stars into groups by mass?

2) What do all low-mass stars have in common? Why do they differ in their levels of surface activity? What are flare stars?

3) When a star exhausts its core hydrogen fuel, the core contracts but the star as a whole expands. Why? 4) What is the helium fusion reaction, and why does it require much higher temperatures than hydrogen fusion? Why will helium fusion in the Sun begin with a helium flash?

5) Why does the H-R diagram of a globular cluster show a horizontal branch? What are the characteristics of the stars on the horizontal branch?

6) What happens to a low-mass star after it exhausts its core helium? Why can't it fuse carbon into heavier elements?

7) How do red giants manufacture carbon-rich dust grains, and why are these important to life?

8) What is a planetary nebula? What happens to the core of a star after a planetary nebula occurs?

9) What will happen to Earth as the Sun changes in the future?

10) Summarize the stages of life that we see on the Sun's life track in Figure 17.8. Be sure to explain both the changes that occur in the Sun's core with each stage and the changes that are observable from outside the Sun. 11) In broad terms, explain how the life of a high-mass star differs from that of a low-mass star.

How do intermediate-mass stars fit into this picture?

12) Describe some of the nuclear reactions that can occur in high-mass stars after they exhaust their core helium. Why does this continued nuclear fusion occur in high-mass stars but not in low-mass stars?

13) Why can't iron be fused to release energy?

14) Summarize some of the observational evidence supporting our ideas about how heavy elements form in massive stars.

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15) What event initiates a supernova, and why is a neutron star or a black hole left behind? What observational evidence supports our understanding of supernovae?

16) Why can the lives of close binary stars differ from those of single stars? Describe the Algol paradox and its resolution.

Does It Make Sense?

Decide whether or not each of the following statements makes sense (or is clearly true or false).

Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

Example: I walked east from our base camp at the North Pole.

Solution: The statement does not make sense because east has no meaning at the North Pole–all directions are south from the North Pole.

17) The iron in my blood came from a star that blew up more than 4 billion years ago.

18) Humanity will eventually have to find another planet to live on, because one day the Sun will blow up as a supernova.

19) I sure am glad hydrogen has a higher mass per nuclear particle than many other elements. If it had the lowest mass per nuclear particle, none of us would be here.

20) I just discovered a 3.5MSun main-sequence star orbiting a 2.5MSun red giant. I'll bet that red giant was more massive than the 3.5MSun star when it was a main-sequence star.

21) If the Sun had been born 4-1/2 billion years ago as a high-mass star rather than as a low-mass star, Jupiter would have Earth-like conditions today, while Earth would be hot like Venus.

22) If you could look inside the Sun today, you'd find that its core contains a much higher proportion of helium and a lower proportion of hydrogen than it did when the Sun was born.

23) Most of the supernova explosions that occur in a star cluster happen during its first 100 million years.

24) Globular clusters generally contain lots of white dwarfs.

25) After hydrogen fusion stops in a low-mass star, its core cools off until the star becomes a red giant.

26) The oxygen in my lungs comes from supernova explosions.

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Quick Quiz 27) Which of these stars has the hottest core? A) a white main-sequence star B) an orange main-sequence star C) a red main-sequence star Answer: No Correct Answer Was Provided. 28) Which of these stars has the hottest core? A) a blue main-sequence star B) a red supergiant C) a red main-sequence star Answer: No Correct Answer Was Provided. 29) Which of these stars does not have fusion occurring in its core? A) a red giant B) a red main-sequence star C) a blue main-sequence star Answer: No Correct Answer Was Provided. 30) After the helium flash in a low-mass star, the star's luminosity A) goes up. B) goes down. C) stays the same. Answer: No Correct Answer Was Provided. 31) What would stars be like if hydrogen had the smallest mass per nuclear particle? A) Stars would be brighter. B) All stars would be red giants. C) Nuclear fusion would not occur in stars of any mass. Answer: No Correct Answer Was Provided. 32) What would stars be like if carbon had the smallest mass per nuclear particle? A) Supernovae would be more common. B) Supernovae would never occur. C) High-mass stars would be hotter. Answer: No Correct Answer Was Provided. 33) What would you be most likely to find if you returned to the solar system in 10 billion years? A) a neutron star B) a white dwarf C) a black hole Answer: No Correct Answer Was Provided.

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34) Which of these stars has the shortest life expectancy?

A) an isolated 1MSun star B) a 1MSun star in a close binary system with a 0.8MSun star C) a 1MSun star in a close binary system with a 2MSun star Answer: No Correct Answer Was Provided.

35) What happens to the core of a high-mass star after it runs out of hydrogen?

A) It shrinks and heats up.

B) It shrinks and cools down.

C) Helium fusion begins right away.

Answer: No Correct Answer Was Provided.

36) Which of these elements can't be made by nuclear fusion in the core of a star?

A) calcium B) uranium C) oxygen Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

37) Connections to the Stars.

In ancient times, many people believed that our lives were somehow influenced by the patterns of the stars in the sky. Modern science has not found any evidence to support this belief, but instead has found that we have a connection to the stars on a much deeper level: We are "star stuff." Discuss in some detail our real connections to the stars, as established by modern astronomy. Do you think these connections have any philosophical implications for how we view our lives and our civilization? Explain.

38) Group Discussion: Inclusion of Amateurs. Observations and careful record keeping by amateur observers of the sky have been enormously beneficial to professional researchers, and particularly to studies of variable and exploding stars. a) Section 17.3 discusses how historical supernova observations, particularly those made by the Chinese, have assisted modern research on supernova remnants. Should we consider those ancient observers to be amateurs or professionals? Is the distinction meaningful? Defend your opinion. b) The American Association of Variable Star Observers (AAVSO) is a large community that has made observations and kept records of stellar brightness variations, including supernova explosions, for more than a century. The vast majority of these observations were made by amateurs, not professionals. Do you think amateur observations can be trusted? Why or why not? c) Imagine you are a professional astronomer. How might you go about verifying that observations made by amateurs are sufficiently accurate to support professional research? d) What do you think of the term amateur in this context? What does it mean to you? Do you think it could be biasing your viewpoint on the work of amateur astronomers?

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The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

Predicting the Sun's Future.

39) Models of stellar evolution make detailed predictions about the fate of the Sun. Describe a piece of evidence that supports each of the following model predictions: a) The Sun cannot continue supplying Earth with light and heat forever. b) The Sun will become a red giant before the end of its life. c) The Sun will leave behind a white dwarf after it dies.

40) Blue Stragglers. Notice in Figure 17.6b the small number of stars that appear to be on the main sequence but above the cluster's main-sequence turnoff point. These stars are known as blue stragglers, because they are bluer and more massive than the stars we expect to find on the cluster's main sequence. a) Why is the presence of blue stragglers surprising? b) One possible explanation for blue stragglers is that there is something wrong with our theory of stellar evolution, but astronomers instead suspect that a blue straggler forms through the merger of two smaller stars. Why do astronomers prefer the merger explanation? Can you think of a way in which the merger explanation could be tested?

41) Group Activity: Comparing Models of Stars with Data. Work as a group to evaluate each of the following predictions. You will find it useful to study the star cluster data in Figures 15.18, 15.19, and 15.20. Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39. a) Type O stars have shorter lives than type G stars. b) Type K supergiant stars produce iron before they explode as supernovae. c) Type F stars become much more luminous near the ends of their lives than they were as main-sequence stars. d) Type O stars do not become more luminous near the ends of their lives but do become redder. e) Type M stars should have longer lives than type K stars. f) Stars similar to the Sun reach a maximum size of about 100 solar radii during the red giant stage. g) Type K main-sequence stars will become red giants when their cores run out of hydrogen. h) Some stars become white dwarfs at the ends of their lives. i) White dwarfs cool with time but do not change much in radius.

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Investigate Further 42) Homes to Civilization? We do not yet know how many stars have Earth-like planets, nor do we know the likelihood that such planets might harbor advanced civilizations like our own.

However, some stars can probably be ruled out as candidates for having planets that support advanced civilizations. For example, given that it took a few billion years for humans to evolve from the first life forms on Earth, it seems unlikely that advanced life would have had time to evolve on a planet around a star that is only a few million years old. For each of the following stars, decide whether you think it is possible that it could have a planet that harbors an advanced civilization. Explain your reasoning in one or two paragraphs. a) A 10MSun main-sequence star b) A 1.5MSun main-sequence star c) A 1.5MSun red giant d) A 1MSun helium core-fusion star e) A red supergiant f) A flare star 43) Rare Elements. Lithium, beryllium, and boron are elements with atomic numbers 3, 4, and 5, respectively. Despite their being three of the five simplest elements, Figure 17) 15 shows that they are rare compared to many heavier elements. Suggest a reason for their rarity. (Hint: Consider the process by which helium fuses into carbon.)

44) Future Skies. As a red giant, the Sun will have an angular size in Earth's sky of about 30°.

What will sunset and sunrise be like? About how long will they take? Do you think the color of the sky will be different from what it is today? Explain.

45) Research: Historical Supernovae. Historical accounts describe supernovae in the years 1006, 1054, 1572, and 1604. Choose one of these supernovae and learn more about historical records of the event. Did the supernova influence human history in any way? Write a two to three-page summary of your research findings.

46) Picturing Star Birth and Death. Photographs of stellar birthplaces (i.e., molecular clouds) and death places (e.g., planetary nebulae and supernova remnants) can be strikingly beautiful, but only a few such photographs are included in this chapter. Search the web for additional images. Look not only for photos taken in visible light, but also for those taken in other wavelengths. Put the photographs you find into a personal online journal, along with a one-paragraph description of what each photograph shows. Include at least 20 images.

47) Humanity in A.D. 5,000,000,000. Do you think it is likely that humanity will survive until the Sun begins to expand into a red giant 5 billion years from now? Why or why not?

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Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

48) Density of a Red Giant.Near the end of the Sun's life, its radius will extend nearly to the distance of Earth's orbit. Estimate the volume of the Sun at that time, using the formula for the volume of a sphereπr 4 3 / 3. Using that result, estimate the average matter density of the Sun at that time. How does that density compare with the density of water (1g/cm3)? How does it compare with the density of Earth's atmosphere at sea level (about 10-3 g/cm3)?

49) Escape Velocity from a Red Giant. What is the escape velocity from a red giant with a mass of 1MSun and a radius of 100RSun? How does that velocity compare with the escape velocity from the Sun? Describe how your results help account for the fact that red giants have strong stellar winds.

50) Roasting the Earth. During its final days as a red giant, the Sun will reach a peak luminosity of about 3000LSun. Earth will therefore absorb about 3000 times as much solar energy as it does now, and it will need to radiate 3000 times as much thermal energy to keep its surface temperature in balance. Estimate the temperature Earth's surface will need to attain in order to radiate that much thermal energy. You will need to use the formula for emitted power per unit area in Mathematical Insight 15.5.

51) Supernova Betelgeuse. The distance of the red supergiant Betelgeuse is approximately 600 light-years. If it were to explode as a supernova, it would be one of the brightest stars in the sky.

Right now, the brightest star other than the Sun is Sirius, with a luminosity of 26LSun and a distance of 8.6 light-years. How much brighter than Sirius would the Betelgeuse supernova be in our sky if it reached a maximum luminosity of 1010 LSun? 52) Construction of Elements. Using the periodic table in Appendix D, determine which elements are made by the following nuclear fusion reactions. (You can assume the total number of protons in the reaction remains constant.) a) fusion of a carbon nucleus with another carbon nucleus b) fusion of a carbon nucleus with a neon nucleus c) fusion of an iron nucleus with a helium nucleus 53) Expansion of the Crab Nebula. Observations of the Crab Nebula taken over several decades show that gas blobs that are now 100 arcseconds from the center of the nebula are moving away from the center by about 0.11 arcsecond per year. Use that information to estimate the year in which the explosion ought to have been observed. How does that year compare with the year in which the supernova that produced the nebula was actually observed?

54) Algol's Orbital Separation. The Algol binary system consists of a 3.7MSun star and a 0.8MSun star with an orbital period of 2.87 days. Use Newton's version of Kepler's third law to calculate the orbital separation of the system. How does that separation compare with the typical size of a red giant star?

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55) The Speed of Supernova Debris. In the year 2001, debris moving outward from Supernova 1987A was seen hitting a preexisting ring of gas at a distance of 0.7 light-year around the star that exploded. Calculate the speed of that debris. How does the speed you find compare with the speed of light?

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter 18 The Bizarre Stellar Graveyard Section 18.1 1) What is a white dwarf?

A) the exposed core of a dead star, supported by electron degeneracy pressure B) the exposed core of a dead star, supported by neutron degeneracy pressure C) a hot, but very small main-sequence star with a mass of less than 1.4 solar masses D) a cool and very small main-sequence star with a mass of less than 1.4 solar masses E) the name for the singularity at the center of a black hole Answer: A 2) How much would a teaspoon of white dwarf material weigh on Earth?

A) the same as a teaspoonful of Earthlike material B) about the same as Mt. Everest C) about the same as Earth D) a few tons E) a few million tons Answer: D 3) Which of the following is closest in mass to a white dwarf?

A) the Moon B) Earth C) Jupiter D) the Sun Answer: D 4) Which of the following is closest in size (radius) to a white dwarf?

A) Earth B) a small city C) a football stadium D) a basketball E) the Sun Answer: A 5) Which of the following is always true about a 1.2-solar mass white dwarf compared to a 1.0 solar mass white dwarf?

A) The more massive white dwarf has a larger radius.

B) The more massive white dwarf has a smaller radius.

C) The more massive white dwarf has a higher surface temperature.

D) The more massive white dwarf has a lower surface temperature.

E) The more massive white dwarf is supported by neutron degeneracy pressure, rather than electron degeneracy pressure.

Answer: B

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6) What is the ultimate fate of an isolated white dwarf? A) It will cool down and become gradually dimmer. B) As gravity overwhelms the electron degeneracy pressure, it will explode as a nova. C) As gravity overwhelms the electron degeneracy pressure, it will explode as a supernova. D) As gravity overwhelms the electron degeneracy pressure, it will become a neutron star. E) The electron degeneracy pressure will eventually overwhelm gravity and the white dwarf will slowly evaporate. Answer: A 7) What kind of pressure prevents a white dwarf from collapsing?

A) neutron degeneracy pressure B) electron degeneracy pressure C) thermal pressure D) radiation pressure Answer: B 8) What is the upper limit to the mass of a white dwarf?

A) There is no upper limit.

B) There is an upper limit, but we do not yet know what it is.

C) about 2.8 solar masses D) about 1.4 solar masses E) about 1 solar mass Answer: D 9) Which of the following hypothetical observations would contradict current understanding of the nature of white dwarfs?

A) discovery of a white dwarf with a mass of 1.5 MSun B) discovery of a white dwarf with a surface temperature of 6000 K C) discovery of a white dwarf at the center of a planetary nebula D) discovery of a white dwarf with a 1.5 MSun mass main-sequence companion Answer: A 10) In which wavelength regions is it easiest to identify and study the accretion disk around a white dwarf in a binary system?

A) visible and infrared light B) infrared and radio light C) x-ray and ultraviolet light D) gamma ray and x-ray light E) radio and visible light Answer: C

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11) Which of the following statements about novae is true?

A) A star system that undergoes a nova may have another nova sometime in the future.

B) A nova involves fusion taking place on the surface of a neutron star.

C) A nova is the most powerful type of explosion ever seen in the universe.

D) When a star system undergoes a nova, it brightens as much as a star system undergoing a supernova.

E) A nova can occur on a single, isolated star.

Answer: A 12) After our Sun dies, will it ever undergo a nova explosion? Why or why not?

A) no, because it does not have a close binary companion B) no, because our Sun will become a neutron star, not a white dwarf C) yes, because our Sun will become a white dwarf D) yes, because all stars undergo novae explosions after they die Answer: A 13) You observe a nova. Based on current understanding, you can also conclude that you are observing A) a close binary system in which one member is a white dwarf.

B) a close binary system in which one member is a neutron star.

C) the complete destruction of a white dwarf.

D) a close binary system in which both members are white dwarfs.

Answer: A 14) Which of the following is most likely to undergo a white dwarf supernova?

A) a main-sequence star of spectral type O B) a main-sequence star like our Sun C) an isolated white dwarf (not in a binary system) D) a close binary system in which both stars are white dwarfs E) a pulsar Answer: D 15) Observationally, a key distinction between a white dwarf supernova and a massive-star supernova is that the spectrum of a massive-star supernova shows prominent hydrogen lines, while the spectrum of a white dwarf supernova does not. Why don't we see prominent hydrogen lines in the spectrum of a white dwarf supernova?

A) White dwarf supernovae occur so far away that their hydrogen lines become too dim to see.

B) During a white dwarf supernova, all the white dwarf's hydrogen fuses instantly to make helium.

C) White dwarfs are stellar corpses that contain little or no hydrogen, so they do not show hydrogen lines when they explode.

D) The hydrogen lines of white dwarf supernovae are so redshifted that we cannot detect them.

Answer: C

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16) What does Einstein's general theory of relativity tell us about two white dwarfs orbiting one another very close together? A) They will emit gravitational waves that carry energy and angular momentum away from the system, causing the white dwarfs to gradually spiral inward toward each other and eventually merge. B) They will emit gravitational waves that carry energy and angular momentum toward the system, causing the white dwarfs to gradually spiral away from one another and eventually fly far apart. C) They will emit gravitational waves that carry energy and angular momentum away from the system, causing the white dwarfs to quickly spiral away from one another. D) They will orbit one another forever. Answer: A 17) Which of the following best describes why a white dwarf cannot have a mass greater than the 1.4-solar-mass limit?

A) The upper limit to a white dwarf's mass is something we have learned from observations, but no one knows why this limit exists.

B) Electron degeneracy pressure depends on the speeds of electrons, which approach the speed of light as a white dwarf's mass approaches the 1.4-solar-mass limit.

C) White dwarfs get hotter with increasing mass, and above the 1.4-solar-mass limit, they would be so hot that even their electrons would melt.

D) White dwarfs are made only from stars that have masses less than the 1.4-solar-mass limit.

Answer: B Section 18.2 1) How much would a paperclip made of neutron star material weigh on Earth?

A) about the same as an actual paperclip B) a few tons C) about as much as Mt. Everest D) about as much as the Moon E) about as much as the Sun Answer: C 2) Which of the following is closest in size (radius) to a neutron star?

A) Earth B) a city C) a football stadium D) a basketball E) the Sun Answer: B

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3) How might a pulsar appear to an observer?

A) It may appear to flash several times per second or more with near perfect regularity.

B) It may slowly get dimmer and brighter with a period ranging from several days to several weeks.

C) It may appear to flash at random intervals.

D) It may appear to change color very rapidly, from blue to red and back again, with near perfect regularity.

E) Its spectrum may alternate between showing blueshifts and redshifts very rapidly with near perfect regularity.

Answer: A 4) What is a pulsar?

A) a star that alternately expands and contracts in size B) a rapidly rotating neutron star C) a neutron star or black hole that happens to be in a binary system D) a binary system that happens to be aligned so that one star periodically eclipses the other E) a star that is burning iron in its core Answer: B 5) What causes the pulses of a pulsar?

A) vibrations on the pulsar's surface B) As the pulsar rotates, beams of radiation along its magnetic axis sweep through space.

C) The pulsar undergoes periodic explosions of nuclear fusion that generate strong emission.

D) The pulsar is periodically eclipsed by an orbiting companion.

E) The pulses are peaks and valleys of gravitational waves.

Answer: B 6) What is the ultimate fate of an isolated pulsar (not in a binary system)?

A) It will spin ever faster, becoming a millisecond pulsar.

B) As gravity overwhelms the neutron degeneracy pressure, it will explode as a supernova.

C) As gravity overwhelms the neutron degeneracy pressure, it will become a white dwarf.

D) It will gradually cool, and it will become too dim to detect.

E) It will gradually turn into a white dwarf.

Answer: D 7) How do we know that pulsars must be very small in size (radius)?

A) We have measured their radii directly in high-resolution telescope images.

B) Their small sizes are inferred from careful study of their spectra.

C) Some spin so rapidly that if they were larger in size, their surfaces would be moving faster than the speed of light.

D) Only very small objects can beam radiation into space.

E) This is only a theory that has not yet been confirmed by observations.

Answer: C

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8) You observe an x-ray burster. Based on current understanding, you can also conclude that you are observing A) a close binary system in which one member is a white dwarf.

B) a close binary system in which one member is a neutron star.

C) a close binary system in which one member is a black hole.

D) a close binary system in which one member is a neutron star and the other is a black hole.

Answer: B Section 18.3 1) After a massive-star supernova, what is left behind?

A) always a white dwarf B) always a neutron star C) always a black hole D) either a white dwarf or a neutron star E) ether a neutron star or a black hole Answer: E 2) Which of the following correctly describes how light will be affected as it tries to escape from the vicinity of a massive object like a black hole?

A) Light doesn't have mass; therefore, it is not affected by gravity.

B) The light will be redshifted.

C) The light will be blueshifted.

D) The visible light will be redshifted, but higher frequencies, such as x-rays and gamma rays, will not be affected.

Answer: B 3) How does a black hole form from a massive star?

A) During a supernova, if a star is massive enough for its gravity to overcome neutron degeneracy of the core, the core will be compressed until it becomes a black hole.

B) Any star that is more massive than 8 solar masses will undergo a supernova explosion and leave behind a black-hole remnant.

C) If enough mass is accreted by a white dwarf star so that it exceeds the 1.4-solar-mass limit, it will undergo a supernova explosion and leave behind a black-hole remnant.

D) If enough mass is accreted by a neutron star, it will undergo a supernova explosion and leave behind a black-hole remnant.

E) A black hole forms when two massive main-sequence stars collide.

Answer: A

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4) Which of the following statements about black holes is true for an object falling into a black hole? A) From an outside observer's viewpoint, the object falls quickly through the event horizon. B) From an outside observer's viewpoint, light from the object will become increasingly blueshifted as it approaches the event horizon. C) In the object's reference frame, the object will slow down as it falls and come to a stop just before it reaches the event horizon. D) Once the object crosses the event horizon, it can no longer communicate with anything outside the black hole. Answer: D 5) Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice?

A) Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.

B) Time on the clock will run faster as it approaches the black hole, and light from the clock will be increasingly blueshifted.

C) The clock will fall faster and faster, reaching the speed of light as it crosses the event horizon.

D) The clock will fall toward the black hole at a steady rate, so that you'll see it plunge through the event horizon within just a few minutes.

Answer: A 6) Which of the following black holes has the largest event horizon? (Ignore any effects of rotation.)

A) a 5 solar mass black hole B) a 10 solar mass black hole C) All black holes have the same size event horizon.

Answer: B 7) Which of the following systems is most likely to contain a black hole?

A) an x-ray binary in which the unseen companion has a mass 2 MSun B) an x-ray binary in which the unseen companion has a mass 10 MSun C) an x-ray binary that undergoes frequent x-ray bursts D) a binary system in which novae have been observed to occur E) a system that has recently undergone a white dwarf supernova Answer: B Section 18.4 1) Evidence indicates that many gamma-ray bursts are produced by A) supernovae of very high-mass stars.

B) the explosion of a white dwarf.

C) bursts of fusion occurring on the surfaces of white dwarfs in binary systems.

D) bursts of fusion occurring on the surfaces of neutron stars in binary systems.

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2) Evidence indicates that at least some gamma-ray bursts, including those detected with gravitational waves, are produced by A) the merger of two neutron stars. B) the explosion of a white dwarf. C) bursts of fusion occurring on the surfaces of white dwarfs in binary systems. D) bursts of fusion occurring on the surfaces of neutron stars in binary systems.

Answer: A 3) Some gamma-ray bursts last only a few seconds and do not appear to come from supernovae. What is the leading hypothesis for the origin of short these gamma-ray bursts? A) new stars forming in the Milky Way B) bursts of fusion occurring on the surfaces of white dwarfs in binary systems C) bursts of fusion occurring on the surfaces of neutron stars in binary systems D) The explosion of a white dwarf E) The mergers of two neutron stars Answer: E 4) Two neutron stars in a close binary are predicted to spiral into each other and eventually merge because they lose orbital energy through A) the emission of gravitational waves. B) strong x-ray emission of their accretion disks. C) friction as they orbit through the gas of a surrounding accretion disk. D) the exchange of mass from one to the other. Answer: A 5) Scientists have detected mergers of black holes that have spiraled into each other by A) detecting gravitational waves from the merger. B) observing distant systems that produce unusually strong gamma-ray emission. C) observing distant systems that produce unusually strong radio emission. D) detecting rapidly pulsating sources of gamma-ray emission. Answer: A General Chapter Questions 1) Consider our Moon, a white dwarf, and a neutron star. Which of the following lists them in the correct order of size (diameter), from smallest to largest? A) neutron star, Moon, white dwarf B) Moon, white dwarf, neutron star C) white dwarf, neutron star, Moon Answer: A

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2) Degeneracy pressure is the source of the pressure that stops the crush of gravity in all the following except A) a brown dwarf. B) a white dwarf. C) a neutron star. D) the core of a high-mass main-sequence star. Answer: D 3) The surface of the neutron star RXJ2015 has a temperature of 10 million K. This neutron star emits radiation most strongly in A) xray light. B) visible light. C) radio light. D) infrared light. Answer: A 4) If you were to come back to our Solar System in 8 billion years, what might you expect to find? A) a red giant star B) a white dwarf C) a rapidly spinning pulsar D) a black hole E) Everything will be pretty much the same as it is now. Answer: B

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5) The figure above shows how the luminosity of supernovae changes over time. About how long does it take a white dwarf supernova to decrease in luminosity by a factor of 100 from its peak?

A) 300 days B) 200 days C) 100 days D) 25 days Answer: B 6) The figure above shows how the luminosity of supernovae changes over time. Which type of supernova is the most luminous one year after the explosion?

A) massive star supernova B) white dwarf supernova C) impossible to tell from the graph Answer: A 7) What would happen if Jupiter was suddenly replaced by a black hole with the same mass as Jupiter?

A) The orbits of the planets in our solar system would be unaffected.

B) The other planets would slowly be pulled into Jupiter, but the Sun would be unaffected.

C) The other planets and the Sun would slowly be pulled into Jupiter.

D) The entire solar system would rapidly be sucked into the black hole.

Answer: A

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8) Consider a main-sequence star, a white dwarf, a neutron star, and the singularity of a black hole. Which of the following lists them in the correct order of increasing density (low to high)? A) main sequence star, white dwarf, neutron star, black hole singularity B) main sequence star, neutron star, white dwarf, black hole singularity C) black hole singularity, main sequence star, white dwarf, neutron star D) main sequence star, black hole singularity, neutron star, white dwarf Answer: A 9) The formula for the Schwarzschild radius of a black hole of mass M is: Rs = 3 ×

km

What is the Schwarzschild radius of a 100 million-solar-mass black hole?

A) 3 km B) 30 km C) 3000 km D) 300 million km E) 3 million km Answer: D Short Answer Questions 1) Could our Sun ever undergo a nova or a white dwarf supernova event? Why or why not?

Answer: No, because both events occur on white dwarfs in close binary systems. Even after our Sun becomes a white dwarf, such events won't occur because our Sun is not part of a close binary.

2) Why do white dwarf supernovae all have approximately the same maximum luminosity?

Answer: White dwarf supernovae occur when the mass of the white dwarf has just exceeded 1.4 MSun. Since the mass is the same and the entire object–made of degenerate matter–explodes at once, the maximum luminosity is the same.

3) What would happen if a small piece (say the size of a paper clip) of neutron star material suddenly appeared on Earth?

Answer: The extremely dense material could not be supported by the ordinary material on Earth, and it would plunge to the center of Earth under the action of gravity. Its momentum would carry it past the center, back to the other side, and it would continue to oscillate back and forth through Earth creating a small hole each time (that would rapidly fill with molten rock) until friction finally brought it to rest at the center of Earth.

4) Briefly describe what you would see if your friend plunged into a black hole. Assume you are watching from orbit of the black hole and have a telescope powerful enough so that you can see him clearly as he falls.

Answer: As he approached the black hole, he would be stretched by tidal forces, his time would run slow, and light coming from him would be redshifted. The closer he got to the event horizon, the slower time would run. You would never see him cross the event horizon, but he would disappear from view when his light became redshifted out of the range of detection.

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5) Why would Earth's orbit be unaffected were the Sun to suddenly become a black hole (with no change in its mass)?

Answer: Earth's orbit (and those of the other planets) would remain unchanged since they are far enough away that Newton's law of gravity applies and the gravitational force depends only on the masses of the objects and the distance between them, not on their composition or density.

Reading Quiz Questions 1) A white dwarf is A) what most stars become when they die.

B) a precursor to a black hole.

C) an early stage of a neutron star.

D) a brown dwarf that has exhausted its fuel for nuclear fusion.

Answer: A 2) A typical white dwarf is A) as massive as the Sun but only about as large in size as Earth.

B) as large in diameter as the Sun but only about as massive as Earth.

C) about the same size and mass as the Sun but much hotter.

D) as massive as the Sun but only about as large in size as Jupiter.

Answer: A 3) If you had something the size of a sugar cube that was made of white dwarf matter, it would weigh A) as much as a truck.

B) about 5 pounds.

C) as much as the entire Earth.

D) as much as an average person.

Answer: A 4) The maximum mass of a white dwarf is A) about the mass of our Sun. B) about 1.4 times the mass of our Sun. C) limitless; there is no theoretical limit to the maximum mass of a white dwarf.

D) about 3 times the mass of our Sun. Answer: B 5) What is an accretion disk?

A) a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole B) any flattened disk in space, such as the disk of the Milky Way Galaxy C) a stream of gas flowing from one star to its binary companion star D) a disk of material found around every white dwarf in the Milky Way Galaxy Answer: A

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6) According to our modern understanding, what is a nova?

A) an explosion on the surface of a white dwarf in a close binary system B) the explosion of a massive star at the end of its life C) the sudden formation of a new star in the sky D) a rapidly spinning neutron star Answer: A 7) What would happen if a white dwarf gained enough mass to reach the 1.4 solar-mass white dwarf limit?

A) The white dwarf would explode completely as a white dwarf supernova.

B) The white dwarf would collapse in size, becoming a neutron star.

C) The white dwarf would undergo a nova explosion.

D) The white dwarf would collapse to become a black hole.

Answer: A 8) A neutron star is A) the remains of a star that died in a massive star supernova (if no black hole was created).

B) the remains of a star that died by expelling its outer layers in a planetary nebula.

C) a star made mostly of elements with high atomic mass numbers, so that they have lots of neutrons.

D) an object that will ultimately become a black hole.

Answer: A 9) A typical neutron star is more massive than our Sun and about the size (radius) of A) a small asteroid (10 km in diameter).

B) Earth.

C) the Moon.

D) Jupiter.

Answer: A 10) If you had something the size of a sugar cube that was made of neutron star matter, it would weigh A) about as much as a large mountain.

B) about 50 pounds.

C) as much as the entire Earth.

D) about as much as a truck.

Answer: A

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11) Which of the following statements about electron degeneracy pressure and neutron degeneracy pressure is true? A) Electron degeneracy pressure is the main source of pressure in white dwarfs, while neutron degeneracy pressure is the main source of pressure in neutron stars. B) Both electron degeneracy pressure and neutron degeneracy pressure help govern the internal structure of a main-sequence star. C) The life of a white dwarf is an ongoing battle between electron degeneracy pressure and neutron degeneracy pressure. D) In a black hole, the pressure coming from neutron degeneracy pressure is slightly greater than that coming from electron degeneracy pressure. Answer: A 12) Pulsars are thought to be A) accreting white dwarfs. B) rapidly rotating neutron stars. C) unstable high-mass stars. D) accreting black holes. Answer: B 13) How is an x-ray burst (in an x-ray binary system) similar to a nova? A) Both involve explosions on the surface of stellar corpse. B) Both typically recur every few hours to every few days. C) Both are thought to involve fusion of hydrogen into helium. D) Both result in the complete destruction of their host stars. Answer: A 14) What is the basic definition of a black hole?

A) an object with gravity so strong that not even light can escape B) a dead star that has faded from view C) any object made from dark matter D) a compact mass that emits no visible light Answer: A 15) Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly A) 0.5 solar masses. B) 1.4 solar masses. C) 3 solar masses. D) 10 solar masses. Answer: C 16) What do we mean by the event horizon of a black hole? A) It is the point beyond which neither light nor anything else can escape. B) It is the center of the black hole. C) It is the distance from the black hole at which stable orbits are possible. D) It is the place where x-rays are emitted from black holes. Answer: A

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17) Imagine that our Sun were magically and suddenly replaced by a black hole of the same mass (1 solar mass). How would Earth's orbit change?

A) Earth would almost instantly be sucked into oblivion in the black hole.

B) Earth would orbit faster, but at the same distance.

C) Earth would slowly spiral inward until it settled into an orbit about the size of Mercury's current orbit.

D) It would not change; Earth's orbit would remain the same.

Answer: D 18) What do we mean by the singularity of a black hole?

A) It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.

B) It is the "point of no return" of the black hole; anything closer than this point will not be able to escape the gravitational force of the black hole.

C) It is the edge of the black hole, where one could leave the observable universe.

D) The term is intended to emphasize the fact that an object can become a black hole only once, and a black hole cannot evolve into anything else.

Answer: A 19) What makes us think that the star system Cygnus X-1 contains a black hole?

A) It emits x-rays as we expect from a system with an accretion disk, but the unseen star in the system is too massive to be a neutron star.

B) No light is emitted from this star system, so it must contain a black hole.

C) The fact that we see strong x-ray emission tells us that the system must contain a black hole.

D) Cygnus X-1 is a powerful x-ray burster, so it must contain a black hole.

Answer: A 20) The Schwarzschild radius of a black hole depends on A) only the mass of the black hole.

B) the observationally measured radius of the black hole.

C) the way in which the black hole formed.

D) both the mass and chemical composition of the black hole.

Answer: A 21) Observational evidence indicates that at least some gamma ray bursts are produced by A) the supernovas of very massive stars that leave behind black holes.

B) the central black hole of the Milky Way Galaxy.

C) the same types of close binary systems that produce x-ray bursts.

D) white dwarf supernovas in distant galaxies.

Answer: A

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22) Suppose two neutron stars or two black holes are closely orbiting one another. What do scientists predict will eventually happen to them, and why? A) Their orbits will spiral inward until the two objects merge because of energy lost through gravitational waves. B) Their orbits will gradually grow larger because of the centrifugal force involved as they circle around each other. C) Their orbits will spiral inward as a result of friction with the surrounding gas until all the gas clears, after which their orbits will remain stable. D) The orbits would remain stable unless there were a third object orbiting along with them.

Answer: A 23) When did scientists first detect gravitational waves from mergers of compact objects (pairs of neutron stars or black holes)? A) 2015 B) 1543 C) 1990 D) Gravitational waves have not yet been detected, though scientists hope to detect them soon. Answer: A 24) According to current models, where does the element gold come from? A) mergers of neutron stars B) the supernova explosions of high-mass stars C) the late stages of fusion in low-mass stars D) the Big Bang Answer: A Concept Quiz Questions 1) Which of the following statements about degeneracy pressure is not true? A) Degeneracy pressure can continue to support an object against gravitational collapse even if the object becomes extremely cold. B) Degeneracy pressure arises from a quantum mechanical effect that we don't notice in our daily lives. C) Black holes form when gravity overcomes neutron degeneracy pressure. D) Degeneracy pressure can arise only from interactions among electrons. Answer: D 2) The more massive a white dwarf, the A) smaller its radius. B) higher its temperature. C) larger its radius. D) higher its luminosity.

Answer: A

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3) Which of the following best describes why a white dwarf cannot have a mass greater than the 1.4-solar-mass limit? A) Electron degeneracy pressure depends on the speeds of electrons, which approach the speed of light as a white dwarf's mass approaches the 1.4solar-mass limit. B) White dwarfs get hotter with increasing mass, and above the 1.4solar-mass limit they would be so hot that even their electrons would melt. C) White dwarfs are made only from stars that have masses less than the 1.4-solar-mass limit. D) The upper limit to a white dwarf's mass is something we have learned from observations, but no one knows why this limit exists. Answer: A 4) The white dwarf that remains when our Sun dies will be mostly made of A) hydrogen. B) helium. C) carbon. D) neutrons.

Answer: C 5) Which statement about accretion disks is not true? A) The primary factor determining whether a white dwarf has an accretion disk is the white dwarf's mass. B) The gas in the inner parts of the disk travels faster than the gas in the outer parts of the disk. C) The gas in the inner parts of the disk is hotter than the gas in the outer parts of the disk. D) Accretion disks are made primarily of hydrogen and helium gas. Answer: A 6) According to present understanding, a nova is caused by A) hydrogen fusion on the surface of a white dwarf. B) carbon fusion in the core of a white dwarf. C) hydrogen fusion on the surface of a neutron star. D) a white dwarf that gains enough mass to exceed the 1.4-solar-mass limit.

Answer: A 7) Which of the following is not true about differences between novae and supernovae? A) Supernovae eject gas into space, but novae do not. B) Novae are much less luminous than supernovae. C) Novae occur only in binary star systems, while supernovae can occur both among single stars and among binary star systems. D) The same star can undergo novae explosions more than once, but can undergo only a single supernova. Answer: A

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8) Will our Sun ever undergo a white dwarf supernova explosion? Why or why not?

A) no, because it is not orbited by another star B) yes, right at the end of its double shell-fusion stage of life C) yes, about a million years after it becomes a white dwarf D) no, because the Sun's core will never be hot enough to fuse carbon and other heavier elements into iron Answer: A 9) Which of the following best describes what would happen if a 1.5-solar-mass neutron star, with a diameter of a few kilometers, were suddenly (for unexplained reasons) to appear in your home town?

A) The entire Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star.

B) It would rapidly sink to the center of Earth.

C) The combined mass of Earth and the neutron star would cause the neutron star to collapse into a black hole.

D) It would crash into Earth, throwing vast amounts of dust into the atmosphere that, in turn, would cool the Earth; this is probably what caused the extinction of the dinosaurs.

Answer: A 10) Each Voyager spacecraft carries a "postcard" designed to be understandable to any aliens that might someday encounter it. On the "postcard," scientists pinpointed the location of Earth by triangulating it between pulsars. Why did the scientists choose pulsars rather than some other type of star?

A) Pulsars are easy to identify by their almost perfectly steady periods of pulsation.

B) Pulsars are very bright and therefore easy to find.

C) Several pulsars are located within a dozen light-years of our solar system, making them useful for finding our solar system.

D) We're pretty sure that aliens will have only radio telescopes and not optical telescopes, so they'll have a better chance of seeing pulsars than ordinary stars.

Answer: A 11) Which statement about pulsars is true?

A) Pulsars can form only in close binary systems.

B) All pulsars are neutron stars, but not all neutron stars are pulsars.

C) Pulsars can "pulse" no more than about once a day.

D) All neutron stars are pulsars.

Answer: B 12) How does an accretion disk around a neutron star differ from an accretion disk around a white dwarf?

A) The accretion disk around a neutron star is much hotter and emits higher-energy radiation.

B) The accretion disk around a neutron star is made mostly of helium, while the accretion disk around a white dwarf is made mostly of hydrogen.

C) The accretion disk around a neutron star is more likely to give birth to planets.

D) The accretion disk around a neutron star always contains much more mass.

Answer: A

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13) Which statement about Schwarzschild radii is true?

A) The Schwarzschild radius of a black hole can be defined only in binary systems.

B) The more massive the black hole, the smaller the Schwarzschild radius.

C) The Schwarzschild radius of a black hole depends only on its mass.

D) The Schwarzschild radius is used to describe the size of neutron stars.

Answer: C 14) Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice?

A) Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.

B) Time on the clock will run faster as it approaches the black hole, and light from the clock will be increasingly blueshifted.

C) The clock will fall toward the black hole at a steady rate, so that you'll see it plunge through the event horizon within just a few minutes.

D) The clock will fall faster and faster, reaching the speed of light as it crosses the event horizon.

Answer: A 15) Which of statement below about black holes is not true?

A) A spaceship passing near a 10 solar-mass black hole is much more likely to be destroyed than a spaceship passing at the same distance from the center of a 10 solar-mass main-sequence star.

B) We have strong observational evidence that black holes really exist.

C) If you watch someone else fall into a black hole, you will never see the person cross the event horizon; you'll only see the person fade from view as the light he or she reflects (or emits) becomes more and more redshifted.

D) If you fell into a black hole, you would experience time to be running normally as you plunged rapidly across the event horizon.

Answer: A 16) When we see x-rays from an accretion disk in a binary system, we can't immediately tell whether the accretion disk surrounds a neutron star or a black hole. Suppose we then observe each of the following phenomena in this system. Which one would rule out the possibility of a black hole?

A) intense x-ray bursts B) spectral lines from the companion star that alternately shift to shorter and longer wavelengths C) visible and ultraviolet light from the companion star D) bright x-ray emission that varies on a time scale of a few hours Answer: A 17) Which of the following observatories is most likely to discover a black hole in a binary system?

A) an x-ray observatory in space B) the Hubble Space Telescope C) an infrared observatory in space D) a radio telescope array on the ground Answer: A

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18) What types of events have scientists so far been able to detect with gravitational wave observatories, such as LIGO? A) x-ray bursts B) supernovae C) mergers of closely-orbiting pairs of neutron stars or black holes D) the gradual decay over many years of the orbits of binary neutron stars Answer: C 19) Imagine an advanced civilization living on a planet orbiting at a distance of 10 AU (1.5 billion kilometers) from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole. The black hole is surrounded by an accretion disk. Sometime within the next million years or so, the civilization's planet is likely to be doomed because A) the red giant will probably supernova within the next million years.

B) jets of material shot out of the accretion disk will shoot down their planet.

C) the red giant star, which provides most of energy the civilization needs to exist, will soon be destroyed in the accretion disk.

D) tidal forces from the black hole will rip the planet apart.

E) the planet's orbit will gradually decay, as it is sucked in by the black hole.

Answer: A 20) Imagine an advanced civilization living on a planet orbiting at a distance of 10 AU (1.5 billion kilometers) from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole. The black hole is surrounded by an accretion disk. One foolhardy day, a daring individual in their space force (let's call him Major Tom) decides to become the first of his species to cross the event horizon of the black hole. To add to the drama, he decides to go in wearing only a thin space suit, which offers no shielding against radiation, no cushioning against any forces, and so on. Which of the following is most likely to kill him first (or at least to start the process of killing him first)?

A) x-rays from the accretion disk B) tidal forces due to the black hole C) the crush of gravity at the singularity embedded within the black hole D) the sucking force from the black hole, which will cause his head to explode Answer: A

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21) Imagine an advanced civilization living on a planet orbiting at a distance of 10 AU (1.5 billion kilometers) from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole. The black hole is surrounded by an accretion disk. Through a bizarre (and scientifically unexplainable) fluctuation in the space-time continuum, a copy of a book from that civilization arrives on your desk; it is titled Iguoonos: How We Evolved. In the first chapter, you learn that these beings evolved from organisms that lived 5 billion years ago. Which of the following statements should you expect to find as you continue to read this book?

A) They evolved on a different planet in a different star system and moved to their current location.

B) As a result of traumatic experiences to their evolutionary ancestors, they dislike television.

C) Their immediate ancestors were chimpanzees.

D) They believe that the presence of two stars in their system was critical to their evolution.

Answer: A Visual Quiz Questions 1) Sirius, the brightest star in the night sky, is actually a binary star system. Sirius A is mainsequence star and Sirius B is a white dwarf. Nearly all the visible light we see from Sirius comes from Sirius A. But when we photograph the system with x-ray light, as shown here, Sirius B is the brighter of the two stars. Why?

A) As a white dwarf, Sirius B is much hotter than Sirius A and thus emits more x-rays.

B) As a white dwarf, Sirius B is too small to emit visible light but not too small to emit x-rays.

C) Sirius B is brighter in x-rays because it is a nova.

D) Sirius B is brighter in x-rays because it is a white dwarf supernova.

Answer: A

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2) Based on the curves shown here, which of the following statements best describes how we can distinguish a massive star supernova from a white dwarf supernova?

A) A white dwarf supernova is brighter at its peak and over a few months fades more steadily than a massive star supernova.

B) For a few seconds, a white dwarf supernova is brighter than a massive star supernova, but after that the two look virtually identical.

C) The spectrum of a massive star supernova has two major spectral lines, while a white dwarf supernova has only one.

D) The two types of supernovae behave so similarly that we can only distinguish them by studying photographs of the systems taken before the explosions.

Answer: A

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3) Which of the following diagrams best represents the scale of Earth in comparison to a neutron star?

A)

B)

C)

D)

Answer: A

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4) Which of the following represents the true shape of a black hole as you'd see it (or measure it) as you flew past it in a spaceship?

A)

B)

C)

D)

Answer: A

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5) A spacecraft is on a trajectory that happens to be taking it near a black hole. Which diagram shows how the spacecraft's orbit will be affected?

A)

B)

C)

D)

Answer: A 6) This series of images shows the pulsar at the center of the Crab Nebula looking bright every 0.033 second. Based on these data and current theory of pulsars, what can we conclude?

A) The pulsar is a neutron star that makes one full rotation every 0.033 second.

B) The pulsar is a neutron star whose magnetic field switches on and off every 0.033 second.

C) The pulsar is part of a close binary star system in which the two stars complete an orbit every 0.033 second.

D) The pulsar has an accretion disk that undergoes an x-ray burst every 0.033 second.

Answer: A

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7) This painting shows an accretion disk around a black hole in a close binary star system. What physical law explains why matter flowing from the companion star orbits rapidly as it nears the black hole?

A) the law of conservation of angular momentum B) Newton's third law of motion C) Kepler's second law of planetary motion D) Einstein's general theory of relativity Answer: A 8) A binary system that looks like the one depicted in this painting should shine brightly in xrays. Which one of the four labeled regions is the source of most of the x-rays?

A) Region I B) Region II C) Region III D) Region IV Answer: B

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9) This graph shows data collected by a gamma ray telescope. What kind of event is it showing?

A) a gamma ray burst from a distant galaxy B) a burst of gamma rays from a flare on a blue supergiant star C) a burst of gamma rays from an explosion on the surface of a neutron star D) the on and off gamma-ray light changes of a pulsar Answer: A 10) This graph shows a gravitational wave signal recorded by the gravitational wave detectors (called LIGO) in two locations. What do models indicate was the cause of this signal?

A) the merger of two black holes in a distant galaxy B) a supernova in a distant galaxy C) the sudden explosion of an entire galaxy D) an x-ray burst on the surface of a neutron star Answer: A

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End-of-Chapter Questions Visual Skills Check Use the following questions to check your understanding of some of the many types of visual information used in astronomy

Figure 18.5, repeated to the right, shows how the luminosities of supernovae change with time.

Answer the following questions using the information provided in the figure.

1) At peak brightness, the white dwarf supernova is approximately ________ times as luminous as the massive star supernova at peak brightness. A) 1.5 B) 3 C) 10 D) 100 Answer: B 2) The luminosity of the white dwarf supernova 175 days after it reaches peak brightness is about ________ of its luminosity at peak brightness. A) 30% B) 10% C) 3% D) 1% Answer: D

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3) Approximately how many days does it take for a white dwarf supernova to decline to 10% of its peak brightness? A) 3 days B) 30 days C) 170 days D) 300 days Answer: B 4) Approximately how many days does it take for a massive star supernova to decline to 10% of its peak brightness? A) 10 days B) 30 days C) 100 days D) 300 days Answer: C 5) Approximately how many days does it take for a massive star supernova to decline to 1% of its peak brightness? A) 3 days B) 30 days C) 170 days D) 300 days Answer: D Chapter Review Questions 1) What is degeneracy pressure, and how is it important to the existence of white dwarfs and neutron stars? What is the difference between electron degeneracy pressure and neutron degeneracy pressure?

2) Describe the mass, size, and density of a typical white dwarf. How does the size of a white dwarf depend on its mass? 3) What happens to the electron speeds in a more massive white dwarf, and how does this behavior lead to the white dwarf limit for mass? 4) What is an accretion disk? Describe how an accretion disk can provide a white dwarf with a new source of energy. 5) What is a nova? Describe the process that creates a nova and what a nova looks like. 6) What processes may cause a white dwarf supernova?

Observationally, how do we distinguish white dwarf and massive star supernovae? 7) Describe the mass, size, and density of a typical neutron star. What would happen if a neutron star came to your hometown?

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8) How do we know that pulsars are neutron stars? Are all neutron stars also pulsars? Explain.

9) Explain how the presence of a neutron star can make a close binary star system appear to us as an x-ray binary. Why do some of these systems appear to us as x-ray bursters?

10) In what sense is a black hole like a hole in the observable universe? Define the event horizon and the Schwarzschild radius, and describe the three basic properties of a black hole.

11) What do we mean by the singularity of a black hole? How do we know that our current theories are inadequate to explain what happens at the singularity?

12) Suppose you are falling into a black hole. How will you perceive the passage of your own time? How will outside observers see time passing for you? Briefly explain why your trip into a stellar-mass black hole is likely to be lethal.

13) Why do we think that supernovae should sometimes form black holes? What observational evidence supports the existence of black holes?

14) What are gamma-ray bursts, and how do we think they are produced?

15) Why can emission of gravitational waves lead to mergers of white dwarfs, neutron stars, and black holes?

16) Describe the event that led to the first detection of gravitational waves. How was it detected?

Does It Make Sense?

Decide whether or not each of the following statements makes sense (or is clearly true or false).

Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

Example: I walked east from our base camp at the North Pole.

Solution: The statement does not make sense because east has no meaning at the North Pole–all directions are south from the North Pole.

17) The white dwarf at the center of the Helix Nebula has a mass three times the mass of our Sun. 18) I observed a white dwarf supernova occurring at the location of a single (not binary) white dwarf.

19) If you want to find a pulsar, you should look near the remnant of a supernova described by ancient Chinese astronomers.

20) It's the year 2030, and scientists have just learned that there is a 10MSun black hole lurking near Pluto's orbit.

21) If your spaceship flew within a few thousand kilometers of a black hole, you and your ship would be rapidly sucked into it.

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22) We can detect black holes with x-ray telescopes because matter falling into a black hole emits x-rays after it smashes into the event horizon.

23) From your point of view, an object falling toward a black hole will never cross the event horizon.

24) The best way to search for black holes is to look for small black circles in the sky.

25) Gamma-ray bursts are more likely to be observed in galaxies that are rapidly forming new stars than in galaxies containing only old stars.

26) Gravitational waves are best observed with the Hubble Space Telescope.

Quick Quiz

27) Which of these objects has the smallest radius? A) a 1.2MSun white dwarf B) a 0.6MSun white dwarf C) Jupiter Answer: No Correct Answer Was Provided.

28) What happens if a white dwarf reaches the 1.4MSun limit?

A) It explodes as a white dwarf supernova.

B) It collapses to become a neutron star.

C) It collapses to become a black hole.

Answer: No Correct Answer Was Provided.

29) If we see a nova, we know that we are observing A) a rapidly rotating neutron star.

B) a gamma ray—emitting supernova.

C) a white dwarf in a binary system.

Answer: No Correct Answer Was Provided.

30) A pulsar is A) an unstable high-mass star.

B) an accreting white dwarf.

C) a rapidly rotating neutron star.

Answer: No Correct Answer Was Provided.

31) What would happen if the Sun suddenly became a black hole without changing its mass?

A) The black hole would quickly suck in Earth.

B) Earth would gradually spiral into the black hole.

C) Earth would remain in the same orbit.

Answer: No Correct Answer Was Provided.

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32) What makes us think that Cygnus X-1 contains a black hole? A) We can directly observe that one member of the system emits no light. B) The unseen object orbited by a luminous star is too massive to be a neutron star. C) The strong x-ray emission from the system means it must contain a black hole. Answer: No Correct Answer Was Provided.

33) Viewed from a distance, how would a flashing red light appear as it fell into a black hole?

A) It would appear to flash more quickly.

B) Its flashes would appear bluer.

C) Its flashes would shift to the infrared part of the spectrum.

Answer: No Correct Answer Was Provided.

34) Which of these black holes exerts the weakest tidal force on an object near its event horizon?

A) a 10MSun black hole B) a 100MSun black hole C) a 106MSun black hole Answer: No Correct Answer Was Provided.

35) Current evidence indicates that most gamma-ray bursts come from A) supernovae that leave a black hole behind.

B) unusually massive x-ray bursters.

C) the merger of two black holes.

Answer: No Correct Answer Was Provided.

36) Why do some pairs of neutron stars collide and merge?

A) Occasionally a neutron star moving through space will collide head-on with another neutron star.

B) andGravitational waves from close neutron star binary systems carry away orbital energy

angular momentum.

C) Electromagnetic waves from pulsars carry away angular momentum.

Answer: No Correct Answer Was Provided.

37) Black Holes in Popular Culture.Expressions such as "it disappeared into a black hole" are Inclusive Astronomy now common in popular culture. Give a few other examples of popular expressions in which the Use tonot reflect on participation in science. isquestions used but is meant to be taken literally.

In what ways are these uses correct termthese black hole in their analogies to real black holes? In what ways are they incorrect? Why do you think such an esoteric scientific idea as that of a black hole has captured the public imagination?

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38) Group Discussion: Graduate Students and Nobel Prizes. A few astronomers have had the opportunity to do Nobel Prize—winning research while still in graduate school. This discussion asks you to reflect on the reasons some of these people have actually received the Nobel Prize and others have not. a) The Extraordinary Claims boxon page 569 briefly discusses Subrahmanyan Chandrasekhar, who won the 1983 Nobel Prize in Physics in part for his discovery in 1930 of the white dwarf limit. He did that work while still a graduate student, and at the time he was ridiculed by the famed astronomer Sir Arthur Stanley Eddington. Chandrasekhar, who was from India, thought that at least some of the ridicule may have been racially motivated. Do you find this possibility plausible? Why or why not? Do you think it could have played a role in the delay of more than 50 years between the time of Chandrasekhar's discovery and his eventual Nobel Prize? b) The Extraordinary Claims box and Section 18.2 discuss the work of Jocelyn Bell Burnell, who discovered the first pulsar in 1967 while working as a graduate student under the mentorship of Antony Hewish. Hewish had led work on the design and construction of the radio telescope that Burnell used in making her discovery. The 1974 Nobel Prize in Physics was awarded based on the pulsar discovery, but it was awarded only to Hewish rather than jointly to him and Burnell.

Do you think Burnell should have shared the prize? Why or why not? In what sense could Hewish have been more deserving? Could cultural biases have influenced the Nobel Committee?

Do you think similar cultural biases could still be influential today? Why or why not?

The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

39) Too Strange to Be True?Despite strong theoretical arguments for the existence of neutron stars and black holes, many scientists rejected the possibility that such objects could exist until they were confronted with very strong observational evidence. Some people claim that this type of scientific skepticism demonstrates an unwillingness on the part of scientists to give up their deeply held scientific beliefs. Others claim that this type of skepticism is necessary for scientific advancement. What do you think? Defend your opinion.

40) Gravitational-Wave Detection. Go to the website for the Laser Interferometer Gravitational Observatory. Look up the detections of gravitational waves that it has made. How many detections have there been so far? What kinds of objects have made the detected waves? What are the approximate distances of those objects? Write a short report on what you learn.

41) Unanswered Questions. We have seen in this chapter that theoretical models make numerous predictions about the nature of black holes but leave many questions unanswered. Briefly describe one important but unanswered question related to black holes. If you think it will be possible to answer that question in the future, describe how we would find an answer, being as specific as possible about the evidence necessary to answer the question. If you think the question will never be answered, explain why you think it is impossible to answer.

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42) Group Activity: A Closer Look at Cygnus X-1. The close binary system Cygnus X-1 contains a supergiant of spectral type O and a black hole with a mass of about 15MSun. Consider this system and the artist's conception of it in Figure 18.15 as you work together as a group to do the following. Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39. a) Use the H-R diagram in Figure 15.10 to estimate the radius of the supergiant star. b) Use Mathematical Insight 18.1 to estimate the Schwarzschild radius of the black hole.

How much larger is the supergiant star than the black hole? c) The supergiant star in Figure 18.15 is drawn to be about 5 centimeters across. Use your result from part b to determine approximately how large (in radius) the black hole would be in this figure if it were drawn to scale. How far off is the figure from this approximate size? d) Discuss whether it is easy or difficult for matter to fall directly into a black hole of this size. e) Notice that the stream of matter from the supergiant star in Figure 18.15 is flowing off to the side of the black hole and onto an accretion disk, rather than going straight into the black hole.

Take turns proposing reasons the stream goes off to the side, and then come to a group consensus on the reason for this flow of matter. f) Given your prior results, discuss whether or not the rendering of Figure 18.15 is an 43) You are a white dwarf whose mass is M 0.8 Sun. "accurate enough" portrayal of the system. What, if anything, would your group do differently if you Sun. 44) wereYou are a neutron star whose mass isM1.5 asked to make an artist's rendering of the Cygnus X-1 system?

Investigate 45) You are aFurther black hole whose mass isM10 Sun. Life Stories of Stars. Write a one- to two-page life story for the scenarios in Problems 43 through 46. aEach story should be detailed and scientifically correctmatter but also creative.

That 46) You are white dwarf in a close binary system that is accreting from its companion is, it should be entertaining and at the same time prove that you understand stellar star. evolution. Be sure to state whether "you" are a member of a binary system.

47) Census of Stellar Corpses. Which kind of object do you think is most common in our galaxy: white dwarfs, neutron stars, or black holes? Explain your reasoning.

48) Fate of an X-Ray Binary. The x-ray bursts that happen on the surface of an accreting neutron star are not powerful enough to accelerate the exploding material to escape velocity. Predict what will happen in an x-ray binary system in which the companion star eventually feeds more than 3 solar masses of matter into the neutron star's accretion disk.

49) Why Black Holes Are Safe. Explain why the principle of conservation of angular momentum makes it very difficult to fall into a black hole.

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50) Surviving the Plunge. The tidal forces near a black hole with a mass similar to that of a star would tear a person apart before that person could fall through the event horizon. Black hole researchers have pointed out that a fanciful "black hole life preserver" could help counteract those tidal forces. The life preserver would need to have a mass similar to that of an asteroid and would need to be shaped like a flattened hoop placed around the person's waist. In what direction would the gravitational force from the hoop pull on the person's head? In what direction would it pull on the person's feet? Based on your answers, explain in general terms how the gravitational forces from the "life preserver" would help to counteract the black hole's tidal forces.

51) Research: Black Holes. Andrew Hamilton, a professor at the University of Colorado, maintains a website with a great deal of information about black holes and what it would be like to visit one. Visit his site and investigate some aspect of black holes that you find to be of particular interest. Write a short report on what you learn.

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

52) A Black Hole I? You've just discovered a new x-ray binary, which we will call Hyp-X1 ("Hyp" for hypothetical). The system Hyp-X1 contains a bright, B2 main-sequence star orbiting an unseen companion. The separation of the stars is estimated to be 20 million kilometers, and the orbital period of the visible star is 4 days. a) Use Newton's version of Kepler's third law to calculate the sum of the masses of the two stars in the system. (Hint: See Mathematical Insight 15.4.) Give your answer in both kilograms and solar masses . b) Determine the mass of the unseen companion. Is it a neutron star or a black hole? Explain.

(Hint: A B2 main-sequence star has a mass of about 10MSun.) 53) A Black Hole II? You've just discovered another new x-ray binary, which we will call HypX2 ("Hyp" for hypothetical). The system Hyp-X2 contains a bright, G2 main-sequence star orbiting an unseen companion. The separation of the stars is estimated to be 12 million kilometers, and the orbital period of the visible star is 5 days. a) Use Newton's version of Kepler's third law to calculate the sum of the masses of the two stars in the system. (Hint: See Mathematical Insight 15.4.) Give your answer in both kilograms and solar masses . b) Determine the mass of the unseen companion. Is it a neutron star or a black hole? Explain.

(Hint: A G2 main-sequence star has a mass of 1MSun.) 54) White Dwarf Density. A typical white dwarf has a mass of about 1.0MSun and the radius of Earth (about 6400 kilometers). Calculate the average density of a white dwarf, in kilograms per cubic centimeter. How does this compare to the mass of familiar objects?

55) Neutron Star Density. A typical neutron star has a mass of about 1.5MSun and a radius of 10 kilometers. a) Calculate the average density of a neutron star, in kilograms per cubic centimeter. b) Compare the mass of 1cm3 of neutron-star material to the mass of Mount Everest .

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56) Schwarzschild Radii. Calculate the Schwarzschild radius (in kilometers) for each of the following. a) A 108MSun black hole in the center of a quasar b) A 5MSun black hole that formed in the supernova of a massive star c) A mini—black hole with the mass of the Moon d) A mini—black hole formed when a super-advanced civilization decides to punish you (unfairly) by squeezing you until you become so small that you disappear inside your own event horizon 57) The Crab Nebula Pulsar Winds Down. Theoretical models of the slowing of pulsars predict that the age of a pulsar is approximately equal to p/2r, where p is the pulsar's current period and r is the rate at which the period is slowing with time. Observations of the pulsar in the Crab Nebula show that it pulses 30 times a second, so p = 0.0333 second, but the time interval between pulses is growing longer by 4.2 x 10-13 second with each passing second, so r = 4.2 x 10-13 second per second. Using that information, estimate the age of the Crab Nebula pulsar.

How does your estimate compare with the true age of the pulsar, which was born in the supernova observed in A.D. 1054?

58) A Water Black Hole. A clump of matter does not need to be extraordinarily dense in order to have an escape velocity greater than the speed of light, as long as its mass is large enough. You 4 πR3S, inside can use the formula for the Schwarzschild radius RS to calculate the volume, the event horizon of a black hole of mass M. What does the mass of a black hole need to be in order for its mass divided by its volume to be equal to the density of water ?

59) Energy of a Supernova. In a massive star supernova explosion, a stellar core collapses to form a neutron star roughly 10 kilometers in radius. The gravitational potential energy released in such a collapse is approximately equal to GM2 / r, where M is the mass of the neutron star, r is its radius, and G = 6.67 × 10-11 m3 / is the gravitational constant. Using this formula, estimate the amount of gravitational potential energy released in a massive star supernova explosion. How does it compare with the amount of energy released by the Sun during its entire main-sequence lifetime?

60) Energy of a Black Hole Merger. Models indicate that the gravitational-wave signal shown in Figure 18.20 came from the merger of two black holes with masses of 29MSun and 36MSun, a merger that resulted in a single black hole with a mass of 62MSun. The difference in total mass between the start and finish of the merger corresponds to the amount of energy carried away in the form of gravitational waves. Use Einstein's formula E = mc2 to calculate this amount of energy. Be sure to put the mass in units of kilograms and the speed of light in meters per second, so that the resulting energy has units of joules. Compare your answer to the energy released by a supernova (from Problem 59) and to the energy released by the Sun during its entire mainsequence life.

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61) Challenge Problem: A Neutron Star Comes to Town. Suppose a neutron star suddenly appeared in your hometown. How thick a layer would Earth form as it wrapped around the neutron star's surface? Assume that the layer formed by Earth has the same average density as the neutron star. (Hint: Consider the mass of Earth to be distributed in a spherical shell over the surface of the neutron star and then calculate the thickness of such a shell with the same mass as Earth. The volume of a spherical shell is approximately its surface area times its thickness: Vshell = 4πr2 × thickness. Because the shell will be thin, you can assume that its radius is the radius of the neutron star.)

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter 19 Our Galaxy Section 19.1 1) What is the approximate diameter of the disk of the Milky Way? A) 100 light-years B) 1000 light-years C) 10,000 light-years D) 100,000 light-years E) 1,000,000 light-years Answer: D 2) The thickness of the disk of the Milky Way is about ________ the diameter of the disk.

A) 1/10th of B) 1/1000th of C) 1/1,000,000th of D) the same as Answer: B 3) Approximately how far is the Sun from the center of the galaxy?

A) 27 light-years B) 270 light-years C) 2700 light-years D) 27,000 light-years E) 27 million light-years Answer: D 4) Which of the following are typically found in the halo of our galaxy?

A) open star clusters B) stars of spectral types O and B C) globular star clusters D) cold gas and dust E) all of the above Answer: C 5) Which of the following typically orbit within the disk of our galaxy?

A) stars of spectral types O and B B) globular star clusters C) the Large and Small Magellanic Clouds D) other galaxies Answer: A

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6) Which of the following represent the oldest stars of the Milky Way? A) the Sun and other stars with similar mass B) O stars C) red giant stars in spiral arms D) Cepheid variable stars E) stars in globular clusters Answer: E 7) How did Harlow Shapley conclude that the Sun was not in the center of the Milky Way Galaxy?

A) by looking at the shape of the "milky band" across the sky B) by mapping the distribution of stars in the galaxy C) by mapping the distribution of globular clusters in the galaxy D) by mapping the distribution of gas clouds in the spiral arms E) by looking at other nearby spiral galaxies Answer: C 8) Why can't we see the center of our galaxy with our eyes?

A) We are in the center of the Milky Way. B) There are no stars in the center of the Milky Way, just a supermassive black hole.

C) Interstellar dust and gas absorbs and scatters visible light before it can reach us from the center.

D) The center of the Milky Way does not emit enough visible light.

E) Stars are too densely spaced in the plane of our galaxy to see all the way to the center.

Answer: C 9) Approximately how long does it take stars in our region of the Milky Way to complete an orbit around the center of the Milky Way Galaxy?

A) 20,000 years B) 200,000 years C) 2 million years D) 200 million years E) 2 billion years Answer: D 10) Which of the following best describes the orbits of stars in the halo?

A) Halo stars have orbits confined to a relatively thin plane.

B) Halo stars have elliptical orbits, with random orientations.

C) Halo stars have elliptical orbits, but all orbiting in the same direction.

D) Some halo stars orbit around the galactic center but others never pass through the plane of the galactic disk.

E) Halo stars orbit the centers of their globular clusters, but do not orbit the center of the galaxy.

Answer: B

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11) What observational evidence supports the conclusion that most of the mass of the Milky Way is in the form of dark matter? A) There is no observational evidence; it is only a theoretical idea based on computer models indicating that the Milky Way could not have formed unless it had much more mass than it contains in stars and gas. B) Although dark matter emits no visible light, we have detected it with radio telescopes. C) We can see the dark matter as dark blotches in the sky, such as the dark lanes that are visible within the Milky Way in the night sky. D) The orbital speeds of stars and gas clouds far from the galactic center are surprisingly high, suggesting that these stars and gas clouds are feeling gravitational effects from unseen matter. Answer: D 12) The average orbital speed of a star around the center of our galaxy depends primarily on A) the mass of the Milky Way's central black hole. B) the mass of the entire galaxy, including the halo. C) whether the star is in the disk or halo of the galaxy. D) the mass of the galaxy inside its orbit.

Answer: D Section 19.2 1) What does the interstellar medium consist of? A) many individual stars of varying mass B) intermediate mass stars C) asteroids, comets, and planets D) gas and dust Answer: D 2) What do astronomers typically refer to as "heavy elements"? A) elements that are heavier than iron B) elements that are heavier than carbon C) elements that are heavier than uranium D) elements besides hydrogen and helium Answer: D 3) How can we see through the interstellar medium? A) by observing in high-energy wavelengths such as x-rays and long wavelengths of light such as radio waves and infrared B) by observing only the brightest visible sources C) by using only the biggest telescopes D) by using telescopes above the Earth's atmosphere E) We cannot see through the interstellar medium. Answer: A

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4) If you were to take a voyage across the Milky Way, in what kind of material would you spend most of your time?

A) empty space–a pure vacuum B) dusty molecular clouds C) star clusters D) warm clouds of atomic hydrogen Answer: D 5) What is a superbubble?

A) a very low-density region of interstellar space, formed by the merger of supernova remnants B) a region filled with hot gas from a single supernova explosion C) It is another name for the large volume of space in the galaxy's halo.

D) It is the roundish region cleared out by the expanding gas of a planetary nebula.

E) a dense cloud of gas that can form many new stars Answer: A 6) What is a shock front?

A) a wave of pressure that moves faster than the speed of sound B) a wave of pressure that moves slightly slower than the speed of sound C) a wave of pressure that moves faster than the speed of light D) a wave of electromagnetic energy that can create electrical shocks E) the wave created when protons slam into electrons Answer: A 7) What are cosmic rays?

A) subatomic particles that travel close to the speed of light B) a form of light with shorter wavelength than gamma rays C) fast-moving dust particles in the interstellar medium D) any light waves from space Answer: A 8) According to the "galactic fountain" model, what can create a superbubble emerging into the halo?

A) winds and jets from newly formed protostars B) a supernova occurring in the halo C) multiple supernovae occurring near one another around the same time D) the combined effect of spiral density waves E) molecular clouds expelled from the galactic center Answer: C

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9) What is the galactic fountain model?

A) a model in which interstellar water vapor can condense to form liquid water rain drops falling toward the galactic disk B) a model that assumes the Milky Way is a spiral galaxy and looks like a whirlpool from above C) a model in which jets of hot, ionized gas blow out of the galactic center and leave the galaxy forever D) a model in which superbubbles blow hot, ionized gas out of the galactic disk and into the halo, where the gas cools down and falls back into the disk Answer: D 10) Where are most elements heavier than hydrogen and helium made?

A) in the gas between the stars B) in the cores of stars, in supernovae, and during neutron star mergers C) All elements were made about 5 minutes after the universe began.

D) in the cool atmospheres of red giant stars E) in white dwarfs Answer: B 11) What is the primary way in which stars with masses similar to the Sun ultimately return some of their material to the interstellar medium?

A) by exploding as white dwarf supernovae B) through jets they may have during the protostar stage C) through planetary nebulae D) through the winds they have during their main-sequence lives E) Stars with masses similar to the Sun do not recycle any material back into the interstellar medium.

Answer: C 12) What is the primary way in which high-mass stars recycle material into the interstellar medium?

A) through stellar winds and supernova explosions B) through the formation of planetary nebulae C) through protostar formation D) through stellar winds alone Answer: A 13) How are interstellar bubbles of hot, ionized gas made?

A) by the collapse of a gas cloud to form stars B) by the ejection of planetary nebulae from low-mass stars C) by the winds of massive stars and supernovae explosions D) by collisions between the Milky Way and satellite dwarf galaxies E) by the rapidly rotating magnetic fields of pulsars Answer: C

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14) Sound waves in the interstellar medium A) cannot exist at all.

B) travel so slowly that they are undetectable.

C) cannot be heard by human ears, but do indeed travel through the gas.

D) travel extremely quickly and are therefore very loud.

Answer: C 15) Which of the following indicates the presence of high-mass stars in a region of recent star formation?

A) closely spaced low-mass stars B) ionization nebulae C) globular clusters D) planetary nebulae Answer: B 16) How should we expect that the interstellar medium of the Milky Way will be different in 50 billion years?

A) The total amount of gas will be much less than it is today.

B) Thanks to the recycling of the star-gas-star cycle, the interstellar medium should look about the same in 50 billion years as it does today.

C) The total amount of gas will be much greater, since many stars will undergo supernovae between now and then.

D) The total amount of gas will be about the same, but it will contain a much higher percentage of elements heavier than hydrogen and helium.

Answer: A 17) What produces the 21-cm line that we use to map out the Milky Way Galaxy?

A) atomic hydrogen B) ionized hydrogen C) molecular hydrogen D) carbon monoxide E) helium Answer: A 18) Where do most dust grains form?

A) in supernovae B) in the winds of red giant stars C) in planetary nebulae D) in molecular clouds Answer: B

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19) The image of our galaxy in radio emission from CO, mapping the distribution of molecular clouds, is closest to the image of our galaxy in

A) radio emission from H atoms.

B) short wavelength infrared light from most of the stars.

C) visible light, which is closest to how the night sky appears from Earth.

D) x-rays from hot gas bubbles in the disk.

E) gamma ray from cosmic-ray collisions with gas atoms.

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20) Where does most star formation occur in the Milky Way today?

A) in the halo B) in the bulge C) in the spiral arms D) in the galactic center E) uniformly throughout the Galaxy Answer: C 21) Where would you least expect to find an ionization nebula?

A) in the galactic disk B) near a hot, young star C) in a spiral arm D) in the halo of our galaxy Answer: D Section 19.3 1) Which of the following best describes stars in the halo of our Galaxy?

A) young, red, and dim with low abundance of heavy elements B) young, blue, and bright with high abundance of heavy elements C) old, red, and dim with low abundance heavy elements D) old, red, and dim with high abundance of heavy elements E) old, red, and bright with low abundance of heavy elements Answer: C 2) Which of the following provides the strongest evidence that halo stars are older, on average, than disk stars?

A) Halo stars orbit in random directions, but disk stars have more ordered orbits.

B) There are no blue halo stars.

C) There are no red disk stars.

D) Theories of galaxy formation tell us that the halo formed earlier than the disk.

Answer: B 3) Which of the following would you expect to be very common in globular clusters?

A) high-mass main-sequence stars B) low-mass main-sequence stars C) supernova remnants D) x-ray binaries E) supergiant stars Answer: B

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4) Which of the following statements about the disk of the Milky Way is generally true? A) The typical age of disk stars is greater than that of halo stars. B) Disk stars are all younger than 5 billion years. C) Disk stars have a higher proportion of heavy elements, on average, than halo stars. D) Disk stars orbit around the Galactic center in orbits of random inclination and direction. E) The diameter of the disk is about 1 million times its thickness.

Answer: C 5) Which of the following statements is generally true for stars in the halo? A) Halo stars all orbit the center of the galaxy in approximately the same plane. B) Halo stars always remain in the halo, never passing through the disk of the galaxy. C) All halo stars will end their main-sequence lives within the next 100 million years. D) Halo stars have a much smaller proportion of elements besides hydrogen and helium than our Sun. E) Halo stars come in a wide variety of ages. Answer: D 6) Why are stars no longer forming in the galactic halo? A) Frequent supernovae blew all the halo gas out into intergalactic space. B) There was never star formation in the halo; its stars were ejected from the disk. C) Only low-mass stars formed in the halo, so there were no stellar winds or supernovae to recycle gas. D) The cold molecular gas required for star formation is in the galactic disk. Answer: D 7) What evidence suggests that some of the stars in the halo of the Milky Way Galaxy came from mergers with other galaxies? A) Globular clusters are typically found only in the halo.

B) Some halo stars with similar and distinct chemical compositions move together in organized streams. C) Some halo stars are older than our Galaxy. D) Halo stars have randomly oriented orbits. Answer: B 8) Suppose you were able to measure the composition of the first star that formed in the Milky Way's history. What would you expect it to have been made of at the time of its birth? A) 100% hydrogen B) 75% hydrogen, 25% helium, and less than 0.1% other elements C) 98% hydrogen, 2% helium, and less than 0.1% other elements D) 70% hydrogen, 28% helium, and 2% other heavy elements Answer: B

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Section 19.4 1) The galactic center lies in the direction of which constellation?

A) Orion B) the Big Dipper C) Leo D) Sagittarius E) Taurus Answer: D 2) Which of the following provides the strongest evidence to support the claim that there is a black hole at the center of our galaxy?

A) We observe many sources of x-rays near the center of our galaxy.

B) Infrared and radio observations show a large concentration of gas near the center of our galaxy.

C) The motions of the gas and stars indicate that the galactic center has a mass millions of times that of the Sun within a region less than 1 light-year across.

D) We observe a large, dark object that absorbs all light at the center of our galaxy.

Answer: C 3) What is SgrA*?

A) a source of bright x-ray emission coming from the entire constellation of Sagittarius B) a source of bright radio emission in the center of our galaxy C) a source that is bright in the visible wavelengths in the center of our galaxy D) the brightest star in the constellation Sagittarius E) the bulge at the center of our galaxy Answer: B 4) How do we study the region near the center of our own galaxy, the Milky Way? A) high-resolution images acquired with visible light telescopes B) observations at radio, infrared and x-ray wavelengths C) We cannot directly observe the galactic center, so we infer its characteristics from theoretical models.

D) We look at the centers of other galaxies and assume that our Milky Way's center is similar.

E) We can study the galactic center only by observing gravitational waves.

Answer: B 5) What is thought to be the origin of enormous x-ray flares that occasionally come from the direction of Sgr A*?

A) x-ray binary systems that lie close to the galactic center B) massive-star supernovae near the galactic center C) comet-sized objects being torn apart by the tidal forces of the black hole near the galactic center D) supernova shock waves plowing into dense regions of the interstellar medium near the galactic center Answer: C

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General Chapter Questions 1) How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?

A) It should have a higher fraction of elements heavier than hydrogen and helium.

B) It should orbit the galactic center at a much higher rate of speed.

C) It should be higher in mass.

D) It should be moving very fast relative to the Earth.

Answer: A Short Answer Questions 1) Briefly explain why stars that formed early in the history of the galaxy contain a smaller proportion of heavy elements than stars that formed more recently.

Answer: The star-gas-star cycle gradually enriches the interstellar medium with heavy elements.

Therefore, stars that formed early in the history of the galaxy were formed before much enrichment from supernova events could take place. Stars that formed more recently were formed from material that had been enriched by the many previous generations of stars.

2) Explain why, even though there are sound waves in space, it is still true that "no one can hear you scream" (assuming that the sound of your scream travels only through space, not through radio communication).

Answer: Screams, and any sound that we make, are waves of gas atoms and molecules. We perceive sound when these waves of gas particles strike our eardrums and cause them to vibrate, but it takes many trillions of such particles to move them noticeably. In interstellar space, where the gas density is extremely low, there are too few (if any) collisions with our eardrums for us to hear sound.

3) Why do spiral arms tend to be blueish in color?

Answer: They are regions of enhanced star formation, and in general, only young stars are blue.

4) Briefly describe how we can use the orbital characteristics of stars at many distances from the galactic center to determine the distribution of mass in the Milky Way. Answer: Using the orbital velocity law, related to Newton's version of Kepler's third law, we can determine the mass of the galaxy that lies within a particular orbit (and thus distance from the center) if we know the average orbital velocity of stars or clouds at that distance. Therefore, by applying the orbital velocity law to the orbits of stars at many distances from the galactic center, we can determine how much mass lies within each radius.

Reading Quiz Questions 1) How does the diameter of the disk of the Milky Way Galaxy compare to its thickness?

A) The diameter is about 100 times as great as the thickness.

B) The diameter and thickness are roughly equal.

C) The diameter is about 10 times as great as the thickness.

D) The diameter is about 100,000 times as great as the thickness.

Answer: A

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2) What do we call the bright, sphere-shaped region of stars that occupies the central few thousand light-years of the Milky Way Galaxy?

A) the galaxy's disk B) the galaxy's bulge C) a globular cluster D) the galaxy's halo Answer: B 3) The Sun's location in the Milky Way Galaxy is A) near the galactic center.

B) in the galactic disk, roughly halfway between the center and the outer edge of the disk.

C) in the halo of the galaxy, about 27,000 light-years above the galactic disk.

D) at the outer edge of the galactic disk.

Answer: B 4) What do we mean by the interstellar medium?

A) the gas and dust that lies in between the stars in the Milky Way galaxy B) the dust that fills the halo of the Milky Way galaxy C) the middle section of the Milky Way galaxy D) the name of an oracle who can channel messages from beings that live near the star called Vega Answer: A 5) What are the Magellanic Clouds?

A) two small galaxies that orbit the Milky Way Galaxy B) two nebulae located in the disk of the Milky Way galaxy and visible only from the Southern Hemisphere C) star-forming clouds found in the constellation Orion D) the clouds of dust and gas found interspersed in many places throughout the Milky Way Galaxy Answer: A 6) How do disk stars orbit the center of the galaxy?

A) They all orbit in roughly the same plane and in the same direction.

B) They have orbits randomly inclined and in different directions relative to the galactic center.

C) They follow spiral paths along the spiral arms.

D) They follow orbits that move up and down through the disk, typically taking them about 50,000 light-years above and below the disk on each orbit.

Answer: A

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7) How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path?

A) by applying Newton's version of Kepler's third law (or the equivalent orbital velocity formula) to the Sun's orbit around the center of the Galaxy B) by counting the number of stars visible in this region of the galaxy C) by estimating the amount of gas and dust in between the stars D) by using the law of conservation of angular momentum to calculate the orbital speeds of nearby stars Answer: A 8) Elements heavier than hydrogen and helium constitute about ________ of the mass of the Milky Way's interstellar medium.

A) 0.002 % B) 2 % C) 70 % D) 98% Answer: B 9) What do we mean by the star-gas-star cycle?

A) It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium.

B) It is the idea that stars in close binary systems can exchange gas with one another.

C) It is the set of nuclear reactions by which heavy elements are produced in the cores of massive stars.

D) It describes the orbits of the stars and interstellar medium around the center of the galaxy.

Answer: A 10) What are cosmic rays?

A) subatomic particles that travel close the speed of light B) another name for gamma rays and x-rays C) fast moving dust particles in the interstellar medium D) the highest-energy form of light E) lasers used as weapons by extraterrestrials Answer: A 11) The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with A) radio telescopes observing at a wavelength of 21 centimeters.

B) ground-based visible-light telescopes.

C) space-based ultraviolet telescopes.

D) x-ray telescopes.

Answer: A

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12) Which of the following models best explains why our galaxy has spiral arms?

A) The spiral arms are a wave of star formation caused by a wave of density propagating outward through the disk of the galaxy.

B) The spiral arms are composed of groups of stars that are bound together by gravity and therefore always stay together as the galaxy rotates.

C) The spiral arms were imprinted on the galaxy at its birth. Ever since, like a coiling rope, the spiral arms have been wound tighter with each galactic rotation.

D) No model can explain the existence of the arms, which rotate with the galaxy like the fins of a giant pinwheel toy.

Answer: A 13) What do we mean by a protogalactic cloud?

A) a cloud of hydrogen and helium that contracts to become a galaxy B) a term once used historically to refer to any galaxy C) a cloud-like halo that surrounds the disks of spiral galaxies D) a cloud of gas that was once a galaxy Answer: A 14) Most stars in the Milky Way's halo are A) very old.

B) found inside molecular clouds.

C) very young.

D) blue or white in color.

Answer: A 15) What is an ionization nebula?

A) a region of hot, low-density gas surrounding a recent supernova B) a clump of gas that will soon give birth to a new star C) a colorful cloud of gas that glows because it is heated by light from nearby hot stars D) a name sometimes used to describe spiral galaxies besides the Milky Way Answer: C 16) Where does most star formation occur in the Milky Way Galaxy?

A) in the spiral arms B) everywhere throughout the galactic disk C) in the central bulge D) within the halo Answer: A 17) What do halo stars do differently from disk stars?

A) Halo stars orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane.

B) Halo stars remain stationary, quite unlike disk stars that orbit the galactic center.

C) Halo stars explode as supernovae much more frequently than disk stars.

D) Halo stars orbit the center of the galaxy at much lower speeds than disk stars.

Answer: A

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18) Based on observations, which of the following statements about stars in the Milky Way is generally true?

A) the older the star, the lower its abundance of heavy elements B) the older the star, the bluer its color C) the older the star, the faster its orbital speed D) the younger the star, the higher its mass E) the less massive the star, the older it is Answer: A 19) What kind of object do we think lies in the center of the Milky Way Galaxy? A) a black hole of about 4 million solar masses B) a gigantic x-ray binary system C) a dense cluster of young, hot stars D) an enormous collection of dark matter, explaining why we detect no light at all from the galactic center Answer: A Concept Quiz Questions 1) If we could see our own galaxy from 2 million light-years away, it would appear A) as a flattened disk with a central bulge and spiral arms, spanning a few degrees across the sky. B) as a faintly glowing band of light stretching all the way around the sky. C) to fill the sky with widely spaced stars. D) like a single, dim star. Answer: A 2) How does the interstellar medium affect our view of most of the galaxy? A) It prevents us from seeing most of the galactic disk with visible and ultraviolet light. B) It absorbs all wavelengths of light. C) It produces so much visible light that it blocks our view of anything beyond it. D) It has no effect on visible-light observations, but prevents us from studying the galactic center with radio waves or x-rays.

Answer: A 3) Applying Newton's version of Kepler's third law (or the orbital velocity law) to the a star orbiting 40,000 light-years from the center of the Milky Way galaxy allows us to determine A) the mass of the Milky Way Galaxy that lies within 40,000 light-years of the galactic center. B) the total mass of the entire Milky Way Galaxy. C) the mass of the black hole thought to reside in the center of the galaxy. D) the percentage of the galaxy's mass that is made of dark matter. Answer: A

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4) How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?

A) It should have a higher fraction of elements heavier than hydrogen and helium.

B) It should be higher in mass.

C) It should be much more luminous.

D) It should orbit the galactic center at a much higher rate of speed.

E) All of the above differences would be expected.

Answer: A 5) Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not?

A) The 10% figure is too high because there are not enough heavy elements to make that much dust.

B) It is reasonable because we already know that interstellar dust obscures our view through the disk of the galaxy.

C) The 10% figure is too low because most of the mass of the galaxy is in the form of interstellar dust.

D) It seems reasonable as long as we assume that red giant stars, which produce dust grains in their stellar winds, are more common than we thought.

Answer: A 6) The most common form of gas in the disk of the Milky Way galaxy is A) atomic hydrogen gas.

B) molecular hydrogen.

C) gas in hot bubbles.

D) gas in stellar winds.

Answer: A 7) How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today?

A) The total amount of gas will be much less than it is today.

B) The total amount of gas will be about the same, but the percentage of elements heavier than hydrogen and helium will have risen from the current 2% to more than 50%.

C) The total amount of gas will be much greater because many stars will undergo supernovae between now and then.

D) Thanks to the recycling of the star-gas-star cycle, the interstellar medium should look about the same in 50 billion years as it does today.

Answer: A 8) Over time, the star-gas-star cycle leads the gas in the Milky Way to A) have a greater abundance of heavy elements.

B) have a lower abundance of heavy elements.

C) become denser and have a greater abundance of heavy elements.

D) become denser and hotter.

Answer: A

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9) Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?

A) an x-ray telescope in space B) a visible-light telescope on a mountain summit C) an infrared telescope in space D) the Hubble Space Telescope Answer: A 10) If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?

A) gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles B) gas that changes only in very slow and steady ways, so that the movie would in fact be quite boring C) The entire disk of the Milky Way would pulsate in and out as it contracts to form stars and then blows out in supernovae and then contracts to form stars again and so on.

D) The movie would alternate back and forth between being very bright when there is a lot of gas and very dark when there is very little gas.

Answer: A 11) What observational evidence supports the galactic fountain model, which describes how gas cycles between the disk of the galaxy and regions high above the disk?

A) We see hot gas high above the region of the disk near our solar system, along with cool gas that appears to be raining down from the halo.

B) We have discovered a jet of ionized gas shooting out of the bulge of our galaxy.

C) We have discovered that the entire galactic disk is being uniformly "rained on" by cool gas coming from the halo.

D) We have observed a lot of water molecules in the interstellar medium.

Answer: A 12) All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except A) globular clusters.

B) young stars.

C) x-ray binaries.

D) high-mass, red supergiant stars.

Answer: A 13) Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found A) only in or near star-forming clouds.

B) in the halo.

C) only in the central bulge.

D) also evenly spread throughout the galactic disk.

Answer: A

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14) Which of the following statements comparing halo stars to our Sun is not true?

A) Most stars in the halo have either died or are in their final stages of life, while the Sun is only in about the middle of its lifetime.

B) Most stars in the halo have cooler surface temperatures than the Sun. C) Most stars in the halo are less luminous than the Sun. D) Most stars in the halo contain a much lower percentage of heavy elements than the Sun. Answer: A 15) Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed?

A) It is probably a halo star that is currently passing through the disk.

B) It is a very young star, recently formed.

C) It has been pushed to high speed by the shock wave from a nearby supernova.

D) It is a high mass star.

Answer: A 16) What evidence suggests that most of the mass of the Milky Way is in the form of dark matter?

A) The orbital speeds of stars far from the galactic center are surprisingly high.

B) Although dark matter emits no visible light, we have detected its radio emissions.

C) Theoretical models of galaxy formation suggest that a galaxy cannot form unless it has at least 10 times as much matter as we see in the Milky Way disk.

D) Our view of distant galaxies is often obscured by dark blotches, which are presumably made of dark matter.

Answer: A 17) Spiral arms appear bright because A) they contain more hot young stars than other parts of the disk.

B) they contain far more stars than other parts of the galactic disk.

C) they contain more molecular clouds than other parts of the disk.

D) they are the only places where we find stars within the disk of the galaxy.

Answer: A 18) How did star formation likely proceed in the protogalactic cloud that formed the Milky Way? A) The stars that formed first could orbit the center of the galaxy in any direction at any inclination.

B) The stars that formed first eventually settled into a galactic disk, circling the center of the galaxy.

C) The protogalactic cloud gradually formed stars, starting from the center of the galaxy working outward.

D) The protogalactic cloud gradually formed stars, starting from the outer edges of the spiral arms and working inward.

Answer: A

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19) If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen?

A) Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out.

B) The spiral arms will seem to "wind up," wrapping more and more tightly around the center of the Galaxy.

C) The spiral arms will quickly dissipate and fade away, because they are a temporary phenomenon that should only last for a million years or so.

D) The spiral arms will eventually unwind, as centripetal forces send the stars flying outward into intergalactic space.

Answer: A 20) What is the best evidence for an extremely massive black hole in the center of the Milky Way? A) The orbits of stars in the center of the galaxy indicate that the presence of an approximately 4 million solar-mass object in a region no larger than our solar system.

B) Huge amounts of x-rays are pouring out of the center of the galaxy.

C) The center of our galaxy hosts a pulsar that is spinning so fast that it could only be a black hole.

D) We observe stars vanishing in the center of the Galaxy as they are sucked into the black hole.

Answer: A 21) Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy?

A) It is by far the brightest source of visible light lying in the direction of the galactic center.

B) It is thought to harbor a black hole of about 4 million solar masses.

C) It is a source of x-ray emission that we have observed with telescopes in space.

D) It is a source of bright radio emission.

Answer: A

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Visual Quiz Questions

1) This diagram represents an edge-on view of our Milky Way Galaxy. Of the four labeled points, which one is located in what we call the halo of the galaxy?

A) point 1 B) point 2 C) point 3 D) point 4 Answer: D 2) This diagram represents an edge-on view of our Milky Way Galaxy. Of the four labeled points, which one could represent the Sun's position in the galaxy?

A) point 1 B) point 2 C) point 3 D) point 4 Answer: A 3) This diagram represents an edge-on view of our Milky Way Galaxy. Of the four labeled points, which one is located closest to a place where evidence suggests we would find a 4 million solar-mass black hole?

A) point 1 B) point 2 C) point 3 D) point 4 Answer: B

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4) This photograph shows an interstellar bubble about 10 light-years in diameter. If you could photograph this same region about 100 years from now, how would you expect it to look different?

A) The bubble will be slightly larger.

B) The bubble will be slightly smaller.

C) The bubble will have burst as a supernova.

D) The bubble's shape will have become much more like an oval rather than a sphere.

Answer: A

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5) This photograph shows x-ray emission from a supernova remnant. What is the source of the xrays?

A) hot gas expanding outward from the site of a supernova B) cold gas that will soon give birth to new stars C) accretion disk around a black hole near the center of the supernova remnant D) spots within the remnant where hot gas is crashing into and destroying planets Answer: A

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6) What are the dark blobs (indicated by the red arrows) in this photograph from the Hubble Space Telescope?

A) cold, dense molecular clouds in which stars are forming B) warm atomic hydrogen gas, much like most of the gas in the galaxy C) tall interstellar bubbles blown out by supernovae or stellar winds D) jets of hot gas shot out from young binary star systems Answer: A

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7) The top panorama shows our view of the Milky Way in all directions as it appears in visible light. The bottom panorama shows the same view, but in a different wavelength of light. What wavelength band are we seeing in the bottom photo, and how do you know?

A) infrared light, because the dust that appears dark in the visible light photo glows in infrared light B) x-ray light, because only x-rays allow us to see through the disk of our galaxy C) radio waves, because clouds of gas in the disk of our galaxy appear pinkish when we look at them with radio telescopes D) infrared light, because clouds of gas and dust appear pinkish when we look at them with infrared telescopes Answer: A

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8) Notice the well-defined spiral arms in this photograph of the galaxy M51. What makes the spiral arms so much brighter than regions between the arms?

A) the presence of many massive young stars in the spiral arms B) the presence of many old, red giant stars in the spiral arms C) the presence of many hot interstellar bubbles in the spiral arms D) The spiral arms are bright because nearly all the galaxy's stars are located within them.

Answer: A 9) What law of nature explains why the galaxy began to rotate rapidly and flatten out as it shrunk in size?

A) the law of conservation of angular momentum B) Newton's universal law of gravitation C) Newton's third law of motion D) Einstein's special theory of relativity Answer: A

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10) This figure shows observations that have allowed astronomers to determine the orbits of several stars around Sgr A* (the central object). What can we learn about Sgr A* by analyzing these orbits?

A) its mass B) its age C) its chemical composition D) all of the above Answer: A

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End-of-Chapter Questions Visual Skills Check Use the following questions to check your understanding of some of the many types of visual information used in astronomy.

The images to the right show what the central region of our galaxy looks like from our viewpoint in different wavelengths of light. Each image shows the same region of the sky. Use the following questions to check your understanding of what these images show and what we can learn by comparing them.

1) The image of radio emission uses different colors to represent different levels of intensity.

Which color represents the brightest radio emission? Which color represents the lowest levels of brightness?

Answer: Brightest: white; lowest levels of brightness: black/dark blue 2) The image of x-ray emission uses different colors to represent different levels of brightness.

The dark blue color represents the least bright x-ray emission. Which color represents the brightest x-ray emission?

Answer: White 3) How do regions showing strong radio emission from molecules look in the visible-light image? Are they bright or are they dark?

Answer: Regions with strong radio emissions are dark in the visible-light image.

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4) How do regions showing strong radio emission from molecules look in the infrared-light image? Are they bright or are they dark?

Answer: Regions with strong radio emissions are dark in the infrared-light image.

Note:( students may find this a bit difficult to see at the scale shown in the images, but the darkest infrared regions clearly correspond to strong radio emissions.)

5) Based on a comparison of the radio, infrared, and visible-light images, we can conclude that gas clouds containing molecules A) absorb roughly equal amounts of infrared starlight and visible starlight B) absorb substantial amounts of visible starlight but no infrared starlight C) absorb infrared starlight less effectively than they absorb visible starlight Answer: C 6) Compare the radio and x-ray images. Can you conclude from this comparison that gas clouds containing molecules absorb x-ray light? Answer: Yes Chapter Review Questions 1) Draw simple sketches of our galaxy as it would appear face-on and edge-on.

Identify the disk, bulge, halo, and spiral arms, and indicate the galaxy's approximate dimensions. 2) What are the Large and Small Magellanic Clouds, and the Sagittarius and Canis Major Dwarfs? 3) Describe the basic characteristics of stars' orbits in the disk, halo, and bulge of our galaxy. 4) How can we use orbital properties to learn about the mass of the galaxy? What have we learned? 5) Summarize the star—gas—star cycle shown in Figure 19.3. 6) What creates a bubble of hot, ionized gas? What happens to the gas in the bubble over time? 7) Why do star clusters make superbubbles? What happens to those bubbles when they expand beyond the galactic disk? 8) How does a galactic fountain help circulate new elements within the Milky Way? 9) What are cosmic rays? Where do they come from? 10) What do we mean by atomic hydrogen gas? How common is it, and how do we map its distribution in the galaxy? 11) Briefly summarize the different types of gas present in the disk of the galaxy, and describe how they appear when we view the galaxy in different wavelengths of light.

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12) What are ionization nebulae, and why are they found near hot, massive stars?

13) How do we know that spiral arms do not rotate like giant pinwheels? What makes spiral arms bright?

14) What triggers star formation within a spiral arm? How do we think spiral arms are maintained?

15) Briefly describe the characteristics that distinguish disk stars from halo stars.

16) What evidence suggests that the Milky Way formed from the merger of smaller protogalactic clouds?

17) What is Sgr A*? What evidence suggests that it contains a massive black hole?

Does It Make Sense?

Decide whether or not each of the following statements makes sense (or is clearly true or false).

Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

Example: I walked east from our base camp at the North Pole.

Solution: The statement does not make sense because east has no meaning at the North Pole–all directions are south from the North Pole.

18) We did not understand the true size and shape of our galaxy until NASA launched satellites into the galactic halo, enabling us to see what the Milky Way looks like from the outside.

19) Planets like Earth probably didn't form around the very first stars because there were so few heavy elements back then.

20) If I could see infrared light, the galactic center would look much more impressive.

21) Many spectacular ionization nebulae are seen throughout the Milky Way's halo.

22) The carbon in my diamond ring was once part of interstellar dust grains.

23) The Sun's velocity around the Milky Way tells us that most of our galaxy's dark matter lies near the center of the galactic disk.

24) We know that a black hole lies at our galaxy's center because numerous stars near it have vanished over the past several years, telling us that they've been sucked in.

25) If we could watch a time-lapse movie of a spiral galaxy over millions of years, we'd see many stars being born and dying within the spiral arms.

26) The star—gas—star cycle will keep the Milky Way looking just as bright in 100 billion years as it looks now.

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27) Halo stars orbit the center of our galaxy much faster than the disk stars.

Quick Quiz 28) What is the shape of the Milky Way's halo? A) round like a ball B) flat like a disk C) flat like a disk but with a hole in the center Answer: No Correct Answer Was Provided. 29) Where are most of the Milky Way's globular clusters found? A) in the disk B) in the bulge C) in the halo Answer: No Correct Answer Was Provided. 30) Why do disk stars bob up and down as they orbit the galaxy? A) because the gravitational pull of other disk stars always pulls them toward the disk B) because of friction with the interstellar medium C) because the halo stars keep knocking them back into the disk Answer: No Correct Answer Was Provided. 31) How do we determine the Milky Way's mass outside the Sun's orbit? A) from the Sun's orbital velocity and its distance from the center of our galaxy B) from the orbits of halo stars near the Sun C) from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun Answer: No Correct Answer Was Provided. 32) Which part of the galaxy has gas with the hottest average temperature? A) the disk B) the halo C) the bulge Answer: No Correct Answer Was Provided. 33) What is the typical percentage (by mass) of elements other than hydrogen and helium in stars that are forming right now in the vicinity of the Sun? A) 20% B) 2% C) 0.02% Answer: No Correct Answer Was Provided.

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34) Which of these forms of radiation passes most easily through the disk of the Milky Way? A) red light B) blue light C) infrared light Answer: No Correct Answer Was Provided.

35) Where would you be most likely to find an ionization nebula?

A) in the halo B) in the disk between spiral arms C) in a spiral arm Answer: No Correct Answer Was Provided.

36) Which kind of star is most likely to be found in the halo?

A) an O star B) an A star C) an M star Answer: No Correct Answer Was Provided.

37) We measure the mass of the black hole at the galactic center from A) the orbits of stars in the galactic center.

B) the orbits of gas clouds in the galactic center.

C) the amount of radiation coming from the galactic center.

Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

38) Group Discussion: Alternative Names for the Milky Way. Today, we call our galaxy the Milky Way Galaxy, after the band of light that appeared "milky" to the ancient Greeks. But other cultures had other names for that band of light. a) Working independently, learn at least three other names that have been used for the Milky Way and the stories behind those names. Then gather in small groups and share the names and stories that each of you found. b) Discuss and then vote on the name other than "Milky Way" that your group favors. What does your group think are the strengths of that name? c) Discuss the reasons your group's favorite name did not become widely used as the name of our galaxy, and make a list of cultural factors that may have favored the name "Milky Way Galaxy."

Do you think some of those other names should become better known? Why or why not?

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The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

39) Discovering the Structure of the Milky Way. The story of how we came to learn the structure of the Milky Way (see the Special Topicon page 582) provides an excellent example of how science progresses. What features of the Milky Way's appearance in our sky led scientists to conclude that it is much wider than it is thick? Why did they originally believe that the Sun was near the Milky Way's center? What key observations forced scientists to change their views about the location of the Sun within the Milky Way? 40) Formation of the Milky Way. Figure 19.18 outlines a basic model that accounts for some but not all of the features of the Milky Way. What observational evidence indicates that the Milky Way's protogalactic cloud contained virtually no elements other than hydrogen and helium? What evidence suggests that the halo stars formed first and disk stars formed later? What features of the Milky Way are not explained by this basic model? 41) Galactic Ecosystem. We have likened the star—gas—star cycle in our Milky Way to the ecosystem that sustains life on Earth. Here on our planet, water molecules cycle from the sea to the sky to the ground and back to the sea. Our bodies convert atmospheric oxygen molecules into carbon dioxide, and plants convert the carbon dioxide back into oxygen molecules. How are the cycles of matter on Earth similar to the cycles of matter in the galaxy? How do they differ? Do you think the term ecosystem is appropriate in discussions of the galaxy? 42) Galaxy Stuff.In the chapters on stars, we learned why we are "star stuff." Based on what you've learned in this chapter, explain why we are also "galaxy stuff." Does the fact that the entire galaxy was involved in bringing forth life on Earth change your perspective on Earth or on life in any way? If so, how? If not, why not?

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43) Group Activity: Star Clusters and Milky Way Structure. Use the all-sky diagram in Appendix H and the information about globular and open clusters below to do the following.Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39.

■ Globular clusters are found in the following constellations: Aquarius, Aquila, Canes Venatici, Capricornus, Carina, Centaurus, Columba, Coma Berenices, Hercules, Hydra, Lepus, Lynx, Lyra, Musca, Ophiuchus, Pegasus, Puppis, Sagittarius, Scorpius, Sculptor, Serpens, Tucana, and Vela.

■ Open clusters with ages of 100 million years or less are found in the following constellations: Canis Major, Carina, Cassiopeia, Crux, Cygnus, Monoceros, Norma, Ophiuchus, Perseus, Puppis, Sagittarius, Scorpius, Scutum, Serpens, Taurus, and Vela. a) Split your group into two teams (e.g., two people each), with one team focusing on globular clusters and the other on open clusters. The members of each team should work together to locate on the all-sky map(Appendix H) the constellations in which their clusters are found and to identify patterns in these locations relative to the Milky Way's disk and relative to the galactic center. b) The full group should compare the locations of the two sets of clusters and describe the differences you've found. c) One member of the group should offer a hypothesis to explain the differences. Others can then suggest alternative hypotheses. d) Discuss the group's set of hypotheses and list possible observations that astronomers could perform to determine which hypothesis is more likely to be correct.

Investigate Further 44) Unenriched Stars. Suppose you discovered a star made purely of hydrogen and helium. How old do you think it would be? Explain your reasoning.

45) Enrichment of Star Clusters. The gravitational pull of an isolated globular cluster is weak enough that a single supernova explosion can blow all of the cluster's interstellar gas out of the cluster. How might this fact be related to observations indicating that stars ceased to form in globular clusters long ago? How might it be related to the fact that globular clusters are deficient in elements heavier than hydrogen and helium? Summarize your answers in one or two paragraphs.

46) High-Velocity Star. The average speed of stars in the solar neighborhood relative to the Sun is about 20 km/s. Suppose you discover a star in the solar neighborhood that is moving relative to the Sun at a much higher speed–say, 200 km/s. What kind of orbit does this star probably have around the Milky Way? In what part of the galaxy does it spend most of its time? Explain.

47) Future of the Milky Way. Imagine that you could make a time-lapse movie from outside the Milky Way Galaxy over the next 100 billion years. How would you expect the appearance of our galaxy to change over that time period? Explain.

48) Orbits at the Galactic Center. Using the information in Figure 19.20, identify the two stars that reach the highest speeds, based on the positions of the color-coded dots representing the yearly motions of each star. Explain how the orbits of those two stars illustrate Kepler's first two laws.

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49) A Nonspinning Galaxy. How would the development of the Milky Way Galaxy have been different if it had collapsed from protogalactic clouds that had no net angular momentum?

Describe how you think our galaxy would look today, and explain your reasoning. 50) Gas Distribution in the Milky Way. Make a sketch of the gas distribution in the plane of the Milky Way, based on the photographs in Figure 19.12. In your sketch, map out where the molecular clouds are and draw the layer of atomic hydrogen gas that surrounds them.

Then add the locations of the most prominent bubbles of hot gas. Explain why each of these components of the Milky Way's interstellar medium is found in the location where you have drawn it. 51) Research: The Galactic Center. Search for recent images of the center of the Milky Way Galaxy, along with information about the massive black hole thought to reside there. Write a short report, with pictures, giving an update on current knowledge.

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

52) Mass of the Milky Way's Halo.The Large Magellanic Cloud orbits the Milky Way at a distance of roughly 160,000 light-years from the galactic center and a velocity of about 300 km/s. Use these values in the orbital velocity law (Mathematical Insight 19.1) to estimate the Milky Way's mass within 160,000 light-years from the center. (The value you obtain is a fairly rough estimate because the orbit of the Large Magellanic Cloud is not circular.)

53) Mass of the Central Black Hole. Suppose you observe a star orbiting the galactic center at a speed of 1000 km/s in a circular orbit with a radius of 20 light-days. Calculate the mass of the object the star is orbiting.

54) Mass of a Globular Cluster. Stars in the outskirts of a globular cluster are typically about 50 light-years from the cluster's center, which they orbit at speeds of about 10 km/s. Use these data to calculate the mass of a typical globular cluster.

55) Mass of Saturn. The innermost rings of Saturn orbit in a circle with a radius of 67,000 km at a speed of 23.8 km/s. Use the orbital velocity law to compute the mass contained within the orbit of those rings. Compare your answer with the mass of Saturn listed in Appendix E. 56) Pressure vs. Gravity in Hot Ionized Gas. Use the formula in Mathematical Insight 16.1 to estimate the mass of a gas cloud in which pressure balances gravity for a gas temperature of 106 K and a number density of 0.01 particle per cubic centimeter. Use your answer to explain why hot gas must cool down before it can collect into star-forming clouds.

57) Pressure vs. Gravity in Warm Atomic Gas. Use the formula in Mathematical Insight 16.1 to estimate the mass of a gas cloud in which pressure balances gravity for a gas temperature of 104 K and a number density of 1 particle per cubic centimeter. Compare your answer to the 1 million solar masses in a typical giant molecular cloud. Use your answer to explain why additional cooling is necessary in order to create such clouds.

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58) Pressure vs. Gravity in Cool Atomic Gas. Use the formula in Mathematical Insight 16.1 to estimate the mass of a gas cloud in which pressure balances gravity for a gas temperature of 102 K and a number density of 100 particles per cubic centimeter. Use your answer to explain why star-forming clouds typically contain enough gas to form several thousand stars.

59) The Speed of Supernova Debris. The kinetic energy Ekinetic of an amount of mass M traveling at a velocity v is given by the formula Ekinetic = Mv2/2. The total kinetic energy of the matter ejected from a supernova explosion is about 1044 joules. Determine the typical speed at which that matter is ejected from a supernova with a mass of 10MSun. Compare that speed with the Sun's orbital speed around our galaxy. Based on your comparison, do you think the galaxy's gravity would be strong enough to retain the supernova debris if there were no interstellar medium to slow it down? Explain.

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The Cosmic Perspective, 9e (Bennett et al.) Chapter 20 Galaxies and the Foundation of Modern Cosmology Section 20.1 1) Which of the following types of galaxies are closest to being spherical in shape? A) spirals B) ellipticals C) irregulars Answer: B 2) Which of the following types of galaxies are typically reddest in color? A) spirals B) ellipticals C) irregulars Answer: B 3) Which types of galaxies have a halo component? A) spirals only B) ellipticals only C) irregulars only D) both spirals and ellipticals Answer: D 4) Which types of galaxies have a clearly defined disk component? A) spirals only B) ellipticals only C) irregulars only D) both spirals and ellipticals Answer: A 5) How do elliptical galaxies typically compare to spiral galaxies? A) Elliptical galaxies are redder and rounder. B) Elliptical galaxies are redder and flattened. C) Elliptical galaxies are bluer and rounder. D) Elliptical galaxies are bluer and flattened. E) Elliptical galaxies are always much smaller. Answer: A 6) How does a lenticular galaxy differ from a spiral galaxy? A) It has no bulge.

B) It has an elongated bulge resembling a bar more than a sphere. C) It is flatter in shape. D) It has no gas or dust. E) It has no spiral arms. Answer: E

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7) Which of the following is true of elliptical galaxies? A) They usually have significant, ongoing star formation. B) They usually contain abundant clouds of cool gas and dust.

C) They are usually bluish in color. D) They are the most common type of galaxy in the central regions of galaxy clusters. Answer: D 8) Which of the following types of galaxies is the most common type of galaxy found in large clusters? A) spirals B) ellipticals C) irregulars Answer: B 9) Which of the following is true about irregular galaxies? A) They are composed solely of old stars. B) They generally have significant bulge populations.

C) They were more common when the universe was younger. D) They have reddish colors. E) They have well-defined spiral arms. Answer: C

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10) The H-R diagram for stars plots color on the horizontal axis and luminosity on the vertical axis. If we make a similar diagram plotting color and luminosity for galaxies, as shown below, what general trend does it show?

A) Galaxies fill the diagram uniformly, indicating no correlation between luminosity and color.

B) Galaxies form two main clumps on the diagram, one on the bluer, fainter side and one on the redder side, somewhat more luminous.

C) Much as with stars, galaxies fall into several distinct groups representing normal galaxies, giant and supergiant galaxies, and dwarf galaxies.

D) A strong trend for blue galaxies to be much more luminous on average than red galaxies.

Answer: B 11) On a graph of luminosity versus color for galaxies, shown below, many galaxies form a clump known as the "blue cloud." Why are galaxies in the blue cloud blue?

A) They contain hot, young stars that indicate active star formation.

B) They are moving slower than galaxies in the red cloud.

C) They contain lots of dust that cause the bluish color.

D) They contain cold, dying stars that are fading to black.

E) They are all moving toward us, causing them to be blueshifted.

Answer: A

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12) The most massive galaxies in the universe are A) ellipticals.

B) spirals.

C) lenticulars.

Answer: A 13) Why do spiral galaxies appear blue in color?

A) The longer wavelengths emitted by these galaxies are absorbed by interstellar dust.

B) Their rapid rotation causes a blueshift in the light they emit.

C) They contain many white dwarfs, which are usually hot enough to appear blue.

D) Their light output is dominated by hot, massive blue stars.

Answer: D 14) Why do elliptical galaxies appear yellow or red in color?

A) They contain many nebulae that glow red in color.

B) Their population is dominated by old, low-mass stars and old, red giant stars.

C) They are moving away from us so their light is highly redshifted.

D) They contain vast amounts of dust that absorbs blue light.

Answer: B 15) What kind of galaxy is the Milky Way? A) an irregular galaxy B) a spiral galaxy C) an elliptical galaxy Answer: C Section 20.2 1) In astronomy, a standard candle is an object with a known A) luminosity.

B) apparent brightness.

C) distance.

D) surface temperature.

Answer: A 2) Why are Cepheid variables important?

A) Cepheid variables are stars that vary in brightness extremely rapidly as they rotate on their axes.

B) Cepheid variables are stars that brighten and dim with a period that is directly related to the star's luminosity.

C) Cepheids are a type of young galaxy that helps us understand how galaxies form.

D) Cepheids are supermassive stars that we observe because they will soon undergo supernovae.

Answer: B

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3) Leavitt's law is a relationship between A) a Cepheid variable star's period and luminosity.

B) a galaxy's distance and recession speed.

C) a galaxy's distance and redshift.

D) a main-sequence star's color and luminosity.

Answer: A

4) This figure shows the Cepheid period-luminosity relation. What is the approximate luminosity of a Cepheid star that varies in brightness with a period of 10 days?

A) about 1000 times the luminosity of the Sun B) about 3000 times the luminosity of the Sun C) about 10,000 times the luminosity of the Sun D) about 30,000 times the luminosity of the Sun Answer: B 5) This figure shows the Cepheid period-luminosity relation. Suppose that a Cepheid with a period of 30 days has the same apparent brightness as a nearby star with a luminosity identical to our Sun. How many times farther away is the Cepheid? (Hint: First use the graph to find the Cepheid's luminosity, then apply the inverse square law for light.)

A) The Cepheid is 10 times farther away than the nearby star.

B) The Cepheid is 100 times farther away than the nearby star.

C) The Cepheid is 10,000 times farther away than the nearby star.

D) The Cepheid is actually 10 times closer than the nearby star.

Answer: B

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6) White dwarf supernovae are good standard candles for distance measurements because A) all white dwarf supernovae have about the same peak luminosity.

B) all white dwarf supernovae occurred at about the same time in the history of the universe.

C) all galaxies have at least one white dwarf supernova each year.

D) white dwarf supernovae occur only among very massive stars.

E) all white dwarf supernovae start out bright and fade with time.

Answer: A 7) Nearly all galaxies contain Cepheid variable stars. Why, then, do we determine the most distant galaxy distances using white dwarf supernovae rather than Cepheids as standard candles?

A) White dwarf supernovae are much more reliable standard candles than Cepheids.

B) White dwarf supernovae can be observed in galaxies that are too far away for us to see individual Cepheids.

C) White dwarf supernovae are more common than Cepheids.

D) White dwarf supernovae can be detected with a single observation, while it takes many observations to measure a Cepheid's light curve.

Answer: B 8) How did Edwin Hubble measure the distance to the Andromeda Galaxy?

A) He measured its parallax.

B) He used Cepheid variables in the Andromeda Galaxy.

C) He deduced it from its redshift.

D) He used white dwarf supernovae in the Andromeda Galaxy.

Answer: B 9) Which of the following expresses the major idea of Hubble's Law? A) There must have been a Big Bang.

B) All galaxies are moving away from us equally fast.

C) The more distant a galaxy is from us, the faster it moves away from us.

D) The closer a galaxy is to us, the faster it moves away from us.

E) More distant galaxies appear younger.

Answer: C 10) What is the most accurate way to determine the distance to a relatively nearby star (within the Milky Way)?

A) radar ranging B) stellar parallax C) using Cepheid variables D) using white dwarf supernovae E) applying Hubble's law Answer: B

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11) What is the most accurate way to determine the distance to a relatively nearby galaxy?

A) radar ranging B) parallax C) using Cepheid variables D) using white dwarf supernovae Answer: C 12) What is the most accurate way to determine the distance to a distant galaxy (a galaxy that is too far away for us to see individual stars)?

A) radar ranging B) parallax C) using Cepheid variables D) using white dwarf supernovae Answer: D 13) The Sloan Digital Sky Survey includes many thousands of galaxies in its spectroscopic catalogs. How are the distances to most of these galaxies estimated?

A) from their redshifts and the application of Hubble's Law B) by measuring Cepheid variable light curves C) by measuring their parallax angle as the Earth circles the Sun D) by measuring the light from white dwarf supernovae Answer: A 14) Dr. X claims that the Hubble constant is 20 km/s/million light-years while Dr. Y claims it is 24 km/s/million light-years. Which statement below automatically follows?

A) Dr. X concludes that the universe is expanding, but Dr. Y does not.

B) Dr. X will conclude that the Andromeda Galaxy (a member of our Local Group) is moving away from us at a slower speed than will Dr. Y. C) Dr. X will conclude that the universe is older than will Dr. Y. D) Dr. X believes that the universe will someday stop expanding, while Dr. Y believes it will expand forever.

E) Dr. X will conclude that the universe contains more mass than will Dr. Y. Answer: C 15) What two quantities did Edwin Hubble compare for a sample of galaxies to discover the expansion of the universe?

A) velocity and distance B) luminosity and distance C) velocity and temperature D) luminosity and temperature E) age and distance Answer: A

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16) This figure shows Hubble's law. If a galaxy is observed to be moving away from us at 30,000 (3 104) km/s, approximately how far away is it? (Hint: Be sure to read the axis labels carefully.)

A) 30,000 light-years B) 1400 million light-years C) 3000 million light-years D) 1400 light-years Answer: B Section 20.3 1) Which statement is true about the relationship between Hubble's constant and the age of the universe?

A) The larger the value of Hubble's constant, the faster the universe is expanding and the younger the universe must be.

B) The larger the value of Hubble's constant, the slower the universe is expanding and the younger the universe must be.

C) The larger the value of Hubble's constant, the slower the universe is expanding and the older the universe must be.

D) There is no relationship between the value of Hubble's constant and the age of the universe.

Answer: A

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2) Based on current estimates of the value of Hubble's constant, approximately how old is the universe? A) 10 billion years old B) 12 billion years old C) 14 billion years old D) 18 billion years old E) 20 billion years old Answer: C 3) You observe a galaxy that is 100 million light-years away. Which of the following do you see? A) The light from the galaxy as it is today, and it is blueshifted. B) The light from the galaxy as it is today, and it is redshifted. C) The light from the galaxy as it was 100 million years ago, and it is blueshifted. D) The light from the galaxy as it was 100 million years ago, and it is redshifted. E) Nothing: the galaxy lies beyond the cosmological horizon. Answer: D 4) Why can't we see past the cosmological horizon? A) Every galaxy in the entire universe (not just the observable universe) exists within the cosmological horizon, so there's nothing to see beyond it. B) It's impossible to look back to a time before the universe was born. C) We do not have telescopes big enough. D) The cosmological horizon is infinitely far away, and we can't see to infinity. Answer: B 5) Suppose you are observing a distant galaxy whose light has taken 12 billion years to reach us. What is the maximum age that this galaxy could have been when it emitted the light we are observing now? (Assume the age of the universe is about 14 billion years.) A) about 2 billion years old B) about 10 billion years old C) about 4 billion years old D) about 12 billion years old Answer: A General Chapter Questions 1) Stars X and Y both have the same apparent brightness, but star X is 10,000 times as luminous as star Y. How much farther away is star X than star Y? A) Star X is 10 times as far away as star Y. B) Star X is 100 times as far away as star Y. C) Star X is 10,000 times as far away as star Y. D) Star X is 10 times closer to us than star Y. Answer: B

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2) Suppose you measure the brightness and period of a number of Cepheid variable stars in Galaxy A and Galaxy B. You find that all the Cepheid variables in Galaxy A are approximately 25 times as bright (apparent brightness) as Cepheid variables with similar periods in Galaxy B. What can you conclude about the distances of these galaxies?

A) Galaxy A is 25 times as far away as Galaxy B. B) Galaxy A is 5 times as far away as Galaxy B. C) Galaxy B is 25 times as far away as Galaxy A. D) Galaxy B is 5 times as far away as Galaxy A. Answer: D 3) Recall that Hubble's law is written v = H0d, where v is the recession velocity of a galaxy located a distance d away from us, and H0 is Hubble's constant. Suppose H0 = 22 km/s/million light-years. How fast would a galaxy 1000 million light-years away be receding from us?

A) 22 km/s B) 22 million light-years/s C) 22,000 km/s D) 1000/22 km/s E) 0.22 times the speed of light Answer: C 4) Hubble's Law states that v = H0d, and current measurements indicate that H0 ≈ 22 kilometers per second per million light-years. About how far away is a galaxy with a redshift indicating that it is moving away from us at 2200 kilometers per second?

A) 100 million light-years B) 200 million light-years C) 200 light-years D) 100 light-years Answer: A Short Answer Questions 1) Briefly describe how astronomers estimate the number of galaxies in the observable universe.

Answer: Astronomers can count galaxies in small regions of the sky that have been photographed in great detail by powerful telescopes (e.g., with photos like the Hubble eXtreme Deep Field). They can then estimate the total number of galaxies in the observable universe by multiplying the number of galaxies in the small region of the sky by the number of such regions it would take to cover the entire sky.

2) Do individual galaxies expand in size as the universe expands? Why or why not?

Answer: No, because they are bound together by their own gravity.

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Reading Quiz Questions 1) Based on the number of galaxies visible in the Hubble eXtreme Deep Field (Figure 20.1 in your text), the estimated number of galaxies in our observable universe is at least A) 100 billion.

B) 100 million.

C) 50,000.

D) infinity.

Answer: A 2) Which of the following is not one of the three major categories of galaxies?

A) globular galaxies B) elliptical galaxies C) spiral galaxies D) irregular galaxies Answer: A 3) Galaxies with disks but no evident spiral arms are called A) irregular galaxies B) lenticular galaxies C) barred spiral galaxies D) elliptical galaxies Answer: B 4) On a graph plotting galaxy luminosities against galaxy colors, the red sequence represents galaxies that are ________ than galaxies of the blue cloud.

A) larger and more elliptical B) smaller and more irregular in shape C) more likely to have spiral shapes D) hotter and less luminous Answer: A 5) Which of the following best describes the status of the Milky Way in our Local Group of galaxies?

A) It is one of the two largest galaxies in the group.

B) It is one of about a dozen large spiral galaxies in the group.

C) It is by far the largest galaxy in the group.

D) It is quite average among the galaxies in the group.

Answer: A 6) A standard candle is A) a 7-cm-long wax candle.

B) a light source of known luminosity.

C) another name for a main-sequence star.

D) another name for a barred-spiral galaxy.

Answer: B

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7) What is a Cepheid variable?

A) It is a type of luminous star that makes an excellent standard candle.

B) It is a bright source of variable x-ray emission, thought to harbor a supermassive black hole.

C) It is a main-sequence star of spectral type B5.

D) It is a type of galaxy that varies in its light output.

Answer: A 8) What two observable properties of a Cepheid variable are directly related to one another?

A) the period between its peaks of brightness and its luminosity B) the period between its peaks of brightness and its distance C) its luminosity and its mass D) its mass and its distance Answer: A 9) What does Hubble's law tell us?

A) The more distant a galaxy, the faster it is moving away from us.

B) The faster a spiral galaxy's rotation speed, the more luminous it is.

C) The longer the period of a Cepheid variable, the greater its luminosity.

D) For every force, there is an equal and opposite reaction force.

Answer: A 10) Given that white dwarf supernovae are such good standard candles, why don't we use them to measure the distance to all galaxies?

A) They are rare events, so we have observed them in only a tiny fraction of all galaxies.

B) We cannot see them beyond a distance of about 100 million light-years.

C) They can occur only in spiral galaxies, not elliptical galaxies.

D) We would, but we don't have enough telescopes.

Answer: A 11) Overall, what is our most accurate technique for measuring the distance to a nearby star?

A) stellar parallax B) radar ranging C) Hubble's law D) Cepheid variables Answer: A 12) When we use an analogy that represents the expanding universe with the surface of an expanding balloon, what does the inside of the balloon represent?

A) The inside of the balloon does not represent any part of our universe.

B) the center of the universe C) the entire universe D) regions of the universe beyond the Milky Way Galaxy Answer: A

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13) If we say that a galaxy has a lookback time of 1 billion years, we mean that A) its light traveled through space for 1 billion years to reach us.

B) it is now 1 billion light-years away.

C) it was 1 billion light-years away when the light left the galaxy.

D) it is 400 million years old.

Answer: A 14) Cosmological redshift is the result of A) the high speeds at which galaxies move within clusters.

B) the expansion of the universe.

C) old, red stars in distant galaxies.

D) supermassive black holes.

Answer: B 15) Although the entire universe may be much larger than our observable universe, we can see only within our observable universe. The "boundary" of our observable universe is called A) the cosmological horizon.

B) the Big Bang.

C) the lookback time.

D) the Hubble Deep Field.

Answer: A 16) Current estimates place the age of the universe at about A) 10 billion years.

B) 10 million years.

C) 14 billion years.

D) 4-1/2 billion years.

Answer: C 17) The fact that the universe is expanding means that space itself is growing A) between individual galaxies within galaxy clusters.

B) between stars within individual galaxies.

C) between clusters of galaxies.

D) on all size scales; even planets like Earth are expanding with time.

Answer: C Concept Quiz Questions 1) In a photo like the Hubble eXtreme Deep Field (Figure 20.1 in your text), we see galaxies in many different stages of their lives. In general, which galaxies are seen in the earliest (youngest) stages of their lives?

A) the galaxies that are farthest away B) the galaxies that have the most hot, young stars of spectral types O and B C) the galaxies that are the reddest in color D) the galaxies that are nearest to us Answer: A

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2) The most basic difference between elliptical galaxies and spiral galaxies is that A) elliptical galaxies lack anything resembling the disk of a spiral galaxy. B) elliptical galaxies lack anything resembling the halo of a spiral galaxy. C) spiral galaxies lack anything resembling the distribution of stars in an elliptical galaxy. D) elliptical galaxies are old and spiral galaxies are young. Answer: A 3) The fact that large elliptical galaxies are much more common in the central regions of galaxy clusters than elsewhere in the universe suggests that A) collisions play a role in the formation of large elliptical galaxies. B) spiral galaxies cannot form in galaxy clusters. C) elliptical galaxies are older than spiral galaxies. D) elliptical galaxies from the outer regions of clusters must migrate inward. Answer: A 4) Hubble's galaxy classification diagram (the "tuning fork") A) shows how galaxies evolve from one form to another. B) relates galaxies according to their colors and luminosities. C) suggests the existence of black holes. D) relates galaxies according to their shapes.

Answer: D 5) Although it is highly unlikely to happen, suppose that we suddenly discovered that all these years we'd been wrong about the distance from Earth to the Sun, and it is actually 10% greater than we'd thought. How would that affect our estimate of the distance to the Andromeda Galaxy? A) It would mean the distance to the Andromeda Galaxy is also 10% greater than we thought. B) It would not have any effect on our estimate of the distance to the Andromeda Galaxy. C) It would mean the distance to the Andromeda Galaxy is 10% less than we thought. D) It would mean that all the objects we've assumed are standard candles really are not good standard candles, and therefore that we have no idea of the true distance to the Andromeda Galaxy. Answer: A 6) Suppose we observe a Cepheid variable in a distant galaxy. The Cepheid brightens and dims with a regular period of about 10 days. What can we learn from this observation? A) the distance to the galaxy B) the rotation rate of the galaxy C) the mass of the galaxy D) the name of the galaxy Answer: A

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7) Why are white dwarf supernovae more useful than massive star supernovae for measuring cosmic distances?

A) White dwarf supernovae all have roughly the same true peak luminosity, while massive supernovae come in a wide range of peak luminosities.

B) We can see only white dwarf supernovae in distant galaxies, not massive star supernovae.

C) White dwarf supernovae are much more common than massive star supernovae.

D) White dwarf supernovae obey Leavitt's law, while massive supernovae do not.

Answer: A 8) In 1924, Edwin Hubble proved that the Andromeda Galaxy lay far beyond the bounds of the Milky Way, thereby putting to rest the idea that it might have been a cloud within our own galaxy. What key observation enabled him to prove this?

A) He observed individual Cepheid variable stars in Andromeda.

B) He was able to measure the parallax of the Andromeda Galaxy.

C) He found that the universe is expanding, and therefore concluded that Andromeda must lie outside our own galaxy.

D) He was the first person ever to look through a telescope at the object we now call the Andromeda Galaxy.

Answer: A 9) Assume that Hubble's constant is 22 kilometers per second per million light-years. Then we would expect a galaxy 100 million light-years away to be moving (Assume the motion is due only to Hubble's law.)

A) away from us at 2,200 km/s B) away from us at 220 km/s C) toward us at 2,200 km/s D) away from us at 22,000 km/s Answer: A 10) Does Hubble's law work well for galaxies within the Local Group? Why or why not?

A) no, because galaxies in the Local Group are gravitationally bound together B) no, because Hubble did not know the Local Group existed when he discovered his law C) no, because we do not know the precise value of Hubble's constant D) Yes, it works so well that we have never detected any measurable deviations from its predictions.

Answer: A 11) Suppose an elliptical galaxy is so far away that we cannot see even its brightest stars individually. Which of the following techniques might allow us to measure its distance?

A) using a white dwarf supernova as a standard candle B) parallax C) using Cepheid variables as standard candles D) radar ranging Answer: A

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12) You observe the peak brightnesses of two white dwarf supernovae. Supernova A is only 1/4 as bright as Supernova B. What can you say about their relative distances? A) Supernova A is twice as far away as Supernova B. B) Supernova A is four times as far away as Supernova B. C) Supernova B is four times as far away as Supernova A. D) Supernova B is twice as far away as Supernova A. E) We can't say anything about their relative distances because we do not have enough information. Answer: A 13) What is the best way to determine a galaxy's redshift? A) Find the galaxy's apparent distance, and look up the redshift based on Hubble's Law. B) Find the color of the galaxy, and estimate its distance based on how red the galaxy is. C) Take a spectrum of the galaxy, and measure the difference in wavelength of spectral lines from the wavelengths of those same lines as measured in the laboratory. D) Measure the magnitude of the galaxy, estimate its distance, and calculate its redshift using Hubble's Law. Answer: C 14) Spectral lines from Galaxy B are redshifted from their rest wavelengths twice as much as the spectral lines from Galaxy B. According to Hubble's law, what can you say about their approximate relative distances? A) Galaxy A is twice as far as Galaxy B. B) Galaxy B is four times as far as Galaxy A. C) Galaxy B is twice as far as Galaxy A. D) Galaxy A is four times as far as Galaxy B. E) Not enough information to say; you need to know Hubble's constant to answer this question. Answer: C 15) Which statement below correctly describes the relationship between expansion rate and age for the universe? A) the faster the rate of expansion, the younger the age of the universe B) the faster the rate of expansion, the older the age of the universe C) Age is independent of the expansion rate. Answer: A 16) What does cosmological redshift do to light? A) stretches its wavelength B) makes it brighter C) makes it slow down D) makes all light infrared Answer: A

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17) The lookback time of the cosmological horizon is A) the age of the universe.

B) the Big Bang .

C) 1 billion years.

D) 10 billion years.

Answer: A 18) Why can't we see past the cosmological horizon?

A) Beyond the cosmological horizon, we would be looking back to a time before the universe was born.

B) We do not have big enough telescopes.

C) The cosmological horizon is infinitely far away, and we can't see to infinity.

D) Every galaxy in the entire universe (not just the observable universe) exists within the cosmological horizon, so there's nothing to see beyond it.

Answer: A Visual Quiz Questions

1) If you looked at the field of view seen in this Hubble Space telescope with your naked eye, about how big would it appear in the sky?

A) about the size of the Big Dipper B) about the size of the full Moon C) about the size of your little finger held at arm's length against the sky D) about the size of this period – . – viewed at arm's length against the sky Answer: D 2) What are we seeing in this Hubble Space Telescope photograph?

A) Almost every object in the photo is a galaxy, and these galaxies are at many different distances from us.

B) This photo shows the Virgo supercluster of galaxies.

C) Most of the dots in this photo are stars in the Milky Way, but we see a few blotches that represent galaxies beyond the Milky Way. D) This is the Local Group of galaxies.

Answer: A

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3) Is this a photo of a spiral galaxy or an elliptical galaxy, and how do you know?

A) It is a spiral galaxy because we can see a dusty disk going across the center.

B) It is a spiral galaxy, and we know because photos of this same galaxy from above show its spiral arms.

C) It is an elliptical galaxy because it is elliptical in shape.

D) It is an elliptical galaxy because it looks red/yellow in color.

Answer: A 4) This photo shows the Large Magellanic Cloud, or LMC for short. Based on what you have learned in about the LMC from your text, which of the following statements about the LMC is not true?

A) Galaxies similar to the LMC are extremely rare.

B) It is an irregular galaxy.

C) It orbits the Milky Way Galaxy.

D) Its color indicates active star formation.

Answer: A

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5) What is this a picture of?

A) a cluster of galaxies B) a globular cluster C) an elliptical galaxy D) the expanding universe Answer: A

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6) Based on this diagram, "red sequence" galaxies are ________ than "blue cloud" galaxies.

A) redder and more luminous B) redder and more distant C) cooler and more distant D) more luminous and older Answer: A

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7) Each dot on this graph represents an individual Cepheid variable star. Which of the following is true for a Cepheid that is 10,000 times as luminous as the Sun?

A) It varies gradually in brightness, with a peak in brightness about every 30 days.

B) It orbits the center of the galaxy about every 30 days.

C) It orbits its binary companion about every 30 days.

D) It undergoes a massive flare about every 30 days.

Answer: A

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8) The data points in this diagram represent the measured speeds and distances of various galaxies, and the solid line represents a best fit to these data. The trend indicated by the solid line is known as A) Hubble's law.

B) Leavitt's law.

C) the inverse square law for light.

D) Kepler's third law.

Answer: A 9) Suppose that Hubble's constant were 11 km/s/million light-years instead of 22 km/s/million light-years. How would this graph look different?

A) The data would follow a much steeper, almost vertical line.

B) The data would follow a more shallow line, closer to horizontal than the current line.

C) The data would follow a flat, horizontal line.

D) The graph would have to show luminosity on the vertical axis, rather than velocity.

Answer: B

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10) The white arrow points to an object that lies within the disk of the galaxy shown. What is this object?

A) a supernova B) an accretion disk around a supermassive black hole C) an extremely bright supergiant star D) an ionization nebula Answer: A

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End-of-Chapter Questions Visual Skills Check Use the following questions to check your understanding of some of the many types of visual information used in astronomy.

The schematic diagram above (repeated from Figure 20.22) shows the techniques that are best suited to measuring various astronomical distances. Answer the following questions based on the information in the figure.

1) Which distance measurement technique is best suited to measuring objects at a distance of 10 million light-years?

Answer: Cepheids 2) Which distance measurement technique is best suited to measuring objects at a distance of 10 light-years?

Answer: Parallax 3) What range of astronomical distances can we currently measure using both Cepheids and distant standards (white dwarf supernovae)?

Answer: From approximately 10 million to 100 million light-years 4) What range of astronomical distances can we currently measure using Hubble's law?

Answer: From about 10 million light-years to more than 10 billion light-years 5) Which technique is best for measuring the distances of nearby galaxies?

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6) Hubble's law requires calibration with distances measured by standard candle techniques.

Which standard candle technique is best for measuring the distances of very distant galaxies?

Answer: White dwarf supernovae (distant standards) Chapter Review Questions 1) Why do we need to understand the evolution of the universe in order to understand the lives of galaxies?

2) What are the three major types of galaxies, and how do their appearances differ?

3) Describe the differences between normal spiral galaxies, barred spiral galaxies, and lenticular galaxies.

4) Distinguish between spiral and elliptical galaxies in terms of the presence or absence of a disk component and a halo component. How does this difference explain the lack of hot, young stars in elliptical galaxies?

5) Where are the blue cloud and the red sequence on a plot of the relationship between galaxy color and luminosity? On which part of the plot would you tend to find the most luminous galaxies?

6) How do galaxies found in clusters tend to differ in type from isolated galaxies and galaxies found in smaller groups?

7) What do we mean by a standard candle? Explain how we can use standard candles to measure distances.

8) What is Leavitt's law? Explain how Hubble used it to prove that the Andromeda Galaxy lies beyond the bounds of the Milky Way. 9) What is Hubble's law? Explain what we mean when we say that Hubble's constant is between 21 and 23 kilometers per second per million light-years.

10) Summarize each of the major links in the distance chain, and explain how each link builds on the previous ones.

11) What is the Cosmological Principle, and how is it important to our understanding of the universe?

12) How is the expansion of the surface of an inflating balloon similar to the expansion of the universe? Use the balloon analogy to explain why Hubble's constant is related to the age of the universe.

13) What do we mean by the lookback time to a distant galaxy? Briefly explain why lookback times are less ambiguous than distances for discussing objects very far away.

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14) What is the cosmological horizon, and what determines how far away it lies?

15) What do we mean by a cosmological redshift? How does our interpretation of a distant galaxy's redshift differ if we think of it as a cosmological redshift rather than as a Doppler shift?

Does It Make Sense?

Decide whether or not each of the following statements makes sense (or is clearly true or false).

Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

Example: I walked east from our base camp at the North Pole.

Solution: The statement does not make sense because east has no meaning at the North Pole–all directions are south from the North Pole.

16) If you want to find a lot of elliptical galaxies, you'll have better luck looking in clusters of galaxies than elsewhere in the universe.

17) Cepheid variables make good standard candles because they all have exactly the same luminosity.

18) If the standard candles you are using are less luminous than you think they are, then the distances you determine from them will be too small.

19) Galaxy A is moving away from me twice as fast as galaxy B. That probably means it's twice as far away.

20) After measuring a galaxy's redshift, I used Hubble's law to estimate its distance.

21) The center of the universe is more crowded with galaxies than any other place in the universe.

22) The lookback time to the Andromeda Galaxy is about 2.5 million light-years.

23) I'd love to live in one of the galaxies near our cosmological horizon, because then I could see the black void into which the universe is expanding.

24) If someone in a galaxy with a lookback time of 4.6 billion years had a superpowerful telescope, that person could see our solar system in the process of its formation.

25) We can't see galaxies beyond the cosmological horizon because their light is too dim.

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Quick Quiz 26) Which of these galaxies do we see at its oldest age?

A) a galaxy in the Local Group B) a galaxy observed at a distance of 5 billion light-years C) a galaxy observed at a distance of 10 billion light-years Answer: No Correct Answer Was Provided.

27) Which of these galaxies would you most likely find at the center of a large cluster of galaxies?

A) a large spiral galaxy B) a large elliptical galaxy C) a small irregular galaxy Answer: No Correct Answer Was Provided.

28) In which of these galaxies would you be least likely to find an ionization nebula heated by hot young stars?

A) a large spiral galaxy B) a large elliptical galaxy C) a small irregular galaxy Answer: No Correct Answer Was Provided.

29) About how many galaxies are there in a typical cluster of galaxies?

A) about 10 B) a few dozen C) a few hundred Answer: No Correct Answer Was Provided.

30) According to Leavitt's law, we can determine the luminosity of a Cepheid in a distant galaxy by A) measuring its parallax.

B) measuring its period of brightening and dimming.

C) measuring its recession speed.

Answer: No Correct Answer Was Provided.

31) Which kind of object is the best standard candle for measuring distances to extremely distant galaxies?

A) parallax B) a Cepheid variable star C) a white dwarf supernova Answer: No Correct Answer Was Provided.

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32) When the ultraviolet light from hot stars in very distant galaxies finally reaches us, it arrives at Earth in the form of A) x-rays. B) slightly more energetic ultraviolet light. C) visible light. Answer: No Correct Answer Was Provided. 33) Why do virtually all the galaxies in the universe appear to be moving away from our own? A) We are located near where the Big Bang happened. B) We are located near the center of the universe. C) Expansion causes all galaxies to move away from nearly all others. Answer: No Correct Answer Was Provided. 34) If you observed the redshifts of galaxies at a given distance to be twice as large as they are now, then you would determine a value for Hubble's constant that was A) twice as large as its current value. B) equal to its current value. C) half its current value. Answer: No Correct Answer Was Provided. 35) What would be your estimate for the age of the universe if you measured Hubble's constant to be 11 kilometers per second per million light-years? A) 7 billion years B) 14 billion years C) 28 billion years Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

36) Group Discussion: Leavitt's Law.This book follows the recommendation of the American Astronomical Society in referring to what was previously called the Cepheid "period-luminosity relation" asLeavitt's law. a) Working independently, do a search on "Leavitt's law" to learn about the history behind this name. b) Gather into small groups to share what you've learned and to discuss the reasons behind the change from "period-luminosity relation" to "Leavitt's law." Make a list of pros and cons of the change in terminology. c) Have each group member state which name ("period-luminosity relation" or "Leavitt's law") she or he prefers, and why. d) Can you come to a group consensus about which term is preferable? If so, prepare a brief summary to explain your choice to others. If not, make a list of reasons for the disagreement.

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The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

37) The Shapley-Curtis Debate.Section 20.2 briefly summarizes the 1920 debate between Harlow Shapley and Heber Curtis on the nature of "spiral nebulae." Do additional research to learn about the evidence presented by Shapley and Curtis to support their opposing positions.

Which position would you have found more convincing at the time, based only on the evidence presented at the debate? Explain.

38) Deviations from Hubble's Law. Suppose you are measuring distances and velocities of galaxies in order to test Hubble's law. You find that 90% of the galaxies have velocities that are within 200 kilometers per second of the predictions of Hubble's law but 10% have velocities that deviate from the predictions by up to 1000 kilometers per second. Propose a hypothesis that would explain these deviations from Hubble's law and outline a set of observations that you could use to test your hypothesis.

39) Hubble's Revolution. Just a century ago, astronomers were still not sure whether the Milky Way was the only large collection of stars in the universe. Then Edwin Hubble measured the distance of the Andromeda Galaxy, proving once and for all that our Milky Way was just one among many galaxies in the universe. How was this change in our "cosmic perspective" similar to the Copernican revolution? How was it different?

40) Group Activity: Counting Galaxies. The following activity is based on the Hubble eXtreme Deep Field in Figure 20.1. Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39. a) Working individually, estimate the number of galaxies in the image. b) Gather into small groups, where each member presents his or her method and estimate to the others. Do the estimates differ? By how much? c) Discuss any significant differences in the methods and the resulting estimates, with the goal of understanding the pros and cons of different estimation methods. d) As a group, come up with an estimation method that incorporates the best features of the team members' individual methods, and apply this method to come up with your group's final estimate for the number of galaxies in the image. Then use the fact that this image shows about 1/30,000,000 of the entire sky to come up with an estimate for the total number of galaxies in the observable universe. Compare your estimate with those of other groups in your class.

Investigate Further 41) Deep Fields with Hubble. The Hubble eXtreme Deep Field in Figure 20.1 was chosen for observation in large part because it is a completely ordinary part of the sky. Why do you think astronomers would want to devote so much precious telescope time to observing totally ordinary regions of the sky in such great detail? Explain.

42) Supernovae in Other Galaxies. In which type of galaxy would you be more likely to observe a massive star supernova: a giant elliptical galaxy or a large spiral galaxy? Explain.

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43) Hubble's Galaxy Types. How would you classify the following galaxies using the system illustrated in Figure 20.8? Justify your answers. a) Galaxy NGC 4594 (Figure 20.3) b) Galaxy M101 (Figure 20.2a) c) Galaxy NGC 4414 (Figure 20.2b) d) Galaxy NGC 1300 (Figure 20.4) e) Galaxy M87 (Figure 20.6a) 44) Color and Luminosity. The luminosity of our Milky Way galaxy is about 1.5 x 1010 LSun. Describe how the Milky Way's light output compares with those of the most luminous galaxies in the blue cloud, based on the information in Figure 20.9. Approximately how much more luminous than the Milky Way are the most luminous galaxies in the red sequence?

45) Distance Measurements. The techniques astronomers use to measure distances are not so very different from the ones you use every day. Describe how standard candle measurements are similar to the way you estimate the distance to an oncoming car at night.

46) Cepheids as Standard Candles. Suppose you are observing Cepheids in a nearby galaxy. You observe one Cepheid with a period of 8 days between peaks in brightness and another with a period of 35 days. Use Leavitt's law (Figure 20.14) to estimate the luminosity of each Cepheid.

47) Galaxies at Great Distances. The most distant galaxies that astronomers have observed are much easier to study with infrared light than with visible light. Explain why that is the case.

48) Universe on a Balloon. In what ways is the surface of a balloon a good analogy for the universe? In what ways is this analogy limited? Explain why a miniature scientist living in a polka dot on the balloon would observe all other dots to be moving away, with more distant dots moving away faster.

49) Lookback Time and the Ages of Galaxies. Suppose you observe a distant galaxy with a lookback time of 10 billion years. What was the maximum possible age of that galaxy when the light we are now observing from it began its journey to Earth? Explain your reasoning. (Hint: Assume the galaxy was born less than a billion years after the Big Bang.)

50) Research: Greatest Lookback Time. Look for recent discoveries of objects with the largest lookback times. What is the current record for the most distant known object? How was it observed, and what kind of object is it?

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Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

51) Cepheids in M100.Consider these actual data for three Cepheids in the galaxy M100; the luminosities were determined with Leavitt's law from the observed periods (measured with observations from the Hubble Space Telescope).

Cepheid 1: luminosity = 3.9 x 1030 watts apparent brightness = 9.3 x 10-19 watt / m2 Cepheid 2: luminosity = 1.2 x 1030 watts apparent brightness = 3.8 x 10-19 watt / m2 Cepheid 3: luminosity = 2.5 x 1030 watts apparent brightness = 8.7 x 10-19 watt / m2 Compute the distance to M100 with data from each of the three Cepheids. Do the three distance computations agree? Based on your results, estimate the uncertainty in the distance you have found.

52) Redshift and Hubble's Law. Imagine that you have obtained spectra for three galaxies and found the following measurements for the observed wavelength of a hydrogen emission line that has a rest wavelength of 656.3 nanometers: Galaxy 1: 659.6 nanometers Galaxy 2: 664.7 nanometers Galaxy 3: 679.2 nanometers a) Calculate the redshift, z, for each of these galaxies. b) From its redshift, calculate the speed at which each of the galaxies is moving away from us.

Give your answers both in kilometers per second and as a fraction of the speed of light. c) Use Hubble's law to estimate the distance to each galaxy. Assume that H0 = 22km / s / Mly. 53) Estimating the Universe's Age. Suppose that Hubble's constant were H0 = 33km / s / Mly (which is not its actual value). What would the approximate age of the universe be in that case?

54) Hubble's First Attempt. Edwin Hubble's first attempt to measure the universe's expansion rate was flawed because the standard candles he was using were not properly calibrated. Look at Figure 20.21. Estimate the value of H0 corresponding to the solid line in the figure. Remember that 106 parsecs = 3.26 million light-years. What is the approximate age of the universe indicated by that erroneous value of H0?

55) Extremely Distant Galaxies. The most distant galaxies observed to date have a redshift of approximately z = 10. How does the wavelength of the light we observe from those galaxies compare with its original wavelength when it was emitted?

56) Stretching of the Universe. The most distant white dwarf supernova observed as of 2018 had a redshift of z = 1.9. How does the average distance between galaxies now compare with the average distance between galaxies at the time the supernova exploded?

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter 21 Galaxy Evolution Section 21.1 1) Which statement accurately describes how observations of distant galaxies help us learn about galaxy evolution?

A) By observing galaxies with different lookback times, we learn how galaxies appeared at different stages of their evolution.

B) We can observe the birth of galaxies.

C) We can observe the evolution of a single galaxy over time.

D) We can watch as two galaxies merge and view the result, helping us learn how mergers affect evolution.

E) We can see what our galaxy used to look like and therefore theorize about the physical processes that led to its current appearance.

Answer: A 2) In what sense do telescopes function as "time machines"?

A) They allow us to observe individual stars and galaxies as they change with time.

B) Some of the oldest telescopes are still in use today.

C) They allow us to see distant objects as they were in the distant past.

D) They allow us to see the Milky Way Galaxy as it was when it was much younger.

Answer: C 3) Which of the following are the two key starting assumptions of theoretical models of galaxy evolution?

A) (1) The beginning of the universe can be modeled as a giant supernova explosion and (2) this supernova created all the elements in the proportions we find them today.

B) (1) Hydrogen and helium gas, along with dark matter, filled all of space and (2) some regions of the universe were slightly denser than others.

C) (1) Hydrogen and helium gas, along with dark matter, filled all of space and (2) the distribution of this material was perfectly uniform everywhere.

D) (1) Hydrogen gas, along with dark matter, filled all of space and (2) all the other elements came from stars.

Answer: B 4) Galaxy A and galaxy B formed at the same time. Galaxy A is much farther from us than galaxy B. Which of the following is true?

A) Galaxy A will appear younger to us than galaxy B. B) Galaxy A will appear older to us than galaxy B. C) Galaxy A will appear to be the same age as galaxy B. Answer: A

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Section 21.2 1) Consider a scale on which our entire Milky Way Galaxy is about the size (diameter) of a grapefruit. Which of the following best describes the size and distance of the Andromeda Galaxy on the same scale?

A) a grapefruit about 3 meters away B) a grapefruit about 3 centimeters away C) a beach ball about the length of a football field away D) a peanut about 3 centimeters away E) a grapefruit about 3 kilometers away Answer: A 2) Which of the following protogalactic clouds is most likely to form an elliptical galaxy?

A) a very low-density cloud with very little angular momentum B) a dense cloud with very little angular momentum C) a low-density cloud with quite a bit of angular momentum D) a very massive cloud with any density and a lot of angular momentum Answer: B 3) Which of the following protogalactic clouds is most likely to form a spiral galaxy?

A) a very low-density cloud with very little angular momentum B) a dense cloud with very little angular momentum C) a low-density cloud with quite a bit of angular momentum D) a very massive cloud with any density and a lot of angular momentum Answer: C 4) What is one idea why a relatively dense protogalactic cloud more likely to produce an elliptical galaxy than a spiral galaxy?

A) The force of gravity from the higher gas density can pull the material into a more spherical shape.

B) The more frequent collisions between particles in the high-density gas randomize the particle orbits.

C) The thickness of the dense cloud prevents a disk from forming.

D) The higher gas density allows more rapid star formation, leaving little gas to form a disk.

Answer: D 5) What observational evidence supports the hypothesis that elliptical galaxies are born from relatively dense protogalactic clouds?

A) Elliptical galaxies have stars packed more densely together than spiral galaxies.

B) Elliptical galaxies are generally larger than spiral galaxies.

C) Even very distant elliptical galaxies tend to lack young, blue stars, indicating their stars formed early and rapidly.

D) Elliptical galaxies have more and denser gas than spiral galaxies.

E) The stars in elliptical galaxies are denser than those in spiral galaxies.

Answer: C

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6) What observational evidence supports the idea that a collision between two spiral galaxies might lead to the formation of a single elliptical galaxy?

A) Some elliptical galaxies surrounded by shells of stars that probably formed from stars stripped out of smaller galaxies.

B) Elliptical galaxies dominate the galaxy populations at the cores of dense clusters of galaxies.

C) Elliptical galaxies with stars and gas clouds that orbit differently from the other stars in the galaxy.

D) There are giant elliptical galaxies at the center of dense clusters that may have grown by consuming other galaxies.

E) All of the above are correct.

Answer: E 7) How might two colliding spiral galaxies tend to create an elliptical galaxy?

A) The orbits of the stars get randomized during the collision, while their gas disks collide and form stars.

B) The collision creates a larger galaxy and elliptical galaxies are always larger.

C) The gas in the disks of the galaxies is expelled into intergalactic space by gravitational interactions.

D) The heat created by collisions between stars causes the disks to puff out into a spherical shape.

Answer: A 8) What is a central dominant galaxy?

A) a galaxy around which many other smaller galaxies orbit B) a giant spiral galaxy that exerts large tidal forces on other nearby galaxies C) a spiral galaxy from which many smaller galaxies form when it is stripped apart by tidal forces D) a giant galaxy with several bright clumps of stars at the center of a dense cluster E) a hypothesized galaxy type that no longer exists, but once dominated the structure of the universe Answer: D 9) Why are collisions between galaxies common, but collisions between stars rare?

A) Stars are very small compared to the distances between them, but galaxies are not.

B) Stars repel each other with stellar winds, but galaxies do not.

C) Stars are much less common than galaxies, so there are fewer of them to collide.

D) Stars do not survive as long as galaxies, so they have less time to collide.

Answer: A 10) A starburst galaxy gets this name because A) it contains far more x-ray bursters than our Milky Way Galaxy.

B) it has many stars bursting out of its disk toward intergalactic space.

C) it is forming stars so fast that the starburst phase of life must be temporary.

D) it contains so many stars that the galaxy's gravity cannot retain them in their orbits.

Answer: C

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11) Why do we conclude that a starburst must represent only a temporary stage in galaxy evolution?

A) We observe starbursts to last only a few years in any individual galaxy before the starburst ends.

B) The star formation rate of a starburst would consume all available gas for star formation in a time much shorter than the age of the universe.

C) We don't see any starbursts occurring in nearby galaxies.

D) Galaxies with starbursts are all very far away, indicating that these starbursts occurred only very long ago.

Answer: B 12) Starburst galaxies are best observed in which wavelength range?

A) x-rays B) ultraviolet C) visible D) infrared E) gamma rays Answer: D 13) What evidence suggests that some small galaxies in our Local Group have undergone two or more starbursts in the past?

A) We observe several small galaxies currently undergoing massive star formation.

B) We see small galaxies in which many stars have one age and many others have another age that is billions of years older.

C) We see evidence that small galaxies in our Local Group have experienced several collisions in the past.

D) We see evidence that several small galaxies were shot out of larger galaxies in our Local Group during a period of starburst activity.

Answer: B Section 21.3 1) What is a quasar?

A) a starlike object that actually represents a bright patch of gas in the Milky Way B) a very large galaxy thought to be formed by the merger of several smaller galaxies, resulting in a quick burst of star formation C) the name given to the largest objects in the Kuiper Belt D) the extremely bright center of a distant galaxy E) another name for very bright stars of spectral type O Answer: D

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2) How do we know that quasars are very distant objects? A) from their high luminosities B) from the fact that they can vary in luminosity in short time periods C) by observing Cepheid variables within them D) from their large redshifts E) by measuring their parallax angles Answer: D 3) Which of the following statements best supports the hypothesis that active galactic nuclei are powered by supermassive black holes? A) Orbits of gas clouds within them indicate billions of solar masses of material in a small volume of space. B) They are found in the centers of galaxies. C) They have jets and accretion disks. D) They emit radiation at all wavelengths, from the radio to the gamma rays. Answer: A 4) Which of the following provides observational evidence that quasars have power sources that are fairly small in size? A) Some quasars are more than a thousand times as luminous as the Milky Way. B) Quasars are always located very far away. C) Telescopic images show the very small power sources of quasars. D) The galaxies in which quasars are embedded often show disruption from the central energy source. E) Some quasars can change significantly in apparent brightness in just a few hours. Answer: E 5) Suppose that an object doubles its luminosity in 10 hours. What can we conclude? A) The object must be a quasar. B) The object must be a Cepheid variable star. C) The object is no more than about 10 light-hours across. D) The object must be very far away, making it one of the most luminous objects in the universe. E) The object is being orbited by another object with an orbital period of 10 hours. Answer: C 6) According to the supermassive black hole hypothesis, how is the energy that powers active galactic nuclei produced? A) by nuclear fusion near the central black hole B) by magnetic fields that trap and accelerate charged particles C) by the impact of jets being shot out of the central black hole D) by converting the gravitational potential energy of infalling matter to kinetic energy and thermal energy E) by matter—antimatter annihilation Answer: D

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7) Quasars and other active galactic nuclei are often intense sources of x-rays. According to the supermassive black hole hypothesis, where are these x-rays produced?

A) in hot gas in an accretion disk around the central black hole B) in ionization nebulae heated by supernovae near the black hole C) in dust-rich molecular clouds that are forming new stars near the black hole D) in the central black hole itself Answer: A 8) All of the following are true. Which of these gives evidence that quasars were more common when the universe was young than they are today?

A) They are more common at very great distances.

B) They are very bright.

C) They are active galactic nuclei.

D) We don't see them in every galaxy.

Answer: A 9) Approximately what percentage of mass-energy of infalling matter can be converted to thermal energy when the matter is falling into a supermassive black hole?

A) about the same as that converted by hydrogen fusion B) much less than that converted by hydrogen fusion C) much more that converted by hydrogen fusion Answer: C Section 21.4 1) Which of the following observations would tell us that there are intergalactic clouds of cool gas between a distant quasar and us?

A) We see hydrogen emission lines at redshifts smaller than that of the quasar.

B) We see hydrogen absorption lines at redshifts smaller than that of the quasar.

C) We see hydrogen emission lines at redshifts greater than that of the quasar.

D) We see hydrogen absorption lines at redshifts greater than that of the quasar.

E) We see that the emission lines from the quasar are lessened by intervening gas and dust.

Answer: B 2) Quasar spectra often show the same hydrogen absorption line with several different redshifts.

What do these lines represent?

A) clouds of gas at different distances between us and the quasar B) different elements in the accretion disk around the quasar C) different elements in the Earth's atmosphere D) a single cloud of gas with clumps moving at different speeds E) hydrogen gas orbiting the quasar's central black hole at high speed Answer: A

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General Chapter Questions 1) Which of the following claims about the very first stars in the universe would be impossible given our basic picture of the history of the universe?

A) Many of them have exploded as supernovae by now.

B) Some of them formed in large clusters.

C) Some of them formed in isolation from one another.

D) Some may have left behind neutron stars or black holes that still exist in the Milky Way today.

E) Some of them had rocky planets around them as they formed.

Answer: E Short Answer Questions 1) Explain why galaxy collisions are far more common than star-star collisions.

Answer: Galaxies are much closer together, relative to their size, than stars are. If the Sun were made the size of a grapefruit, the nearest star would be on the other side of the continent.

However, if the Milky Way (or any other galaxy) were made the size of a grapefruit, there would likely be another galaxy within a few feet of it. Thus, galaxy interactions are far more common than star-star interactions and must be taken into account when considering galaxy evolution.

2) In what sense are telescopes "time machines" that allow us to study the formation and evolution of galaxies?

Answer: Telescopes see objects as they were when the light left them. For distant objects, this "lookback time" can be billions of years, which means in some cases we can literally see galaxies in the process of formation. Moreover, by looking to different distances (lookback times), we see other galaxies in various stages of life between formation and the present day.

Reading Quiz Questions 1) Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in A) infrared light.

B) visible light.

C) radio waves.

D) x-rays.

Answer: A 2) Which of the following is an important starting assumption in models of galaxy formation?

A) Some regions in the universe start out denser than others.

B) Galaxies form first, then black holes.

C) All galaxies start out as spiral galaxies.

D) Black holes form first, seeding the formation of galaxies.

Answer: A

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3) According to observations, which type of galaxy was much more common when the universe was 2 billion years old than it is today? A) irregular galaxies B) elliptical galaxies C) spiral galaxies D) We can't say because humans were not alive at that time. Answer: A 4) Collisions between galaxies typically unfold over a period of A) several days. B) several months. C) thousands of years. D) hundreds of millions of years. Answer: D 5) Why are collisions between galaxies more likely than collisions of stars within galaxies? A) Relative to their sizes, galaxies are much closer together than stars. B) Galaxies are much larger than stars. C) Galaxies travel through space much faster than stars. D) Galaxies have higher redshifts than stars. Answer: A 6) Which of the following statements correctly explains why galaxy collisions should have been more common in the past than they are today? A) Galaxies were closer together in the past, because the universe was smaller. B) Galaxies were more active in the past and therefore would have collided with each other more frequently. C) Galaxies were much bigger in the past, because they had not contracted completely. D) Galaxies attracted each other more strongly in the past, because they had not yet turned most of their mass into stars. Answer: A 7) Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) A) may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies. B) is always determined by the angular momentum of the protogalactic cloud that formed it. C) is always determined by whether the galaxy is located in a cluster where collisions are likely or outside a cluster where collisions are less likely. D) may either be a result of the mass of the protogalactic cloud that formed it or the result of the heavy element abundance in that cloud. Answer: A

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8) Which of the following is not a feature of central dominant galaxies? A) They're found in clusters of galaxies. B) They are spiral galaxies. C) They often have multiple galactic nuclei.

D) They are thought to form through the mergers of smaller galaxies. Answer: B 9) The distinguishing feature of a starburst galaxy is A) a rate of star formation that may be 100 or more times greater than that in the Milky Way. B) the presence of an unusually large number of binary star systems containing x-ray bursters. C) a large luminosity compared to the total luminosity of the Milky Way. D) strong radio emission from "lobes" of material well outside the visible boundaries of the galaxy. Answer: A 10) The unusually bright centers found in some galaxies are called A) active galactic nuclei. B) halos. C) supermassive black holes. D) starbursts. Answer: A 11) According to current understanding, what is a quasar? A) an active galactic nucleus that is particularly luminous B) a large galaxy thought to be formed by the merger of several smaller galaxies, typically found in the center of a galaxy cluster C) Any object with an extremely large redshift is called a quasar. D) a galaxy with an unusually high rate of star formation Answer: A 12) Which of the following observational phenomena is probably not a direct consequence of the presence of a supermassive black hole? A) the energy output of quasars B) the radio emission from radio galaxies C) the huge jets seen emerging from the centers of some galaxies D) the large numbers of stars found in globular clusters Answer: D 13) The mass of a supermassive black hole thought to power a quasar is typically A) 3 solar masses. B) 10 solar masses. C) 1 trillion solar masses. D) 1 billion solar masses. Answer: D

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14) According to the model in which active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of A) light emitted by hot gas in an accretion disk that swirls around the black hole.

B) intense radiation emitted by the black hole itself.

C) the combined light of thousands of young, high-mass stars that orbit the black hole.

D) radio waves emitted from radio lobes found on either side of the galaxy we see in visible light.

Answer: A 15) According to the model in which that active galactic nuclei are powered by supermassive black holes, the energy released as light comes from A) gravitational potential energy released by matter that is falling toward the black hole.

B) nuclear fusion in the accretion disk surrounding the black hole.

C) matter-antimatter annihilation occurring just outside the event horizon of the black hole.

D) jets emerging from the accretion disk.

Answer: A 16) Observations have shown that when the mass of the central black hole is large, then A) the mass of the halo and disk of the host galaxy is also large.

B) the host galaxy is eventually completely consumed by the black hole.

C) the galaxy is always a spiral galaxy.

D) the mass of the bulge of the host galaxy is also large.

Answer: D 17) Which of the following observations provides direct evidence that galaxies containing active galactic nuclei have central, supermassive black holes?

A) the high orbital velocities of stars and gas clouds at the very centers of these galaxies B) jet velocities that approach the speed of light observed in gas at great distances from the galaxy centers C) large quantities of high-energy emission such as x-rays and gamma rays coming from these galaxies D) huge dark regions in the centers of these galaxies Answer: A 18) Intergalactic hydrogen clouds are easiest to study by looking at A) 21 cm radio emission.

B) radio emission from molecular hydrogen.

C) x-ray emission.

D) absorption lines in quasar spectra.

Answer: D

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Concept Quiz Questions 1) We can study how galaxies evolve because A) we can watch as they interact in real time. B) the farther away we look, the further back in time we see. C) galaxies are transparent to visible light. D) we are really smart astronomers. Answer: B 2) Which of the following statements is not an assumption used in models of galaxy formation? A) The very early universe had an almost perfectly uniform distribution of hydrogen and helium gas. B) Some regions in the universe were slightly more dense than others. C) The universe is expanding. D) Gas contracted to form the disks of galaxies before any stars were born. Answer: D 3) One possible explanation for a galaxy's type invokes the angular momentum of the protogalactic cloud from which it formed. Suppose a galaxy forms from a protogalactic cloud with a lot of angular momentum. Assuming its type has not changed as a result of other interactions, we'd expect this galaxy to be A) an irregular galaxy. B) an elliptical galaxy. C) a spiral galaxy. D) a torn and incoherent galaxy.

Answer: C 4) Two ways in which the starting conditions in a protogalactic cloud might cause it to become an elliptical (rather than spiral) galaxy are if the cloud begins with either A) relatively little angular momentum or relatively high density. B) relatively high angular momentum or relatively low density. C) relatively high mass and a relatively low abundance of heavy elements. D) relatively low mass and a relatively high abundance of heavy elements. Answer: A 5) Which of the following phenomena is not thought to be a result of collisions or other interactions between galaxies? A) the presence of large, central dominant galaxies in clusters of galaxies B) the fact that elliptical galaxies are more common in clusters of galaxies than outside clusters C) starbursts D) the fact that spiral galaxies have both disk and halo components Answer: D

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6) If the Andromeda Galaxy collided with the Milky Way, what would most likely happen to Earth? A) nothing B) The Sun and all its planets would be crushed by collisions with stars and planets from the Andromeda Galaxy. C) The Sun would not be disturbed, but the solar system would be ripped apart. D) The Sun would die, causing Earth to die as well.

Answer: A 7) Interactions among galaxies also are thought to influence a galaxy's type in at least some cases. Which of the following does not support the idea that interactions can shape galaxies? A) the fact that more distant galaxies have larger redshifts B) computer modeling of collisions between galaxies C) the fact that galaxies with distorted appearances are more common at great distances than nearby D) the presence of features such as "tails" extending out of galaxies, bridges between galaxies, and rings of stars around galaxies Answer: A 8) Observations indicate that over billions of years, galaxies in general tend to change from A) smaller and bluer to larger and redder. B) larger and redder to smaller and bluer. C) smaller and redder to larger and bluer. D) larger and bluer to smaller and redder. Answer: A 9) Which is not a general characteristic of starburst galaxies? A) The observed features that cause us to classify a galaxy as a "starburst" galaxy must be only temporary phenomena in the galaxy's history. B) The galaxy's rate of star formation is many times higher than the rate of star formation in the Milky Way. C) Supernovae occur so frequently that their effects combine to drive a galactic wind that blows material into intergalactic space. D) The "starburst" is thought to be caused by the presence of a supermassive black hole in the galaxy's center. Answer: D 10) A quasar's spectrum is hugely redshifted. What does this large redshift tells us about the quasar? A) the composition of the quasar B) the distance to the quasar C) the mass of the quasar's central, supermassive black hole D) the type of host galaxy in which the quasar resides Answer: B

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11) Most active galactic nuclei are found at large distances from us, with relatively few nearby. What does this imply? A) Active galactic nuclei exist tend to become less active as they age. B) Supermassive black holes existed only when the universe was young, and no longer exist today. C) Active galactic nuclei can form only at large distances from the Milky Way. D) The jets seen in many active galactic nuclei must cause them to move far away from us. Answer: A 12) Suppose we observe a source of x-rays that varies substantially in brightness over a period of a few days. What can we conclude? A) The x-ray source is no more than a few light-days in diameter. B) The x-ray source is a quasar. C) The x-ray source contains a black hole with an accretion disk. D) The x-ray source must have a strong, rapidly varying magnetic field. Answer: A 13) All of the following observations are real. Which one does not support the model in which active galactic nuclei are powered by accretion disks around massive black holes? A) The most luminous active galactic nuclei have huge redshifts. B) X-ray emission from active galactic nuclei can vary significantly in times as short as a few days. C) The total luminosity of an active galactic nucleus can be as high as about 10 billion times that of the Sun. D) Spectra of active galactic nuclei show that clouds of gas are orbiting a central object at a high speed. Answer: A 14) Roughly what percentage of the mass-energy of in-falling matter can be converted to other forms of energy as the matter falls into a black hole? A) 0.7% B) 1-4% C) 10-40% D) 100% Answer: C 15) The observed relationship between the masses of central black holes and the bulge masses of galaxies implies that A) galaxy formation and supermassive black hole formation must be related. B) the black hole will eventually suck in the rest of the galaxy.

C) the biggest galaxies have the most luminous quasars. D) quasars were more common 10 billion years ago than they are today. Answer: A

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16) Quasar spectra often show many absorption lines that all appear to be as a result of the same electron transition (such as level 1 to level 2 in hydrogen) but that fall at different wavelengths in the spectrum. Why do we think this is the case?

A) We are seeing absorption lines from clouds of gas that lie between us and the quasar, and therefore each cloud has a different redshift.

B) Quasars are rotating rapidly, and this rotation produces spectral lines with a wide variety of Doppler shifts.

C) The lines fall at different wavelengths because they are produced by different chemical elements.

D) No one knows; it remains perhaps the greatest mystery about quasars.

Answer: A Visual Quiz Questions 1) The three cubes represent three stages in a computer simulation of the formation of protogalactic clouds; they represent a region about 500 million light-years across. Which of the following statements about this model is true?

A) Although the gas looks uniform in the first frame, the model assumes there were small regions with slightly higher densities than other regions.

B) The gas in the third frame looks disorganized, but the model indicates that all this gas soon became the spiral arms of a large spiral galaxy.

C) Although the gas in the three frames looks fairly stationary, the model assumes that the entire blob of gas must have been rotating rapidly.

D) Although the model shows only visible gas, it assumes that there were supermassive black holes randomly distributed throughout the universe.

Answer: A

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2) This diagram shows several stages in a computer simulation of a collision between two galaxies. What is being simulated?

A) Two spiral galaxies merge to become an elliptical galaxy surrounded by debris.

B) Two elliptical galaxies merge to become a spiral galaxy.

C) Two galaxies collide, sending their stars flying off into space while their centers merge to become a supermassive black hole.

D) Two spiral galaxies collide, ultimately merging to make a larger spiral galaxy.

Answer: A 3) This large fuzzy blob in the center of this image is the central dominant galaxy of the cluster that surrounds it. The arrow points to several large clumps of stars near the center of the galaxy.

What do those clumps probably represent?

A) Each clump was probably once the center of an individual galaxy.

B) The clumps are regions of extremely strong x-ray emission.

C) Each clump is the endpoint of one of the galaxy's spiral arms.

D) The clumps are globular clusters orbiting the center of the galaxy.

Answer: A

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4) This photo shows the central region of a cluster of galaxies. Based on what you have learned from your text, which of the following statements about this photo is not true?

A) The oldest stars in these galaxies are much older than the oldest stars in our own Milky Way. B) Half for more of the large galaxies we see in this photo are elliptical galaxies.

C) Many of the galaxies in this photo have probably suffered collisions in the past.

D) These galaxies are held together by their mutual gravitational attraction.

Answer: A

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5) The photo shows a galaxy known as Arp 220, and the inset shows this galaxy's spectrum.

What can you infer?

A) Most of the light from this galaxy is infrared, indicating active star formation.

B) Most of the light from this galaxy is visible light, and it comes from an active galactic nucleus.

C) The broad peak is a spectral line indicating the presence of massive amounts of hydrogen gas.

D) The spectrum peaks in the infrared, indicating that this galaxy has a huge redshift and must be far away.

Answer: A

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6) This visible-light photo shows a starburst galaxy. What is going on with the reddish gas that seems to be emerging on both sides of it?

A) It is gas being blown out in a galactic wind.

B) It is gas from a protogalactic cloud that is still falling inward toward the galactic center.

C) The gas represents jets being shot out from the region of a supermassive black hole.

D) The gas consists of many individual stars that are bursting out from the central regions of the galaxy.

Answer: A 7) This Hubble Space Telescope photo shows the bright active galactic nucleus in the center of galaxy M87. What is the long bluish streak coming out of it?

A) a "jet" of material being shot out at high speed B) the blueshifted light of gas that is moving toward us C) a stream of material that is falling in toward an accretion disk D) a spiral arm filled with regions of active star formation Answer: A 18 Copyright © 2020 Pearson Education, Inc.

8) The bright object in this photo is a quasar (named 3C 273). According to current understanding, about how big is the source of the bright light?

A) about the size of our solar system B) about a million light-years across C) about the size of the central bulge of the Milky Way Galaxy (that is, about 10,000 light-years across) D) about 500 light-years across Answer: A 9) This image combines a visible-light view of the galaxy Hercules A with an image showing radio wave emission (the reddish colors) in its vicinity. The likely explanation for the two huge clumps of radio emission far to either side of the visible galaxy is

A) the radio emission comes from gas shot out into space by jets powered by a supermassive black hole in the galactic center.

B) the radio emission comes from gas that was left behind in the halo when the galaxy collapsed from a protogalactic cloud.

C) the radio emission comes from gas pulled out of the galaxy when it collided with another galaxy in the distant past.

D) this galaxy must have suffered two collisions in the past, with each collision pulling out gas in one direction.

Answer: A

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10) Based on this graph, if you can measure the mass of a galaxy's central bulge, then you will also know the approximate

A) mass of the galaxy's supermassive black hole.

B) mass of the galaxy as a whole.

C) diameter of the galaxy.

D) diameter of the central bulge.

Answer: A

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End-of-Chapter Questions Visual Skills Check Use the following questions to check your understanding of some of the many types of visual information used in astronomy.

In the figure above (repeated from Figure 21.20), the scale bar shows a distance of 500,000 light-years. Use the information in the figure to answer the following questions.

1) This image combines observations made with visible-light and radio telescopes. Which color in the image represents the radio emission?

Answer: Purple to pinkish-white 2) What is the approximate distance from the far edge of one lobe to the far edge of the other?

Answer: Approximately 2,500,000 light-years 3) What is the approximate size of the visible galaxy producing the jets?

Answer: Approximately 200,000 light-years in diameter 4) Suppose we looked at this image without its overlay of radio emission (that is, we looked only at the visible-light image). What features of the image would no longer be visible? Select all that apply.

A) the background galaxies B) the central galaxy C) the jets D) the lobes Answer: C, D

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Chapter Review Questions 1) What do we mean by galaxy evolution? How do telescopic observations allow us to study galaxy evolution? How do theoretical models help us study galaxy formation?

2) What are the starting assumptions for models of galaxy formation? Describe how a spiral galaxy is thought to form.

3) Describe two ways in which conditions in a protogalactic-cloud system might lead to the birth of an elliptical rather than a spiral galaxy.

4) What happens when two galaxies collide? How might elliptical galaxies form from collisions of spiral galaxies? What evidence supports the idea that galaxy collisions sometimes occur?

5) Describe some of the consequences of galaxy collisions. Why were collisions more common in the past?

6) What is a starburst galaxy? Explain how we know that starbursts must be only temporary stages in a galaxy's life.

7) What is a galactic wind? What causes it? How is it similar to a superbubble in the Milky Way, and how is it different?

8) Define active galactic nuclei and quasars. How can we use variations in luminosity to set limits on the sizes of their emitting regions?

9) What evidence suggests that supermassive black holes really exist?

10) How are the masses of supermassive black holes related to the masses of the bulges of their surrounding galaxies? What does this suggest about the role of supermassive black holes in galaxy evolution?

11) What is a radio galaxy? How can radio galaxies affect the gas surrounding them?

12) Briefly describe how studies of absorption lines in quasar spectra allow us to study the gas outside of galaxies. Why is that gas important to galaxy evolution?

Does It Make Sense?

Decide whether or not each of the following statements makes sense (or is clearly true or false).

Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

Example: I walked east from our base camp at the North Pole.

Solution: The statement does not make sense because east has no meaning at the North Pole–all directions are south from the North Pole.

13) Galaxies that are more than 10 billion years old are too far away for us to see even with our most powerful telescopes.

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14) Elliptical galaxies are more likely to form in denser regions of space.

15) If the Andromeda Galaxy someday collides and merges with the Milky Way, the resulting galaxy may be elliptical.

16) NGC 9645 is a starburst galaxy that has been forming stars at the same furious pace for 10 billion years.

17) The energy from supernova explosions can drive a large proportion of the interstellar gas out of a small galaxy.

18) Astronomers proved that quasar 3C 473 contains a supermassive black hole when they discovered that its center is completely dark.

19) The black hole at the center of our own galaxy may once have powered an active galactic nucleus.

20) Radio galaxies emit only radio waves and no visible light.

21) Activity around a galaxy's central black hole can lead to heating and disruption of gas more than a million light-years from its center.

22) Quasar spectra can tell us about intergalactic gas clouds that might otherwise remain invisible.

Quick Quiz

23) When we observe a distant galaxy whose photons have traveled for 10 billion years before reaching Earth, we are seeing that galaxy as it was when the universe was about A) 10 billion years old.

B) 7 billion years old.

C) 4 billion years old.

Answer: No Correct Answer Was Provided.

24) Which of these statements is a key assumption in our most successful models for galaxy formation?

A) The distribution of matter was perfectly uniform early in time.

B) Some regions of the universe were slightly denser than others.

C) Galaxies formed around supermassive black holes.

Answer: No Correct Answer Was Provided.

25) A collision between two large spiral galaxies is likely to produce A) a large elliptical galaxy.

B) a large spiral galaxy.

C) one large spiral galaxy and one large elliptical galaxy.

Answer: No Correct Answer Was Provided.

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26) A collision and merger of two large elliptical galaxies will eventually produce A) a large elliptical galaxy. B) a large spiral galaxy. C) a large irregular galaxy. Answer: No Correct Answer Was Provided. 27) Starburst galaxies are especially bright in A) visible light. B) ultraviolet light. C) infrared light. Answer: No Correct Answer Was Provided. 28) The rate at which supernovae explode in a starburst galaxy that is forming stars 10 times as fast as the Milky Way is A) about the same as the rate in the Milky Way. B) about 10 times the rate in the Milky Way. C) about 0.1 times the rate in the Milky Way. Answer: No Correct Answer Was Provided. 29) The luminosity of a quasar is generated in a region the size of A) the Milky Way. B) a star cluster. C) the solar system. Answer: No Correct Answer Was Provided. 30) The primary source of a quasar's energy is A) chemical energy. B) nuclear energy. C) gravitational potential energy. Answer: No Correct Answer Was Provided. 31) Observations indicate that galaxies with more massive central black holes tend also to have A) a greater mass of stars in their central bulges. B) a greater overall luminosity. C) a more elliptical shape. Answer: No Correct Answer Was Provided.

32) Which of the following quasars would you expect to have the largest number of hydrogen absorption lines in its spectrum? A) a quasar with a lookback time of 1 billion years B) a quasar with a lookback time of 8 billion years C) a quasar with a lookback time of 13 billion years Answer: No Correct Answer Was Provided.

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Inclusive Astronomy Use these questions to reflect on participation in science.

33) Group Discussion: Who Gets to Use JWST?As this book goes to press, the James Webb Space Telescope (JWST) is scheduled for launch in 2021. It is a joint project of NASA, the European Space Agency, and the Canadian Space Agency. Once in space, JWST will be the world's premier facility for many astronomical observations, including those relating to galaxy evolution. Demand for telescope time will be enormous, and the people in charge of the proposal process will want to maximize the telescope's scientific productivity while simultaneously minimizing the impact of cultural biases on who gets access. a) Working independently, learn about the process of submitting proposals for observations with JWST. Then gather into small groups to share what you've learned. b) Imagine that your group were in charge of reading the proposals and deciding who should be awarded telescope time. What kinds of people would you consult to determine which proposals had the greatest scientific merit and whether those proposals were feasible, given the technological capabilities of JWST? c) Do you think there is a risk that the judgments of the people you consult might be culturally biased in any way? Make a list of potential cultural biases that could in principle affect their judgments. d) Overall, how would your group structure the process to minimize the effects of cultural bias?

Do you think your process would be foolproof, or might there still be biases?

The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

34) The Case for Supermassive Black Holes.

The case for supermassive black holes at the centers of galaxies is strong, but it is based on evidence from orbiting objects, because the black holes themselves emit no light. How compelling do you find this evidence? Would you say that astronomers have proved the case for these black holes "beyond a reasonable doubt"? Defend your opinion.

35) Black Holes and Galaxy Evolution. Astronomers do not yet know the cause of the relationship between the mass of a galaxy's supermassive black hole and the total mass of stars in its bulge (see Figure 21.19), but this chapter discussed the hypothesis that it arises because energetic outbursts from the black hole periodically disrupt a galaxy's star—gas—star cycle.

Suggest possible observations that could be made to test this hypothesis.

36) Unanswered Questions. Briefly describe one important but unanswered question related to galaxy evolution. If you think it will be possible to answer that question in the future, describe how we might find an answer, being as specific as possible about the evidence necessary.

If you think the question will never be answered, explain why you think it is impossible to answer.

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37) Group Exercise: Testing Galaxy Evolution. Imagine that astronomers discovered galaxies with the following characteristics; note that these are not real discoveries: • an extremely large elliptical galaxy with a lookback time of 13 billion years • a small irregular galaxy at a distance of 10 million light-years from Earth • a large spiral galaxy, 20 million light-years from Earth, consisting entirely of stars made only of hydrogen and helium • a small spiral galaxy, 5 million light-years from Earth, with one of the most massive black holes ever discovered at its center • a spiral galaxy approximately the size of the Milky Way with a lookback time of 13 billion years Work in small groups to decide whether each discovery would be easy or difficult to explain in terms of our current ideas about how galaxies form and evolve. After discussing all of the hypothetical discoveries, rank them in order from easiest to explain to most difficult to explain, and give your reasons. Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39.

Investigate Further 38) Life Story of a Spiral. Imagine that you are a spiral galaxy. Describe your life history from birth to the present day. Your story should be detailed and scientifically consistent, but also creative. That is, it should be entertaining while at the same time incorporating current scientific ideas about the formation of spiral galaxies.

39) Life Story of an Elliptical. Imagine that you are an elliptical galaxy. Describe your life history from birth to the present. There are several possible scenarios for the formation of elliptical galaxies, so choose one and stick to it. Be creative while also incorporating scientific ideas that demonstrate your understanding.

40) The Color of an Elliptical Galaxy. Explain how the color of an old elliptical galaxy has changed during the last 10 billion years. How might you be able to use the galaxy's color to determine when it formed?

41) Very Early Collisions. Suppose two protogalactic clouds collide while they are still made primarily of gas, before forming many stars. Could the resulting system end up as a spiral galaxy? Explain your reasoning.

42) Life in Colliding Galaxies. Suppose the Milky Way were currently undergoing a collision with another large spiral galaxy. Do you think this collision would affect life on Earth? Why or why not? How would the night sky look if our galaxy were in the midst of such a collision?

43) A Small-Scale Starburst. The Large Magellanic Cloud, a small companion galaxy to the Milky Way, is currently undergoing a small-scale starburst. Inspect the picture of the Large Magellanic Cloud in Figure 20.7a for evidence of widespread star formation. Describe the evidence you find.

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44) Orbits Around Supermassive Black Holes. The data in Figure 21.18b show orbital velocities of gas clouds at distances of 0.4 to 0.7 light-year from the center of the galaxy M106, derived from the Doppler shifts of emission lines. Suppose you observed emission lines from gas clouds orbiting more than 1 light-year from the center. How would you expect the Doppler shifts of those lines to be different, assuming that the gas really is orbiting a supermassive black hole?

45) Absorption Lines in Quasar Spectra. Based on your understanding of galaxy evolution, what patterns would you expect to see among the redshifts of absorption lines from elements other than hydrogen? Would you expect those absorption lines to have redshifts similar to those of galaxies along the line of sight to the quasar, or would their redshifts be evenly distributed throughout intergalactic space? Would you expect to see any absorption lines with redshifts greater than that of the quasar? Explain your reasoning.

46) Research: Greatest Redshift. As of summer 2018, the most distant quasar known had a redshift of z = 7.54, meaning that the wavelengths of its spectral lines are 1 + 7.54 = 8.54 times their rest wavelengths. Find the current record holder for the largest redshift. Write a few paragraphs describing the object and its discovery.

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

47) Distances Between Galaxies.

If you were to divide the present-day universe into cubes with sides 10 million light-years long, each cube would contain, on average, about one galaxy similar in size to the Milky Way. Now suppose you traveled back in time, to an era when the average distance between galaxies was one quarter of its current value, corresponding to a cosmological redshift of z = 3. How many galaxies similar in size to the Milky Way would you expect to find, on average, in cubes of that same size? In order to simplify the problem, assume that the total number of galaxies of each type has not changed. Based on your answer, would you expect collisions to be much more frequent at that time or only moderately more frequent?

48) A Nearby Starburst. The galaxy M82, shown in Figure 21.11, is one of the nearest starburst galaxies.

Even though it is considerably smaller than the Milky Way, it is currently forming stars more quickly, converting approximately 10MSun of gas into stars each year. The total amount of gas in its interstellar medium is currently about 109MSun. About how much longer can the starburst in M82 last? Based on your answer, describe how the appearance of M82 will change with time.

49) Your Last Hurrah. Suppose you fall into an accretion disk that sweeps you into a supermassive black hole. On your way down, the disk radiates 10% of your mass-energy, E = mc2. a) What is your mass in kilograms? (Recall that 27 1 kg = 2.2lb on Earth.) Calculate how much radiative energy will be produced by the accretion as a result of your fall into the black Copyright © 2020 Pearson disk Education, Inc. hole. b) thisCalculate approximately how long a 100-watt light bulb would have to shine to radiate

50) The Black Hole in M106. The molecular clouds circling the active nucleus of the galaxy M106 orbit at a speed of about 1000 km/s, with an orbital radius of 0.49 light-year = 4.8 x 1015 meters. Use the orbital velocity law (see Mathematical Insight 21.2) to calculate the mass of the central black hole. Give your answer both in kilograms and in solar masses (1MSun = 2.0 x 1030 kg).

51) The Black Hole in M31. Measurements of star motions at the center of the Andromeda Galaxy, also known as M31, show that stars about 3 light-years from the center are orbiting at a speed of 400 km/s. These stars are suspected to be orbiting a supermassive black hole. Use the orbital velocity law from Mathematical Insight 21.2 to estimate the mass of this black hole.

52) Building a Supermassive Black Hole. The black hole in the galaxy M87 has a mass of about 3 billion solar masses. Let us assume that most of that mass flowed into the black hole through an accretion disk that radiated 10% of the mass-energy passing through it. In that case, what would be the total amount of energy radiated by the accretion disk during the history of the black hole? What would be the average luminosity of the accretion disk, if it continuously radiated that energy over a period of 10 billion years? How does that average luminosity compare with the luminosity of the Milky Way? 53) Stars and Clouds in Colliding Galaxies. When two spiral galaxies collide, the stars generally do not run into each other, but the gas clouds do collide, triggering a burst of new star formation. a) Estimate the probability that our Sun would collide with another star in the Andromeda Galaxy if a collision between the Milky Way and the Andromeda Galaxy were happening at the present time. To simplify the problem, assume that each galaxy has 100 billion stars exactly like the Sun spread evenly over a circular disk with a radius of 100,000 light-years. (Hint: First calculate the total area of 100 billion circles with the radius of the Sun and then compare that total area to the area of the galactic disk.) b) Estimate the probability that a gas cloud in our galaxy would collide with another gas cloud in the Andromeda Galaxy. To simplify the problem, assume that each galaxy contains 100,000 clouds of warm hydrogen gas, that each cloud has a radius of 300 light-years, and that these clouds are spread evenly over a circular disk with a radius of 100,000 light-years. (Hint: Use the same method as in part a.) c) Compare the probabilities you found in parts a and b and use them to explain what you see in Figure 21.5.

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter 22 The Birth of the Universe Section 22.1 1) To date, physicists have investigated the behavior of matter and energy at temperatures as high as those that existed in the universe as far back as ________ after the Big Bang.

A) 1 million years B) 300,000 years C) 300 years D) 3 minutes E) 10-10 second Answer: E 2) What is the main idea of the Big Bang theory?

A) The universe formed from the collapse of a previous universe.

B) All matter and energy in the universe was in an incredibly hot, dense state and space itself began expanding.

C) The universe as we see it formed from the singularity in a black hole.

D) A massive explosion about 14 billion years ago created all matter and energy that rushed out to fill the surrounding space.

Answer: B 3) When we say that the electromagnetic and weak forces "freeze out" from the electroweak force at 10-10 seconds after the Big Bang, what do we mean?

A) These forces are important only at temperatures below the freezing point of water–a temperature that the universe reached at an age of about 10-10 seconds.

B) "Freezing out" was a term coined by particle physicists who think that the Big Bang theory is really cool.

C) Prior to this time, the electromagnetic and weak forces could not be distinguished from each other, but they possessed separate identities following this time.

D) Following this time, neither the electromagnetic nor the weak force was ever important in the universe again.

E) Following this time, the electromagnetic and weak forces starting cooling the universe, which is why it is so cold today.

Answer: C 4) What direct evidence do we have that the weak and electromagnetic forces were once unified as a single electroweak force?

A) The most advanced telescopes are able to see back to this era in the universe.

B) Detectors on Earth have received photons and high-energy particles from this era.

C) Temperatures in the center of the Sun can reproduce the conditions during this era.

D) Particle accelerators on Earth can recreate the conditions of this era and have produced particles predicted by the electroweak theory.

E) We have no direct evidence of the electroweak force.

Answer: D

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5) What happened to the quarks that existed freely during the particle era?

A) They combined to make protons, neutrons, and their antiparticles.

B) They froze out of the soup of particles at the end of the era.

C) They evaporated.

D) They combined to make electrons and neutrinos.

E) They combined to make W and Z bosons.

Answer: A 6) If the electromagnetic force is stronger than gravity, then why is it overpowered by gravity on large scales?

A) The electromagnetic force only works on scales of about the size of an atomic nucleus.

B) The electromagnetic force follows an inverse cube law with distance, rather than an inverse square law.

C) Electrical charge is canceled out by mass.

D) Most objects are electrically neutral.

Answer: D 7) This figure shows the relative strength of the four fundamental forces at different temperatures. Based on this graph, at what temperature does the strong force become equal in strength to the electroweak force?

A) about 1015 K B) about 1027 K C) about 1032 K D) The strong force always remains distinct and stronger than the electroweak force at all temperatures. Answer: B

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8) Which force binds together protons and neutrons in a nucleus?

A) strong force B) gravity C) electromagnetic force D) weak force Answer: A 9) Our current theories are unable to describe the conditions of the universe during the Planck era. About how long after the Big Bang did the Plank era end?

A) 10-10 second B) 10-35 second C) 10-43 second D) 3 minutes E) 380,000 years Answer: C 10) Why can't current theories describe what happened during the Planck era?

A) We do not yet have a theory that links quantum mechanics and general relativity.

B) We do not understand the properties of antimatter.

C) We do not know how much energy existed during that time.

D) It was a time period from which we cannot receive radiation.

E) The Planck era was the time before the Big Bang, and we cannot describe what happened before that instant.

Answer: A 11) If grand unified theories are correct, what caused inflation at the end of the GUT era?

A) Gravity was an extremely weak force at this period in time.

B) Large amounts of matter and antimatter annihilated at this time.

C) There wasn't enough matter present to slow down the expansion at that time.

D) The universe was too small and needed to grow quickly.

E) An enormous amount of energy was released when the strong force froze out (became distinct) from the GUT force.

Answer: E 12) Approximately how long after the Big Bang did the era of nucleosynthesis end, marking the time when the basic chemical composition of the universe was determined?

A) 10-10 second B) 0.001 second C) 5 minutes D) 380,000 years E) 5 million years Answer: C

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13) What kinds of atomic nuclei were present when fusion ceased at the end of the era of nucleosynthesis?

A) only hydrogen B) only helium C) hydrogen, helium, and trace amounts of deuterium and lithium D) roughly equal amounts of each of the following: hydrogen, helium, deuterium, and lithium E) nuclei of all the chemical elements Answer: C 14) Why did fusion cease only about 5 minutes after the Big Bang (at the end of the era of nucleosynthesis)? A) The density of the universe became too low. B) There was no long any more hydrogen available for fusion reactions. C) Too many heavy elements were produced. D) Neutrinos carried off too much energy. Answer: A 15) What event made it possible for photons to begin to travel freely through the universe when the universe was about 380,000 years old? A) Magnetic fields had weakened to the point that they no longer interfered with light. B) The universe had expanded and cooled enough for electrons to combine with nuclei to form neutral atoms. C) Atomic nuclei were finally able to escape the plasma of the early universe. D) Atoms started to combine to make the first molecules. E) Fusion of hydrogen to make helium ceased. Answer: B 16) Which of the following happened first (after the Big Bang)? A) Molecules formed. B) Neutral atoms formed. C) Protons formed. D) Helium formed. Answer: C

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17) Order these cosmological events in order from earliest to latest.

A) formation of the first atomic nuclei, formation of the first neutral atoms, formation of the first subatomic particles, formation of the first galaxies B) formation of the first subatomic particles, formation of the first atomic nuclei, formation of the first neutral atoms, formation of the first galaxies C) formation of the first galaxies, formation of the first neutral atoms, formation of the first atomic nuclei, formation of the first subatomic particles D) formation of the first subatomic particles, formation of the first neutral atoms, formation of the first atomic nuclei, formation of the first galaxies E) formation of the first neutral atoms, formation of the first subatomic particles, formation of the first atomic nuclei, formation of the first galaxies F) formation of the first atomic nuclei, formation of the first subatomic particles, formation of the first neutral atoms, formation of the first galaxies Answer: B 18) According to the Big Bang theory, why do we live in a universe that is made of matter rather than antimatter?

A) Einstein's famous equation E = mc2 tells us that energy can turn into matter, but does not tell us that it can turn into antimatter.

B) GUT theories predict that under the conditions that prevailed in the early universe, the normal laws of physics would have been suspended so that only matter particles were created, and no particles of antimatter.

C) The fact that we live in a universe made of matter is not surprising because antimatter has never been shown to exist for real.

D) During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the very slight excess of matter particles.

Answer: D Section 22.2 1) Which of the following lists the two key predictions of the Big Bang model that have been verified by observations?

A) (1) the existence of the cosmic microwave background and (2) the amount of helium relative to hydrogen in the universe B) (1) the existence of the cosmic microwave background and (2) the existence of dark matter C) (1) the existence of the cosmic microwave background and (2) the near-critical density of the universe D) (1) the predominance of matter over antimatter and (2) the near-critical density of the universe E) (1) the predominance of matter over antimatter and (2) the large-scale structure of galaxies Answer: A

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2) When did the photons in the cosmic microwave background first travel freely through the universe?

A) the moment of the Big Bang B) about 10-10 second after the Big Bang C) about 5 minutes after the Big Bang D) about 380,000 years after the Big Bang E) At the time, the first stars began to shine in the universe.

Answer: D 3) Why does the Big Bang theory predict that the cosmic microwave background should have an essentially perfect thermal radiation spectrum?

A) This radiation came from the heat of the universe itself.

B) The spectrum of pure hydrogen is a perfect thermal radiation spectrum.

C) The spectrum of 75 percent hydrogen and 25 percent helium is a perfect thermal radiation spectrum.

D) The light from all the stars and gas in the sky averaged over the entire universe is a perfect thermal radiation spectrum.

E) The first stars that formed in the universe must all have had the same temperature.

Answer: A 4) The cosmic microwave background is almost perfectly uniform in all directions, except for very small deviations in its temperature. What do scientists think these small deviations represent?

A) regions of slightly higher density that made it possible for galaxies to form B) small increases in temperature caused by individual stars C) The warmer spots represent regular matter and the cooler spots represent dark matter.

D) regions of slightly higher helium abundance in the early universe E) varying redshifts and blueshifts due to motions of gas in the early universe Answer: A 5) Suppose you repeated the famous COBE and Planck observations of the cosmic microwave background, with a slight different observational setup, to check their results. Which of the following observations would be surprising and contradict earlier findings?

A) You find that cosmic microwave background has a temperature of 2.73 K. B) You find that cosmic microwave background has an almost perfect thermal spectrum.

C) You find that cosmic microwave background contains redshifted emission lines of hydrogen and helium.

D) You find that cosmic microwave background looks essentially the same in all directions.

E) You find that cosmic microwave background has tiny temperature fluctuations at the level of about 1 part in 100,000.

Answer: C

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6) Approximately how long have cosmic microwave background photons been traveling through space? A) 5000 years B) 380,000 years C) 14 billion years D) It depends on which direction they come from. Answer: C 7) In what sense is the cosmic microwave background evidence for the Big Bang? A) because Einstein predicted that the universe should be expanding B) because this radiation would have been necessary to cause the Big Bang C) because explosions are always associated with dangerous levels of radiation D) because it was predicted to exist if the Big Bang really occurred Answer: D 8) Our Sun is made of about 28% helium (by mass). Where did this helium come from? A) Fusion in stars B) The Big Bang only C) Mostly from fusion in stars with a small contribution from the Big Bang D) Mostly from the Big Bang with a small contribution from fusion in stars E) Radioactive decay of heavier elements only Answer: D 9) Why did the Big Bang not produce a significant proportion of elements heavier than helium? A) High-energy photons did not affect helium, but blasted apart the nuclei of heavier elements as soon as they formed. B) By the time helium nuclei could survive, the temperature had become too low for heavier elements to form. C) They did, but radioactive decay caused these elements to disappear again. Answer: B Section 22.3 1) Why do scientists think tiny quantum ripples should have been present in the very early universe? A) The shock wave of the Big Bang caused ripples that expanded outward with time. B) The energy released when the strong force froze out of the GUT force caused shock waves that produced ripples in the universe.

C) Matter and antimatter particles that spontaneously formed from high-energy photons caused disruptions in the radiation field. D) The annihilation of matter and antimatter particles caused tiny explosions that disrupted the radiation field. E) Quantum mechanics tells us that the energy fields at any point in space be continually fluctuating as a result of the uncertainty principle. Answer: E

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2) What is hypothesized to have caused a sudden inflation of the early universe?

A) the annihilation of matter and antimatter B) the separation of the electromagnetic and weak forces C) the "freezing out" of the strong force from the GUT force D) the energy released in the fusion of protons and neutrons to produce helium E) giant quantum fluctuations Answer: C 3) The temperature variations of the cosmic microwave background observed by the Planck telescope agree well with our model predictions. This provides persuasive evidence in favor of what?

A) both the Big Bang and inflation B) only the Big Bang, but not inflation C) not the Big Bang, but inflation D) neither the Big Bang nor inflation Answer: A 4) How does the hypothesis of inflation explain the near-uniformity of the cosmic microwave background?

A) The expanding universe would have cooled.

B) Matter expanded into regions of space that had no matter, and thus ended up at the same temperature.

C) Matter was near the critical-density for a universal collapse, which smoothed out the differences in temperatures.

D) Prior to rapid inflation, all regions of space were close enough to bounce radiation back and forth and reach the same temperature.

Answer: D 5) Which of the following hypothetical observations would demonstrate that the inflation model was incorrect?

A) The geometry of the universe is curved, not flat.

B) The helium abundance in a galaxy is 29%.

C) The rate of expansion of the universe has changed over time.

D) Stars began to shine only a few hundred million years after the Big Bang.

Answer: A Section 22.4 1) The fact that the sky is dark at night proves that the universe cannot be A) eternal and unchanging with an infinite number of stars.

B) expanding.

C) more than 20 billion years old.

D) infinite in size.

Answer: A

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General Chapter Questions 1) Which of the following observations is not a piece of evidence that lends support to aspects of the Big Bang theory?

A) the darkness of the night sky B) the expansion of the universe C) the observed helium abundance in the universe D) the relative motions of galaxies in the Local Group Answer: D 2) The spectrum of the cosmic microwave background peaks at a wavelength of about 1 mm, and the universe has a temperature of about 3 K. If the spectrum instead peaked at a wavelength of 10 millimeters, what would its temperature be?

A) the same temperature of about 3 K B) less than 3 K C) more than 3 K Answer: B 3) This chapter discusses the "critical density" of the universe. More generally, when we say that something is a "critical value" in science, we mean A) a number that signifies some boundary or change.

B) a value that has been heavily criticized.

C) an idea, representing a value system, that is very important.

D) any number that has been very precisely measured.

Answer: A Short Answer Questions 1) Briefly explain how Hubble's discovery of a relationship between galactic distance and recession velocity led to the idea of the Big Bang.

Answer: Hubble's discovery showed that the universe is expanding. If the universe is expanding, then logically it seems that it should have been smaller in the past. Extrapolating back in time, there must have been a time when everything was in one place–which we call the Big Bang.

2) Briefly describe two key pieces of evidence that support the Big Bang theory.

Answer: The two key pieces of evidence that support the Big Bang theory are the cosmic microwave background and the observed helium content of the universe. The cosmic microwave background consists of photons arriving at Earth directly from the end of the era of nuclei. The radiation came from the heat of the universe and should have a thermal radiation spectrum with a peak wavelength corresponding to 3000 K, the temperature the universe was at the end of that era. Since the universe has expanded by a factor of about 1000 since that time, we observe the radiation with a peak wavelength of about a millimeter, corresponding to a temperature of 2.73K. The Big Bang theory predicts that the universe should have had a composition of 75 percent hydrogen and 25 percent helium by mass at the end of the era of nucleosynthesis. The Milky Way's helium fraction is about 28 percent, and no galaxy has a helium fraction lower than 25 percent.

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Reading Quiz Questions 1) In general, what happens to the density and temperature of a gas as it expands?

A) It gets less dense and cooler.

B) It gets denser and cooler.

C) It gets less dense and hotter.

D) It gets denser and hotter.

Answer: A 2) In the distant past, the temperature of the universe was A) hotter than it is today.

B) cooler than it is today.

C) absolute zero (0 K).

D) 3 K. Answer: A 3) What is antimatter?

A) It consists of particles that are just like those of ordinary matter except with opposite properties, such as opposite charge.

B) It is matter that we infer to be in space by its gravitational influence, but that we have not yet identified.

C) It is a form of matter that has been hypothesized to exist but for which there is not yet any experimental evidence.

D) It is the material that makes up a complete antiuniverse that is identical to our own except that it is made from antimatter instead of matter.

Answer: A 4) What happens when a particle of matter meets its corresponding antiparticle of antimatter?

A) The combined mass of the two particles is completely transformed into energy (photons).

B) They can form a complete atom.

C) They fuse to make a heavier particle.

D) The question makes no sense, because antimatter does not really exist.

Answer: A 5) Experiments allow physicists today to reproduce (on very small scales) energy and temperature conditions thought to have prevailed in the early universe as far back in time as about A) one ten-billionth (10-10) of a second after the Big Bang.

B) 380,000 years after the Big Bang.

C) 10-45 seconds after the Big Bang.

D) 10 billion years ago.

Answer: A

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6) The four fundamental forces that operate in the universe today are A) strong force, weak force, electromagnetic force, and gravity.

B) strong force, weak force, electric force, and magnetic force.

C) nuclear force, electromagnetic force, gravity, and tidal force.

D) nuclear force, gravity, electric force, and magnetic force.

Answer: A 7) What is the significance of Planck time?

A) Before it, conditions were so extreme that our current understanding of physics is insufficient to predict what might have occurred.

B) It is the time at which inflation is thought to have occurred.

C) It is the time when the cosmic microwave background was released.

D) It is the amount of time required for two protons to fuse to make deuterium.

Answer: A 8) A grand unified theory (GUT) refers to a type of theory intended to unify A) the strong force with the electromagnetic and weak forces.

B) gravity with the strong and weak forces.

C) the electromagnetic and weak forces.

D) all four forces together.

Answer: A 9) In terms of the "eras" that scientists use to describe different times in the history of the universe, we live in the A) era of galaxies.

B) era of nucleosynthesis.

C) era of nuclei.

D) Planck era.

Answer: A 10) What do we mean by the term inflation?

A) a sudden and extremely rapid expansion of the universe that occurred in a tiny fraction of a second during the universe's first second of existence B) the expansion of the universe that we still observe today C) the sudden release of photons when a particle and antiparticle annihilate one another D) quantum fluctuations by high speed, relativistic particles in a state of false vacuum that caused disturbances in the space—time continuum leading to the process described in the question to which this answer refers Answer: A

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11) Which of the following statements correctly summarizes the events in the early universe according to the Big Bang theory?

A) The universe began with the forces unified. During the first fraction of a second, the forces separated and there was a brief but important episode of inflation. Subatomic particles of both matter and antimatter then began to appear from the energy present in the universe. Most of the particles annihilated to make photons, but some became protons, neutrons, electrons, and neutrinos. The protons and neutrons underwent some fusion during the first three minutes, thereby determining the basic chemical composition of the universe.

B) An episode of what we call inflation initiated the event of the Big Bang. Once the Big Bang got underway, particles and forces began to appear one by one. The forces produced protons, which fused to make hydrogen and helium until the universe was about 380,000 years old. Then gravity began to act, turning the hydrogen and helium into galaxies.

C) Forces and various subatomic particles began to appear during the first second after the Big Bang. For reasons not understood, the particles were all made of ordinary matter and none were made of antimatter, thus explaining why we live in a universe made of matter. The particles underwent some fusion for the first 380,000 years after the Big Bang, at which time the first stars were born.

D) The Big Bang began with the initiation of what we call inflation, which gradually slowed to the current expansion rate of the universe. Forces came to exist for a different reason, having to do with quantum fluctuations in the space—time continuum. Particles came to exist as a result of cracks made when forces froze. Once there were particles, gravity brought them together to make stars, and the stars then turned the particles into hydrogen, helium, and other elements.

Answer: A 12) Which statement about the cosmic microwave background is not true?

A) It is the result of a mixture of radiation from many independent sources, such as stars and galaxies.

B) Its spectrum has the shape of an essentially perfect thermal radiation spectrum.

C) With the exception of very small variations, it appears essentially the same in all directions in which we look into space.

D) It is thought to be radiation that began its journey to our telescopes when the universe was about 380,000 years old.

Answer: A 13) What is the approximate temperature of the universe (as a whole) today?

A) 3K B) 3000K C) 300K D) The universe cannot be said to have a single temperature.

Answer: A

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14) The Big Bang theory is supported by two major lines of evidence that alternative models have not successfully explained. What are they? A) (1) the existence and specific characteristics of the observed cosmic microwave background (2) the observed overall chemical composition of the universe B) (1) the early universe was hot and dense (2) we see distant galaxies as they were in the distant past C) (1) the episode of inflation thought to have occurred in the early universe (2) the separation of gravity and the other forces at the end of the Planck era D) (1) the universe is expanding (2) the observed ratio of spiral to elliptical galaxies in the universe Answer: A 15) Which of the following observations cannot be explained by the Big Bang theory, unless we assume that an episode of inflation occurred? A) the fact that the temperature of the cosmic microwave background is almost the same everywhere B) the fact that about 25% of the ordinary matter in the universe consists of helium C) the existence of the cosmic microwave background D) the fact that the universe is expanding Answer: A 16) The critical density of the universe is the A) total density of matter and energy needed to give the universe a "flat" geometry (in spacetime).

B) measured average density of matter in the universe. C) density of matter necessary for stars and galaxies to form. D) density value at which the total density of matter precisely equals the total density of antimatter. Answer: A 17) Models of the Big Bang that include inflation predict that the overall geometry of the universe should be "flat" (in spacetime).

This prediction A) agrees with data found by studying the cosmic microwave background.

B) has been verified by observing the paths of light coming from distant galaxies. C) has been shown to be incorrect by study of the expansion pattern of the universe. D) cannot be tested at this time. Answer: A 18) Which of the following statements cannot be tested by science today? A) Prior to the Planck time, our universe sprouted from another universe. B) Our universe is flat. C) The universe is 14 billion years old. D) The expansion of the universe is now accelerating. Answer: A

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19) Olbers's paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the question?

A) Why is the sky dark at night?

B) What would it be like to ride on a beam of light?

C) How many stars are in the universe?

D) Can we measure the position and momentum of an electron at the same time?

Answer: A Concept Quiz Questions 1) How do we determine the conditions that existed in the early universe?

A) We work backward from current conditions to calculate what temperatures and densities must have been when the observable universe was much smaller in size.

B) We look all the way to the cosmological horizon, where we can see the actual conditions that prevailed all the way back to the first instant of the Big Bang.

C) The conditions in the very early universe must have been much like those found in stars today, so we learn about them by studying stars.

D) We can only guess at the conditions, because we have no way to calculate or observe what they were.

Answer: A 2) Why do scientists assume that the early universe was much hotter and denser than the universe of today?

A) The fact that the universe is expanding implies that objects were closer together in the past, and compressing material makes it hotter and denser.

B) All objects begin their existence in a hot and dense state.

C) The fact that the universe is filled with stars and galaxies implies that matter must once have been hot to have made these objects.

D) Observations of distant galaxies show that they are much hotter than galaxies of today, suggesting that the whole universe was hotter in the past.

Answer: A 3) Why can't current theories describe what happened during the Planck era?

A) We do not yet have a theory that links quantum mechanics and general relativity.

B) We do not know how hot or dense the universe was during that time.

C) We do not understand the properties of antimatter.

D) The Planck era was the time before the Big Bang, and we cannot describe what happened before that instant.

Answer: A

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4) Which of the following statements best explains what we mean when we say that the electroweak and strong forces "froze out" at 10-38 second after the Big Bang?

A) These two forces first became distinct at this time.

B) These forces are important only at temperatures below the freezing point of water, which is a temperature that the universe reached at an age of about at 10-38 second.

C) Freezing out was a term coined by particle physicists who think that the Big Bang theory is really cool.

D) Following this time, neither the strong nor electroweak forces were ever important in the universe again.

Answer: A 5) According to the Big Bang theory, how many forces, and which ones, operated in the universe during the GUT era?

A) two forces: gravity and a single force that later became the strong, weak, and electromagnetic forces B) one force that represented the unification of all four forces that operate today C) three forces: gravity, the strong force, and the electroweak force D) two forces: the strong force and the electroweak force Answer: A 6) Laboratory experiments conducted with particle accelerators confirm predictions made by the theory that unifies A) the electromagnetic and weak forces into the electroweak force.

B) the strong, weak, and electromagnetic forces into the GUT force.

C) the unification of all four forces into a single "superforce."

D) the strong and weak forces into the combined nuclear force.

Answer: A 7) What was the significance of the end of the era of nucleosynthesis, when the universe was about 5 minutes old?

A) The basic chemical composition of the universe had been determined.

B) The proportions of dark matter and luminous matter had been determined.

C) It marks the time at which the first stars formed.

D) It marks the time at which the expansion of the universe had settled down to its current rate.

Answer: A

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8) According to the Big Bang theory, why do we live in a universe that is made of almost entirely of matter rather than antimatter?

A) During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything was annihilated except the slight excess of matter particles.

B) GUT theories predict that under the conditions that prevailed in the early universe, the normal laws of physics would have been suspended, so that only matter particles were created and no particles of antimatter.

C) The fact that we live in a universe made of matter is not surprising, because antimatter has never been shown to exist for real.

D) Einstein's famous equation E = mc2 tells us that energy can turn into matter but does not tell us that it can turn into antimatter.

Answer: A 9) Which of the following is not an observed characteristic of the cosmic microwave background?

A) It contains prominent spectral lines of hydrogen, the primary chemical ingredient of the universe.

B) It has a perfect thermal radiation spectrum.

C) Its temperature is the same everywhere, except for small variations at the level of 1 part in 100,000.

D) Its temperature is a little less than 3 Kelvin (3 degrees above absolute zero).

Answer: A 10) In principle, if we could see all the way to the cosmological horizon we could see the Big Bang taking place. However, our view is blocked for times before about 380,000 years after the Big Bang. Why? A) Before that time, the universe was too crowded with stars.

B) Before that time, the gas in the universe was dense and ionized and therefore did not allow light to travel freely.

C) Before that time, the universe was dark so there was no light to illuminate anything.

D) 380,000 years after the Big Bang marks the time when stars were first born, producing the light by which we can see the universe.

Answer: B 11) If observations had shown that the cosmic microwave background was perfectly smooth (rather than having slight variations in temperature), then we would have no way to account for A) the relationship between the strong and the weak force.

B) the fact that our universe is expanding.

C) how galaxies came to exist.

D) the existence of helium in the universe.

Answer: C

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12) In stars, helium can sometimes be fused into carbon and heavier elements (in their final stages of life). Why didn't the same fusion processes produce carbon and heavier elements in the early universe?

A) By the time stable helium nuclei had formed, the temperature and density had already dropped too low for helium fusion to occur.

B) Helium fusion occurred, but the carbon nuclei that were made were later destroyed by the intense radiation in the early universe.

C) Temperatures in the early universe were never above the roughly 100 million Kelvin required for helium fusion.

D) No one knows; this is one of the major mysteries in astronomy.

Answer: A 13) How does the hypothesis of inflation account for the existence of the "seeds" of density from which galaxies and other large structures formed?

A) Inflation would have caused random, microscopic quantum fluctuations to grow so large in size that they became the seeds of structure.

B) Inflation predicts that gravity would have been very strong and thereby would have concentrated mass into seeds.

C) Inflation tells us that the universe should have a "flat" overall geometry, and this led to the flat disks of galaxies.

D) Inflation predicts that temperatures and densities should have become nearly equal throughout the universe.

Answer: A 14) Suppose that inflation did not occur. In that case, the fact that the cosmic microwave background has the same temperature in opposite directions of the sky would be considered A) surprising, because those locations would never have been close enough to have any light or matter exchanged between them.

B) surprising, because we would not expect those locations to have the same age.

C) natural, because everything in the universe is naturally expected to have the same temperature.

D) natural, because quantum fluctuations in the early universe would have ensured that the whole universe had precisely the same temperature.

Answer: A 15) Which analogy best explains why inflation predicts that the overall geometry of the observable universe should appear to be flat?

A) Earth is so big that it the part explored by an ant appears flat.

B) As a balloon expands, dots that you have drawn on it become farther apart with time.

C) If you stretch a wavy line infinitely, it becomes straight.

D) No city on Earth is any more the "center" of Earth's surface than any other city.

Answer: A

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16) Suppose that the universe were infinite in both extent and age. In that case, we would expect the night sky to be A) uniformly bright.

B) much larger.

C) filled with about 1,000 times as many stars.

D) expanding at a more rapid rate.

Answer: A 17) The Big Bang theory seems to explain how elements were formed during the first few minutes after the Big Bang. Which hypothetical observation (these are not real observations) would call our current theory into question?

A) the discovery of a galaxy with a helium abundance of only 10% by mass B) the discovery of a star-like object made almost entirely of carbon and oxygen C) the discovery of a planet that with no helium in its atmosphere D) the discovery of a galaxy with 27% helium by mass Answer: A

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Visual Quiz Questions

1) What does this graph show?

A) It shows calculations indicating that the temperature of the universe began quite high but is now quite low.

B) It shows that the temperature of the universe began quite low but is now quite high.

C) It shows that the temperature of the universe has changed diagonally with time.

D) It shows evidence that the Big Bang really happened.

Answer: A 2) On this graph, the time corresponding to the point at the far lower right of the diagonal line represents A) the age of the universe today, in seconds.

B) age of the universe at the time the cosmic microwave background was released.

C) the Planck time.

D) the amount of time required for nucleosynthesis in the early universe.

Answer: A

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3) What does this diagram represent?

A) a particle and antiparticle colliding and converting all their mass into photons B) two photons spontaneously turning into a particle and an antiparticle C) a blue electron and a red electron turning into to pink light rays D) a particle of ordinary matter and a particle of dark matter colliding and becoming two photons of dark energy Answer: A 4) Study this diagram that summarizes the eras of the universe. Which of the following statements is not true?

A) The electroweak era was the longest lasting of all the eras in the history of the universe.

B) The total time during which the universe went through the Planck era, GUT era, electroweak era, and particle era was less than the time it takes you to blink your eyes.

C) Whole atoms with both electrons and nuclei were not stable until the universe was about 380,000 years old.

D) Most of the helium in the universe was produced before the universe was about five minutes old.

Answer: A

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5) Which of the following isnot implied by this diagram?

A) There are six distinct forces known to operate in the universe, and a seventh force called the "super force" might also exist.

B) At very high temperatures, the electromagnetic and weak forces are no longer distinct but instead act as a single electroweak force.

C) At the current temperature of the universe, gravity is the weakest of the forces.

D) At the current temperature of the universe, the strong force is the strongest of the forces.

Answer: A

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6) The data points on this graph represent the measured spectrum of the cosmic microwave background, while the solid curve represents a theoretically calculated thermal radiation spectrum for a temperature of 2.73 K. What is the significance of the near-perfect match between the data and the theoretical spectrum?

A) It shows that the cosmic microwave background has a thermal radiation spectrum, just as the Big Bang theory predicts it should.

B) It shows that all objects in the universe emit thermal radiation.

C) It tells us the average temperature of gas in the Milky Way Galaxy.

D) It demonstrates that the universe must contain more dark matter than ordinary matter.

Answer: A

7) This figure shows an all-sky map of the cosmic microwave background recorded by the Planck telescope. How long did this light travel through space before it reached the telescope?

A) about 14 billion years B) about 380,000 years C) about a million years D) Depending on what portion of the map we're looking at, the light traveled for times between just a few minutes and a few billion years.

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8) This figure shows an all-sky map of the cosmic microwave background recorded by the Planck mission. What do the dark and bright variations represent?

A) regions of the sky with very slightly different density at the time the radiation was emitted B) regions of the sky with different proportions of hydrogen and helium at the time the radiation was emitted C) the present distribution of galaxies in the universe D) regions of the sky with different amounts of dark energy Answer: A 9) This diagram represents what we call saddle-shaped geometry. In the context of this chapter, what is its significance?

A) It represents a two-dimensional analogy to one possible geometry for our universe.

B) It shows what our universe really looks like if the density is less than the critical density.

C) It shows the general shape of the cosmic microwave background.

D) Its dark color is meant to indicate that dark energy and dark matter are the primary constituents of the universe.

Answer: A

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10) This graph is made from measurements of the cosmic microwave background (dots) and a model based on the hypothesis of inflation. What does it show?

A) Measurements of the separation between regions of different temperature (in the cosmic microwave background) agree with predictions made by the hypothesis of inflation.

B) It shows that the temperature of the cosmic microwave background is about 3 Kelvin.

C) It shows that some regions of the early universe must have been hundreds of times as dense as others.

D) It shows that the universe has no center and no edges.

Answer: A

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End-of-Chapter Questions Visual Skills Check Use the following questions to check your understanding of some of the many types of visual information used in astronomy.

This graph (repeated from Figure 22.1) shows how the temperature of the universe has declined with time; the graph spans many orders of magnitude for both temperature and time. Answer the following questions based on the information provided in the graph.

1) What was the approximate temperature of the universe at an age of 1015 s? A) about 1 K B) about 100 K C) about 105 K D) about 1015 K Answer: B

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2) What was the approximate temperature of the universe at an age of 5 minutes? A) about 300 K B) about 106 K C) about 109 K D) about 1012 K Answer: C 3) How much cooler is the universe now (at an age of 4 × 1017 s) than it was at an age of 1 second?

A) Its current temperature is one hundred-millionth (10-8) the temperature at an age of 1 second.

B) 1 Its current temperature is one hundred-thousandth (10-5) the temperature at an age of second.

C) Its current temperature is one-hundredth (10-2) the temperature at an age of 1 second.

D) Its current temperature is one ten-billionth (10-10) the temperature at an age of 1 second.

Answer: D Chapter Review Questions 1) Explain what we mean by the Big Bang theory.

2) What is antimatter? How were particle-antiparticle pairs created and destroyed in the early universe?

3) What are the four forces that operate in the universe today? Why do we think there were fewer forces operating in the early universe?

4) Make a list of the major eras in the history of the universe, summarizing the important events thought to have occurred during each era.

5) Why can't our current theories describe the conditions that existed in the universe during the Planck era?

6) What are grand unified theories? According to these theories, how many forces operated during the GUT era? How are these forces related to the four forces that operate today?

7) What do we mean by inflation, and when do we think it occurred?

8) Why do we think there was slightly more26matter than antimatter in the early universe?

Copyright © 2020 Pearson Education, Inc. What happened to all the antimatter, and when?

11) Briefly describe how the cosmic microwave background was discovered. How do the existence and nature of this radiation support the Big Bang theory?

12) How does the chemical abundance of helium in the universe support the Big Bang theory?

Explain.

13) Describe three key questions about the universe that are answered by inflation, and explain how inflation answers each of them.

14) What observational evidence supports the hypothesis of inflation? Be sure to explain how observations of the cosmic microwave background can tell us about the universe at the much earlier time when inflation occurred.

15) What is Olbers's paradox, and how is it resolved by the Big Bang theory?

Does It Make Sense?

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

Example: I walked east from our base camp at the North Pole.

Solution: The statement does not make sense because east has no meaning at the North Pole–all directions are south from the North Pole.

16) According to the Big Bang theory, the early universe had nearly equal amounts of matter and antimatter.

17) According to the Big Bang theory, the cosmic microwave background was created when energetic photons ionized the neutral hydrogen atoms that originally filled the universe.

18) Observed characteristics of the cosmic microwave background can be explained by assuming that it comes from individual stars and galaxies.

19) According to the Big Bang theory, most of the helium in the universe was created by nuclear fusion in the cores of stars.

20) According to the hypothesis of inflation, large-scale structures in the universe may have originated as tiny quantum fluctuations.

21) According to the hypothesis of inflation, the "flat" geometry of the universe most likely arose by chance.

22) Inflation is a nice idea, but there are no known ways to test whether it really happened.

23) In the distant past, what is now the cosmic microwave background consisted primarily of infrared light.

24) The main reason the night sky is dark is that stars are so far away.

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25) Patterns in the cosmic microwave background tell us about conditions in the early universe that ultimately led to galaxy formation.

Quick Quiz 26) The current temperature of the universe as a whole is A) absolute zero.

B) a few K. C) a few thousand K. Answer: No Correct Answer Was Provided.

27) The charge of an antiproton is A) positive.

B) negative.

C) neutral.

Answer: No Correct Answer Was Provided.

28) When a proton and an antiproton collide, they A) repel each other.

B) fuse together.

C) convert into two photons.

Answer: No Correct Answer Was Provided.

29) Which of the following does not provide strong evidence for the Big Bang theory?

A) observations of the cosmic microwave background B) observations of the amount of hydrogen in the universe C) observations of the ratio of helium to hydrogen in the universe Answer: No Correct Answer Was Provided.

30) When the universe was 380,000 years old, its thermal radiation spectrum consisted mostly of A) radio and microwave photons.

B) visible and infrared photons.

C) x-ray and ultraviolet photons.

Answer: No Correct Answer Was Provided.

31) Which of the following does inflation help to explain?

A) the uniformity of the cosmic microwave background B) the amount of helium in the universe C) the temperature of the cosmic microwave background Answer: No Correct Answer Was Provided.

32) Which of the following does inflation help to explain?

A) the origin of hydrogen B) the origin of galaxies C) the origin of atomic nuclei Answer: No Correct Answer Was Provided.

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33) Which of these pieces of evidence supports the idea that inflation really happened?

A) the enormous size of the observable universe B) the large amount of dark matter in the universe C) the apparently "flat" geometry of the universe Answer: No Correct Answer Was Provided.

34) What is the earliest time from which we observe light in the universe?

A) a few hundred million years after the Big Bang B) a few hundred thousand years after the Big Bang C) a few minutes after the Big Bang Answer: No Correct Answer Was Provided.

35) Which of the following best explains why the night sky is dark?

A) The universe is not infinite in space.

B) The universe has not always looked the way it looks today.

C) The distribution of matter in the universe is not uniform on very large scales.

Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

36) Group Discussion: Astronomy and Technological Development.

The stated mission of the (IAU), an organization of professional astronomers from International Astronomical Union around the world, is to "promote and safeguard the science of astronomy in all its aspects through international cooperation." As part of this mission, the IAU promotes astronomy as a means of broadly enhancing technological research and education in the developing world. a) Working independently, go to the website of the IAU's Office of Astronomy for Development (OAD) and learn about at least two of the activities it is supporting. b) Gather into small groups and tell each other about the OAD's activities. Make a list of impacts that those activities might have on the countries in which they are happening. c) How do you think the IAU should determine whether the OAD's efforts are successful? What processes would you recommend for deciding if or how particular activities should be changed? d) Within your group, briefly debate the relative merits for developing countries of spending money on astronomy versus spending money on sciences with more direct practical applications. Summarize the outcome of your debate.

The Process of Science These questions may answered individually in short-essay form or discussed in groups, except where identified as group-only.

37) Unanswered Questions.

Briefly describe one important but unanswered question about the events that happened shortly after the Big Bang. If you think it will be possible to answer that question in the future, describe how we might find an answer, being as specific as possible about the evidence necessary. If you think the question will never be answered, explain why you think it is impossible to answer.

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38) Darkness at Night. Imagine that you were Johannes Kepler, pondering the darkness of the night sky without any knowledge of the Big Bang or the expanding universe. Come up with a hypothesis for the darkness of the night sky that would have been plausible in Kepler's time. Propose an experiment that scientists might be able to perform today to test that hypothesis.

39) The Moment of Creation. The timeline of the universe discussed in this textbook takes us back only to within 10-43 second of creation, not to the moment of creation itself. Do you think it will ever be possible for science to consider the moment of creation, or to answer questions such as why the Big Bang happened? Defend your opinions.

40) Discussion: The Big Bang. How convincing do you find the evidence for the Big Bang theory? really What are its strengths? What does it fail to explain? Do you think the Big Bang happened? Defend your opinion.

41) Group Activity: Testing the Big Bang Theory. Listed below are five hypothetical observations that are not predicted by the Big Bang theory. For each one, imagine what would

happen if it were actually observed–whether it could be explained with the existing Big Bang theory, abandoncould be explained with a revision to the Big Bang theory, or would force us to the Big Bang theory. Write down your team's reasoning for each case. Note: You may wish

to do this activity using the four roles described in Chapter 1, Exercise 39.

• Hypothetical observation 1: a star cluster with an age of 15 billion years •42) Hypothetical 2: athe galaxy with of anaage of 10 million years Life Story ofobservation a Proton. Tell life story proton from its formation shortly after the Big •Bang Hypothetical observation 3: a galaxy at a distance of 10 billion light-years whose to its presence in the nucleus of an oxygen atom you have just inhaled. Your story should be creative and imaginative, but it should also demonstrate your scientific understanding of as spectrum is many stages in the proton's life as possible.

You can draw on material from the entire book, and blueshifted your story should be three to pages long.

• Hypothetical observation 4: five a galaxy containing 90% hydrogen and 10% helium •background Hypothetical observation 5: evidence for an increase in the by cosmic microwave 43) Creative History of the Universe. The story of creation as envisioned the Big Bang theory temperature with but timeit is usually told in a fairly straightforward, scientific way. Tell the story is quite dramatic, more dramatically, Investigate Furtherin the form of a short story, play, or poem. Be as creative as you wish, but be sure to remain true to the science as it is understood today.

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44) Re-creating the Big Bang. Particle accelerators on Earth can push particles to extremely high speeds. When these particles collide, the amount of energy associated with the colliding particles is much greater than the mass-energy these particles have when at rest.

As a result, these collisions can produce many other particles out of energy. Explain in your own words how the conditions that occur in these accelerators are similar to the conditions that prevailed shortly after the Big Bang. Also, point out some of the differences between what happens in particle accelerators and what happened in the early universe. 45) Betting on the Big Bang Theory. If you had $100, how much money would you wager on the proposition that we have a reasonable scientific understanding of what the universe was like when it was 1 minute old? Explain your bet in terms of the scientific evidence presented in this chapter. 46) "Observing" the Early Universe. Explain why we will never be able to observe the era of nucleosynthesis through direct detection of the radiation emitted at that time. How do we learn about this era? 47) Element Production in the Big Bang.

Nucleosynthesis in the early universe was unable to produce more than trace amounts of elements heavier than helium. Using the information in Figure 17.14, which shows the mass per nuclear particle for many different elements, explain why producing elements like lithium (3 protons), boron (4 protons), and beryllium (5 protons) was so difficult. 48) Evidence for the Big Bang. Make a list of at least seven observed features of the universe that are satisfactorily explained by the Big Bang theory and the idea of inflation.

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

The total annual U.S. power consumption is about 2 × 1020 joules.

49) Energy from Antimatter.

Suppose you could supply that energy by combining pure matter with pure antimatter.

Estimate the total mass of matter-antimatter fuel you would need to supply the United States with energy for 1 year. How does that mass compare with the amount of matter in your car's gas tank? (A gallon of gas has a mass of about 4 kilograms.)

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50) Gravity vs. the Electromagnetic Force. The amount of electromagnetic force between two charged objects can be computed with an inverse square law similar to Newton's universal law of gravitation; for the electromagnetic force, the law is F=k× In this formula, the charges must be given in units of coulombs (abbreviated C), the distance d between the objects' centers must be in meters, and the constant k = 9 × 109 kg × m3/(C2 × s2). a) Compute the gravitational force between your body and Earth using Newton's universal law of gravitation (see Section 4.4 or Appendix B). b) Now suppose all the electrons suddenly disappeared from Earth, making it positively charged, and all the protons in your body suddenly changed into neutrons, making you negatively charged. Compute the strength of the electromagnetic force between the electrons in your body and the protons in Earth. Assume that the charge per unit of mass of both you and Earth is 5 × 107 C/kg. c) Compare the electromagnetic force from part b to the gravitational force from part a. Use that result to explain why gravity is considered weaker than the electromagnetic force.

51) Background Radiation During Galaxy Formation. What was the peak wavelength of the background radiation at the time light left the most distant galaxies we can currently see?

Assume those galaxies have a cosmological redshift of z = 10. What is the temperature corresponding to that peak wavelength?

52) Expansion Since the Era of Nucleosynthesis. Compare the peak wavelength of the radiation in the universe at the end of the era of nucleosynthesis with the peak wavelength of the radiation in the universe currently. Assume the temperature at the end of the era of nucleosynthesis was 109 K. How much have the wavelengths of the photons in the universe been stretched since that time?

53) Temperature of the Universe. What will the temperature of the cosmic microwave background be when the average distances between galaxies are twice as large as they are today?

54) Uniformity of the Cosmic Microwave Background. The temperature of the cosmic microwave background differs by only a few parts in 100,000 across the sky. Compare this level of uniformity to that of the surface of a table that is 1 meter square. How big would the largest bumps on that table be if its surface were smooth to one part in 100,000? Could you see bumps of that size on the table's surface?

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55) Daytime at "Night." According to Olbers's paradox, the entire sky would be as bright as the surface of a typical star if the universe were infinite in space, unchanging in time, and the same everywhere. However, conditions would not need to be quite that extreme for the "nighttime" sky to be as bright as the daytime sky. a) Using the inverse square law for light from Mathematical Insight 15.1, determine the apparent brightness of the Sun in our sky. b) Using the inverse square law for light, determine the apparent brightness our Sun would have if it were at a distance of 10 billion light-years. c) From your answers to parts a and b, estimate how many stars like the Sun would need to exist at a distance of 10 billion light-years for their total apparent brightness to equal that of our Sun. d) Compare your answer to part c with the estimate of the total number of stars in our observable universe from Mathematical Insight 1.3. Use your answer to explain why the night sky is much darker than the daytime sky. How much larger would the total number of stars need to be for "night" to be as bright as day?

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter 23 Dark Matter, Dark Energy, and the Fate of the Universe Section 23.1 1) Why is dark matter called "dark matter"?

A) It does not interact strongly with light, but it does have gravity like other matter.

B) It is made from black holes.

C) It absorbs all light and exerts gravity.

D) The name is misleading because it is pure energy rather than matter.

Answer: A Section 23.2 1) How do we know that there is much more mass in the halo of our galaxy than in the disk?

A) There are so many globular clusters in the halo that their total mass is greater than the mass of stars in the disk.

B) Stars and gas clouds in the outskirts of the Milky Way orbit the galaxy at much higher speeds than we would expect if all the mass were concentrated in the disk.

C) We have discovered that the halo is filled with an enormous number of brown dwarfs that have a combined mass much greater than the disk.

D) Laboratory experiments have proven that the halo must be full of subatomic dark matter particles.

Answer: B 2) What evidence suggests that the Milky Way contains dark matter?

A) We observe stars and gas clouds far from the galactic center orbiting the galaxy at higher speeds than we would expect without dark matter.

B) We see many lanes of dark material blocking out the light of stars behind them along the band of the Milky Way. C) We see many dark voids between the stars in the halo of the Milky Way. D) When we observe in different wavelengths, such as infrared or radio, we see objects that don't appear in visible-light observations.

E) When we look at the galactic center, we are able to observe a large black hole that is composed of dark matter.

Answer: A 3) Which of the following best describes the distribution of the dark matter in a spiral galaxy?

A) approximately spherical and about the same size as the galaxy halo B) approximately spherical and about 10 times the size of the galaxy halo C) flattened in a disk and about the same size as the stellar disk D) flattened in a disk but about 10 times larger than the stellar disk E) predominantly concentrated in the spiral arms Answer: B

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4) How do we determine the amount of dark matter in elliptical galaxies? A) We measure the orbital velocities of star-forming gas clouds around the outer portions of the galaxy. B) We measure the range of speeds of collections of stars at different distances from the galactic center and determine how much mass is interior to their orbits. C) We count the number of stars in the galaxy and determine its volume, so that we can calculate the galaxy's density. D) We search for dark lanes of dust and black holes within the galaxy. E) We measure how fast the galaxy rotates as a whole. Answer: B 5) How are rotation curves of spiral galaxies determined for orbital distances beyond where starlight can be detected?

A) by extrapolation of the measured rotation curve B) by observations of the 21 cm line of atomic hydrogen C) by observations of spectral lines emitted by dark matter D) by watching the galaxies rotate over a period of decades E) by measuring the broadening of the galaxy's absorption lines Answer: B 6) Why can't astronomers use the 21-cm line of neutral hydrogen gas to measure the masses of elliptical galaxies? A) All elliptical galaxies are too far away for us to detect the 21-cm line. B) The disorderly motion of the material in elliptical galaxies prevents us from measuring different velocities at different sides of the galaxy. C) Elliptical galaxies have little gas, so they do not have strong 21-cm line emission. D) Supermassive black holes in the center of elliptical galaxies disturb measurements of the 21cm line. Answer: C 7) We observe stars orbiting at distances of 25,000 light-years from the centers of Galaxy X and Galaxy Y. For this orbital distance, stars in Galaxy X have higher orbital speeds than stars in Galaxy Y. What can we conclude?

A) Galaxy X contains more total mass within 25,000 light-years of its center than Galaxy Y. B) Galaxy Y contains more total mass within 25,000 light-years of its center than Galaxy X. C) Galaxy X contains more dark matter than Galaxy Y. D) Galaxy Y contains more dark matter than Galaxy X. E) The stars of Galaxy X are more massive than those of Galaxy Y. Answer: A 8) Which of the following provides important evidence for the existence of dark matter in clusters of galaxies? A) careful counts of the numbers of galaxies in clusters B) the relative proportions of spiral and elliptical galaxies in clusters C) the high temperature and distribution of gas between the galaxies in clusters D) the total luminosity in visible light coming from cluster galaxies Answer: C

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9) Which of the following methods used to determine the mass of a cluster of galaxies does not depend on Newton's law of gravity? A) measuring the orbital velocities of galaxies in the cluster B) measuring the temperature of x-ray gas in the intracluster medium C) measuring the amount of distortion caused by gravitational lensing Answer: C 10) How do x-ray measurements help us measure the amount of dark matter in galaxy clusters? A) The gravity of dark matter causes intracluster gas to be heated to very high temperatures, and the higher the temperature, the stronger the x-ray emission we will detect from the gas. B) Dark matter absorbs x-rays, so we can map the dark matter distribution by looking for absorption lines in x-ray spectra. C) Collisions between dark matter particles emit x-rays, allowing us to map dark matter with x- ray images. Answer: A 11)

This figure shows the x-ray emission from hot gas (red) in two colliding galaxy clusters and the distribution of mass found by gravitational lensing (blue). Which of the following three conclusions can be reasonably drawn from this image?

I) The mass found by gravitational lensing in these colliding galaxy clusters is in a separate physical location from the hot gas that resulted from the collision.

II) Dark matter does not strongly interact with regular matter, other than through gravity.

III) Hot intracluster gas makes up the majority of the mass of galactic clusters.

A) Only I B) I and II C) I and III D) I, II, and III Answer: B

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12) Which of the following best describes what we mean by gravitational lensing?

A) A massive object bends light beams that are passing nearby.

B) A massive object scatters away the light of more distant objects.

C) Gravity causes matter that would otherwise be dark to become visible.

D) Telescope lenses are distorted by gravity.

Answer: A 13) Which of the following is not evidence for dark matter?

A) the flat rotation curves of spiral galaxies B) x-ray observations of hot gas in galaxy clusters C) gravitational lensing around galaxy clusters D) the expansion of the universe Answer: D 14) Which of the following is an example of ordinary (baryonic) matter?

A) you B) photons of light C) electrons and positrons produced by pair production D) weakly interacting massive particles (WIMPs) E) neutrinos Answer: A 15) Neutrinos travel at very high speeds (close to the speed of light). How does this fact imply that neutrinos cannot make up the bulk of the dark matter in galaxies?

A) Their high speeds mean neutrinos cannot have much mass.

B) Their high speeds tell us that neutrinos interact only through the weak force and gravity.

C) Their speeds of neutrinos mean that neutrinos would escape from galaxies.

Answer: C 16) Based on current evidence, which of the following is the most likely candidate to make up the majority of dark matter?

A) brown dwarfs B) Jupiter-size objects C) weakly interacting massive particles (WIMPs) D) faint red stars E) black holes Answer: C 17) The more ordinary (baryonic) matter there is in the universe, the lower the ratio of deuterium to hydrogen. Therefore, if Harry measures a higher ratio of deuterium to hydrogen than Sally, Harry infers A) the same density of ordinary matter as Sally.

B) a lower density of ordinary matter than Sally.

C) a higher density of ordinary matter than Sally.

Answer: B

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18) Why do we expect WIMPs to be distributed throughout galactic halos, rather than settled into the galaxy's disk?

A) WIMPs are lightweight enough that they have expanded out into the halo.

B) WIMPs were produced by stars in the early stages of galaxy evolution and therefore are only found in globular clusters in the halo.

C) WIMPS rarely interact with other particles and therefore cannot settle into galactic disks.

D) Shock waves from generations of supernovae have blown the WIMPs out into the halo.

E) WIMPs annihilate when they come into contact with ordinary matter, such as stars.

Answer: C 19) Based on the observed amount of deuterium in the universe, we can conclude that A) ordinary (baryonic) matter makes 75 percent of the mass of the universe.

B) neutrons outnumber protons 7 to 1 in the universe.

C) most of the deuterium that was created in the early universe has since been destroyed.

D) the density of ordinary (baryonic) matter is about 5 percent of the critical density.

E) the density of the universe equals the critical density.

Answer: D Section 23.3 1) Galaxies sometimes have recession speeds that deviate somewhat from the exact speed predicted by Hubble's law. What do these so-called peculiar velocities reveal?

A) velocities due to the gravitational tugs of nearby galaxies or clusters of galaxies B) deviations in the rate at which the universe has been expanding at different times in the past C) These velocities indicate serious discrepancies with Hubble's law and with our view of an expanding universe.

D) These velocities indicate that the expansion rate differs for different directions in space.

Answer: A 2) How do astronomers create three-dimensional maps of the universe?

A) through comparison of computer models of the structure formation with observations B) by measuring the sky positions and redshifts of many galaxies C) by measuring the sky positions and apparent brightnesses of many galaxies D) by measuring the colors and luminosities of many galaxies E) by carefully measuring the parallax of many galaxies Answer: B 3) Which of the following best describes how galaxies are distributed on large scales in the universe?

A) Galaxies are uniformly distributed throughout space.

B) Galaxies are randomly distributed throughout space.

C) Galaxies are distributed in large, elongated chains radiating outward from a common center.

D) Galaxies appear to be distributed in chains and sheets that surround great voids.

E) Galaxies are distributed in a series of concentric spherical shells around a common center.

Answer: D

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Section 23.4 1) Given the observed average distance between galaxies today, which statement is true?

A) The faster the past expansion rate, the younger the age of the universe.

B) The faster the past expansion rate, the older the age of the universe.

C) The age of the universe depends only on the average distance between galaxies, not on the rate of expansion.

Answer: A 2) Which model of the universe gives the youngest age for the present size of the universe?

(Assume that the present expansion rate is the same for all of these models. The different models have different expansion histories.)

A) a recollapsing universe B) a coasting universe C) a critical universe D) an accelerating universe E) All models give the same age.

Answer: A 3) Why were astronomers in the 1990s surprised to learn that the expansion rate of the universe is getting faster with time?

A) They did not expect the universe to be expanding at all.

B) They assumed that the expansion rate should never change.

C) They expected gravity to be slowing the expansion rate with time.

Answer: C 4) Why isn't the space within our solar system or the Milky Way expanding according to Hubble's law?

A) Hubble's law only applied early in the history of the universe.

B) As we are inside our solar system and the Milky Way, we cannot observe their expansion.

C) The universe is not old enough for the solar system or Milky Way to have begun their expansion.

D) The gravity exerted by the solar system and the Milky Way is strong enough to hold them together against the expansion of the universe.

Answer: D 5) Which of the following hypothetical universes would be the youngest, assuming that all are expanding at the same rate?

A) the one that has the most dark matter B) the nearly empty universe with no dark matter C) the one that has the most dark energy D) All of these universes would have the same age.

Answer: A

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6) Based on current understanding, what exactly is causing the expansion of the universe to accelerate? A) WIMPs B) neutrinos C) white dwarf supernovae D) Gravity E) We don't know, but we call it "dark energy." Answer: E

7) The graph above shows 4 models for how the average distance between galaxies could change with time, from the past (left) to now (middle) to the future (right-hand side). The graph also shows real data, based on white dwarf supernovae. Which model(s) predict(s) that galaxies are getting farther apart NOW? A) accelerating B) coasting C) critical D) recollapsing E) all of them Answer: E

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8) The graph above shows 4 models for how the average distance between galaxies could change with time, from the past (left) to now (middle) to the future (right-hand side). The graph also shows real data, based on white dwarf supernovae. Which model(s) predict(s) that galaxies will eventually get closer together? A) accelerating B) coasting C) critical D) recollapsing E) all of them Answer: D 9) The graph above shows 4 models for how the average distance between galaxies could change with time, from the past (left) to now (middle) to the future (right-hand side). The graph also shows real data, based on white dwarf supernovae. Which model predicts that galaxies had the largest separations in the past? A) accelerating B) coasting C) critical D) recollapsing Answer: A 10) The graph above shows 4 models for how the average distance between galaxies could change with time, from the past (left) to now (middle) to the future (right-hand side). The graph also shows real data, based on white dwarf supernovae. Which model is most strongly supported by the data? A) critical B) accelerating C) recollapsing D) coasting Answer: B 11) Based on current evidence and models, visible, luminous matter (such as the stars within galaxies) amounts to approximately what percentage of the total mass and energy of the universe?

A) less than 1 percent B) 10 percent C) 25 percent D) 50 percent Answer: A

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12) Based on current evidence and models, the total amount of matter (both luminous matter and dark matter) represents approximately what percentage of the total mass and energy of the universe? A) about 1 percent B) about 10 percent C) about 30 percent D) about 50 percent E) 100 percent Answer: C 13) In a mass-energy census of the universe, what are the approximate proportions of normal matter, dark matter, and dark energy? A) 68% dark matter, 27% dark energy, 5% normal matter B) mostly normal matter, with only trace amounts of dark matter and dark energy C) 27% dark matter, 68% dark energy, 5% normal matter D) 33% dark matter, 33% dark energy, 33% normal matter Answer: C General Chapter Questions 1) What is the approximate mass-to-light ratio for the inner region of the Milky Way Galaxy, in units of solar masses per solar luminosity? A) 1000 B) 600 C) 100 D) 6 E) 0.1 Answer: D 2) If all the "dark matter" in the Universe were somehow instantaneously removed, which of the following would happen? A) The Solar System would fly apart. B) All the stars in the Milky Way would collapse into the central black hole.

C) Clusters of galaxies would no longer be gravitationally bound together. D) Atoms would split into nuclei and free electrons. Answer: A 3) If a galaxy's overall mass-to-light ratio is 100 solar masses per solar luminosity, and its stars account for only 5 solar masses per solar luminosity, how much of the galaxy's mass must be dark matter? A) 100 percent B) 95 percent C) 80 percent D) 50 percent E) 5 percent Answer: B

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4) What does Einstein's cosmological constant represent in his equations of general relativity?

A) the value of the expansion rate of the universe B) the value of the acceleration of the universe C) the value that measures the strength of gravity across the universe D) the size of the cosmological horizon E) a repulsive force that can counteract gravity Answer: E Short Answer Questions 1) How do we know that there is an insignificant amount of dark matter in our solar system?

Answer: We can measure the distribution of mass in the solar system through analysis of the orbital speeds of planets orbiting the Sun (i.e., making a rotation curve). This analysis tells us vast majority of the solar system's mass in the Sun, which means there cannot be much mass of any kind–including dark matter–in the rest of the solar system.

Reading Quiz Questions 1) Which of the following best summarizes what we mean by dark matter?

A) matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light B) matter that may inhabit dark areas of the cosmos where we see nothing at all C) matter consisting of black holes D) matter for which we have theoretical reason to think it exists, but no observational evidence for its existence Answer: A 2) Which of the following best summarizes what we mean by dark energy?

A) It is a name given to whatever is causing the expansion of the universe to accelerate with time.

B) It is the energy contained in dark matter.

C) It is the energy of black holes.

D) It is a type of energy that is associated with the "dark side" of The Force that rules the cosmos.

Answer: A 3) The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg. This refers to the idea that A) dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way. B) luminous matter emits white light, much like the light reflected from icebergs.

C) black holes are much more interesting than ordinary stars that give off light.

D) the luminous matter of the Milky Way is essentially floating on the surface of a great sea of dark matter.

Answer: A

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4) What is a rotation curve?

A) a graph showing how orbital velocity depends on distance from the center for a spiral galaxy B) a precise description of the shape of a star's orbit around the center of the Milky Way Galaxy C) a curve used to decide whether a star's orbit places it in the disk or the halo of a spiral galaxy D) a graph that shows a galaxy's mass on the vertical axis and size on the horizontal axis Answer: A 5) What is the primary way in which we determine the mass distribution of a spiral galaxy?

A) We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.

B) We calculate its mass-to-light ratio.

C) We apply Newton's version of Kepler's third law to the orbits of globular clusters in the galaxy's halo.

D) We count the number of stars we can see at different distances from the galaxy's center.

Answer: A 6) What do we mean when we say that the rotation curve for the Milky Way Galaxy is "flat"?

A) Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.

B) The amount of light emitted by stars at different distances is about the same throughout the galaxy.

C) The disk of a spiral galaxy is quite flat rather than spherical like the halo.

D) All the galaxy's mass is concentrated in its flat, gaseous disk.

Answer: A 7) Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?

A) Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies.

B) Elliptical galaxies probably contain far less dark matter than spiral galaxies.

C) Elliptical galaxies probably contain far more dark matter than spiral galaxies.

D) Unlike the broad distribution of dark matter in spiral galaxies, elliptical galaxies probably contain dark matter only near their centers.

Answer: A 8) In general, when we compare the mass of a galaxy or cluster of galaxies to the amount of light it emits (that is, when we look at its mass-to-light ratio), we expect that A) the higher the amount of mass relative to light (higher mass-to-light ratio), the greater the proportion of dark matter.

B) the higher amount of mass relative to light (higher mass-to-light ratio), the lower the proportion of dark matter.

C) the amount of light should be at least one solar luminosity for each solar mass of matter (mass-to-light ratio less than or equal to 1).

D) the higher the amount of mass relative to light (higher mass-to-light ratio), the older the galaxy or cluster.

Answer: A

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9) Which of the following is not one of the three major strategies used to measure the mass of a galaxy cluster?

A) measuring the speeds of galaxies orbiting the cluster's center B) studying x-ray emission from hot gas inside the cluster C) observing how the cluster bends light from galaxies located behind it D) measuring the temperatures of stars in the halos of the galaxies Answer: D 10) When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?

A) It bends or distorts the light coming from galaxies located behind it.

B) It magnifies the effects of gravity that we see in the cluster.

C) It is an unusually large cluster that has a lot of gravity.

D) The overall shape of the cluster is that of a lens.

Answer: A 11) Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?

A) Dark matter is the dominant form of mass in both clusters and in individual galaxies.

B) Dark matter is present between galaxies in clusters, but not within individual galaxies.

C) Dark matter is present in individual galaxies, but there is no evidence that it can exist between the galaxies in a cluster.

D) Within individual galaxies, dark matter is always concentrated near the galactic center, and within clusters it is always concentrated near the cluster center.

Answer: A 12) What is the distinguishing characteristic of what we call ordinary (or baryonic) matter?

A) It consists of atoms or ions with nuclei made from protons and neutrons.

B) It emits a great deal of light.

C) It can attract other matter through the force of gravity.

D) It is made of subatomic particles that scientists call WIMPs. Answer: A 13) Measuring the amount of deuterium in the universe allows us to set a limit on A) the density of ordinary (baryonic) matter the universe.

B) the total amount of mass in the universe.

C) the acceleration of the universe.

D) the current age of the universe.

Answer: A

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14) What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?

A) They respond to the weak force but not to the electromagnetic force, which means they cannot emit light. B) The light that they emit is so weak that it is undetectable to our telescopes. C) They are only weakly bound by gravity, which means they can fly off and escape from galaxies quite easily. D) They interact with other matter only through the weak force and not through gravity or any other force. Answer: A 15) Which of the following best sums up current scientific thinking about the nature of dark matter? A) Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.

B) Dark matter consists primarily of a mysterious form of energy that is causing the expansion of the universe to accelerate. C) There is no longer any doubt that dark matter is made mostly of WIMPs. D) Dark matter probably does not really exist, and rather indicates a fundamental problem in our understanding of gravity. Answer: A 16) When we speak of the large-scale structure of the universe, we mean A) the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe. B) the structure of any large galaxy.

C) the structure of any individual cluster of galaxies. D) the overall shape of the observable universe. Answer: A 17) The primary evidence that has led astronomers to conclude that the expansion of the universe is accelerating comes from A) observations of white dwarf supernovae. B) observations of the speeds of individual galaxies in clusters. C) measurements of the rotation curve for the universe. D) measurements of how galaxy speeds away from the Milky Way have increased during the past century. Answer: A 18) Which of the following best sums up current scientific thinking about the nature of dark energy?

A) Dark energy most likely consists of a form of photons that we can't see or detect. B) Dark energy is most likely made up of weakly interacting particles that do not interact with light. C) Dark energy probably exists, but we have little (if any) idea what it is. D) Dark energy is the source of the mind weapon used by Sith Lords in Star Wars. Answer: C

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19) Based on evidence from measurements of the acceleration of the expansion rate and from careful study of the cosmic microwave background, about what percentage of the universe's total mass and energy takes the form of ordinary atomic matter (protons, neutrons, and electrons)? A) 5% B) 0.5% C) 27% D) 68% Answer: A 20) According to current understanding, if the universe continues to expand forever, the last major source of light will come from A) evaporation of black holes. B) the last supernovas. C) collisions between galaxies. D) gamma-ray bursts. Answer: A Concept Quiz Questions 1) Why do we call dark matter "dark"? A) It emits no radiation that we have been able to detect. B) It is dark brown or dark red in color. C) It blocks out the light of stars in a galaxy. D) It contains large amounts of dark-colored dust. Answer: A 2) Although most astronomers assume dark matter really exists, there is at least one other possible explanation for the phenomena attributed to dark matter. What is it? A) There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales. B) The so-called dark matter is really just ordinary stars that are enshrouded in clouds of dust. C) We could just be having a hard time understanding the observations, because they involve very distant galaxies. D) There could be something wrong with our understanding of how atoms produce light. Answer: A 3) Spiral galaxy rotation curves are generally fairly flat out to large distances. Suppose that spiral galaxies did not contain dark matter. How would their rotation curves be different? A) The orbital speeds would fall off sharply with increasing distance from the galactic center. B) The orbital speeds would rise upward with increasing distance from the galactic center, rather than remaining approximately constant. C) The rotation curve would be a straight, upward sloping diagonal line, like the rotation curve of a merry-go-round. D) The rotation curve would look the same with or without the presence of dark matter. Answer: A

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4) The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter. Do they tell us anything about where the dark matter is located within the galaxy?

A) Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center.

B) Yes, they tell us that dark matter is concentrated near the center of the galaxy.

C) Yes, they tell us that dark matter is spread uniformly throughout the galactic disk.

D) No, we cannot determine anything about the location of dark matter from the rotation curve.

Answer: A 5) It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why? A) Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.

B) Elliptical galaxies contain much less dark matter than spiral galaxies, so it's much more difficult to measure.

C) Stars in elliptical galaxies are dimmer, making them harder to study.

D) We cannot observe spectral lines for elliptical galaxies.

Answer: A 6) How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?

A) We detect this gas with x-ray telescopes.

B) We infer its existence by observing its gravitational effects on the galaxy motions.

C) The hot gas shows up as bright pink in visible-light photos of galaxy clusters.

D) We can observe the frictional effects of the hot gas in slowing the speeds of galaxies in the clusters.

Answer: A 7) Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?

A) The temperature tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass.

B) Temperature is always directly related to mass, which is why massive objects are always hotter than less massive objects.

C) The temperature of the gas tells us the gas density, so we can use the density to determine the cluster's mass.

D) The question is nonsense; gas temperature cannot possibly tell us anything about mass.

Answer: A

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8) How does gravitational lensing tell us about the mass of a galaxy cluster?

A) Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it.

B) The lensing allows us to determine the orbital speeds of galaxies in the cluster, so that we can determine the mass of the cluster from the orbital velocity law.

C) The lensing broadens spectral lines, and we can use the broadening to "weigh" the cluster.

D) Newton's universal law of gravitation predicts how mass can distort light, so we can apply Newton's law to determine the mass of the cluster.

Answer: A 9) If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?

A) They travel at speeds close to the speed of light.

B) They are subatomic particles.

C) They can neither emit nor absorb light.

D) They tend to orbit at large distances from the galactic center.

Answer: A 10) Is space expanding within clusters of galaxies? Why or why not?

A) no, because their gravity is strong enough to hold them together even while the universe as a whole expands B) no, because the universe is not old enough yet for these objects to have begun their expansion C) no, because expansion of the universe affects only empty space, not space in which matter is present D) Yes, and that is why clusters tend to grow in size with time.

Answer: A 11) Which of the following statements about large-scale structure is probably not true?

A) Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.

B) Galaxies and clusters have grown around tiny density enhancements that were present in the early universe.

C) Voids between superclusters began their existence as regions in the universe with a slightly lower density than the rest of the universe.

D) Many cluster and superclusters are still in the process of formation as their gravity gradually pulls in new members.

Answer: A 12) Based on current evidence, a supercluster is most likely to have formed in regions of space where A) the density of dark matter was slightly higher than average when the universe was very young.

B) there was an excess concentration of hydrogen gas when the universe was very young.

C) supermassive black holes were present in the very early universe.

D) the acceleration of the expansion was proceeding faster than elsewhere.

Answer: A

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13) Imagine that it turns out that dark matter (not dark energy) is made up of an unstable form of matter and that all of it suddenly decays tomorrow into photons or other forms of energy. Based on current understanding, which of the following would begin to occur? A) Stars would begin to stop having nuclear fusion in their cores. B) Planetary systems would begin to expand and disperse. C) The galaxies in clusters would begin to fly apart. D) The expansion of the universe would begin coming to a halt. Answer: C 14) Based on current evidence, how does the actual average density of matter in the universe compare to the critical density? A) The actual average density of matter, even with dark matter included, is only about a quarter of the critical density. B) If we include dark matter, the actual average density of matter equals the critical density. C) The actual average density of matter less than 1% of the critical density. D) The actual density of average matter is about twice the critical density. Answer: A 15) Hubble's constant is a "constant" in that its value A) is the same across all of space and does not change on human time scales. B) stays the same at all times. C) is a value calculated from models, and therefore not affected by observations.

D) always stays the same unless the expansion rate is accelerating. Answer: A 16) Hubble's constant is related to the age of the universe, but the precise relationship depends on the way in which the expansion rate changes with time. For a given value of Hubble's constant (such as 22 km/s/Mly), the age of the universe is oldest if A) the expansion rate has been increasing with time (an accelerating universe). B) the expansion rate has remained nearly constant with time (a coasting universe). C) the expansion rate slows ever more gradually with time (a critical universe). D) the expansion rate is slowing enough that it will eventually reverse (a recollapsing universe). Answer: A 17) Current evidence suggests the universe will continue to expand forever, but some people would prefer if we lived in a recollapsing universe that would eventually stop expanding and start contracting. Based on current understanding, which of the following would have to be true for our universe to fit a recollapsing model? A) Dark energy does not exist and there is much more matter than current evidence suggests. B) Dark energy is the dominant form of energy in the cosmos. C) The density of dark matter turns out to be greater than the density of dark energy. D) The total density of matter equals the critical density. Answer: A

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18) Scientists do not know what dark energy is, yet they claim that it is the dominant constituent of the mass-energy of the universe. What (if any) evidence supports this claim?

A) There are two lines of evidence: (1) measurements of the accelerating expansion and (2) the universal geometry inferred from the cosmic microwave background.

B) There are two lines of evidence: (1) the fact that the universe is expanding and (2) the observed ratio of deuterium to hydrogen.

C) The only evidence comes from observations of white dwarf supernovae that indicate the expansion rate is accelerating.

D) The claim comes from models of the Big Bang, but it is not backed up by any observational evidence.

Answer: A 19) Which of the following statements best describes current understanding of the eventual fate of the universe?

A) Although the accelerating expansion suggests that the universe will continue to expand forever, it is possible that future discoveries will indicate a different fate.

B) The universe will continue to expand forever, because only gravity could reverse the expansion, and it is not strong enough to do so.

C) The acceleration of the expansion will eventually lead to a Big Rip that will break all matter apart.

D) The universe will gradually decay as it continues to expand, with protons eventually decaying and black holes ultimately evaporating, leaving the universe cold and dark.

Answer: A

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Visual Quiz Questions 1) Consider a spinning disk of pizza dough, as shown here. What would the rotation curve for the spinning dough look like?

A)

B)

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C)

D)

Answer: A

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2) This graph shows the rotation curves of four different spiral galaxies. Based on these curves, what do all four galaxies have in common?

A) Their most distant stars all orbit at about the same speed as stars located about 30,000 light-years from their centers.

B) Their fastest-moving stars are always located within about 1,000 light-years of their centers.

C) They all have supermassive black holes near their centers.

D) All their stars take about 200 million years to complete a single orbit, no matter how far they are from the galactic center.

Answer: A

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3) Notice the blue ovals (such as those indicated by the arrows) in this image of a galaxy cluster.

The oval structures are not really located where they appear to be, but instead are multiple images of a single galaxy that lies directly behind the cluster. What do we call the process that creates these multiple images?

A) gravitational lensing B) cluster image distortion C) the arc effect D) dark energy Answer: A

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4) Notice the distorted galaxy images, such as the large arc-shaped structure, in this image of a galaxy cluster. What can astronomers learn by carefully measuring the distortions in this image?

A) the total mass of the cluster B) the amount of dark energy in the universe C) the precise distance to the galaxy cluster D) the age of the galaxy cluster Answer: A

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5) This image shows a colliding pair of galaxy clusters known together as the Cluster. The Bullet blue region represents a map of the cluster's dark matter. How was this blue map made?

A) The blue region was inferred from studies of how the cluster causes gravitational lensing of objects located behind it.

B) The blue region represents X-ray emission from hot gas within the cluster.

C) The blue region represents previously unknown radio emission from the dark matter.

D) The blue region comes from the combined light of many young stars drawn together by the cluster's dark matter.

Answer: A

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6) Study this graph, focusing on the red curve and reddish horizontal swath. Which statement correctly interprets what the graph shows?

A) The measured abundance of deuterium agrees with the theoretically predicted abundance only if we make the prediction with a model of the universe in which ordinary matter makes up about 5% of the critical density.

B) The abundance of deuterium in the universe has rapidly decreased with time.

C) The actual abundance of deuterium in the universe has never changed, but the predicted abundance has decreased with time.

D) The abundance of deuterium in the universe is high in galaxies near the Milky Way but much lower in galaxies that are much farther away.

Answer: A

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7) This figure shows a "slice of the universe" from the Sloan Digital Sky Survey. What is the Sloan Great Wall (indicated by the arrow)?

A) a huge collection of galaxies extending a billion light-years in length B) a giant wall of hot gas extending a billion light-years in length C) our cosmological horizon, the point beyond which we cannot see D) a hypothesized structure built of the particles responsible for creating dark energy in the universe Answer: A

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8) These diagrams represent four possible models for the universe. Each model shows how the size of the observable universe changes with time. Of the four models, which one gives the universe the oldest age at present?

A) accelerating universe B) coasting universe C) critical universe D) recollapsing universe E) The age of the universe would be the same no matter which model is correct.

Answer: A 9) These diagrams represent four possible models for the universe. Which model presumes the existence of some type of dark energy in the universe?

A) accelerating universe B) coasting universe C) critical universe D) recollapsing universe E) All four models assume the existence of dark energy.

Answer: A

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10) This graph shows how the average distances between galaxies changes with time in the four models for the universe, and also shows data points measured from white dwarf supernovae.

Which of the following statements best describes what the data are telling us?

A) The data indicate that we live in an accelerating universe.

B) The data prove beyond doubt that we live in an accelerating universe that will continue to expand forever.

C) The data strongly suggest that our universe is made 95% of dark matter and only 5% of ordinary matter.

D) The data prove beyond doubt that the universe will never collapse.

Answer: A

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End-of-Chapter Questions Visual Skills Check Use the following questions to check your understanding of some of the many types of visual information used in astronomy.

The schematic figure to the left shows a more complicated expansion history than the four idealized models in Figure 23.17. Answer the following questions, using the information given in this figure.

1) At time A, is the expansion of the universe accelerating, coasting, or decelerating?

Answer: Accelerating 2) At time B, is the expansion of the universe accelerating, coasting, or decelerating?

Answer: Accelerating 3) At time C, is the expansion of the universe accelerating, coasting, or decelerating?

Answer: Coasting 4) At time D, is the expansion of the universe accelerating, coasting, or decelerating?

Answer: Decelerating

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Chapter Review Questions 1) Define dark matter and dark energy, and clearly distinguish between them. What types of observations have led scientists to propose the existence of each of these unseen influences?

2) Describe how orbital speeds in the Milky Way depend on distance from the galactic center.

How does this relationship indicate the presence of large amounts of dark matter?

3) How do orbital speeds depend on distance from the galactic center in other spiral galaxies, and what does this tell us about dark matter in spiral galaxies?

4) How do we measure the masses of elliptical galaxies? What do these masses lead us to conclude about dark matter in elliptical galaxies?

5) Briefly describe the three different ways of measuring the mass of a cluster of galaxies. Do the results from the different methods agree? What do they tell us about dark matter in galaxy clusters?

6) What is gravitational lensing? Why does it occur? How can we use it to estimate the masses of lensing objects?

7) Briefly explain why the conclusion that dark matter exists rests on the assumption that we understand gravity correctly. Is it possible that our understanding of gravity is not correct?

Explain.

8) In what sense is dark matter "dark"? Briefly explain why objects like you, planets, and even dim stars qualify as dark matter.

9) What evidence indicates that most of the matter in the universe cannot be ordinary (baryonic) matter?

10) Explain what we mean when we say that a neutrino is a weakly interacting particle. Why can't the dark matter in galaxies be made of neutrinos?

11) What do we mean by WIMPs? Why does it seem likely that dark matter consists of these particles, even though we do not yet know what they are?

12) Briefly explain why dark matter is thought to have played a major role in the formation of galaxies and larger structures in the universe. What evidence suggests that larger structures are still forming?

13) What do the large-scale structures of the universe look like? Explain why we think these structures reflect the density patterns of the early universe.

14) Describe and compare the four general patterns for the expansion of the universe: recollapsing, critical, coasting, and accelerating. Observationally, how can we decide which of the four general expansion models best describes the present-day universe?

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15) How do observations of distant supernovae provide evidence for dark energy?

16) How do observations of the cosmic microwave background provide evidence for dark energy?

17) Based on current evidence, what is the overall inventory of the mass-energy content of the universe?

18) What implications does the evidence for dark energy have for the fate of the universe?

Does It Make Sense?

Decide whether or not each of the following statements makes sense (or is clearly true or false).

Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

Example: I walked east from our base camp at the North Pole.

Solution: The statement does not make sense because east has no meaning at the North Pole–all directions are south from the North Pole.

19) Strange as it may sound, most of both the mass and the energy in the universe may take forms that we are unable to detect directly.

20) A cluster of galaxies is held together by the mutual gravitational attraction of all the stars in the cluster's galaxies.

21) We can estimate the total mass of a cluster of galaxies by studying the distorted images of galaxies whose light passes through the cluster.

22) Clusters of galaxies are the largest structures that we have so far detected in the universe.

23) The primary evidence for an accelerating universe comes from observations of young stars in the Milky Way. 24) There is no doubt remaining among astronomers that the fate of the universe is to expand forever.

25) Dark matter is called "dark" because it blocks light from traveling between the stars.

26) Dark energy is the energy associated with the motion of particles of dark matter.

27) Evidence that the expansion of the universe is accelerating comes from observations showing that the average distance between galaxies is increasing faster now than it was 5 billion years ago.

28) If dark matter consists of WIMPs, then we should be able to observe photons produced by collisions between these particles.

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Quick Quiz 29) Dark matter is inferred to exist because A) we see lots of dark patches in the sky.

B) it explains how the expansion of the universe can be accelerating.

C) we can observe its gravitational influence on visible matter.

Answer: No Correct Answer Was Provided.

30) Dark energy has been hypothesized to exist in order to explain A) observations suggesting that the expansion of the universe is accelerating.

B) the high orbital speeds of stars far from the center of our galaxy.

C) explosions that seem to create giant voids between galaxies.

Answer: No Correct Answer Was Provided.

31) Measurements of how orbital speeds depend on distance from the center of our galaxy tell us that stars in the outskirts of the galaxy A) orbit the galactic center just as fast as stars closer to the center.

B) rotate rapidly on their axes.

C) travel in straight, flat lines rather than elliptical orbits.

Answer: No Correct Answer Was Provided.

32) Strong evidence for the existence of dark matter comes from observations of A) our solar system.

B) the center of the Milky Way. C) clusters of galaxies.

Answer: No Correct Answer Was Provided.

33) A photograph of a cluster of galaxies shows distorted images of galaxies that lie behind it at greater distances. This is an example of what astronomers call A) dark energy.

B) spiral density waves.

C) gravitational lensing.

Answer: No Correct Answer Was Provided.

34) Based on the observational evidence, is it possible that dark matter doesn't really exist?

A) No, the evidence for dark matter is too strong for us to think it could be in error.

B) yes, but only if there is something wrong with our current understanding of how gravity should work on large scales C) yes, but only if all the observations themselves are in error Answer: No Correct Answer Was Provided.

35) Based on current evidence, which of the following is considered a likely candidate for the majority of the dark matter in galaxies?

A) subatomic particles that we have not yet detected in particle physics experiments B) swarms of relatively dim red stars C) supermassive black holes Answer: No Correct Answer Was Provided.

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36) Which region of the early universe was most likely to become a galaxy?

A) a region whose matter density was lower than average B) a region whose matter density was higher than average C) a region with an unusual concentration of dark energy Answer: No Correct Answer Was Provided.

37) The major evidence for the idea that the expansion of the universe is accelerating comes from observations of A) white dwarf supernovae.

B) the orbital speeds of stars within galaxies.

C) the evolution of quasars.

Answer: No Correct Answer Was Provided.

38) Which of the following possible types of universe would not expand forever?

A) a critical universe B) an accelerating universe C) a recollapsing universe Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

39) Group Discussion: Vera Rubin and the Nobel Prize.

As of 2018, no one had yet received a Nobel Prize for any of the discoveries implying that dark matter exists. Vera Rubin would have seemed a likely candidate for such a prize, for reasons briefly noted in the Extraordinary Claims boxon page 679. Unfortunately, however, she died in 2016, and Nobel Prizes can be awarded only to people who are still living. a) Working independently, read at least one article about whether Vera Rubin should have won a Nobel Prize. b) Gather into small groups and discuss the articles that you read. What are the arguments in favor of a Nobel Prize for Rubin? Did you come across anyone arguing that a Nobel Prize would not have been appropriate? c) Many eminent female astronomers in the 21st century already view Rubin as a pioneer and role model. Do you think receiving a Nobel Prize would have further increased her positive impact on women's participation in astronomy? Explain your reasoning. d) When considering candidates for the Nobel Prize, would it be appropriate for the Prize Committee to consider potential impacts of the award on diversity and inclusion in science?

Make a list of the pros and cons of considering these impacts.

The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

40) Dark Overall, how convincing do you consider the case for the existence of dark Matter. matter? Write a short essay in which you explain what we mean by dark matter, describe the evidence for its existence, and discuss your opinion about the strength of the evidence.

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41) Alternative Gravity. Suppose someone proposed a new theory of gravity that claimed to explain observations of motion in galaxies and clusters of galaxies without the need for dark matter. Briefly describe at least one other test that you would expect the new theory to be able to pass if it was, in fact, a better theory of gravity than general relativity, which is currently our best explanation of how gravity works. 42) The Nature of Dark Matter. Find and study recent reports on the possible nature of dark matter. Write a report or hold a group discussion on the latest ideas about what dark matter is made of. 43) Dark Energy.

Overall, how convincing do you consider the case for the existence of dark energy? Write a short report or hold a group discussion on what we mean by dark energy, the evidence for its existence, and your opinion about the strength of the evidence.

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44) Group Activity: Dark Matter and Distorted Galaxies. Study the gravitational lensing shown in Figure 23.9. Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39. a) Notice in Figure 23.9 how gravitational lensing causes the image of a background galaxy to shift to a position farther from the center of the cluster. Individually, draw simple sketches of how you'd expect the lensed image of a spherical galaxy to look. As a group, briefly discuss your sketches. Do they agree? If not, do you think that one prediction is better than the others? b) Make a copy of the diagram below on a large piece of paper. Use a straight edge to make sure that the lines are straight and that they all intersect at the same place; the circle should be close to the point of intersection, as shown. Note that the point at which the lines intersect represents the center of a galaxy cluster, and the circle represents the true position and shape of a spherical galaxy at a much greater distance from Earth.

c) Now work together to determine the effect of the cluster's lensing shift on the galaxy's image as thefollows. same Step 1: For each dot on the circle, draw another dot farther to the right along line, so that the distance between the two dots is equal to the length of the line labeled "lensing shift." Step 2: Connect the new dots to see the shape of the lensed image. d) How do your results from part c compare to the predictions you made in part a? How do the results for any compare with the lensed galaxy images in Figure 23.10? Discuss possible reasons discrepancies you find.

Investigate Further 45) The Future Universe. Based on current evidence concerning the growth of structure in the universe, briefly describe what you would expect large-scale structures in the universe to look like about 10 billion years from now. 46) Dark Matter and Life. State and explain at least two reasons one might argue that dark matter is (or was) essential for life to exist on Earth.

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47) Orbital Speed vs. Radius. For each of the following three hypothetical galaxies, draw a graph showing how orbital speed depends on distance from the galactic center. Make sure the horizontal axis has approximate distances labeled. a) a galaxy with all its mass concentrated at its center b) a galaxy with constant mass density within 20,000 light-years of its center, and zero density beyond that distance c) a galaxy with constant mass density within 20,000 light-years of its center, and beyond that an enclosed mass that increases proportionally with distance from the center 48) Dark Energy and Supernova Brightness. When astronomers began measuring the brightnesses and redshifts of distant white dwarf supernovae, they expected to find that the expansion of the universe was slowing down. Instead they found that it was speeding up. Were the distant supernovae brighter or fainter than expected? Explain why. (Hint: In Figure 23.18, the position of a supernova on the vertical axis depends on its redshift. Its position on the horizontal axis depends on its brightness–supernovae seen further back in time are not as bright as those seen closer in time.)

49) What Is Dark Matter? Describe at least three possible constituents of dark matter. Explain how we would expect each to interact with light, and how we might go about detecting its existence.

50) Alternative Gravity. How would gravity have to be different in order for us to be able to explain the rotation curves of galaxies without the need for dark matter? Would gravity at very large distances need to be stronger or weaker than expected? Explain.

51) Photo Journal: Gravitational Lenses. Gravitational lensing occurs in numerous astronomical situations. Compile a catalog of examples from the web, with photos of lensed stars, quasars, and galaxies. Give a one-paragraph explanation of what is shown in each photo.

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

52) White Dwarf M/L.What is the mass-to-light ratio of aM1Sun white dwarf with a luminosity of 0.001L Sun? 53) Supergiant M/L.What is the mass-to-light ratio of aM 30Sun supergiant star with a L luminosity of 300,000Sun? 54) Solar System M/L.What is the mass-to-light ratio of the solar system?

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55) Mass from Orbital Velocities. Study the graph of orbital speeds for the spiral galaxy NGC 7541, which is shown in Figure 23.4. a) Use the orbital velocity law to determine the mass (in solar masses) of NGC 7541 enclosed within a radius of 30,000 light-years from its center. (Hint: 1 light-year = 9.461 × 1015 m.) b) Use the orbital velocity law to determine the mass of NGC 7541 enclosed within a radius of 60,000 light-years from its center. c) Based on your answers to parts a and b, what can you conclude about the distribution of mass in this galaxy?

56) Weighing a Cluster. A cluster of galaxies has a radius of about 5.1 million light-years (4.8 × 1022 m) and an intracluster medium with a temperature of 6 × 107 K. Estimate the mass of the cluster. Give your answer in both kilograms and solar masses. Suppose that the combined luminosity of all the stars in the cluster is 8 × 1012LSun. What is the cluster's mass-to-light ratio?

57) Cluster Mass from Hot Gas. The gas temperature of the Coma Cluster of galaxies is about 9 × 107 K. What is the mass of this cluster within 15 million light-years of the cluster center?

58) From Newton to Dark Matter. Show that the equation Mr = r × v2/G from Mathematical Insight 23.2 is equivalent to Newton's version of Kepler's third law from Mathematical Insight 4.3. Assume that one mass is much larger than the other mass and that the orbit is circular. (Hint: What is the mathematical relationship between period p and orbital velocity v and orbital radius r for a circular orbit?)

59) 10100 Years. Based on current understanding, the final stage in the history of a perpetually expanding universe would come about 10100 years from now. Such a large number is easy to write but difficult to understand. This problem investigates some of the incredible properties of very large numbers. a) The current age of the universe is around 1010 years. How much longer is a trillion years than this current age? How much longer is 1015 years? 1020 years? b) Suppose protons decay with a half-life of 1032 years. When will the number of remaining protons be half its current amount? When will it be a quarter of its current amount? How many half-lives will have gone by when the universe reaches an age of 1034 years? What fraction of the original protons will remain at this time? Is it reasonable to conclude that all protons in today's universe will be gone by the time the universe is 1040 years old? Explain. (Hint: See Mathematical Insight 8.1.)

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter 24 Life in the Universe Section 24.1 1) How do scientists study the history of life on Earth?

A) by studying fossils of ancient life only B) by comparing DNA among living organisms only C) both by studying fossils of ancient life and by using telescopes to study habitable worlds elsewhere D) both by studying fossils of ancient life and comparing DNA among living organisms E) by using telescopes to study habitable worlds elsewhere only Answer: D 2) If scientists excavate two fossils at the Grand Canyon, one of which was found at a much deeper rock layer than the other, what can we assume about the fossils?

A) The fossil found in a deeper layer is older.

B) The fossil found in a deeper layer is younger.

C) The fossil found in a deeper layer is not as well-preserved.

D) The fossil found in a deeper layer has a higher carbon-12 abundance.

Answer: A 3) What evidence leads scientists to conclude that life was present on Earth within a billion years of its formation?

A) Such early existence of life is the only way to explain the variety of life-forms seen today.

B) Fossil evidence of life dates back to this time.

C) Studies of DNA indicate that life must have been present by that time.

Answer: B

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4) The figure above shows the geological time scale of Earth. Animals have been present on land for approximately ________ of the Earth's history. A) 1/10 B) 1/4 C) 1/2 D) 3/4 Answer: A 5) The figure above shows the geological time scale of Earth. Life has existed for ________ of the Earth's history. A) all B) about 90% C) about 50% D) about 25% Answer: B 6) The figure above shows the geological time scale of Earth. The time since dinosaurs went extinct (with the K-T event) represents approximately ________ of the Earth's history. A) 0.15% B) 1.5% C) 15% D) 50% Answer: B

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7) The figure above shows the geological time scale of Earth. The K-T event that marks the extinction of the dinosaurs occurred approximately ________ years ago. A) 65 million B) 10 million C) 650 million D) 100 million Answer: A 8) About when did the extinction of the dinosaurs occur? A) 3.5 billion years ago B) 2 billion years ago C) 540 million years ago D) 65 million years ago E) less than 1 million years ago Answer: D 9) What was the Cambrian explosion?

A) the impact of the meteor that led to mass extinction of the dinosaurs B) the initial spark of lightning that created the first life-form from amino acids C) the eruption of the volcano known as Cambria on the ancient supercontinent of Pangaea D) a dramatic diversification of life that began about 542 million years ago E) the largest mass extinction in Earth's history Answer: D 10) Which of the following contributed to the gradual evolution of life via natural selection? A) Organisms tend to produce more offspring than the local environment can support. B) Individuals in a population vary in many heritable traits. C) Individuals with traits that help them survive and reproduce are more likely to pass those traits on to succeeding generations. D) All of the above are correct. Answer: D 11) Which of the following can cause mutations in living organisms? A) natural selection B) a changing climate C) reproduction D) ultraviolet light and toxic chemicals E) the greenhouse effect Answer: D 12) The oldest carbon isotope evidence of life indicates that life on Earth arose at least A) 4.5 billion years ago. B) 3.95 billion years ago. C) 2 billion years ago. D) 542 million years ago. E) 65 million years ago. Answer: B

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13) Which of the following living organisms most resemble the common ancestor of all life according to genetic testing?

A) bacteria such as E. coli B) organisms living deep in the oceans around seafloor volcanic vents C) plankton that use sunlight as an energy source through photosynthesis D) viruses E) stromatolites Answer: B 14) Why are relatively few details known about the life that existed on Earth prior to a few hundred million years ago?

A) Nearly all life at that time was microscopic, making it much more difficult to identify.

B) Older rocks are rarer than younger rocks because of geologic processes.

C) Many rocks have been transformed by high heat and pressure, destroying any fossils they contained.

D) All of the above are correct.

Answer: D 15) Which of the following offers evidence to support the hypothesis that life arises relatively easily under the conditions that existed on the early Earth?

A) Small animals like Tardigrades can survive exposure to the near-vacuum of space.

B) Life is currently present in extreme environments like deep ocean volcanic vents.

C) Life was present on Earth by about the time that the heavy bombardment ended.

D) All organisms on Earth use DNA as their genetic material.

Answer: C 16) Which of the following provides direct evidence that the chemical building blocks of life (such as amino acids and DNA bases) could have formed naturally under the conditions that were present on the early Earth?

A) Fossil remains of life date to fairly early in Earth's history.

B) These chemicals have been formed in laboratory experiments that recreate conditions of the early Earth.

C) DNA studies show that all known life on Earth evolved from a common ancestor.

D) We have found evidence of complex carbon molecules on Saturn's moon Titan.

Answer: B 17) Which of the following provides evidence that all life on Earth shares a common ancestor?

A) All life on Earth shares certain biochemical similarities, including use of DNA as its hereditary molecule.

B) All life on Earth requires energy and nutrients to survive.

C) All life on Earth requires liquid water to survive.

D) All life on Earth shares similarities in appearance that could not possibly be a coincidence.

Answer: A

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18) What are the three key necessities for all life on Earth? A) (1) nutrients, (2) energy, (3) liquid water B) (1) liquid water, (2) carbon, (3) oxygen C) (1) nutrients, (2) liquid water, (3) oxygen D) (1) DNA, (2) mutations, (3) liquid water E) (1) DNA, (2) liquid water, (3) oxygen Answer: A 19) What is not necessary for all living organisms on Earth to survive? A) energy B) nutrients C) oxygen D) liquid water Answer: C Section 24.2 1) Which of the following worlds is the one for which we have sent the most space missions to learn about the potential of life beyond the Earth? A) the Moon B) Venus C) Mars D) Europa E) Titan Answer: C 2) What evidence tells us that the so-called "Martian meteorites" actually came from Mars? A) Their chemical composition matches that of known Martian rocks. B) Their ages match the known age of Mars. C) Their color is the same as most Martian rocks. D) Their composition tells us they formed farther from the Sun than Earth. Answer: A 3) Which of the following explains why we do not consider the surface of Mars to be habitable today? A) Mars has too much carbon dioxide in its atmosphere. B) Mars is located far outside the Sun's habitable zone. C) The atmospheric pressure is too low for liquid water to be stable on the surface. D) Mars is bombarded too often with asteroids and comets. E) Mars has too little gravity for its surface to be habitable. Answer: C

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4) Which of the following best describes the findings of the Viking landers in the experiments they conducted to search for life on Mars?

A) Some experiments were potentially suggestive of life, but their results could also be explained by chemical reactions not associated with life.

B) Absolutely no evidence indicating life was found in the Martian soil.

C) The landers detected organic material and evidence for microbial organisms in the soil.

D) The experiments failed to operate properly and therefore did not provide any results.

Answer: A 5) Why didn't the Viking landers find organic molecules in the soil on Mars?

A) The Martian soil contains perchlorate, which destroyed the organic molecules when heated during the experiments.

B) There are no organic molecules on Mars because they were carried away by the solar wind.

C) The Viking landers were not designed to detect organic molecules.

D) Organic molecules exist too deep below the surface of Mars to be detected.

Answer: A Section 24.3 1) Which of the following gases, which we might be able to detect in infrared spectra, would be a strong indicator of life on another planet?

A) carbon dioxide B) ozone C) methane D) water vapor E) sulfuric acid Answer: B 2) Which of the following has been the most important for maintaining the Earth's stable climate over the time it took for large organisms to evolve?

A) impacts of water-bearing planetesimals B) the ending of the heavy bombardment C) plate tectonics D) the tides E) deep sea volcanic vents Answer: C 3) Why is a global magnetic field thought to be necessary for life to exist on the surface of a planet?

A) A magnetic field shields the planet from harmful ultraviolet light.

B) A magnetic field helps form ozone in the upper atmosphere.

C) A magnetic field helps keep the planet's atmosphere from being stripped away by stellar winds.

D) A magnetic field helps warm the planet's surface through the greenhouse effect.

Answer: C

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4) What defines the habitable zone around a star?

A) the region around a star where rocky planets can form B) the region around a star where humans can survive C) the region around a star where liquid water can exist on planetary surfaces D) the region around a star where its ultraviolet radiation is too weak to destroy biological organisms on a planetary surface E) the region around a star outside of its hot, tenuous corona Answer: C 5) Jovian planets are thought to form beyond the frost line of their star systems. How, then, might it be possible for a large moon around a jovian planet to have a habitable surface with liquid water?

A) The moon would need to be locked in synchronous rotation with its planet.

B) The jovian planet would need to have migrated inward to the habitable zone of its star.

C) The moon would need to have a magnetic field.

D) The moon would need to have plate tectonics.

E) The moon would need to have a high density.

Answer: B Section 24.4 1) Which of the following best describes the goal of SETI (Search for Extraterrestrial Intelligence)?

A) to detect signals originating from civilizations from outside the solar system B) to detect life of any kind, anywhere besides Earth C) to estimate the number of civilizations in the galaxy D) to find habitable planets around other stars Answer: A 2) In 1974, a radio message was sent out from the Arecibo observatory in Puerto Rico. About how far has it gotten by now?

A) just beyond our Solar System B) not even to the nearest stars C) a small fraction of the distance across the Milky Way D) almost to the center of the Milky Way E) beyond the Milky Way, to the Andromeda galaxy Answer: C

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3) Which of the following statements best reflects our current knowledge about the term flife in the equation Number of Civilizations = NHP × flife × fcivilization × fnow?

A) The value of flife is either 0 or 1, but not anything in between.

B) The value of flife is between 0 and 1.

C) The value of flife is less than 0.0000001.

D) The value of flife is equal to 1/2.

E) The value of flife is greater than 0.9.

Answer: B 4) What do we mean by fnow in the Drake equation (Number of Civilizations = NHP × flife × fcivilization × fnow)?

A) the fraction of planets with civilizations on which the civilizations exist now B) the fraction of time since the Big Bang represented by the present era C) the fraction of all species ever to exist that we currently are aware of D) the fraction of planets in the galaxy on which a civilization could theoretically develop now E) the fraction of civilizations in the universe that currently are sending messages to us Answer: A 5) Which factor(s) in the Drake equation (Number of Civilizations = NHP × flife × fcivilization × fnow) can we estimate based on discoveries of extrasolar planets?

A) NHP B) flife C) fcivilization D) fnow E) all of the above Answer: A Section 24.5 1) Which of the following best explains why an interstellar ramjet could, in theory, achieve continuous acceleration?

A) It uses fuel that is more efficient than any other known fuel.

B) It collects its fuel as it goes, rather than having to carry the weight of fuel along with it.

C) It takes advantage of theoretically possible loopholes in Einstein's theory of relativity.

D) It has such a large fuel tank that it will essentially never use up all the fuel it carries.

E) Its speed always gets faster because time dilation changes the rate at which time flows.

Answer: B

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2) At about what fraction of the speed of light do our fastest current spacecraft travel?

A) 1/2 B) 1/10 C) 1/100 D) 1/1000 E) 1/10,000 Answer: E 3) If we develop spacecraft that can take humans to nearby solar systems at a few percent of the speed of light, how long would it be before we could conceivably populate all habitable planets in the entire Milky Way? A) a few thousand years B) a few million years C) a few billion years D) We could never colonize the galaxy unless we had ships that could travel very close to the speed of light.

Answer: B 4) Which of the following is not a possible solution to the question of why we have not yet found evidence of other civilizations?

A) We are the only intelligent civilization to have ever arisen in the Milky Way. B) Of all of the intelligent civilizations in the Milky Way, none has chosen to create a galactic civilization.

C) There is a galactic civilization, but we are currently unable to detect or recognize it.

D) There is a galactic civilization, but it is being actively hidden from us.

E) Given the current age of our galaxy, there has not been enough time for a galactic civilization to develop.

Answer: E 5) Which of the following best describes Fermi's Paradox?

A) Galactic civilizations seem forbidden by the laws of physics.

B) Interstellar travel is possible yet would take an infinite amount of time because of relativistic time dilation.

C) We would be unable to detect an Earthlike planet even at a distance of a few light-years.

D) Reasonable assumptions predict that a galactic civilization should have already arisen in the Milky Way, yet we have absolutely no evidence for it.

E) The Drake equation predicts that there should be no intelligent life in the Milky Way, yet we exist.

Answer: D

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General Chapter Questions 1) Which of the following statements about ozone (in Earth's stratosphere) is not true?

A) The presence of ozone was crucial to the origin of life.

B) If the ozone layer is depleted, we can expect the rate of mutations in land-based life-forms to increase significantly.

C) Ozone absorbs ultraviolet light from the Sun. D) The human-made chemicals known as CFCs can cause a loss of ozone in the atmosphere.

Answer: A Short Answer Questions 1) Why is ozone so crucial to the continued well-being of life on Earth?

Answer: Ozone (O3) absorbs ultraviolet photons from the Sun and prevents them from reaching the ground where these high-energy photons would harm biological life.

2) What evidence do we have that life arose fairly quickly on Earth?

Answer: Because of the period of heavy bombardment, it would have been difficult for life to survive more than about 4 billion years ago. Combined with evidence from fossils of life 3.5 billion years ago and carbon isotope evidence dating back to 3.95 billion years, it seems that it only takes a few hundred million years for life to spring into existence.

3) Why do we generally rule out stars with masses more than a few times that of the Sun when we consider star systems where life could be possible?

Answer: Their stellar lifetimes are too short to allow enough time for life to evolve.

Reading Quiz Questions 1) Fossil evidence points to the presence of life on Earth as far back as ________ ago.

A) almost 4 billion years B) about 2.0 billion years C) about 545 million years D) about 65 million years Answer: A 2) What is the significance of stromatolites?

A) They appear to be fossils of structures made by microbes long ago.

B) They indicate that microbes in the distant past were much larger than bacteria of today.

C) They show that the oxygen content of Earth's atmosphere was much smaller in the past.

D) They are the earliest known fossils of animals.

Answer: A

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3) Why do scientists say that evolution is a "theory"?

A) because it explains a great deal about life and is supported by an enormous body of evidence B) because they are not very confident that it really happened C) because it's really just a guess about how life developed on Earth D) because it is supported by only a small amount of evidence Answer: A 4) What is a mutation?

A) a change in a living cell's DNA B) a change in an organism that turns it into a different species C) a change in the type of food an organism consumes D) a change in the physical appearance of a living organism Answer: A 5) Based on DNA studies, it seems that all life on Earth A) shares a common ancestor.

B) belongs to one of just two kingdoms: plants and animals.

C) arose from one of five distinct ancestors that lived about 2 billion years ago.

D) requires oxygen to survive.

Answer: A 6) Which of the following is considered by biologists to be a likely place where life might have first arisen on Earth?

A) in hot water near undersea volcanoes B) on meteorites that landed on Earth C) on land surfaces that got moderately heavy rainfall D) deep in Earth's core Answer: A 7) How did oxygen (O2) get into Earth's atmosphere?

A) It was released by life through the process of photosynthesis.

B) It was captured from the solar nebula.

C) It was outgassed from volcanoes.

D) It came from chemical reactions with surface rocks.

Answer: A 8) Which of the following is not considered crucial for life to exist on some world?

A) a liquid, such as liquid water B) a source of energy that can be used by life C) a source of nutrients D) an atmosphere Answer: D

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9) Which of the following best describes what we mean by a habitable world?

A) a planet or moon that could support life, if any life happened to be on it B) a planet or moon with life C) a planet or moon that lies within its star's habitable zone D) a planet or moon on which humans could survive if we happened to go there Answer: A 10) Which of the following places is not generally considered a potential home for life in our solar system?

A) Jupiter's atmosphere B) Mars C) Europa D) Titan Answer: A 11) The Sun's habitable zone A) extends from some place a little beyond the orbit of Venus to some place near the orbit of Mars.

B) consists only of Earth, because Earth is the only planet known to be inhabited.

C) extends from the orbit of Earth to the orbit of Jupiter.

D) extends from just beyond the orbit of Mercury to just beyond Earth's orbit.

Answer: A 12) Why don't we expect to find life on planets orbiting high-mass stars?

A) The lifetime of a high-mass star is too short.

B) The stars are too hot to allow for life.

C) Planets cannot have stable orbits around high-mass stars.

D) The high-mass stars emit too much ultraviolet radiation.

Answer: A 13) In the context of this chapter, what is an orphan planet?

A) a planet-size object that does not orbit a star B) a planet orbiting a star that is not a member of a galaxy C) a planet that has microbial life but no larger organisms D) a planet with no moons, such as Mercury or Venus Answer: A 14) At present, what is the primary way that the search for extraterrestrial intelligence (SETI) is carried out?

A) by using radio telescopes to search for signals from extraterrestrial civilizations B) by analyzing high-resolution images of nearby stars in search of evidence for structures that could not have developed naturally C) by searching for planets around distant stars D) by using x-ray telescopes to search for exhaust from interstellar spacecraft E) by seeking access to the secret records and alien corpses kept at the military's Area 51 in Nevada Answer: A

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15) In the Drake equation (Number of Civilizations = NHP × flife × fciv × fnow), what do we mean by fnow?

A) the fraction of planets with civilizations at the present time (as opposed to only in the past or future) B) the fraction of planets in the galaxy on which a civilization could theoretically develop right now C) the fraction of civilizations in the universe that currently are sending messages to us D) the fraction of all species ever to exist that we currently are aware of Answer: A 16) We have sent several spacecraft on trajectories that will ultimately take them into interstellar space (including Pioneer 10 and 11, Voyager 1 and 2, New Horizons). How long will it take these spacecraft to travel as far as the nearest stars?

A) tens of thousands of years B) a few hundred years C) a few decades D) never, because they will rust and fall apart E) about a thousand years Answer: A 17) Einstein's theory of relativity tells us that travelers who make a high-speed trip to a distant star and back will A) age less than people who stay behind on Earth.

B) age more than people who stay behind on Earth.

C) have more fun than people who stay behind on Earth.

D) never be able to make the trip within their lifetimes.

Answer: A 18) If there are other civilizations at present in the Milky Way Galaxy, which statement is almost undoubtedly true?

A) They are far more technologically advanced than we are.

B) They are anatomically much like us, with two arms, two legs, two eyes, and two ears.

C) They have social structures that are completely different from our own; for example, different types of "family" units, and so on.

D) For fun, they enjoy "buzzing" to Earth and temporarily abducting people, showing a clear preference for people located in less-developed rural areas.

Answer: A

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Concept Quiz Questions 1) Why are fossils of early life on Earth more rare than fossils of plants and animals from the past few hundred million years?

A) Early organisms lacked skeletons and other hard structures that are most likely to be fossilized.

B) Life was far less abundant before a few hundred million years ago.

C) Fossils could not form before there was oxygen in the atmosphere.

D) We find fossils in sedimentary layers, and no sediments were deposited until just a few hundred million years ago.

Answer: A 2) Which of the following best describes natural selection?

A) It is the idea that organisms with genetic traits that improve their ability to reproduce are more likely to pass those traits on to future generations.

B) It is the idea that the strong survive and the weak die off.

C) It is a guess made by scientists about how life develops, but it has no hard evidence to support it.

D) It is the idea that organisms naturally increase in complexity and intelligence with time.

Answer: A 3) Which of the following is not key evidence in support of the idea that all life today shares a common ancestor?

A) We have identified fossils of the first life forms that ever existed on Earth.

B) All life uses DNA and the same genetic code.

C) Mapping of gene sequences shows how life is all related.

D) All life builds proteins from the same amino acids and uses ATP to store energy in cells.

Answer: A 4) Which of the following best describes the predominant scientific view of the origin of life on Earth?

A) We may never know precisely how life arose, but current evidence suggests that life probably can arise naturally under the conditions that prevailed on the early Earth.

B) Life probably migrated to Earth from some other world.

C) Life arose through a series of extremely unlikely chemical coincidences, making it seem almost miraculous that life ever came to exist at all.

D) We can describe with great certainty the precise steps by which life arose on Earth.

Answer: A

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5) According to current science, why didn't oxygen begin to accumulate in the atmosphere for more than a billion years after life appeared on the Earth? A) Oxygen was removed from the atmosphere by chemical reactions with surface rocks as quickly as it was released by life. B) Early forms of animal life consumed the oxygen released by plants during the first billion years of life on Earth. C) Early life did not release oxygen, and oxygen releasing organisms didn't evolve for a billion years after the earliest life. D) Oxygen was removed from the atmosphere by dissolving in the ocean as quickly as it was released by life.

Answer: A 6) When we analyze whether a world is a possible home to life, the key thing we look for is A) the past or present existence of liquid water. B) evidence of atmospheric oxygen. C) the presence of organic molecules such as amino acids. D) surface coloration changes that could indicate vegetative growth. Answer: A 7) Which of the following best describes the current status of the evidence concerning whether life has ever existed on Mars? A) Mars probably once had a habitable surface, but we do not know if it had life. B) Clear evidence strongly suggests that life once existed on Mars. C) Clear evidence indicates that Mars never had conditions favorable to life. D) Although current evidence is mixed, ongoing studies should ensure that we will have a definitive answer to the question of whether life ever existed on Mars within about five years. Answer: A 8) Why is Europa considered a good candidate for the possible existence of life? A) Strong evidence suggests that it has a deep, subsurface ocean of liquid water. B) The Galileo spacecraft found strange seasonal changes on its surface that look like they could be due to life. C) It is located within our Sun's habitable zone. D) It has a thick atmosphere with a surface pressure greater than that on Earth. Answer: A 9) In general, how does the size and location of a star's habitable zone depend on the star's mass? A) the smaller (less massive) the star, the smaller and the closer-in the habitable zone B) the smaller (less massive) the star, the larger and the closer-in the habitable zone C) the smaller (less massive) the star, the larger and the farther-out the habitable zone D) The habitable zone is always about the same size, but its location moves inward for smaller stars. Answer: A

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10) We are not yet capable of detecting life on planets around other stars. But as our technology develops, our first real chance of detecting such life will probably come from A) examining spectral lines from the atmospheres of distant planets.

B) sending spacecraft to study the planets up close.

C) examining high-resolution images of the planets made by orbiting telescopes.

D) determining the orbital properties of the planets.

Answer: A 11) Some scientists speculate that super-Earths and water worlds might be habitable at greater distances from their stars than Earth-size planets. Why? A) Their larger sizes might allow them to retain hydrogen atmospheres.

B) Their larger sizes mean they might have more of the ingredients needed for life.

C) For the water worlds, at least, they are certain to have oceans no matter how far they are from a star.

D) These planets may retain more internal heat than Earth, which might drive greater geological activity.

Answer: A 12) Which of the following best describes how the Drake equation is useful?

A) It helps us understand what we need to know to determine the likelihood of finding other civilizations.

B) It has allowed us to determine the number of civilizations in the Milky Way Galaxy.

C) It allows us to calculate the masses of planets orbiting other stars.

D) It tells us what wavelengths of light will be most useful to examine in the search for extraterrestrial intelligence.

Answer: A 13) In the Drake equation (Number of Civilizations = NHP × flife × fciv × fnow), we expect the term fciv to be small if A) most civilizations destroy themselves within just a few hundred years of arising.

B) most of the civilizations that have ever existed are still out and about in the galaxy.

C) primitive life is common but intelligent life is rare.

D) most habitable planets never actually get life on them.

Answer: C 14) Suppose it turns out that one in 1 million stars has a planet that at some point in its history is home to an advanced civilization. Then the total number of civilizations that have arisen in our galaxy would be closest to ________. Assume there are about 100 billion stars in the Milky Way galaxy.

A) 100,000 B) 10,000 C) 1,000 D) 100 E) 10 F) 1 Answer: A

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15) Which of the following describes a major danger of interstellar travel at near-light speed?

A) Atoms and ions in interstellar space will hit a fast-moving spacecraft like a flood of dangerous cosmic rays.

B) Any interstellar journey will take much longer than the lives of the crew members.

C) Time dilation will slow the heart beats of the crew to a dangerously low rate.

D) Asteroid fields floating in interstellar space will present a navigational challenge.

Answer: A 16) Which of the following statements about matter—antimatter engines is not true?

A) Matter—antimatter engines would be great in theory, but to date we have no evidence that antimatter even exists.

B) Matter—antimatter reactions represent the most efficient possible reactions in terms of energy release.

C) Spacecraft powered by matter—antimatter engines could probably reach speeds of more than half the speed of light.

D) One of the major challenges to developing matter—antimatter engines is finding a way to store antimatter after it is produced.

Answer: A 17) Which of the following is not considered a potential solution to the question of why we lack any evidence of a galactic civilization?

A) The galactic civilization probably is undetectable because they operate under different laws of physics from the ones we know.

B) There is no galactic civilization because we are the first species ever to achieve the ability to study the universe.

C) The galactic civilization is deliberately avoiding contact with us.

D) There is no galactic civilization because all civilizations destroy themselves before they achieve the ability to colonize the galaxy.

Answer: A

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Visual Quiz Questions 1) What are the different colored horizontal layers that we see in this photo of a wall in the Grand Canyon?

A) They are sediments deposited at different times in the past, so that layers near the bottom are the oldest and layers near the top are the youngest.

B) They are different types of volcanic rock that cooled at different rates after a single, large volcanic eruption.

C) They formed as the Colorado River carved out the canyon, so that the layers near the top are the oldest and the layers at the bottom are the youngest.

D) They got their coloration from ancient civilizations that painted the rock.

Answer: A

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2) Suppose you had a time machine and traveled back in time to Earth about 3 billion years ago.

Based on studying this diagram of the geological time scale, what would have caused you the most difficulty as you tried to walk around?

A) a lack of breathable air B) the danger posed by carnivorous dinosaurs C) the possibility of being harmed by poisonous plants D) a lack of liquid water Answer: A

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3) This diagram shows the "tree of life" based on genetic studies of relationships between living organisms. Which statement about the tree of life is not true?

A) It shows that plants and animals are the two most dominant forms of life on Earth today.

B) It shows that all organisms alive today bear genetic similarities that suggest they evolved from a common ancestor.

C) It shows that, based on genetic relationships, there are three major branches of life on Earth.

D) It shows that there is greater genetic diversity in microscopic organisms than among animals.

Answer: A

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4) This photo shows an undersea volcanic vent. What is it, and what is its significance to the study of the origin of life?

A) Genetic studies suggest that life may have originated around vents like this one.

B) It is the place where we have found the oldest living organisms on Earth.

C) It is the place where we have found the oldest fossils on Earth.

D) It shows that fires can burn underwater.

Answer: A

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5) This photo shows apparatus used in an experiment in which materials thought to have been present on the early Earth were sparked with energy. What have we learned from this type of experiment?

A) These experiments have shown that the building blocks of life form easily and naturally under conditions that existed on the early Earth.

B) These experiments have told us the precise composition of Earth's early atmosphere.

C) These experiments have shown us how life originated on Earth.

D) These experiments have provided strong evidence for the idea that life first arose near volcanic vents at the bottom of the ocean.

Answer: A

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6) These two diagrams contrast the habitable zones around the Sun and around a much lower mass star. They are shown to scale. Based on what you see here, what can you conclude about the possibility of finding life around the lower-mass star compared to finding life around a Sunlike star?

A) There is a higher probability that an orbiting planet will be in the habitable zone for a Sun-like star than for a lower-mass star.

B) Sun-like stars must always have more life around them than lower-mass stars.

C) A Sun-like star always has at least two habitable planets, while a lower-mass star may have only one habitable planet.

D) There is virtually no chance of finding life around a lower-mass star.

Answer: A

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7) What does this graph tell us about humans compared to other animals?

A) Humans have a brain that is unusually large for an animal of our size.

B) Humans have the largest brains of any animal.

C) Unlike the brains of other animals, human brains cannot be described by a straight line.

D) Based on averages determined from other animals, human brains are precisely the size we'd expect for animals of our weight.

Answer: A 8) This photo shows a radio telescope that is being used in the search for life beyond Earth. How is it being used in this search?

A) It is used to listen for signals deliberately broadcast by another civilization.

B) It is used to obtain spectra of planets around other stars so that we can look for chemicals, like oxygen, that might indicate the presence of life.

C) It is used to make detailed radar maps of Mars, from which we may be able to determine whether cities ever existed on our planetary neighbor.

D) It is used to communicate with spacecraft that we have launched toward distance stars in search of life on planets that my orbit them.

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9) This painting shows a futuristic spacecraft powered by the detonation of small hydrogen bombs. If we actually had such a spacecraft, what would it allow us to do?

A) It would allow us to make a one-way trip to the nearest star besides the Sun in about a century.

B) It would allow us to travel roundtrip to almost anywhere in the Milky Way Galaxy in just a few years of ship time.

C) It would allow us to travel from Earth to Mars in less than an hour.

D) It would allow us to travel up to several times faster than the speed of light.

Answer: A 10) This photo shows a boy playing with sand on the beach. If we think about the grains of sand, how are they significant to the question of life beyond Earth?

A) There are about as many stars in the universe as there are grains of sand on all the beaches on Earth, so there are a lot of possible homes to life.

B) There are about as many life-bearing planets in the universe as there are grains of sand on all the beaches on Earth.

C) There are about as many habitable planets orbiting other stars in the universe as there are grains of sand on all the beaches on Earth.

D) We think that life must always arise on or near an ocean, so this scene on the beach probably looks much like what we'd expect to find on any other planet with life on it.

Answer: A

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End-of-Chapter Questions Visual Skills Check Use the following questions to check your understanding of some of the many types of visual information used in astronomy.

The figure above, which repeats Figure 24.4, shows the geological time scale. Use this figure to answer the following questions.

1) List the following events in the order in which they occurred, from first to last. a) earliest humans b) earliest animals c) impact causes extinction of dinosaurs d) earliest mammals e) earliest plants living on land f) first time there is significant oxygen in Earth's atmosphere g) first life on Earth Answer: g, f, b, e, d, c, a 2) List the following time frames in order of how long they lasted, from longest to shortest. a) Hadean eon b) Proterozoic eon c) Paleozoic era d) Cretaceous period Answer: b, a, c, d

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3) Which time frame(s) do we live in today? (More than one may apply.) A) Quaternary period B) Tertiary period C) Cenozoic era D) Phanerozoic eon E) Paleozoic era Answer: A, C, D 4) How long did the Cambrian explosion last? A) less than 1 year B) about a decade C) about 10,000 years D) about 20 million years E) about 500 million years Answer: D 5) When did the heavy bombardment end? A) about 4.5 billion years ago B) between about 4.3 and 4.5 billion years ago C) between about 3.8 and 4.0 billion years ago D) exactly 3.95 billion years ago Answer: C 6) How long have mammals been present on Earth? A) about 1 million years B) about 65 million years C) about 225 million years D) about 510 million years Answer: C Chapter Review Questions 1) What is astrobiology, and what type of research does it involve? 2) How do we study the history of life on Earth? Describe the geological time scale and a few of the major events along it. 3) Summarize the evidence pointing to an early origin of life on Earth. How far back in Earth's history did life exist? 4) Why is the theory of evolution so critical to our understanding of the history of life on Earth? Explain how evolution proceeds by natural selection, and what happens to DNA that allows species to evolve. 5) How are laboratory experiments helping us study the origin of life on Earth? Explain.

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6) Give a brief overview of the history of life on Earth. What evidence points to a common ancestor for all life? How and when did oxygen accumulate in Earth's atmosphere? When did larger animals diversify on Earth?

7) Is it possible that life migrated to Earth from elsewhere? Explain.

8) Describe the range of environments in which life thrives on Earth. What three basic requirements apply to life in all these environments?

9) What is a habitable world? Which worlds in our solar system seem potentially habitable, and why?

10) Briefly summarize the current status of the search for life on Mars.

11) What do we mean by a star's habitable zone? What key factors have given Earth long-term surface habitability, and do they seem likely on other worlds? Explain.

12) What types of worlds might support surface habitability? What types of worlds might have subsurface habitability? Explain.

13) What is SETI? Describe the capabilities of current SETI efforts.

14) What is the Drake equation? Define each of its factors, and describe the current state of understanding about the potential values of each factor.

15) Why is interstellar travel so difficult? Describe a few technologies that might someday make it possible.

16) What is Fermi's paradox? Describe several potential solutions to the paradox and the implications of each to our civilization.

Fantasy or Science Fiction?

Based on our present understanding of science, decide whether each of the following futuristic scenarios is plausible or whether it is unlikely to be possible. Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

17) The first human explorers on Mars discover the ruins of an ancient civilization, including remnants of tall buildings and temples.

18) The first human explorers on Mars drill a hole into a Martian volcano to collect a sample of soil from several meters underground. On analyzing the soil, they discover that it holds living microbes resembling terrestrial bacteria but with a different biochemistry.

19) In 2040, a spacecraft lands on Europa and melts its way through the ice into the Europan ocean. It finds numerous strange, living microbes, along with a few larger organisms that feed on the microbes.

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20) It's the year 2075. A giant telescope on the Moon, consisting of hundreds of small telescopes linked together across a distance of 500 kilometers, has just captured a series of images of a planet around a distant star that clearly show seasonal changes in vegetation.

21) A century from now, after completing a careful study of planets around stars within 100 light-years of Earth, astronomers discover that the most diverse life exists on a planet orbiting a young star that formed just 100 million years ago.

22) In 2040, a brilliant teenager working in her garage builds a coal-powered rocket that can travel at half the speed of light.

23) In the year 2750, we receive a signal from a civilization telling us that the Voyager 2 spacecraft recently crash-landed on its planet, which orbits a nearby star.

24) Crew members of the matter-antimatter spacecraft Star Apollo, which left Earth in the year 2165, return to Earth in the year 2450, looking only a few years older than when they left.

25) Aliens from a distant star system invade Earth with the intent to destroy us and occupy our planet, but we successfully fight them off when their technology proves no match for ours.

26) A single great galactic civilization exists. It originated on a single planet long ago but is now made up of beings from many different planets, all assimilated into the galactic culture.

Quick Quiz

27) Fossil evidence suggests that life on Earth arose A) almost immediately after Earth formed.

B) very soon after the end of the heavy bombardment.

C) about a billion years before the rise of the dinosaurs.

Answer: No Correct Answer Was Provided.

28) The theory of evolution is A) a scientific theory backed by extensive evidence.

B) one of several competing scientific models that all seem equally successful in explaining the nature of life on Earth.

C) essentially just a guess about how life changes through time.

Answer: No Correct Answer Was Provided.

29) Plants and animals are A) the two major forms of life on Earth.

B) the only organisms that have DNA. C) just two small branches of the diverse "tree of life" on Earth.

Answer: No Correct Answer Was Provided.

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30) Which of the following is a reason early living organisms on Earth could not have survived on land? A) the lack of an ozone layer B) the lack of oxygen in the atmosphere C) the fact that these organisms were single-celled Answer: No Correct Answer Was Provided.

31) According to current understanding, the key requirement for life is A) photosynthesis.

B) liquid water.

C) an ozone layer.

Answer: No Correct Answer Was Provided.

32) Which of the following worlds is not considered a candidate for harboring life?

A) Europa B) Mars C) the Moon Answer: No Correct Answer Was Provided.

33) How does the habitable zone around a star of spectral type G compare to that around a star of spectral type M? A) It is larger.

B) It is hotter.

C) It is closer to its star.

Answer: No Correct Answer Was Provided.

34) In the Drake equation, suppose that the term flife was equal to 1/2. What would this mean?

A) Half the stars in the Milky Way Galaxy have a planet with life.

B) Half of all life-forms in the universe are intelligent.

C) Half of the habitable worlds in the galaxy actually have life, while the other half don't.

Answer: No Correct Answer Was Provided.

35) The amount of energy that would be needed to accelerate a large spaceship to half the speed of light is A) about 100 times the energy needed to launch the Space Shuttle.

B) more than 2000 times the current annual world energy consumption.

C) more than the amount of energy released by a supernova.

Answer: No Correct Answer Was Provided.

36) According to current scientific understanding, the idea that the Milky Way Galaxy might be home to a civilization millions of years more advanced than ours is A) a virtual certainty.

30 B) extremely unlikely.

2020 Pearson Education, Inc. C) one reasonable answer toCopyright Fermi's © paradox.

Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

37) The Turning Point.The end of this chapter presented the idea that humanity is now at a turning point, and that current generations have a greater responsibility for the future than did any previous generation. Do you agree with this assessment? If so, how can we ensure that we deal with this responsibility wisely, in a way that builds a better future for the entire human race?

If not, why not?

38) Group Discussion: Who Speaks for Earth? Form small groups to discuss the issues that would arise if SETI were to receive a message from an alien civilization. a) The message would presumably be received first by a relatively small group of astronomers.

Work together to write up a short protocol specifying what these scientists should do upon receiving the message. For example, should they announce the discovery immediately and share the message immediately? b) Responding to the message would make our existence known to the aliens. Is this a good idea?

Make a list of pros and cons of sending a response. c) Assume that we have decided to send a response. Given that the response will represent the entire human race, who should write it, and what type of information should it offer about our species and our planet?

The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

39) The Science of Astrobiology.The study of astrobiology is sometimes criticized as being the study of something for which we have no evidence, since we do not yet have evidence of life beyond Earth. Is astrobiology a science or speculation? Defend your opinion.

40) Astrobiology Funding. Imagine that you were a member of Congress and your job included deciding how much government funding to allocate to research in different areas of science.

How would the amount you allotted to the search for life in the universe compare to the amount you allotted to research in other areas of astronomy and planetary science? Why? 41) Breakthrough Starshot. The "Breakthrough Starshot" initiative proposes to send a fleet of tiny, robotic spaceships to the Alpha Centauri star system. Learn about how this effort is intended to work, who is funding it, and what its goals are. Overall, do you think it is realistic?

Do you think it is worthwhile? Defend your opinions.

42) The 100-Year Starship. The "100-Year Starship" is a research project started by NASA and the U.S. Department of Defense to investigate the possibility of sending humans on interstellar trips. Learn about this effort. Based on your findings and the challenges of interstellar flight discussed in this chapter, when (if ever) do you think humans are likely to begin traveling among the stars? Why?

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43) Unanswered Questions. In a sense, this entire chapter was about one big unanswered question: Are we alone in the universe? But as we attempt to answer that "big" question, we encounter many smaller questions that we might wish to answer along the way. Describe one currently unanswered question about life in the universe that we might be able to answer with new missions or experiments over the next couple of decades. What kinds of evidence will we need to answer the question? How will we know when it is answered?

44) Group Discussion: Solution to the Fermi Paradox. Discuss the various possible solutions to the question "Where are the aliens?" Can your group reach a consensus on which possibility is the most likely? If so, write a brief summary of why you chose this solution and what it means for our future. If not, record the number of group members supporting each possibility, and summarize the key points of disagreement.

45) Group Activity: Habitable Planets? Work in small groups to rank the four hypothetical planets described below in order from most likely to support life to least likely to support life.

Assume the planets are all approximately the same size as Earth, and explain your reasons for each ranking.Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39.

Planet 1: orbits a star of spectral type B (approximately 10 solar masses) in a circular orbit within the habitable zone Planet 2: orbits a Sun-like star in a circular orbit at a distance twice Earth's orbital distance from the Sun Planet 3: orbits a star with a luminosity one-quarter the Sun's luminosity in a circular orbit at a distance one-half Earth's orbital distance from the Sun Planet 4: orbits a Sun-like star in an elliptical orbit, with its orbital distance ranging from 1 AU to 10 AU Investigate Further

46) Artificial Selection. Suppose you lived hundreds of years ago (before we knew about genetic engineering) and wanted to breed a herd of cows that would provide more milk than the cows in your current herd. How would you have gone about it? How would this process of "artificial selection" be similar to natural selection? How would it be different?

47) Statistics of One. Much of the search for life in the universe is based on what we know about life on Earth. Write a short essay discussing the pros and cons of basing our general understanding of biology on the example of Earth.

48) Most Likely to Have Life. Suppose you were asked to vote in a contest to name the world in our solar system (besides Earth) "most likely to have life." Which world would you vote for?

Explain and defend your choice in a one-page essay.

49) Likely Suns. Study the stellar data for nearby stars given in Appendix F, Table F.1. Which star on the list would you expect to have the largest habitable zone? The second-largest habitable zone? If we rule out multiple-star systems, which star would you expect to have the highest probability of having a habitable planet? Explain your answers.

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50) Is Life Common? Based on what you have learned in this book, do you think life will ultimately prove to be rare, common, or something in between? Write a one to two-page essay explaining and defending your opinion.

51) What's Wrong with This Picture? Many science fiction stories have imagined the galaxy divided into a series of empires, each having arisen from a different civilization on a different world, that hold one another at bay because they all have about the same level of military technology. Is this a realistic scenario? Explain.

52) Aliens in the Movies. Choose a science fiction movie (or television show) that involves an alien species. Do you think aliens like those depicted could really exist? Write a one to two-page critical review of the movie or show, focusing primarily on the question of whether it portrays the aliens in a scientifically reasonable way.

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

53) Nearest Civilization. a) Suppose there are 10,000 civilizations in the Milky Way Galaxy. If the civilizations were randomly distributed throughout the disk of the galaxy, about how far (on average) would it be to the nearest civilization? (Hint: Start by finding the area of the Milky Way's disk, assuming that it is circular and 100,000 light-years in diameter. Then find the average area per civilization, and use the distance across this area to estimate the distance between civilizations.) b) Repeat part a, but this time assume that there are only 100 civilizations in the galaxy.

54) SETI Search. Suppose there are 10,000 civilizations broadcasting radio signals in the Milky Way Galaxy right now. On average, how many stars would we have to search before we would expect to hear a signal? Assume there are 500 billion stars in the galaxy. How would the answer change if there were only 100 civilizations instead of 10,000?

55) SETI Signal. Consider a civilization broadcasting a signal with a power of 10,000 watts. The Arecibo radio telescope, which is about 300 meters in diameter, could detect this signal if it was coming from as far away as 100 light-years. Suppose instead that the signal is being broadcast from the other side of the Milky Way Galaxy, about 70,000 light-years away. How large a radio telescope would we need to detect this signal? (Hint: Use the inverse square law for light.)

56) Cruise Ship Energy. Suppose we have a spaceship about the size of a typical ocean cruise ship today, which means it has a mass of about 100 million kilograms, and we want to accelerate the ship to a speed of 10% of the speed of light. a) How much energy would be required? (Hint: You can find the answer simply by calculating the kinetic energy of the ship when it reaches its cruising speed; because 10% of the speed of light is still small compared to the speed of light, you can use this formula: kinetic energy = 1/2 × m × v2.) b) How does your answer compare to total world energy use at present, which is about 5 × 1022 joules per year? c) The typical cost of energy today is roughly 5¢ per 1 million joules. At this price, how much would it cost to generate the energy needed by this spaceship?

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57) Matter-Antimatter Engine. Consider the spaceship from Problem 56. Suppose you wanted to generate the energy to get it to cruising speed using matter-antimatter annihilation. How much antimatter would you need to produce and take on the ship? (Hint: When matter and antimatter meet, they turn all their mass into energy equivalent to mc2.)

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter S1 Celestial Timekeeping and Navigation Section S1.1 1) A star is crossing your meridian right now. When will it cross the meridian next?

A) very close to 23 hours, 56 minutes from now B) very close to 24 hours from now C) one year from now D) It depends on the star's position on the celestial sphere.

Answer: A 2) The Sun is crossing your meridian right now. When will it cross the meridian next?

A) very close to 23 hours, 56 minutes from now B) very close to 24 hours from now C) one year from now D) It depends on the phase of the Moon.

Answer: B 3) Why is the Moon's cycle of phases (the synodic month) longer than the Moon's orbital period (the sidereal month)?

A) The Moon completes the cycle of lunar phases before it completes a full orbit around Earth.

B) The Moon has to complete more than one full orbit around Earth to complete the cycle of lunar phases.

C) The Moon orbits Earth faster than Earth orbits the Sun. D) The Moon orbits Earth faster than Earth rotates.

E) The lunar month is based on the Moon's orbit, while the sidereal month is based on Earth's orbit.

Answer: B 4) The Sun is rising in the east and will reach its highest point (on your meridian) in 2 hours.

What is your local apparent solar time?

A) 2 a.m.

B) 2 p.m.

C) 10 a.m.

D) 10 p.m.

E) Noon Answer: C

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5) Our current Gregorian calendar usually has a leap year every 4 years, but there are exceptions in some century years (e.g., 2100 will not be a leap year). Which of the following best explains why the calendar needs these exceptions? A) The length of the tropical year (from one March equinox to the next) is not exactly 365-1/4 days. B) Precession of Earth's axis makes the tropical year (from one March equinox to the next) slightly different from the sidereal year (Earth's actual orbital period). C) Earth's orbital period around the Sun is gradually changing due to tidal forces. D) The exceptions are necessary to account for the difference between a solar and sidereal day. Answer: A 6) What is the basic purpose of having leap years? A) to keep the equinoxes and solstices occurring around the same calendar dates each year B) to keep lunar calendars roughly synchronized with solar calendars C) to make sure that the average length of a solar day remains about 24 hours D) to ensure that there are always 12 months in a year Answer: A Section S1.2 1) The constellation shaped like a big square (the "great square of . . . "), which can be used to help locate the March equinox in the sky, is A) Cassiopeia. B) Pegasus. C) Canis Major. D) Taurus. E) Andromeda. Answer: B 2) In what constellation is the Sun be located at the moment of the March equinox? A) Pisces B) Aries C) Orion D) Taurus E) Andromeda Answer: A 3) How many degrees equivalent to 4 hours of right ascension? A) 60° B) 15° C) 45° D) 30° E) 120° Answer: A

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4) The right ascensions and declinations of stars stay approximately but not perfectly constant with time. What causes these coordinates to gradually change, enough to be noticeable with telescopes in just a few decades? A) the precession of Earth's axis B) changes in Earth's orbit around the Sun C) tidal effects of the Moon D) the motions of stars relative to the Sun Answer: A 5) What determines the amount of time that a particular star remains above your horizon any given day? A) all three of your latitude, longitude, and the star's declination B) your latitude and the star's declination only C) your latitude and the star's right ascension only D) your longitude and the star's declination only E) your longitude and the star's right ascension only Answer: B 6) Suppose you live at latitude 40°N. Which of the following describes the conditions that make a star circumpolar? A) Stars are circumpolar if they have declination > +50°. B) Stars are circumpolar if they have declination > +40°. C) Stars are circumpolar if they have right ascension > 6 hr. D) Stars are circumpolar if they have right ascension < 6 hr. E) No stars are circumpolar at this latitude. Answer: A 7) Suppose you live at latitude 40°N. What is the altitude and direction of the North Celestial Pole in your local sky? A) 50°, due north B) 40°, due north C) It is on your zenith. D) It is on your horizon, due north. E) 50°, due south Answer: B 8) Where would you have to live in order to have no circumpolar stars? A) the equator B) the North or South Pole C) between the Arctic Circle and the tropic of Cancer D) between the Antarctic Circle and the tropic of Capricorn E) There will always be circumpolar stars wherever you live. Answer: A

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9) Suppose the date is March 21 and the Sun passes through your zenith at noon. Where are you?

A) the equator B) the Tropic of Cancer (latitude 23-1/2°N) C) the Tropic of Capricorn (latitude 23-1/2°S) D) the Arctic Circle (latitude 66-1/2°N) E) the Antarctic Circle (latitude 66-1/2°S) Answer: A 10) Suppose the date is March 21 and the Sun crosses your meridian at an altitude of 23-1/2° in the north. Where are you?

A) the equator B) the Tropic of Cancer (latitude 23-1/2°N) C) the Tropic of Capricorn (latitude 23-1/2°S) D) the Arctic Circle (latitude 66-1/2°N) E) the Antarctic Circle (latitude 66-1/2°S) Answer: E 11) Suppose the date is June 21 and the Sun never sets, just touching your northern horizon at midnight. Where are you?

A) the equator B) the Tropic of Cancer (latitude 23-1/2°N) C) the Tropic of Capricorn (latitude 23-1/2°S) D) the Arctic Circle (latitude 66-1/2°N) E) the Antarctic Circle (latitude 66-1/2°S) Answer: D 12) Suppose the date is June 21 and the Sun never rises, just touching your southern horizon at midnight. Where are you?

A) the equator B) the Tropic of Cancer (latitude 23-1/2°N) C) the Tropic of Capricorn (latitude 23-1/2°S) D) the Arctic Circle (latitude 66-1/2°N) E) the Antarctic Circle (latitude 66-1/2°S) Answer: E 13) What can an observer in the tropics (i.e., between latitudes 23-1/2°S and 23-1/2°N) see that an observer outside the tropics can never see?

A) the Sun appearing at the zenith B) the Sun on the horizon C) a full moon D) the Sun remaining above the horizon for more than 24 hours Answer: A

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Section S1.3 1) The south celestial pole appears on your meridian at an altitude of 30° in the south. Where are you?

A) latitude = 30°S B) latitude = 60°S C) latitude = 30°N D) latitude = 60°N E) the South Pole Answer: A 2) What is your approximate latitude if you observe Polaris to be 35° above the horizon?

A) 35° N B) 35° S C) 55° N D) 55° S E) Polaris cannot be used to find your latitude.

Answer: A 3) Toronto, Canada and Santiago, Chile have similar longitudes but very different latitudes.

Which of the following is true about how observers in those cities will view the constellation Orion, which is located approximately on the celestial equator?

A) Orion will cross the meridian at about the same local time in both cities, but at different altitude and direction.

B) Orion will cross the meridian at the same altitude and direction in both cities, but at different local times.

C) Orion will cross the meridian at the same altitude and direction and at about the same local time in both cities.

D) If Orion is visible in the night sky in one city, then the other city will need to wait a few months to see Orion in its night sky.

Answer: A 4) Denver, Colorado and Istanbul, Turkey have similar latitudes but very different longitudes.

Which of the following is true about how observers in those cities will view the constellation Orion, which is located approximately on the celestial equator?

A) Orion will cross the meridian at about the same local time in both cities, but at different altitude and direction.

B) Orion will cross the meridian at the same altitude and direction in both cities, but at different local times.

C) Orion will cross the meridian at the same altitude and direction and at about the same local time in both cities.

D) If Orion is visible in the night sky in one city, then the other city will need to wait a few months to see Orion in its night sky.

Answer: C

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5) The Sun is on your meridian, and you have a UT clock that tells you it is 3 P.M. in Greenwich. What is your longitude? A) 3° west of Greenwich B) 3° east of Greenwich C) 45° west of Greenwich D) 45° east of Greenwich E) 30° west of Greenwich Answer: C 6) The Sun is on your meridian, and you have a UT clock that tells you it is midnight in Greenwich. What is your longitude?

A) 12° west of Greenwich B) 12° east of Greenwich C) 60° west of Greenwich D) 60° east of Greenwich E) 180° of longitude from Greenwich Answer: E General Chapter Questions 1) What is the hour angle of a star that is crossing your meridian right now?

A) −6 hours B) 0 hours C) 6 hours D) It depends on your latitude.

E) It depends on the right ascension of the star.

Answer: B 2) A star has an hour angle of +4 hours. What can you conclude?

A) The star remains above your horizon for only 4 hours each day.

B) The star crossed your meridian 4 hours ago.

C) The star remains below your horizon for only 4 hours each day.

D) The star crosses your meridian at 4 a.m. each day.

E) The star crosses your meridian at 8 p.m. each day.

Answer: B 3) Based on the given right ascensions and declinations, which of the following stars lies closest to the celestial equator on the celestial sphere?

A) Sirius: RA = 6hr 45m, dec = -16°42' B) Canopus: RA = 6hr 24m, dec = -52°41' C) Alpha Centauri: RA = 14hr 40m, dec = -60°50' D) Arcturus: RA = 14hr 16m, dec = +19°11' E) Vega: RA = 18hr 37m, dec = +38°47' Answer: A

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4) Based on the given right ascensions and declinations, which of the following stars lies closest to the north celestial pole on the celestial sphere?

A) Sirius: RA = 6hr 45m, dec = -16°42' B) Canopus: RA = 6hr 24m, dec = -52°41' C) Alpha Centauri: RA = 14hr 40m, dec = -60°50' D) Arcturus: RA = 14hr 16m, dec = +19°11' E) Vega: RA = 18hr 37m, dec = +38°47' Answer: E 5) Based on the given right ascensions and declinations, which of the following stars lies closest to the south celestial pole on the celestial sphere?

A) Sirius: RA = 6hr 45m, dec = -16°42' B) Canopus: RA = 6hr 24m, dec = -52°41' C) Alpha Centauri: RA = 14hr 40m, dec = -60°50' D) Arcturus: RA = 14hr 16m, dec = +19°11' E) Vega: RA = 18hr 37m, dec = +38°47' Answer: C 6) Suppose you live at latitude 20°N. Based on the given right ascensions and declinations, which of the following stars passes closest to your zenith on its daily path through your sky?

A) Sirius: RA = 6hr 45m, dec = -16°42' B) Canopus: RA = 6hr 24m, dec = -52°41' C) Alpha Centauri: RA = 14hr 40m, dec = -60°50' D) Arcturus: RA = 14hr 16m, dec = +19°11' E) Vega: RA = 18hr 37m, dec = +38°47' Answer: D 7) Suppose you live at latitude 20°S. Based on the given right ascensions and declinations, which of the following stars passes closest to your zenith on its daily path through your sky?

A) Sirius: RA = 6hr 45m, dec = -16°42' B) Canopus: RA = 6hr 24m, dec = -52°41' C) Alpha Centauri: RA = 14hr 40m, dec = -60°50' D) Arcturus: RA = 14hr 16m, dec = +19°11' E) Vega: RA = 18hr 37m, dec = +38°47' Answer: A

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8) Suppose you live at latitude 30°N. Based on the given right ascensions and declinations, which of the following stars is never visible in your sky?

A) Sirius: RA = 6hr 45m, dec = -16°42' B) Canopus: RA = 6hr 24m, dec = -52°41' C) Alpha Centauri: RA = 14hr 40m, dec = -60°50' D) Arcturus: RA = 14hr 16m, dec = +19°11' E) Vega: RA = 18hr 37m, dec = +38°47' Answer: C 9) Suppose it is late June. Based on the given right ascensions and declinations, which of the following stars crosses your meridian around midnight? Assume you live at a latitude where all the stars on this list rise above your horizon each day. (Hint: Recall that the Sun's celestial coordinates on the June solstice are RA = 6h, dec = +23-1/2°.)

A) Sirius: RA = 6hr 45m, dec = -16°42' B) Canopus: RA = 6hr 24m, dec = -52°41' C) Alpha Centauri: RA = 14hr 40m, dec = -60°50' D) Arcturus: RA = 14hr 16m, dec = +19°11' E) Vega: RA = 18hr 37m, dec = +38°47' Answer: E 10) Suppose you live at latitude 40°N. Based on the given right ascensions and declinations, which of the following stars is never visible in your sky?

A) Spica: RA = 13hr 25m, dec = -11°09' B) Deneb: RA = 20hr 41m, dec = +45°16' C) Sirius: RA = 6hr 45m, dec = -16°42' D) Alpha Centauri: RA = 14hr 40m, dec = -60°50' E) Vega: RA = 18hr 37m, dec = +38°47' Answer: D 11) Based on the given right ascensions and declinations, which of the following stars appears nearest to the south celestial pole?

A) Spica: RA = 13hr 25m, dec = -11°09' B) Deneb: RA = 20hr 41m, dec = +45°16' C) Sirius: RA = 6hr 45m, dec = -16°42' D) Alpha Centauri: RA = 14hr 40m, dec = -60°50' E) Vega: RA = 18hr 37m, dec = +38°47' Answer: D

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12) Suppose you live at latitude 40°N. Based on the given right ascensions and declinations, which of the following stars remains above your horizon for the longest time each day?

A) Spica: RA = 13hr 25m, dec = -11°09' B) Deneb: RA = 20hr 41m, dec = +45°16' C) Sirius: RA = 6hr 45m, dec = -16°42' D) Alpha Centauri: RA = 14hr 40m, dec = -60°50' E) Vega: RA = 18hr 37m, dec = +38°47' Answer: B 13) Suppose you are looking at the sky at about midnight on the December solstice. Based on the given right ascensions and declinations, which of the following stars will be nearest to your meridian? Assume you live at a latitude where all the stars on this list rise above your horizon each day. (Hint: Recall that the Sun's celestial coordinates on the December solstice are RA = 18h, dec = -23-1/2°.)

A) Spica: RA = 13hr 25m, dec = -11°09' B) Deneb: RA = 20hr 41m, dec = +45°16' C) Sirius: RA = 6hr 45m, dec = -16°42' D) Alpha Centauri: RA = 14hr 40m, dec = -60°50' E) Vega: RA = 18hr 37m, dec = +38°47' Answer: C 14) Suppose you live at latitude 40°N? Based on the given right ascensions and declinations, which of the following stars passes closest to your zenith on its daily path through your sky?

A) Spica: RA = 13hr 25m, dec = -11°09' B) Deneb: RA = 20hr 41m, dec = +45°16' C) Sirius: RA = 6hr 45m, dec = -16°42' D) Alpha Centauri: RA = 14hr 40m, dec = -60°50' E) Vega: RA = 18hr 37m, dec = +38°47' Answer: E 15) Suppose that both the Sun and a distant star cross your meridian at exactly 12:00 noon today.

At about what time will the star cross your meridian tomorrow?

A) 11:56 a.m.

B) 12:00 noon C) 12:10 p.m.

D) 9:00 a.m.

E) 3:00 p.m.

Answer: A

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16) The constellation Orion crosses your meridian at midnight in mid-December. At about what time does Orion cross your meridian in mid-January? A) 10 p.m. B) midnight C) 11:56 p.m. D) 12:04 a.m. E) 1:00 a.m. Answer: A 17) All of the following are true. Which of the following gives evidence that Earth's orbit is not perfectly circular? A) The Sun's angular size changes throughout the year. B) The Sun's daily path through the local sky varies over the course of the year. C) In North America, it is hotter in July than January. D) The Earth's axis precesses gradually with a period of about 26,000 years. Answer: A Reading Quiz Questions 1) Which of the following statements about sidereal and solar days is not true?

A) A sidereal day is the definition of a day on the Earth, and a solar day is the definition of a day on the Sun. B) A solar day is approximately 4 minutes longer than a sidereal day. C) A solar day is the amount of time between successive appearances of the Sun on our meridian. D) A sidereal day is the amount of time between successive appearances of any star on our meridian. Answer: A 2) What is the difference between a synodic month and a sidereal month? A) A synodic month is the time it takes for a cycle of lunar phases, and a sidereal month is the time it takes the Moon to orbit Earth (relative to the stars). B) A synodic month is our 30- or 31-day calendar month and a sidereal month is the time it takes for a cycle of the Moon's phases. C) A synodic month is the time it takes for a cycle of the Moon's phases and a sidereal month is our 30- or 31-day calendar month. D) A synodic month is based on the orbit of some planet around the Sun and a sidereal month is based on the orbit of the Moon around the Earth. Answer: A 3) When we say that Jupiter is at opposition, we mean that it A) appears on the opposite side of the sky from the Sun, so that it is on our meridian at midnight. B) is behind the Sun as seen from Earth, so that it appears close to the Sun in our sky. C) is at its greatest western elongation. D) is at the point in its orbit where it is farthest from the Sun. Answer: A

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4) What kind of time can be read directly from a sundial?

A) apparent solar time B) mean solar time C) standard time D) universal time Answer: A 5) Universal time (UT) is A) the mean solar time in Greenwich, England.

B) another name for standard time.

C) the apparent solar time in Greenwich, England.

D) a timekeeping system based on careful observations of positions of stars throughout the universe.

Answer: A 6) Our modern calendar is also known as A) the Gregorian calendar.

B) the Julian calendar.

C) the Metonic calendar.

D) the Universal Calendar (UC).

Answer: A 7) Which of the following best describes the meaning of declination and right ascension?

A) They are coordinates on the celestial sphere, with declination telling us the angular distance of an object from the celestial equator and right ascension telling us how far around the celestial sphere an object is located from the March equinox.

B) They are coordinates on the celestial sphere that are similar to the coordinates of altitude and direction on Earth.

C) They are used to describe the locations and distances of stars that we see in the night sky.

D) They are terms used by astronomers to describe how the sky changes with the seasons.

Answer: A 8) The Sun's declination on June 21st is A) +23.5 degrees.

B) 0 degrees.

C) -23.5 degrees.

D) different depending on where you are on Earth.

Answer: A

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9) Which of the following best describes the Tropic of Cancer?

A) It is the circle of latitude on Earth for which the Sun appears directly overhead at noon on the June solstice.

B) It is the circle of latitude on Earth for which the Sun appears to remain above the horizon for a full 24 hours on the June solstice.

C) It is the circle of latitude on Earth for which the Sun appears directly overhead at noon on the March and September equinoxes.

D) It is the line of right ascension passing through the constellation of Cancer.

Answer: A 10) Suppose you live at latitude 40ºN. Which of the following describes the path of the celestial equator through your sky?

A) It extends from due east on your horizon, to an altitude of 50º in the south, to due west on your horizon.

B) It extends from due south on your horizon, to your zenith, to due north on your horizon.

C) It extends from due east on your horizon, to an altitude of 40º in the south, to due west on your horizon.

D) It extends from due east on your horizon, to an altitude of 40º in the north, to due west on your horizon.

Answer: A 11) Suppose it is January and the Sun remains above your horizon all day long. Where must you be located?

A) somewhere between the Antarctic Circle and the South Pole B) at the South Pole C) somewhere between the Arctic Circle and the North Pole D) at the North Pole Answer: A 12) From a latitude of 60ºN during the winter, the Sun will A) spend most of each day below the horizon.

B) spend most of each day above the horizon.

C) never be seen above the horizon.

D) spend a few weeks continuously below the horizon.

Answer: A 13) If you were standing on Earth's equator, where would you look to see the north celestial pole?

A) on your horizon due north B) directly overhead C) toward the ecliptic D) due east Answer: A

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14) Suppose you know the declination of a bright star that is now crossing your meridian.

What else must you measure to determine your latitude? A) the star's altitude B) the apparent solar time C) the current universal time D) the star's right ascension Answer: A 15) What is the global positioning system (GPS)? A) a system for determining positions on Earth using signals from a set of satellites in Earth orbit B) a system for determining positions on the Earth by careful observations of the Sun and stars C) a system of determining positions on the Earth by receiving radio signals from many ground- based beacons D) a term used to refer to any type of celestial navigation Answer: A Concept Quiz Questions 1) Which of the following best explains why a solar day is longer than a sidereal day? A) Because Earth orbits the Sun at the same time it rotates, Earth must make slightly more than one full rotation between noon one day and noon the next. B) The precession of Earth's axis gradually changes the length of the solar day while leaving the length of the sidereal day alone. C) The fact that Earth's orbit is not a perfect circle causes the length of the solar day to vary during the year, so that it the solar day is sometimes longer and sometimes shorter than a sidereal day. D) The Sun rotates much as the Earth does, but more slowly, so that a solar day is longer than a sidereal day. Answer: A 2) When should we next expect to observe a transit of the Sun by Mars? A) Never B) 2035 C) 2030 D) 2148 Answer: A 3) A friend says she saw a planet shining on her meridian at midnight. Which planet can you be sure that she did not see? A) Venus B) Mars C) Jupiter D) Saturn Answer: A

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4) Suppose you have a time-keeping device that always reads precisely 12:00 when the Sun crosses your meridian. What kind of time is this clock keeping?

A) apparent solar time B) mean solar time C) standard time D) universal time Answer: A 5) All the following statements are true. Which one explains why mean solar time differs from apparent solar time?

A) The length of a solar day is not always exactly 24 hours.

B) Earth's rotation period is actually about 23 hours and 56 minutes, not 24 hours.

C) Earth's axis precesses with a period of about 26,000 years.

D) The Sun reaches the meridian at different times at different longitudes within the same time zone.

E) The path of the Sun through the sky depends on both latitude and date.

Answer: A 6) As viewed from any place within the continental United States, noon on apparent solar time is defined as the moment when the Sun is A) directly south.

B) directly overhead.

C) ante meridiem.

D) at a right ascension of 0 hours.

Answer: A 7) All the following statements are true. Which one explains why our calendar has leap years?

A) A tropical year is slightly more than 365 days.

B) Earth's axis precesses with a period of about 26,000 years.

C) There is a difference between a sidereal year and a tropical year.

D) Earth's speed varies as it orbits the Sun. Answer: A 8) Consider the following data for four stars: Star Spica Deneb Sirius Alpha Centauri

RA 13hr24m.1 20hr39m.7 6hr44m.2 14hr38m.4

Dec. -11°20' +45°06' -16°42' -60°46'

Which of these stars lies closest to the north celestial pole on the celestial sphere?

A) Deneb B) Alpha Centauri C) Spica D) Sirius Answer: A 14 Copyright © 2020 Pearson Education, Inc.

9) Consider the following data for four stars: Star RA Dec. Spica 13hr24m.1 -11°20' Deneb 20hr39m.7 +45°06' Sirius 6hr44m.2 -16°42' Alpha Centauri 14hr38m.4 -60°46' Suppose you live at latitude 30°S. Which star is circumpolar in your sky?

A) Alpha Centauri B) Deneb C) Spica D) Sirius Answer: A 10) Consider the following data for four stars: Star Spica Deneb Sirius Alpha Centauri

RA 13hr24m.1 20hr39m.7 6hr44m.2 14hr38m.4

Dec. -11°20' +45°06' -16°42' -60°46'

Suppose that you are looking at the sky at about midnight on the December solstice. Which star will be nearest to your meridian?

A) Sirius B) Alpha Centauri C) Spica D) Deneb Answer: A 11) Which of the following statements is not true about the sky at Earth's equator?

A) The Sun will pass directly overhead around noon each day.

B) Over the course of the year, all 88 constellations will at some point be visible in the evening sky.

C) There are no circumpolar stars in the equatorial sky.

D) The north celestial pole is directly on your horizon, due north (with Polaris quite nearby).

Answer: A

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12) Suppose Earth's axis had a greater tilt (more than 23.5 degrees). Which of the following statements would not be true?

A) The celestial sphere would be covered by a different set of constellations than it is with the current axis tilt.

B) Seasonal temperature variations would be more extreme than they are with the current tilt.

C) We would have more daylight in summer and less daylight in winter than we do with the current tilt.

D) The Arctic and Antarctic circles would be located closer to the equator than they are now.

Answer: A 13) Which of the following explains why navigators prior to a few hundred years ago found it much more difficult to determine longitude than latitude?

A) Determining longitude requires an accurate clock, but latitude does not.

B) Determining longitude requires mathematical techniques that were not known at the time.

C) Determining longitude requires much more precise measurements of angles in the sky than does latitude.

D) Determining longitude requires accurate tables of the celestial coordinates of stars, while determining latitude does not.

Answer: A 14) It's the June solstice, the Sun is at your zenith, and you have a UT clock that tells you it is midnight in Greenwich, England. Where are you?

A) on the Tropic of Cancer near the International Date Line B) on the equator near the International Date Line C) on the Tropic of Cancer, due south of Greenwich, England D) at latitude 25°S and longitude 49°E Answer: A 15) The north celestial pole appears 30 degrees above your horizon. The star Vega is on your meridian. By studying your star charts and your clocks, you determine that Vega crossed the meridian in Greenwich (England) 3 hours ago. Where are you?

A) latitude 30°, longitude 45°W B) latitude 30°N, longitude 3°W C) latitude 60°N, longitude 45°W D) It is not possible to determine your longitude from the information given.

Answer: A

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Visual Quiz Questions 1) Which numbered position in the figure shows a planet at ? opposition

A) 1 B) 2 C) 3 D) 4 Answer: D 2) Knowing that the dot near the upper center of the Sun in this photo is the planet Venus, you can conclude that the photo was taken

A) during a transit.

B) when Venus is at greatest western elongation.

C) when Venus is at greatest eastern elongation.

D) when Venus is at superior conjunction.

Answer: A

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3) On this diagram of the celestial sphere, the numbered point(s) that have declination 0º are A) 1 and 3.

B) 1 only.

C) 2 only.

D) 2 and 4.

Answer: A 4) Among the points and circles shown on this diagram, which one(s) have varying right ascension?

A) the celestial equator and the ecliptic B) Earth's equator and the celestial equator C) only the celestial equator D) the north and south celestial poles Answer: A

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5) On this map of Earth, which numbered circle represents the place where the Sun appears directly overhead at noon on the December solstice?

A) 3 B) 1 C) 2 D) 4 E) None of the answers are correct; the Sun can be directly overhead only on the equator.

Answer: A

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6) Suppose you go outside and see the view shown when you look due south. Knowing that the stars of Orion's Belt lie close to the celestial equator, what can you conclude?

A) You are in the Northern Hemisphere.

B) You are in the Southern Hemisphere.

C) It is winter.

D) In a couple of hours, Lepus will be higher in the sky than Orion.

Answer: A

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7) The celestial coordinates of the red dot on this diagram of the celestial sphere are approximately

A) Dec = +60º, RA = 6 hr.

B) Dec = -60º, RA = 6 hr.

C) Dec = −60º, RA = 12 hr.

D) Dec = +60º, RA = 12 hr.

Answer: A 8) This sequence of photos shows the Sun at one-hour intervals throughout the day of the June solstice; directions and times are indicated. Where was this photo taken?

A) the Arctic Circle B) the Antarctic Circle C) the North Pole D) the equator Answer: A

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9) This ancient navigational instrument is called a(n)

A) astrolabe.

B) celestial sphere.

C) cross-staff.

D) analemma.

Answer: A 10) The figure shows the equation of time; notice that the vertical axis shows "apparent solar time minus means solar time." Suppose the date is about Feb. 15 and you have a digital watch that is set to the local mean solar time. If the watch say sit is precisely noon, then

A) the Sun will reach your meridian in about 15 minutes.

B) the Sun is now crossing your meridian.

C) the Sun crossed your meridian about 15 minutes ago.

D) the Sun is now at its highest point in your sky.

Answer: A

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End-of-Chapter Questions Visual Skills Check Use the following questions to check your understanding of some of the many types of visual information used in astronomy.

The six diagrams represent the sky at six different latitudes. Answer the following questions about them.

1) Which diagram represents the paths of stars at the North Pole?

Answer: a 2) Which diagram represents the paths of stars at the South Pole?

Answer: b 3) Which diagrams represent Southern Hemisphere skies?

Answer: b, c, f 4) What latitude is represented in diagram c?

Answer: 66.5°S (the Antarctic Circle)

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5) Which diagram(s) represent(s) a latitude at which the Sun sometimes passes directly overhead?

Answer: d 6) Which diagram(s) represent(s) a latitude at which the Sun sometimes remains below the horizon during a full 24-hour period?

Answer: a, b, c 7) Each diagram shows five star circles. Look at the first circle to the north of the celestial equator on each diagram. Can you characterize the declination of stars on this circle? If so, what is it? Can you characterize the right ascension of stars on this circle? If so, what is it?

Answer: Declination is +30°; right ascension cannot be characterized without further information.

Chapter Review Questions 1) Explain the differences between a (a) sidereal day and solar day, (b) sidereal month and synodic month, (c) sidereal year and tropical year, (d) planet's sidereal period and synodic period.

2) Define opposition, conjunction, and greatest elongation for planets both closer to and farther from the Sun than is Earth.

3) For what planets do we sometimes observe a transit? Why? 4) Distinguish among apparent solar time, mean solar time, standard time, daylight saving time, and universal time.

5) Describe the origins of the Julian and Gregorian calendars. Which one do we use today?

6) What do we mean when we say the equinoxes and solstices are points on the celestial sphere?

How are these points related to the times of year called the equinoxes and solstices?

7) What are declination and right ascension? How are they similar to latitude and longitude?

How are they different?

8) How and why do the Sun's celestial coordinates change over the course of each year?

9) Suppose you are at the North Pole. Where is the celestial equator? Where is the north celestial pole? Describe the daily motion of the sky. Do the same for the equator and for latitude 40°N. 10) Describe the Sun's path through the local sky on the equinoxes and on the solstices for latitude 40°N. Do the same for the North Pole, South Pole, and equator.

11) What is special about the tropics of Cancer and Capricorn? Describe the Sun's path on the solstices at these latitudes. Do the same for the Arctic and Antarctic Circles.

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12) Briefly describe how you can use the Sun or stars to determine your latitude and longitude.

Does It Make Sense?

Decide whether or not each of the following statements makes sense (or is clearly true or false).

Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer. (Hint: For statements that use coordinates such as altitude, longitude, or declination, be sure to check whether the coordinates are used correctly; for example, it would not make sense to describe a location on Earth's surface by an altitude.)

Example: I walked east from our base camp at the North Pole.

Solution: The statement does not make sense because east has no meaning at the North Pole–all directions are south from the North Pole.

13) Last night I saw Venus shining brightly on the meridian at midnight.

14) The apparent solar time was noon, but the Sun was just setting.

15) My mean solar clock said it was 2:00 p.m., but a friend who lives east of here had a mean solar clock that said it was 2:11 P.M. 16) When the standard time is 3:00 p.m. in Baltimore, it is 3:15 p.m. in Washington, D.C. 17) Last night around 8:00 p.m., I saw Jupiter at an altitude of 45° in the south.

18) The latitude of the stars in Orion's belt is about 5°N. 19) Today the Sun is at an altitude of 10° on the celestial sphere.

20) Los Angeles is west of New York by about 3 hours of right ascension.

21) The June solstice is east of the March equinox by 6 hours of right ascension.

22) My UT clock had stopped, but I found my longitude by measuring the altitudes of 14 stars in my local sky.

Quick Quiz

23) The time from one March equinox to the next is the A) sidereal day.

B) tropical year.

C) synodic month.

Answer: No Correct Answer Was Provided.

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24) Jupiter is brightest when it is A) at opposition.

B) at conjunction.

C) closest to the Sun in its orbit.

Answer: No Correct Answer Was Provided.

25) Venus is easiest to see when it is at A) superior conjunction.

B) inferior conjunction.

C) greatest eastern elongation.

Answer: No Correct Answer Was Provided.

26) In the winter, your wristwatch tells A) apparent solar time.

B) standard time.

C) universal time.

Answer: No Correct Answer Was Provided.

27) A star located 30° north of the celestial equator has A) declination = 30°.

B) right ascension = 30°.

C) latitude = 30°.

Answer: No Correct Answer Was Provided.

28) A star's path through your sky depends on your latitude and the star's A) declination.

B) right ascension.

C) both declination and right ascension.

Answer: No Correct Answer Was Provided.

29) At latitude 50°N, the celestial equator crosses the meridian at altitude A) 50° in the south.

B) 50° in the north.

C) 40° in the south.

Answer: No Correct Answer Was Provided.

30) At the North Pole on the June solstice, the Sun A) remains stationary in the sky.

B) reaches the zenith at noon.

C) circles the horizon at altitude °.

Answer: No Correct Answer Was Provided.

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31) If you know a star's declination, you can determine your latitude if you also A) measure its altitude when it crosses the meridian.

B) measure its right ascension.

C) know the universal time.

Answer: No Correct Answer Was Provided.

32) If you measure the Sun's position in your local sky, you can determine your longitude if you also A) measure its altitude when it crosses the meridian.

B) know its right ascension and declination.

C) know the universal time.

Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

33) Pair Discussion: Benjamin Banneker.

The African American astronomer Benjamin Banneker was also a farmer, a surveyor, and an author who corresponded with Thomas Jefferson on the subject of slavery. a) Working independently, spend a little time learning about who Benjamin Banneker was, when he lived, and his area of expertise in astronomy. Also, learn what Thomas Jefferson had to say about him. b) Pair up with another student and share what you learned. How did Banneker's area of expertise in astronomy relate to the subject of this chapter and his work as a surveyor? c) Had either of you heard of Banneker before? Do you think he should be better known? Do you think there are reasons why is not more famous? Explain. d) Discuss what Thomas Jefferson had to say about Banneker. Do you think Jefferson's impressions of Banneker made any difference in his attitude toward slavery?

The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

34) Transits and the Geocentric Universe.

Ancient people could not observe transits of Mercury or Venus across the Sun, because they lacked instruments for viewing a small dark spot against the Sun. But suppose they could have seen transits. How would transit observations have affected the debate over an Earth-centered versus a Sun-centered solar system? Explain.

35) Geometry and Science. As discussed in Mathematical Insight S1.1, Copernicus found that a Sun-centered model led him to a simple geometric layout for the solar system, a fact that gave him confidence that his model was on the right track. Did the mathematics actually prove that the Sun-centered model was correct? Use your answer to briefly discuss the role of mathematics in science.

36) Northern Chauvinism. Why is the writing on maps and globes usually oriented so that the Northern Hemisphere is at the top, even though there is no up or down in space? How does this relate to the fact that the June solstice has traditionally been called the summer solstice? Discuss.

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37) Group Activity: Find Your Way Home. You and your partners are international spies who have been captured by a criminal gang and flown to a secret compound. You escape . . . but all you have is a watch (set to your previous local time), a star chart, and a world map (marked with longitude and latitude). How do you figure out where you are? How could you use celestial navigation to find your way home?Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39.

Investigate Further 38) Opposite Rotation. Suppose Earth rotated in a direction opposite to its orbital direction; that is, suppose it rotated clockwise (as seen from above the North Pole) but orbited counterclockwise. Would the solar day still be longer than the sidereal day? Explain. 39) No Precession. Suppose Earth's axis did not precess. Would the sidereal year still be different from the tropical year? Explain. 40) The Sun from Mars. Mars has an axis tilt of 25.2°, only slightly larger than that of Earth. Compared to that on Earth, is the range of latitudes on Mars for which the Sun can reach the zenith larger or smaller? Is the range of latitudes for which the Sun is circumpolar larger or smaller? Make a sketch of Mars similar to the one for Earth in Figure S1.18. 41) Fundamentals of Your Local Sky. Answer each of the following for your latitude. a) Where is the north (or south) celestial pole in your sky? b) Describe the meridian in your sky, specifying at least three distinct points along it (such as the points at which it meets your horizon and its highest point). c) Describe the celestial equator in your sky, specifying at least three distinct points along it. d) Does the Sun ever appear at your zenith? If so, when? If not, why not? e) What range of declinations makes a star circumpolar in your sky? f) What is the range of declinations for stars that you can never see in your sky? 42) Sydney Sky. Repeat Problem 41 for the local sky in Sydney, Australia (latitude 34°S). 43) Local Path of the Sun. Describe the path of the Sun through your local sky for each of the following days: a) the March and September equinoxes. b) the June solstice. c) the December solstice. d) today. (Hint: You can estimate the Sun's RA and dec for today's date from data in Table S1.1.) 44) Sydney Sun. Repeat Problem 43 for the local sky in Sydney, Australia (latitude 34°S). 45) Calendar History. Investigate the history of the Julian or Gregorian calendar in greater detail. Write a short summary of at least one interesting aspect of the history you learn from your research. (For example, why did Julius Caesar allow one year to have 445 days? How did our months end up with 28, 30, or 31 days?)

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46) Global Positioning System. Learn more about the global positioning system and its uses.

Write a short report summarizing how new uses of GPS may affect our lives over the next 10 years.

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

47) Lost at Sea I.During a vacation, you decide to take a solo boat trip. While contemplating the universe, you lose track of your location. Fortunately, you have some astronomical tables and instruments, as well as a UT clock. You thereby put together the following description of your situation: • It is the March equinox.

• The Sun is on your meridian at altitude 75° in the south.

• The UT clock reads 22:00. a) What is your latitude? How do you know? b) What is your longitude? How do you know? c) Consult a map. Based on your position, where is the nearest land? Which way should you sail to reach it?

48) Lost at Sea II. Repeat Problem 47 for this situation: • It is the day of the June solstice.

• The Sun is on your meridian at altitude ° in the north.

• The UT clock reads 06:00.

49) Lost at Sea III. Repeat Problem 47 for this situation: • Your local time is midnight.

• Polaris appears at altitude 67° in the north.

• The UT clock reads 01:00.

50) Lost at Sea IV. Repeat Problem 47 for this situation: • Your local time is 6 a.m.

• From the position of the Southern Cross, you estimate that the south celestial pole is at altitude 33° in the south.

• The UT clock reads 11:00.

51) Orbital and Synodic Periods. Use each object's given synodic period to find its actual orbital period.

• Saturn, synodic period = 378.1 days • Mercury, synodic period = 115.9 days • An asteroid with synodic period = 429 days 52) Using the Analemma. a) It's February 15 and your sundial tells you the apparent solar time is 18 minutes until noon.

What is the mean solar time? b) It's July 1 and your sundial tells you that the apparent solar time is 3:30 p.m. What is the mean solar time?

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53) HA = LST − RA. a) It is 4 p.m. on the March equinox. What is the local sidereal time? b) The local sidereal time is 19:30. When will Vega cross your meridian? c) You observe a star that has an hour angle of 13 hours (13h) when the local sidereal time is 8:15. What is the star's right ascension? d) The Orion Nebula has declination of about −5.5° and right ascension of 5h25m. If you are at latitude 40°N and the local sidereal time is 7:00, approximately where does the Orion Nebula appear in your sky?

54) Meridian Crossings of the Moon and Phobos. Estimate the time between meridian crossings of the Moon for a person standing on Earth. Repeat your calculation for meridian crossings of the Martian moon Phobos for a person on Mars. Use the Appendixes in the back of the book if necessary.

55)itsMercury's of orbital period, Rotation Period. Mercury's sidereal day is approximately or about 58.6 days. Estimate the length of Mercury's solar day. Compare it to Mercury's orbital period of about 88 days.

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter S2 Space and Time Section S2.1 1) A person watching a train going by says the train is moving 50 km/hr. A person in the train claims the train is not moving. Who is correct?

A) Both are correct in some sense, but should be more specific when describing the motion.

B) The person watching the train going by.

C) The person in the train.

D) Both are wrong.

Answer: A 2) Which of the following is an assumption rather than a prediction of Einstein's special theory of relativity?

A) the constancy of the speed of light B) No material object can reach or exceed the speed of light.

C) time dilation D) the equivalence of mass and energy E) length contraction Answer: A 3) According to the authors of the text, which of the following best describes how our common sense relates to the theory of relativity?

A) Relativity does not violate our common sense because we do not have common sense about motion near the speed of light.

B) Relativity violates common sense because it contradicts logic.

C) Relativity violates common sense because it is counterintuitive.

D) Relativity violates common sense because the theory contradicts itself in many cases.

E) Relativity violates common sense because it is full of paradoxes.

Answer: A 4) Which two things are the same for all observers according to the theory of relativity?

A) (1) the laws of nature and (2) the speed of light B) (1) the laws of nature and (2) the rate at which time proceeds C) (1) the speed of light and (2) the simultaneity of events D) (1) the rate at which time proceeds and (2) the simultaneity of events E) (1) the size of moving objects and (2) the mass of moving objects Answer: A

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Section S2.2 1) Bob is coming toward you at a speed of 75 km/hr. You throw a baseball in his direction at 75 km/hr. What does he see the ball doing? A) He sees the ball coming at him at 75 km/hr. B) He sees the ball going away from him at 75 km/hr. C) He sees the ball coming at him at 150 km/hr. D) He sees the ball going away from him at 150 km/hr. E) He sees the ball remaining stationary. Answer: C 2) Shawn is traveling away from you at a speed of 120 km/hr. He throws a baseball that, according to him, is going at 100 km/hr in your direction. What do you see the ball doing? A) You see the ball traveling away from you at 120 km/hr. B) You see the ball traveling away from you at 100 km/hr. C) You see the ball traveling away from you at 220 km/hr. D) You see the ball traveling toward you at 20 km/hr. E) You see the ball traveling away from you at 20 km/hr. Answer: E 3) Sue is traveling toward you at 90 km/hr. She throws a baseball that, according to her, is going at 90 km/hr in the opposite direction. What do you see the ball doing? A) You see the ball traveling away from you at 90 km/hr. B) You see the ball remaining stationary. C) You see the ball traveling away from you at 180 km/hr. D) You see the ball traveling toward you at 90 km/hr. E) You see the ball traveling toward you at 180 km/hr. Answer: B 4) Carol is going away from you at 75 km/hr, and Sam is going in the opposite direction away from you at 90 km/hr. According to Carol, how fast is Sam going? A) Carol sees Sam going away from her at 75 km/hr. B) Carol sees Sam going away from her at 90 km/hr. C) Carol sees Sam going away from her at 15 km/hr. D) Carol sees Sam going away from her at 165 km/hr. E) Carol sees Sam coming toward her at 15 km/hr. Answer: D

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Section S2.3 1) A spaceship is moving past us at a speed close to the speed of light. What would we conclude about time (clocks) on the spaceship as it goes by?

A) Time is the same for everyone.

B) We would conclude that their clocks are going at the same rate as ours.

C) We would conclude that their clocks are going exactly half as fast as ours.

D) We would conclude that their clocks are going faster than ours.

E) We would conclude that their clocks are going slower than ours.

Answer: E 2) A spaceship is moving past us at a speed close to the speed of light. What would passengers on the spaceship conclude about our clocks?

A) Time is the same for everyone.

B) They would conclude that our clocks going faster than theirs.

C) They would conclude that our clocks going slower than theirs.

D) They would conclude that our clocks going at the same rate as theirs.

E) They can't see our clocks, but we can see theirs.

Answer: C 3) A spaceship is moving past us at a speed close to the speed of light. If we could measure the mass of the spaceship as it goes by, what would it be?

A) Its mass is infinite.

B) Its mass is greater than its rest mass.

C) Its mass is the same as its rest mass.

D) Its mass is less than its rest mass.

Answer: B 4) Al is moving by you at a very high speed (close to the speed of light). You get out a clock and measure 10 seconds going by. If you also watch a clock in Al's ship, how much time will you see it record during your 10 seconds?

A) You will see Al's clock record less than 10 seconds.

B) You will see Al's clock record more than 10 seconds.

C) You will see Al's clock agree that 10 seconds go by.

Answer: A 5) Carol is stationary midway between two light bulbs and observes that they turn on at the exact same time. You are moving near the speed of light toward the light bulb to Carol's left. What do you observe?

A) You observe the light bulb to the left turns on first.

B) You observe the light bulb to the right turns on first.

C) You observe the light bulbs turn on at the same time, like Carol.

D) The order of the events depends on how fast you are moving.

Answer: A

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6) Carol sees a light bulb turn red, then turn blue. If you are moving near the speed of light past Carol, what do you observe?

A) You also see the light bulb turn red, then blue.

B) You observe the light bulb turn blue first, then red.

C) You observe the light bulb turn red and blue at the same time.

D) What you observe depends on how fast you are moving.

Answer: A 7) You make a roundtrip to a star located 1000 light-years away (so the roundtrip distance is 2000 light-years in Earth's reference frame) at an average speed of 0.99c. Based on the time dilation formula below, about how long will the trip take, according to you?

t' = t A) 2.8 years B) 10 years C) 28 years D) 280 years E) 2000 years Answer: D 8) You take a roundtrip to a star located 1000 light-years away (so the roundtrip distance is 2000 lightyears in Earth's reference frame) at an average speed of 0.99c. Based on the time dilation formula below, about how many years will pass on Earth while you are gone?

t' = t A) 2.8 years B) 10 years C) 28 years D) 2000 years E) 280 years Answer: D

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9) The picture below shows a sphere as it looks at rest. Suppose the sphere is moving to the right at about 90 percent of the speed of light.

Which figure below correctly shows the shape of the sphere as you would measure it?

A) Figure A B) Figure B C) Figure C D) Figure D E) Figure E Answer: B 10) If you see Alice going to your left at exactly 0.99c and Bob going to your right (away from Alice) at exactly 0.99c, what will Alice say Bob is doing?

A) going away from her at 1.98c B) going away from her at exactly 0.99c C) going away from her at exactly c D) going away from her at about 0.98c E) going away from her faster than 0.99c, but slower than c Answer: E 11) The predictions of special relativity can sound implausible at first. Which of the following provide observational evidence that the theory is correct?

A) Subatomic particles in accelerators exhibit effects of time dilation and mass increase at high speeds.

B) Light is always observed to travel at the same speed.

C) Time dilation can be observed in airplanes.

D) All of the above are correct.

Answer: D

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12) What did the Michelson—Morley experiment demonstrate?

A) The speed of light that we observe is not affected by Earth's motion around the Sun. B) The speed of light that we observe is different depending on the time of year.

C) The speed of light that we observe is not affected by the gravity of the Moon.

D) The speed of light that we observe is not affected by the medium in which it travels.

Answer: A 13) How fast have we made subatomic particles travel inside particle accelerator?

A) within 0.00001% of the speed of light B) exactly the speed of light C) faster than the speed of light D) no more than half the speed of light E) no more than one-tenth the speed of light Answer: A Section S2.4 1) According to measurements made here on Earth, the distance to Alpha Centauri is about 4 light-years. If a spaceship is traveling toward Alpha Centauri at 0.999c, How far away is Alpha Centauri according to the travelers in the ship?

A) also about 4 light-years B) very slightly more than 4 light-years C) very slightly less than 4 light-years D) quite a bit less than 4 light-years E) quite a bit more than 4 light-years Answer: D Short Answer Questions 1) Suppose a supersonic airplane flies at a speed of 1670 km/hr from Nairobi, Kenya to Quito, Ecuador; note that this is the same speed that Earth rotates, but in the opposite direction.

Describe how this flight would look to an observer on the Moon.

Answer: The airplane would essentially appear to lift straight off the ground and then remain stationary while Earth rotated. The rotation would carry Nairobi away from the airplane and Quito toward it. When Quito arrived, the airplane would drop back down to the ground, completing the trip.

2) Explain why relativity allows us to travel, in principle, to the center of the Milky Way, but we would never be able to tell our friends back home about it.

Answer: If we could build a very fast spacecraft that traveled within a tenth of a percent of the speed of light, then time dilation and length contraction allow us to travel the 28,000 light-years to the center of the Milky Way in what would seem like just a few years to us. However, back home on Earth, the trip would take about 28,000 years to go and another 28,000 years to come back: none of your friends would be around to listen to your tales of adventure when you returned. Note that even if you sent a radio message while you were at the center of the Galaxy, it would still take 28,000 years to arrive at Earth because the radio waves barely move faster than the spacecraft.

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Reading Quiz Questions 1) The primary difference between the special theory of relativity and the general theory of relativity is that the general theory also describes A) the nature of gravity. B) length contraction. C) time dilation. D) the constancy of the speed of light.

Answer: A 2) Which of the following statements best describes what is "relative" in the theory of relativity? A) The theory says that measurements of motion make sense only when we state what they are measured relative to. B) The theory says that everything is relative. C) The theory says that the speed of light is relative and depends on who is measuring it. D) The theory says that truth can never be established in any absolute sense.

Answer: A 3) In relativity, two people share the same reference frame only if they A) are not moving relative to one another. B) are both located in the same place. C) are both located in the same place and are stationary. D) agree on the laws of nature. Answer: A 4) You are in an airplane, traveling relative to the ground at 500 km/hr. You throw a baseball toward the front of the plane at a speed of 50 km/hr. How fast would someone on the ground say the baseball is moving? A) 550 km/hr B) 50 km/hr C) 500 km/hr D) 450 km/hr Answer: A 5) You and Mae are in free-float frames. Mae has just passed you, traveling at a speed of 60 km/hr. You throw a ball toward her at a speed of 60 km/hr. What will Mae see the ball doing? A) She'll see the ball stationary, floating freely in her own reference frame.

B) She'll see the ball coming toward her at 60 km/hr. C) She'll see the ball moving away from her at 60 km/hr. D) She'll see the ball coming toward her at 120 km/hr. Answer: A

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6) The measured value of the speed of light is about 300,000 km/s. Suppose a futuristic space train is traveling at 200,000 km/s with its headlights on. If you could measure the speed of the light from the headlights, you would find it to be A) 300,000 km/s.

B) 200,000 km/s.

C) 100,000 km/s.

D) 500,000 km/s.

Answer: A 7) What do we mean by time dilation in relativity?

A) It is the idea that time really runs slower in reference frames moving relative to you.

B) It is the idea that time seems to run slower in reference frames moving relative to you, but time really is the same for everyone.

C) It is the idea that relativity makes time dilate, or expand, for people who study it.

D) It is a hallucinogenic effect experienced by people who take too much of something.

Answer: A 8) You can see a clock in a spaceship moving past you at 90% of the speed of light. According to you, how much time would pass while the clock in the spaceship ticked through one minute?

A) more than one minute B) less than one minute C) A minute is the same on the ship as it is for you.

D) A minute on the ship is exactly two minutes for you.

Answer: A 9) What do we mean by length contraction in relativity?

A) It is the idea that if you measure the size of an object moving relative to you, you will find that in the direction of motion it is shorter than it would be at rest, while its size in other directions is unchanged.

B) It is the idea that if you measure the size of an object moving relative to you, you will find that it is shorter in every direction than it would be at rest. That is, length, width, and height are all shorter for an object when it is moving by you.

C) It is the idea that moving objects look smaller than nonmoving objects, but their sizes have not really changed.

D) It is the idea that if you travel fast, you'll notice yourself getting shorter.

Answer: A

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10) A man is moving by you at a speed close to the speed of light. Which of the following correctly summarizes the effects you would notice on his mass, size, and time? A) His mass would be increased from its value at rest, his length would be decreased from its rest value but only in the direction in which he is moving, and his time would be running slower than yours. B) His mass would be increased from its value at rest, his length, width, and height would all be decreased no matter what direction he is moving, and his time would be running slower than yours. C) His mass would be decreased from its value at rest, his length would be decreased from its rest value but only in the direction in which he is moving, and his time would be running slower than yours. D) His mass would be increased from its value at rest, his size would be increased, and his time would be running slower than yours. Answer: A 11) A super train is moving along a track at a speed close to the speed of light. You are watching the miletrain from the ground. You observe lightning to strike in two places along the track, a apart, at precisely the same time. What would someone on the train say?

A) two bolts of lightning struck at different times, and they struck at places that are lessThe than

a mile apart.

B) The two bolts of lightning struck at different times, and they struck at places that are more than a mile apart.

C) two bolts of lightning struck at the same time, and they struck at places that are lessThe than

a mile apart.

D) The two bolts of lightning struck at different times, and they struck at places that are precisely a mile apart.

Answer: A

12) Kim is in a spaceship moving past you at half the speed of light. Which of the following correctly describes how you will perceive each other's time?

A) You will say that Kim's time is running slow, and she will say that your time is running slow.

B) You will say that Kim's time is running slow, and she will say that your time is running fast.

C) You will say that Kim's time is running fast, and she will say that your time is running slow.

D) You will both agree that time runs at the same rate for both of you.

Answer: A 13) The lifetime of a π+ meson normally is 18 nanoseconds. In a large particle accelerator, scientists can create π+ mesons moving at speeds close to the speed of light. What do the scientists observe when they measure the lifetime of a π+ meson that is created at high speed?

A) The π+ meson lasts much more than 18 nanoseconds.

B) The π+ meson lasts 18 nanoseconds.

C) The π+ meson lasts much less than 18 nanoseconds.

9 D) The π+ meson no longer decays and remains as Education, a stable particle.

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14) Two twin sisters, Gwen and Jackie, are both 20 years old in the year 2050. Jackie takes off on a round-trip to Vega, 25 light-years away. She travels at an average speed close to the speed of light, say, 0.9999c. According to Gwen back on Earth, about how long does it take Jackie to reach Vega?

A) about 25 years B) about a month C) about 10 years D) about 100 years Answer: A 15) Two twin sisters, Gwen and Jackie, are both 20 years old in the year 2050. Jackie takes off on a round-trip to Vega, 25 light-years away. She travels at an average speed of 0.9999c. Which of the following best describes the situation according to Jackie?

A) Jackie stays still, while Earth rushes away from her at 0.9999c and Vega rushes toward her at 0.9999c. She sees the distance from Earth to Vega shortened considerably from 25 light-years, and therefore it takes far less than 25 years for Vega to reach her.

B) Jackie stays still, while Earth rushes away from her at 0.9999c and Vega rushes toward her at 0.9999c. Because Vega is 25 light-years away, she says it takes Vega about 25 years to reach her.

C) Jackie says that the 25 light-year trip takes only a few months, and therefore concludes that she is traveling faster than the speed of light.

D) It takes her 25 years to reach Vega.

Answer: A 16) Two twin sisters, Gwen and Jackie, are both 20 years old in the year 2050. Suppose that Jackie makes the round-trip to Vega, which is 25 light-years away, at an average speed of 0.9999c. Which of the following correctly describes the situation when Jackie returns to Earth?

(Ages in this problem refer to biological ages, that is, how much time has passed for each sister since she was born.)

A) It's the year 2100. Gwen (who stayed home) is 70 years old, but Jackie is only a little older than her age of 20 when she left.

B) It's the year 2100 and both twin sisters are 70 years old.

C) It's the year 2100. Gwen is 70 years old. Jackie is 19.

D) According to Gwen, Jackie returns to Earth in the year 2100, but according to Jackie, she returns in the year 2051.

Answer: A 17) Which of the following is not true of the special theory of relativity?

A) The theory is valid only at speeds close to the speed of light.

B) The special theory does not apply to situations that involve substantial acceleration or gravity; for that, you need the general theory of relativity.

C) For low speeds, the theory predicts effects that are so small that they cannot be noticed without extremely precise, high-tech measurement.

D) The theory tells us that there is no such thing as absolute time or space because measurements of time and space depend on your reference frame.

Answer: A

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Concept Quiz Questions 1) Each of the following lists two statements. Which two are the basic premises for the special theory of relativity?

A) (1) The laws of nature are the same for everyone.

(2) The speed of light is the same for everyone.

B) (1) The laws of nature are relative.

(2) The speed of light depends on your viewpoint.

C) (1) You can't go faster than the speed of light.

(2) Time is different for different people.

D) (1) Everything is relative.

(2) You can never really tell who is moving.

Answer: A 2) Which of the following is not a prediction made by the theory of relativity?

A) Observers in different reference frames may disagree about the basic laws of nature.

B) No material object sent outward from Earth can reach or exceed the speed of light.

C) E = mc2 D) Observers in different reference frames may disagree about whether two events in two different places occur simultaneously.

Answer: A 3) Why do the predicted consequences of the special theory of relativity seem so strange to most of us?

A) because they are obvious only at speeds that we never experience in our daily lives B) because they are self-contradictory, making it impossible to make any sense of them C) because they contradict the well-tested ideas of Newton's laws of motion D) because they affect only subatomic particles and not big things like people Answer: A 4) What is the primary reason why scientists accept the idea that the speed of light is the same for everyone?

A) because it has been verified by many observations and experiments B) because it makes sense when you think about it as Einstein did C) because no one has come up with a better idea about the speed of light D) because the theory is just so darn cool, it can't be wrong Answer: A

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5) You and Al are both floating freely in your spaceships. Suppose that Al is moving away from you at 85 km/hr. You throw a ball in his direction at a speed of 75 km/hr. According to Al, which of the following is going on? A) He sees you moving away from him at 85 km/hr and the ball moving away from him at 10 km/hr. B) He sees you moving toward him at 85 km/hr and the ball moving toward him at 75 km/hr. C) He sees you moving away from him at 85 km/hr and the ball moving toward him at 10 km/hr. D) He agrees that he is moving at 85 km/hr and the ball is moving at 75 km/hr while you are standing still. Answer: A 6) You are racing away from Earth in a super space ship in which you can continually increase your speed by firing engines that never quit. Which of the following best explains how people on Earth will perceive your speed? A) They will see your speed getting closer and closer to the speed of light but never reaching it. B) They will know you are going fast but will have no way to know if you ever exceed the speed of light. C) They will see you speed up until you reach the speed of light, and then your speed will remain constant after that. D) They will see you going faster and faster until you reach the speed of light, and after that they won't be able to see you any more. Answer: A 7) A spaceship is moving past you at a speed close to the speed of light. If you could somehow watch the people inside it over a long period of time, what would you conclude? A) They are aging more slowly than you. B) They are aging more rapidly than you. C) Their mechanical clocks go slower than yours, but the people are aging at the same rate as you. D) Their time is the same as your time. Answer: A 8) Suppose your normal resting heart rate is 60 beats per minute. Now, suppose you board a spaceship that travels away from Earth at 90% of the speed of light. While on the ship, you measure your heart rate. What will it be? A) It will still be 60 beats per minute. B) It will be 46 beats per minute. C) It will be 74 beats per minute. D) It will be 12 beats per minute.

Answer: A

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9) Bob is in a spaceship going by you at 90% of the speed of light, but in an upright standing position. Bob measures his mass to be 50 kg, his height to be 2.0 meters, and his waist size to be 80 cm. What would you say about his measurements?

A) His mass is more than 50 kg, his height is still 2.0 meters, and his waist size is less than 80 cm.

B) His mass is more than 50 kg, his height is less than 2.0 meters, and his waist size is less than 80 cm.

C) His mass is less than 50 kg, his height is more 2.0 meters, and his waist size is less than 80 cm.

D) You would measure the same mass and size as he does.

Answer: A 10) Suppose you measure the density (mass per volume) of an object moving by you at high speed. How will its density compare to the density it would have at rest in your reference frame?

A) Its density would be higher.

B) Its density would be lower.

C) Its density would be the same.

D) There is no way to know because relativity doesn't tell us anything about density.

Answer: A 11) Tom is going to your left at 90% of the speed of light and Joan is going to your right at 90% of the speed of light. What will Tom say about you and Joan?

A) You are moving away from him at 90% the speed of light. Joan is moving away from him at as speed that is faster than 90% of the speed of light but slower than the whole speed of light.

B) You are moving away from him at 90% the speed of light. Joan is moving away from him at 180% of the speed of light.

C) You are moving away from him at 90% the speed of light. Joan is moving away from him at as speed of light.

D) You and Joan are both moving away from him at 90% the speed of light.

Answer: A 12) How does the explosion of a nuclear bomb provide evidence for the theory of relativity?

A) The bomb converts rest-mass energy into thermal energy in accord with E = mc2, which is part of the theory of relativity.

B) The mass of the bomb when it explodes is much greater than its normal mass.

C) The bomb causes things in its vicinity to move at high speeds, allowing scientists to measure effects of time dilation.

D) The bomb shortens the lifetimes of all people who happen to be near it when it detonates.

Answer: A

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13) In what way do observations of binary star systems support the special theory of relativity?

A) The fact that the stars appear distinct supports the idea that the speed of light is unaffected by the motion of the stars.

B) Stars in binary systems often orbit at speeds close to the speed of light, allowing us to check whether time dilation really occurs.

C) Stars in binary systems are moving, so we can compare their moving masses to their rest masses.

D) Binary systems have two stars, and therefore give us twice the opportunity to test the theory than single star systems.

Answer: A 14) A sprinter named Ben has challenged a beam of light to a race in the 100-meter dash. The race is held in a stadium full of spectators. At the start signal, a laser beam is turned on at the start line, pointed down the track. At the same instant, Ben bursts out of the starting blocks at 99% of the speed of light (0.99c). According to the spectators watching in the stands, what happens? (Assume they could somehow watch a slow-motion replay to see what occurred.)

A) The light beam wins the race, but barely; it is going 1% of the speed of light faster than him.

B) The light beam instantly finishes the race, before Ben even has a chance to start.

C) The light beam wins the race by a large margin because it is going faster than him by the full speed of light.

D) Ben beats the light beam to the finish line, becoming a hero.

Answer: A 15) A sprinter named Ben has challenged a beam of light to a race in the 100-m dash. The race is held in a stadium full of spectators. At the start signal, a laser beam is turned on at the start line, pointed down the track. At the same instant, Ben bursts out of the starting blocks at 99% of the speed of light (0.99c). According to Ben, what happens?

A) The light beam moves out ahead of him at the full speed of light, winning the race easily.

B) The light beam gradually pulls ahead of him and wins the race because it is going 1% of the speed of light faster than him.

C) The light beam instantly finishes the race, before Ben even has a chance to start.

D) Ben wins the race and becomes a hero.

Answer: A 16) Which statement is true?

A) If we could build a spaceship capable of traveling at a speed near the speed of light, passengers on the ship could make a journey to a star that is 500 light-years away within their lifetimes.

B) If we could build a spaceship capable of traveling at a speed near the speed of light, we could send a ship to a star that is 500 light-years away and it would return just a few years after we launched it.

C) Thanks to effects of special relativity, we could build a spaceship that could go close to the speed of light with much less fuel than we would have guessed from Newton's laws of motion.

D) Because special relativity tells us that we cannot reach or exceed the speed of light, it means that distant stars will never be within our reach because the time required to reach them will always be quite long, even with fast spaceships.

Answer: A

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17) Use the appropriate relativity formula given in the text to answer the following. Isabella is moving by you at a speed of 0.8c. Although 1 minute passes on your watch, how much time will you see pass on her watch?

A) 36 seconds B) 1 minute C) 48 seconds D) 100 seconds Answer: A Visual Quiz Questions

1) This figure shows an airplane traveling east to west along Earth's equator at the same speed that Earth rotates from west to east. Viewed from the Moon, what does the airplane do? (Assume the trip is at constant speed, so you can neglect time required for the airplane to accelerate for takeoff or decelerate for landing.)

A) It remains stationary while Earth rotates beneath it.

B) It travels to the west at supersonic speed.

C) It travels at 2 × 1,670 = 3,340 km/hr.

D) You couldn't see the airplane from the Moon.

Answer: A 2) This figure shows an airplane traveling east to west along Earth's equator at the same speed that Earth rotates from west to east. People on Earth would, of course, simply see the airplane fly westward at 1,670 km/hr. What is the significance of the difference in viewpoints between people on Earth and people on the Moon?

A) It shows that motion is relative not absolute.

B) It shows that you can get a wrong answer if you don't view the situation from the correct perspective.

C) It shows that the airplane is traveling in a direction opposite to Earth's rotation.

D) It shows that airplanes can move quite fast if you view them from the right place.

Answer: A

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3) This diagram shows Earth with what, to scale, would have to be a truly gigantic person standing in Australia. For this person, which way is up?

A)

B)

C)

D) All of the interpretations of "up" are valid.

Answer: A

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4) As shown in the diagram, you've just thrown a baseball toward Al at 100 km/hr, while he is moving away from you to the right at 90 km/hr. According to Al, what is the baseball doing?

A) traveling to the right at 10 km/hr B) traveling to the right at 100 km/hr C) staying stationary D) traveling to the right at 190 km/hr Answer: A 5) As shown in the diagram, you've just shined a flash light toward Al, so from your point of view the light is moving to the right at speed c. Al is moving away from you to the right at speed 0.9c. According to Al, what is the light doing?

A) moving to the right at speed c B) gradually catching up to him at speed c0.1 C) moving to the right at speed 1.9c D) moving to the left at 0.1c Answer: A

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6) This diagram shows a person on a planet watching you travel to the right at 0.9c, while you have a flashlight turned on with the light shining to the right. Which of the following statements is not true from your point of view?

A) If you can increase your speed by another 0.1c, you'll be able to keep up with the flashlight beam.

B) The person and planet are moving to your left at 0.9c.

C) Time is running slow for the person on the planet.

D) The light from the flashlight beam is moving to the right at the speed of light c.

Answer: A 7) This diagram is the same as the one in the last question, showing a person on a planet watching you travel to the right at 0.9c, while you have a flashlight turned on with the light shining to the right. Which of the following statements is not true from the point of view of the person on the planet?

A) If you can accelerate enough, you'll soon be going faster than the flashlight beam.

B) The light beam's speed is only 0.1c faster than your speed.

C) Your mass is greater than it would be if you were not moving.

D) The light from the flashlight beam is moving to the right at the speed of light c.

Answer: A

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8) This graph shows how the time dilation factor (t′/t) depends on the speed of an object moving by you. How fast does an object have to be moving for you to see its time running about 20% slower than normal (about 80% of normal)?

A) 0.6c B) 0.2c C) 0.8c D) None of the choices are correct; time must be the same for everyone.

Answer: A

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9) This picture shows a sphere as it looks at rest. Suppose that the sphere is moving to the right at about 90% of the speed of light. Which figure correctly shows the shape of the sphere as you would measure it?

A)

B)

C)

D)

E)

Answer: A

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10) This diagram shows how light from a binary star system would travel to Earth if the speed of light were not constant. What is its observational significance?

A) It tells us that if the speed of light were not the same for everyone, then we would not necessarily see binary stars as distinct objects.

B) It shows that time runs slower for fast-moving objects.

C) It shows that motion is relative.

D) It shows that observers in different reference frames will not necessarily agree when two events are simultaneous.

Answer: A Chapter Review Questions 1) What is the theory of relativity? How does special relativity differ from general relativity?

2) List five major predictions of the special theory of relativity.

3) According to the theory of relativity, what are the two absolutes in the universe? Which one is more surprising, and why?

4) What is a paradox? How can a paradox lead us to a deeper understanding of an issue?

5) What do we mean by a frame of reference? What is a free-float frame?

6) Suppose you see a friend moving by you at some constant speed. Explain why your friend can equally well say that she is stationary and you are moving by her.

7) Construct your own variation on Thought Experiment 6 to prove that no material object can be observed to travel at or above the speed of light.

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8) What is time dilation? Explain how and why your measurements of time will differ from those of someone moving by you.

9) Explain why observers in different reference frames will not necessarily agree about the order of two events that occur in different places.

10) What is length contraction? How will your measurements of the size of a spaceship moving by you differ from your measurements of the same spaceship when it is at rest in your reference frame?

11) What is mass increase? How does the mass of an object moving by you compare to its rest mass?

12) Construct your own variation on Thought Experiment 11 to show why velocities must add differently than we would expect according to our "old common sense."

13) Briefly describe experimental tests of special relativity. 14) Why do tests of E = mc2 also test special relativity? Describe several tests of E = mc2 that you can see for yourself.

15) If you watch a friend moving by you at a speed close to that of light, you'll say that her time is running slowly, her length is contracted, and her mass is greater than her rest mass. How will

she perceive her own time, length, and mass? Why? How will she perceive your time, Example: I walked east from our base camp at the North Pole. length, Solution: The statement does not make sense because east has no meaning at the North Pole–all and mass? directions are south from the North Pole.

16) Supposeproved you could a trip to a distant star at a speed very close to the speed of 17) Einstein that take everything is relative.

18) An object moving by you at very high speed will appear to have a higher density than it has at rest. light.

How 19) You and a friend that you popped a peanut your mouth at precisely the same would relativity makeagree it possible foreach you to make this trip ininto a reasonably short time? What instant, but someone moving past you at high speed may observe that you ate your peanut first. would you find when you returned home?

Does It Make Sense?

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

22

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20) You and a friend agree that you each popped a peanut into your mouth at precisely the same instant, but someone moving past you at high speed may observe that you ate cashews rather than peanuts.

21) Relativity is "only a theory," and we have no way to know whether any of its predictions would really occur at speeds close to the speed of light.

22) The detonation of a nuclear bomb is a test of the special theory of relativity.

23) If you could travel away from Earth at a speed close to the speed of light, you would feel uncomfortably heavy because of your increased mass.

24) Future technology should allow us to build rockets capable of accelerating to speeds much faster than the speed of light.

25) If you see someone's time running slowly in a different reference frame, that person must see your time running fast.

26) If you had a sufficiently fast spaceship, you could leave today, make a round trip to a star 500 light-years away, and return home to Earth in the year 2100.

Quick Quiz 27) Which of the following is not relative in the special theory of relativity?

A) motion B) time C) the speed of light Answer: No Correct Answer Was Provided.

28) Which of the following must be true of a person who shares your reference frame?

A) The person must be sitting right next to you.

B) The person must be the same size as you.

C) The person must not be moving relative to you.

Answer: No Correct Answer Was Provided.

29) Which of the following best describes why your rocket could never reach the speed of light?

A) The absoluteness of the speed of light means you could never keep up with the light coming from you and your rocket.

B) Our technology has not advanced enough to make faster-than-light travel possible.

C) A rocket that could reach the speed of light would have to be bigger than the entire Earth.

Answer: No Correct Answer Was Provided.

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30) Carla is traveling past you at a speed close to the speed of light. According to you, how much time passes for Carla while 1 minute passes for you?

A) 1 minute B) less than 1 minute C) more than 1 minute Answer: No Correct Answer Was Provided.

31) Carla is traveling past you at a speed close to the speed of light. According to her, how much time passes for you while 1 minute passes for her?

A) 1 minute B) less than 1 minute C) more than 1 minute Answer: No Correct Answer Was Provided.

32) A subatomic particle that normally decays in 1 microsecond is created in a particle accelerator and is traveling at close to the speed of light. If you measure its lifetime in that case, you'll find that it is A) less than 1 microsecond.

B) more than 1 microsecond.

C) 1 microsecond.

Answer: No Correct Answer Was Provided.

33) What does the famous formula E = mc2 have to do with special relativity?

A) Nothing; it comes from a different theory.

B) It is one of the two starting assumptions of special relativity.

C) It is a direct consequence of the theory.

Answer: No Correct Answer Was Provided.

34) What provides the strongest evidence that everyone always measures the same speed of light?

A) precise measurements of the speed of light under different circumstances B) thought experiments showing that we would encounter paradoxes if it were not true C) the fact that Einstein said so and he was very smart Answer: No Correct Answer Was Provided.

35) If you observe people moving by you at very high speed, you will say that their time runs slowly, their lengths are contracted in the direction of motion, and their masses are increased from their rest masses. What will they say about you?

A) Your time runs slowly, your length is contracted in the direction of motion, and your mass is increased from your rest mass.

B) Your time runs fast, your length is expanded in the direction of motion, and your mass is decreased from your rest mass.

C) Your time runs fast, but your length and mass are unaffected.

Answer: No Correct Answer Was Provided.

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36) Suppose you had a spaceship so fast that you could make a round-trip journey of 1 million light-years (in Earth's reference frame) in just 50 years of ship time. If you left in the year 2030, you would return to Earth A) in the year 2080. B) in the year 2130. C) a million years from now. Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

37) Pair Discussion: Mileva Maric.We generally credit Einstein for coming up with the theory of relativity, but in fact Mileva Maric certainly played a role. a) Working independently, find out who Mileva Maric was, how she was related to Einstein, and what evidence indicates that she was a serious contributor to the ideas of relativity. b) Pair up with another student to discuss what each of you learned and the significance of Maric's work. Do you think she should be better known for her contributions to relativity? c) Do you think Mileva Maric and Albert Einstein had equal opportunities to contribute to progress in science? Why or why not? d) Does knowing the role of Mileva Maric change your view of Albert Einstein in any way?

Why or why not?

The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

38) A Paradigm Shift.Chapter 3 introduced the idea of a paradigm shift, in which general patterns of scientific thought undergo a major or surprising change. How did the development of relativity theory represent such a shift? Did it mean throwing out old ideas or only modifying them? Defend your opinion.

39) Thought Experiments. Science demands that hypotheses be tested by real experiments or observations, yet Einstein developed his theory of relativity largely through the use of thought experiments. How, then, did relativity still come to be accepted as a scientific theory? Explain.

40) Common Sense. Discuss the meaning of the term common sense. How do we develop common sense? Can you think of other examples, besides the example of the meanings of up and down, of situations in which you've had to change your common sense? Do you think that the theory of relativity contradicts common sense? Why or why not?

41) Group Discussion: Breaking the Light Barrier. People long said that no one would travel faster than the speed of sound, but that barrier was broken. Is it possible that we'll someday find a way to travel faster than the speed of light? Discuss in small groups and come to a group consensus for or against this possibility. Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39.

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Investigate Further 42) Stationary Bike. Suppose you are riding on a stationary bike and the speedometer says you are going 30 km/hr. What does this number mean? What does it tell you about the idea of relative motion?

43) Walking Trip. Suppose you go for a walk down the street. In your own reference frame, you are going quite slowly, probably just a few kilometers per hour. About how fast would you appear to be going according to an astronaut on the Moon? Explain.

44) Relative Motion Practice I. In all the following, assume that you and your friends are in freefloat reference frames. a) Bob is coming toward you at a speed of 75 km/hr. You throw a baseball in his direction at 75 km/hr. What does he see the ball doing? b) Marie is traveling away from you at a speed of 120 km/hr.

She throws a baseball at 100 km/hr (according to her) in your direction. What do you see the ball doing? c) José is traveling away from you at 99% of the speed of light when he turns on a flashlight and points it in your direction. How fast will the beam of light be going when it reaches you?

45) Relative Motion Practice II. In all the following, assume that you and your friends are in free-float reference frames. a) Carol is going away from you at 75 km/hr, and Sam is going away from you in the opposite direction at 90 km/hr. According to Carol, how fast is Sam going? b) Consider again the situation in part a. Suppose you throw a baseball in Sam's direction at a speed of 120 km/hr. What does Sam see the ball doing? What does Carol see the ball doing? c) Cameron is traveling toward you at 99.9999% of the speed of light when he turns on a flashlight and points it in your direction. How fast will the beam of light be going when it reaches you?

46) Moving Spaceship. Suppose you are watching a spaceship go past you at a speed close to the speed of light. a) How do clocks on the spaceship run, compared to your own clocks? b) If you could measure the length, width, and height of the spaceship as it passed by, how would these measurements compare to your measurements of the size of the spaceship if it were stationary? c) If you could measure the mass of the spaceship, how would this value compare to its rest mass? d) How would a passenger on the spaceship view your time, size, and mass?

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47) Relativity of Simultaneity. Consider the situation in Thought Experiment 8, about the green and red flashes of light at opposite ends of Al's spaceship. Suppose your friend Jackie is traveling in a spaceship in the opposite direction from Al. Further imagine that she is also precisely aligned with you and Al at the instant the two flashes of light occur (in your reference frame). a) According to Jackie, is Al illuminated first by the green flash or the red flash? Explain. b) According to you, which flash illuminates Jackie first? Why? c) According to Jackie, which flash occurs first? Explain. How does Jackie's view of the order of the flashes compare to your view and to Al's view?

48) Photon Philosophy. Extend the ideas of time dilation and length contraction to think about how the universe would look if you were a photon traveling at the speed of light. Do you think there's any point in thinking about how a photon "perceives" the universe? If so, discuss any resulting philosophical implications. If not, explain why not.

49) Ticket to the Stars. Suppose that we someday acquire the technology to travel among the stars at speeds near the speed of light. Imagine that many people make journeys to many places.

Discuss some of the complications that would arise from people aging at different rates depending on their travels.

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

50) Time Dilation. a) A student, after learning about the theory of relativity, decides to try to prolong his life by spending the rest of his life traveling in a car at 55 miles per hour (89 km/hr). If he does this while 70 years pass in his house, how much time will pass in the car? (Hint: If you are unable to find a difference, be sure to explain why.) b) A more clever student decides to prolong her life by cruising among the stars at a speed of 0.95c. How much time will pass on her spacecraft during a period in which 70 years pass on Earth? Will she feel as if her life span has been extended? Explain. c) Suppose you stay home on Earth while your twin sister takes a trip to a distant star and back in a spaceship that travels at 99% of the speed of light. If both of you are 25 years old when she leaves and you are 45 years old when she returns, how old is your sister when she gets back?

51) Length Contraction. a) Marta has a spaceship that measures 50 meters from end to end when at rest. If she flies past you at 0.75c, how long will you measure it to be? b) The star Sirius is 8.6 lightyears from Earth (in our Earth-based reference frame). Suppose you traveled from Earth to Sirius at 0.92c. During your trip, how far would you measure the distance from Earth to Sirius to be?

52) Mass Increase. a) A spaceship has a rest mass of 500,000 tons. If you could measure its mass when it was traveling at half the speed of light, what would the value be? b) A fly has a mass of 1 gram at rest. How fast would it have to be traveling to have a mass of 3000 kilograms (about that of a large SUV)?

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53) Time Dilation with Subatomic Particles. A π+ meson produced at rest has a lifetime of 18 nanoseconds (1.8 × 10−8 s). Suppose a π+ meson is produced in a particle accelerator at a speed of 0.998c. How long will scientists see the particle last before it decays? Briefly explain how an experiment like this helps verify the special theory of relativity.

54) Time Dilation on the Space Station. The International Space Station orbits Earth at a speed of about 30,000 km/hr. While 1 hour passes on Earth, how much time passes on the Station?

Assume that both the Station and Earth are in free-float frames, although in reality they are not.

(Hint: Start by converting the Station's speed to km/s.)

55) Super Baseball. For the game of interstellar baseball, teams have used robotic enhancements to allow speed of superhuman feats. Consider a pitcher who can throw a baseball at 80% of the

light and is practicing in a spaceship traveling away from Earth at 95% of the speed of light (on the way to direction thethe team's next game). Assuming that he throws the baseball in the same spacecraft is traveling, how fast will those of us on Earth measure the baseball to be going?

56) Racing a Light Beam I. A long time ago, in a galaxy far, far away, there was a civilization whose inhabitants hosted an Olympic competition every 4 of their years. Unfortunately, the competition had become tarnished by the use of illegal substances designed to aid performance, and many great athletes were disqualified and stripped of their medals. One famed sprinter,

named Jo, finally decided that it was pointless to continue racing humans. Instead, he held a press crowdsconference to announce that he would race a beam of light! Sponsors lined up, gathered, and the event was sold to a pay-per-view audience. Everything was set. The

starting gun was100 fired. Jo raced out of the starting block and shattered the old world record, running meters in 8.7 seconds. The light beam, represented by a flashlight turned on at precisely

the right moment, of course emerged from its "block" at the speed of light. How long did it take the light beam to cover the 100-meter distance? What can you say about the outcome of this race?

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57) Racing a Light Beam II. Following his humiliation in the first race against the light beam (Problem 56), Jo went into hiding for the next 2 years. By that time, most people had forgotten about both him and the money they had wasted on the pay-per-view event. However, Jo was secretly in training during this time. He worked out hard and tested new performance-enhancing substances. One day, he emerged from hiding and called another press conference. "I'm ready for a rematch," he announced. Sponsors were few this time and spectators scarce in the huge Olympic stadium where Jo and the flashlight lined up at the starting line. But those who were there will never forget what they saw, although it all happened very quickly. Jo blasted out of the starting block at 99.9% of the speed of light. The light beam, emitted from the flashlight, took off at the speed of light. The light beam won again–but barely! After the race, TV commentators searched for Jo, but he seemed to be hiding again. Finally, they found him in a corner of the locker room, sulking under a towel. "What's wrong? You did great!" said the commentators. Jo looked back sadly, saying, "Two years of training and experiments, for nothing!" Let's investigate what happened. a) As seen by spectators in the grandstand, how much faster than Jo is the light beam? b) As seen by Jo, how much faster is the light beam than he is? Explain your answer clearly. c) Using your results from parts a and b, explain why Jo can say that he was beaten just as badly as before, while the spectators can think he gave the light beam a good race. d) Although Jo was disappointed by his performance against the light beam, he did experience one pleasant surprise: The 100-meter course seemed short to him. In Jo's reference frame during the race, how long was the 100-meter course?

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter S3 Spacetime and Gravity Section S3.1 1) What do we mean by the straightest possible path along Earth's surface between two points on Earth's surface?

A) a path that actually is a perfect straight line B) a path that follows a circle of latitude C) a path that follows a circle of longitude D) a path that crosses the equator E) the shortest path between the two points Answer: E 2) Which of the following statements is not a prediction of the general theory of relativity?

A) Time runs slightly slower on the surface of the Sun than on the surface of Earth.

B) The universe can be finite without having boundaries or a center.

C) The curvature of spacetime can distort the appearance of distant objects.

D) Different observers can disagree about the fundamental structure of spacetime.

E) A binary star system with two stars orbiting each other rapidly emits gravitational waves.

Answer: D 3) Alice is floating freely in her spacecraft, and you are accelerating away from her with an acceleration of 1g. How will you feel in your spacecraft?

A) You will be floating weightlessly.

B) You will feel weight, but less than on Earth.

C) You will feel weight, but more than on Earth.

D) You will feel the same weight as you do on Earth.

E) You will feel yourself pressed against the back of your spaceship with great force, making it difficult to move.

Answer: D 4) Alice is floating freely in her spacecraft, and you are accelerating away from her with an acceleration of 1g. Suppose you claim that you are feeling the effects of a gravitational field.

How can you explain the fact that Alice is weightless?

A) She is weightless because she is moving at constant velocity.

B) She is weightless because she is in a not moving.

C) She is weightless because she is in free-fall.

D) If you are in a gravitational field, then she cannot be weightless.

Answer: C

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Section S3.2 1) Which of the following correctly describes the conditions under which you will feel weightless, according to general relativity? A) any time you are traveling through space at constant speed B) any time your worldline is perfectly straight C) any time your worldline is following the straightest possible path through spacetime D) any time you are very far from any planet E) You can never be truly weightless. Answer: C 2) How many dimensions of space can you move in if you are on the surface of a sphere, and you cannot leave that surface? A) 1 dimension B) 2 dimensions C) 3 dimensions D) 4 dimensions Answer: B 3) Which of the following would count as a hyperspace? A) a space in which locating a point requires stating four coordinates B) the surface of a sphere C) a line D) a point E) any space that is moving very quickly Answer: A 4) If you draw a spacetime diagram, how will the worldline of an object that is stationary in your reference frame appear? A) vertical B) horizontal C) slanted D) curved E) a circle Answer: A 5) If you draw a spacetime diagram, how will the worldline of an object that is traveling by you at constant speed appear? A) vertical B) horizontal C) slanted D) curved E) a circle Answer: C

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6) If you draw a spacetime diagram, how will the worldline of an object that is accelerating away from you appear? A) vertical B) horizontal C) slanted D) curved E) a circle Answer: D 7) Why can't a worldline on a spacetime diagram be horizontal? A) A horizontal worldline corresponds to something moving infinitely fast. B) A horizontal worldline represents traveling backward in time. C) A horizontal worldline represents moving at exactly the speed of light. D) A horizontal worldline is possible. It corresponds to a constant, finite speed. E) A horizontal worldline is possible. It corresponds to a stationary object. Answer: A 8) Denver, Colorado and Madrid, Spain are at nearly the same latitude (about 40°N). If you travel from Denver to Madrid staying at the same latitude the entire trip, will you be taking the shortest route? A) No. The shortest path on the Earth has to be a great circle. B) No. The shortest path on the Earth must always be a straight line. C) Yes. A flat map of Earth shows this path to be straight, and a path that is straight on a map is the shortest path. D) Yes. Staying at a constant latitude guarantees the shortest path. Answer: A 9) Considering the three general types of geometry (flat, spherical, and saddle-shaped), when do the angles in a triangle add to 180°? A) only in a flat geometry B) always, regardless of the geometry of space C) only in a spherical geometry D) only in a saddle-shaped geometry E) only if the triangle is an equilateral triangle Answer: A 10) Considering the three general types of geometry (flat, spherical, and saddle-shaped). If the spacetime of the universe is infinite, what might the geometry of the universe be? A) either flat or saddle-shaped B) flat, spherical, or saddle-shaped C) only flat D) either flat or spherical E) only spherical Answer: A

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Section S3.3 1) According to general relativity, why does the Earth orbit the Sun? A) The Sun causes spacetime to curve, and Earth follows the straightest path through that curved spacetime.

B) The Earth feels a force directed toward the Sun that pulls it inward.

C) The universe has a spherical geometry, causing all objects within it to take closed orbits.

D) The Earth only appears to be moving in an orbit because of length contraction and time dilation.

Answer: A 2) Why can't an object return to a point in spacetime that it previously occupied?

A) It would require going back in time.

B) It would require going faster than the speed of light.

C) It would require deviating from the straightest possible path.

D) It could be possible, but only if the universe has a spherical geometry.

Answer: A 3) If the Sun was gradually compressed while maintaining its mass in order to form a black hole, how would the curvature of spacetime be affected at the location of the Earth?

A) It would not change because the mass of the Sun is not changing.

B) The curvature would become greater because a black hole has stronger gravity than the Sun. C) The curvature would be reduced because the Sun would get so much smaller.

D) Compressing the Sun would not form a black hole because a black hole needs to be at least 2—3 times the mass of the Sun. Answer: A 4) As viewed by us here on Earth, where does time run slowest?

A) the surface of a white dwarf B) the surface of the Sun C) the surface of the Earth D) the top of Mount Everest on Earth E) in empty space Answer: A

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Section S3.4 1) Consider this image that appears to show four identical galaxies arranged as a cross. What are we really seeing?

A) four galaxies that are nearly identical because they were born at about the same time B) four images of a single background galaxy, created by the gravitational lens of a massive foreground galaxy or cluster C) a large galaxy with four central masses that glow brightly D) a picture taken with a poorly made telescope, so that a single large object appears as four fuzzy dots.

Answer: B 2) In 1919, general relativity was tested by measuring the deflection of starlight by the Sun. What event made this measurement possible?

A) a total solar eclipse B) a rare alignment in which some stars were positioned almost perfectly behind the Sun as viewed from Earth C) the first international treaty in which astronomers from different nations all worked together D) Einstein's personal permission for the measurements to be carried out Answer: A

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3) Hydrogen atoms have a spectral line with a wavelength of 656 nm. Suppose we observe this particular line from hydrogen gas located near the event horizon of a black hole. Assuming the gas has no motion toward or away from us, where would this line appear in the spectrum?

A) at a wavelength larger than 656 nm B) at a wavelength shorter than 656 nm C) at the same wavelength of 656 nm that we observe for this line in the laboratory D) We could not record a spectrum from this gas because its light would be sucked into the black hole and therefore would never reach Earth.

Answer: A 4) The Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first detection of gravitational waves in 2016. Which of the following best describes the discovery?

A) The detection verified that gravitational waves exist as predicted by the general theory of relativity.

B) The detection of gravitational waves was a huge surprise because no one had predicted they would exist prior to their discovery.

C) There is still considerable doubt whether LIGO really detected gravitational waves.

D) The detection of gravitational waves contradicted predictions made by the general theory of relativity, requiring scientists to revise the theory.

Answer: A 5) According to the general theory of relativity, how fast to gravitational waves move through space?

A) the speed of light B) the speed of gravity, which is slower than light C) the speed of gravity, which is faster than light D) Gravitational waves can be felt instantaneously over any distance of space.

E) the speed of sound Answer: A Section S3.5 1) What is a "wormhole"?

A) a hypothetical connection through hyperspace between two points in the universe B) an observationally confirmed feature of the universe that allows interstellar travel and time travel C) another name for the shortest path between two points D) the opposite of a black hole; it ejects mass and energy out of it and nothing can enter Answer: A

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General Chapter Questions 1) Which of the following is false?

A) The speed of light in empty space is constant.

B) Nothing can be accelerated to a speed faster than the speed of light.

C) Observers in different reference frames measure the same speed of light.

D) The absoluteness of the speed of light is an experimentally verified fact.

E) The speed of light slows down near a black hole.

Answer: E 2) Which of the following is false?

A) The order of events can be different for observers in different reference frames.

B) From your point of view, time runs slower in the reference frame of anyone moving relative to you.

C) If one observer measures two events to be simultaneous, all observers must agree on their simultaneity.

D) Time dilation is an observationally verified fact.

E) Time runs slower near a black hole.

Answer: C Short Answer Questions 1) In your own words, explain what we mean by spacetime.

Answer: It is the combination of space and time that all observers will agree upon, even while they may disagree about space and time independently.

2) What is gravitational lensing and why does it occur?

Answer: It is the distortion of the appearance of distant objects due to gravity and occurs because their light passes through regions of space that are curved.

Reading Quiz Questions 1) Einstein's general theory of relativity suggests that gravity is A) caused by curvature of spacetime.

B) a force of attraction that acts at a distance between two masses.

C) F = G(M1M2) /d2.

D) one of four fundamental forces in nature.

Answer: A

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2) Which of the following is not a major idea of Einstein's general theory of relativity?

A) It is possible to travel through wormholes.

B) Time runs slower near strong gravitational fields.

C) Our universe can have no center or boundaries, yet be finite.

D) Black holes can exist in spacetime, and falling into a black hole means leaving the observable universe.

E) Large masses that undergo rapid changes in motion or structure can emit gravitational waves that travel at the speed of light.

Answer: A 3) Einstein's equivalence principle says that A) the effects of gravity are exactly equivalent to the effects of acceleration.

B) everyone measures the speed of light to be equivalent.

C) someone traveling at 0.9c will age at the same rate as someone at 0.99c.

D) all people see themselves at an equivalent distance to the center of the universe.

Answer: A 4) What do we mean by dimension in the context of relativity?

A) the number of independent directions in which movement is possible B) the size of an object C) the number of sides that we can see when we look at an object D) the letter used to represent length mathematically Answer: A 5) What do we mean by hyperspace?

A) It is any space with more than three dimensions.

B) It is another word for spacetime.

C) It is a space through which it is possible to travel fast.

D) It is a space with a hyperbolic geometry.

Answer: A 6) What is spacetime?

A) It's the inseparable combination of space and time.

B) It's time that we measure when traveling in space.

C) It's a graph with four axes.

D) It's a way of viewing books at different angles.

Answer: A 7) If two straight lines start out parallel but eventually cross, then they must be in a A) spherical geometry.

B) flat geometry.

C) saddle-shaped geometry.

D) spacetime geometry.

Answer: A

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8) Which of the following correctly describes what we mean by a great circle on the surface of the Earth?

A) any circle whose center is at the center of the Earth B) any circle of latitude C) any circle larger than 10,000 km in diameter D) a circle of longitude Answer: A 9) What do we mean by the event horizon of a black hole?

A) It is the boundary within which events in the black hole cannot influence events in the outside universe.

B) It is the center of the black hole.

C) It is the "bottomless pit" of the black hole.

D) It is the place where time begins to slow down as you approach a black hole.

Answer: A 10) What do we mean by gravitational time dilation?

A) It is the idea that time runs slower in places where gravity is stronger.

B) It is the idea that clocks run slow for people moving at high speed past you.

C) It is the idea that everyone measures time differently, depending on his or her reference frame.

D) It is the idea that clocks run faster in stronger gravitational fields.

Answer: A 11) In what way do observations of Mercury support Einstein's general theory of relativity?

A) Mercury's orbit slowly precesses in a way that matches the prediction of general relativity but disagrees with the prediction based on Newton's universal law of gravitation.

B) Einstein discovered that time runs slower on Mercury than on Earth, as his theory predicted.

C) Einstein was able to explain the fact that Mercury orbits the Sun exactly twice for every three rotations, and Newton's theory of gravity cannot account for this.

D) We can see that Mercury lies deeper in the spacetime bowl that surrounds the Sun than does Earth.

Answer: A 12) Why do we see a gravitational redshift in the spectrum of the Sun? A) Time runs slower on the Sun than on Earth, making lines in the solar spectrum have lower frequency and hence longer wavelength than normal.

B) Gravity makes light heavy, causing it to appear redder.

C) Because of gravity, the Sun is always moving away from us so we see a redshift.

D) Spacetime curvature allows red photons of light to escape the Sun more easily than blue photons, leading to an apparent redshift.

Answer: A

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13) What is the purpose of the observatory commonly called LIGO? A) to detect gravitational waves B) to look for examples of gravitational lensing C) to test the equivalence principle D) to attempt observations of hyperspace Answer: A 14) Which of the following best describes what we mean by a worm hole?

A) a hypothesized but unproven type of "tunnel" through hyperspace that connects distant points in the real universe B) a proven way of traveling through space faster than the speed of light C) a possible method of traveling through space faster than the speed of light D) a discarded idea about time travel that physicists now know to be impossible Answer: A 15) Which of the following correctly describes the relationship between Newton's theory of gravity and general relativity?

A) Newton's theory of gravity is an approximation to general relativity that works when gravity is relatively weak but breaks down when gravity is strong.

B) Newton's theory of gravity is now known to be false, and we were previously misled to think it was true by measurement errors.

C) Newton's theory of gravity and general relativity give the same answers, but the former tells us to think of gravity as a force and the latter tells us to think of gravity as curvature of spacetime.

D) General relativity applies at the subatomic level, but Newton's theory of gravity does not.

Answer: A Concept Quiz Questions 1) Suppose you launch two probes in opposite directions from the Space Station and they meet as they orbit Earth. According to general relativity, why does this happen?

A) They are following the straightest possible paths through spacetime, and those paths curve so that the probes meet.

B) They are following the allowed orbits according to Newton's universal law of gravitation.

C) The force of gravity tugs on the probes and forces them to orbit the Earth.

D) Objects launched in space always follow circular paths, no matter where they are.

Answer: A 2) Suppose the room in which you are sitting was magically transported off the Earth, and sent accelerating through the universe at 9.8 m/s2. Assuming your doors and windows are sealed and closed, how could you tell that you'd left the Earth?

A) You couldn't; the equivalence principle tells us that you won't be able to tell the difference.

B) You would weigh more than you do on Earth.

C) Your time would run slower than it does on Earth.

D) You'd hear a loud whooshing sound as you careened through space.

Answer: A

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3) Suppose you are in a spaceship accelerating away from Earth at 9.8 m/s2 (1 g), so that you can walk on the floor of your spaceship as though experiencing normal gravity on Earth. Is it possible for you to consider yourself to be at rest? Why or why not?

A) yes, by assuming you are feeling effects of gravity and that Earth is accelerating away from you because it is in freefall B) no, because you feel a force so you must truly be accelerating C) No, because you are accelerating away from Earth, and it's not legitimate to claim that Earth is accelerating away from you.

D) yes, but only if you believe that you are the center of the universe Answer: A 4) In special relativity, we learned that people in different reference frames will measure time differently. According to general relativity, why does this happen?

A) They are experiencing the same spacetime reality, but from different perspectives.

B) People in different reference frames experience different realities.

C) It is an illusion, and time really is the same for everyone.

D) They have different worldlines.

Answer: A 5) Assuming you are sitting still as you take this quiz, how would you draw your own worldline on a spacetime diagram?

A) a straight, vertical line B) You would not have a worldline because you are not moving.

C) a straight, horizontal line D) a diagonal line Answer: A 6) Planes traveling between Seattle and Tokyo often go near Alaska because A) the distance is shorter.

B) there is less air traffic to contend with.

C) planes travel faster in cooler air.

D) the plane can stay over the safety of land for longer.

Answer: A 7) If your worldline is following the straightest possible path through spacetime, then A) you will be weightless.

B) you will feel heavier than your normal weight.

C) you must be accelerating.

D) you must be traveling at constant velocity.

Answer: A

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8) According to general relativity, why does Earth orbit the Sun? A) Earth is following the straightest path possible through spacetime, but this path happens to go around and around the Sun. B) because the mysterious force that we call gravity holds the Earth in orbit C) because the Earth and Sun are connected by a "rope-like" set of invisible, subatomic particles D) Earth orbits the Sun because a spacetime diagram shows the Sun to be a bowl-shaped dip in a rubber sheet.

Answer: A 9) According to general relativity, what is a black hole?

A) a hole in the observable universe B) an object that cannot be seen C) a place where light travels slower than the normal speed of light D) a place where the strength of gravity is infinite Answer: A 10) According to general relativity, how is time affected by gravity?

A) Time runs slower in places where gravity is stronger.

B) Time runs faster in places where gravity is stronger.

C) Time is stopped in places where gravity is strong.

D) Time is not affected by gravity.

Answer: A 11) Suppose you and a friend have precise clocks. You are on the ground floor of a tall building and your friend is on the roof. Which of the following statements is true?

A) You will see your friend's clock ticking faster than yours, and she will see your clock ticking slower than hers.

B) You will see your friend's clock ticking faster than yours, and she will see your clock ticking faster than hers.

C) You will see your friend's clock ticking slower than yours, and she will see your clock ticking slower than hers.

D) You will see your friend's clock ticking slower than yours, and she will see your clock ticking faster than hers.

Answer: A 12) Which of the following provided the first observational in support of the predicted existence of gravitational waves?

A) the orbit of a star system consisting of two neutron stars that is slowly decaying in a way consistent with energy being carried away by gravitational waves B) the energy generated by gravitational waves from the Sun that can be seen as it is absorbed by Jupiter C) gravitational waves detected by the Hubble Space Telescope D) photographs of spacetime that show the gravitational waves as ripples that are clearly visible Answer: A

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13) LIGO can detect strong gravitational waves as they pass by because they cause the L-shaped arms of the detector to A) contract and expand by different amounts.

B) become alternately more and less massive.

C) experience gravitational time dilation.

D) vibrate enough so that the vibrations can be measured with a seismograph.

Answer: A 14) Each of the following is a prediction of the theory of relativity (special or general). Which one is crucial to understanding how the Sun provides light and heat to the Earth?

A) E = mc2 B) Gravity is curvature of spacetime.

C) If you observe someone moving by you, you'll see his or her time running slow.

D) Space is different for different observers. Time is different for different observers. Spacetime is the same for everyone.

Answer: A 15) Suppose that a ship is accelerating through space in such a way that the passengers are experiencing a constant force (resulting from the thrust of the ship) equivalent to the total weight of the ship and passengers on Earth. From the point of view of observers on Earth, how does the ship accelerate?

A) At first, it has a nearly constant acceleration of 9.8 m/s2. But as it approaches the speed of light, its acceleration gradually slows in such a way that it never stops accelerating, but it never reaches the speed of light either.

B) It has a constant acceleration of 9.8 m/s2, therefore increasing its speed by 9.8 m/s with each passing second.

C) At first, it has a large acceleration, much larger than 9.8 m/s2. But when it reaches the speed of light it stops accelerating.

D) It has a constant acceleration of 9.8 m/s2 until it reaches a speed of half the speed of light.

Then its acceleration suddenly slows so that it can't go much faster.

Answer: A 16) Which of the following best describes the current scientific view of possibilities of travel to distant parts of our galaxy (or universe) with such things as hyperspace, wormholes, or warp drive?

A) We do not know if travel in any of these ways is really possible, but they do not contradict any known laws of physics.

B) There is no doubt that such travel is possible, though it remains well beyond our current technology.

C) They sound like fun, but they are not possible because they allow faster-than-light travel.

D) They seemed to be allowed by special relativity, but general relativity does not allow for them.

Answer: A

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Visual Quiz Questions 1) Suppose you are sitting in a closed room that is magically transported off Earth so that, as shown in the diagram, you are accelerating through space at 9.8 m/s2. According to the equivalence principle, how will you know that you've left Earth?

A) You won't know that you've left Earth.

B) by looking out the window C) by the fact that you'll become weightless D) by the fact that you'll feel yourself pressed harder against the floor Answer: A

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2) This graph shows a spacetime diagram for a car. At which of the numbered points is the car stationary?

A) Point 1 B) Point 2 C) Point 3 D) Point 4 E) both Points 2 and 4 Answer: C 3) Your worldline is shown in red on this spacetime diagram. The diagonal orange line could represent the worldline of

A) a flashlight beam that you turned on 2 seconds ago.

B) you, when you start running to your left.

C) a person running backward away from you.

D) a light beam that is shining on you.

Answer: A

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4) What kind of geometry does this surface have?

A) saddle-shaped geometry B) spherical geometry C) flat geometry D) spacetime geometry Answer: A 5) In the geometry shown, the angles of the triangle A) add up to less than 180 degrees.

B) add up to more than 180 degrees.

C) add up to exactly 180 degrees.

D) cannot be measured.

Answer: A

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6) Which of the following shows the actual shape of a holein space? black A)

B)

C)

D)

Answer: A

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7) Look carefully at this map and the two labeled paths from Philadelphia to Beijing. Which statement is true?

A) The map is fine, and the distances seem backward only because it is a flat map of a round Earth.

B) The distances on the two paths are labeled backward; the curved one should be the longer one.

C) The upper curved path makes sense only if we assume that it is a path that is drilled through the center of our planet.

D) The routes shown are impossible because there can be only one correct distance between two cities.

Answer: A

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8) Although it is greatly exaggerated as shown, what real effect does this diagram show?

A) The perihelion and aphelion of Mercury's orbit gradually precesses around the Sun. B) Time runs slow on Mercury.

C) Mercury has a spiral shaped orbit around the Sun. D) Mercury will eventually fly off into space.

Answer: A

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9) This graph shows that the orbit of a binary pulsar is decaying just as general relativity predicts it should (notice the agreement between data and theory). What is the significance of this particular result?

A) It supports the prediction that orbiting stars will radiate energy outward in the form of gravitational waves.

B) It tells us that time runs slower in this binary system than on Earth.

C) It confirms that gravitational lensing can really occur.

D) It shows that general relativity can explain all known natural phenomena.

Answer: A

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10) This photograph shows what appears to be identical four quasars arranged as a cross. What are we really seeing?

A) four images of a single quasar, with the images formed by gravitational lensing because a large galaxy lies between us and the quasar B) four quasars that are nearly identical because they were born at about the same time C) a picture taken with a poorly made telescope so that a single large object appeared as four fuzzy dots D) a large galaxy with four central masses that each glow brightly Answer: A Chapter Review Questions 1) What do we mean by the straightest possible path on Earth's surface?

2) List five major ideas that come directly from the general theory of relativity.

3) What is the equivalence principle? Give an example that clarifies its meaning.

4) What do we mean by dimension? Describe a point, a line, a plane, a three-dimensional space, and a four-dimensional space. What does hyperspace mean?

5) Explain the meaning of the statement "Space is different for different observers. Time is different for different observers. Spacetime is the same for everyone."

6) What is a spacetime diagram? Define worldline and event. What is the significance of whether a worldline is vertical, slanted, or curved?

7) How do rules of geometry differ depending on whether the geometry is flat, spherical, or saddle shaped?

8) Explain how the idea of spacetime geometry means that the universe has no center and no edges.

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9) How can you tell whether you are following the straightest possible path through spacetime? 10) According to general relativity, what is gravity and why does Earth orbit the Sun? Describe both the use and the limitations of the rubber sheet analogy for picturing gravity.

11) What is a black hole? What do we mean by the event horizon of a black hole?

12) What is gravitational time dilation? What determines how much time is slowed in a gravitational field?

13) Briefly describe several observational tests that support general relativity, including Mercury's redshift. orbit, examples of gravitational lensing, and measurements of gravitational 14) What are gravitational waves? Describe both indirect and direct evidence that they exist.

15) What is the current evidence regarding the possibility of travel through hyperspace or wormholes or by means of warp drive?

Does It Make Sense?

Decide whether the statement makes sense (or is clearly true) or does not make sense (or east fromclearly; our base North Pole.definitive answers, so your is clearlyI walked false). Explain notcamp all at of the these have Example: Theisstatement does not make sense because east has no meaning at the North Pole–all explanation more important than your chosen answer.

Solution: directions are south from the North Pole.

16) The equivalence principle tells us that experiments performed on a spaceship accelerating through space at 1g will give the same results as experiments performed on Earth.

17) The equivalence principle tells us that there's no difference at all between a planet and a human-made spaceship.

18) A person moving by you at high speed will measure time and space differently than you will, but you will both agree that there is just a single reality in spacetime.

19) With a sufficiently powerful telescope, we could search for black holes by looking for funnel-shaped objects in space.

20) Time runs slightly more slowly on the surface of the Sun than it does here on Earth.

21) Telescopes sometimes see multiple images of a single object, just as we would expect from the general theory of relativity.

22) When I walk in circles, I am causing curvature of spacetime.

23) Although special relativity deals only with relativity of motion, general relativity tells us that everything is relative.

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24) The shortest distance between two points is always a straight line. 25) General relativity tells us that it is impossible to travel through hyperspace or to use anything like Star Trek's "warp drive."

Quick Quiz 26) Spacetime is A) another name for gravity. B) the combination of time and the three dimensions of space. C) a curved rubber sheet. Answer: No Correct Answer Was Provided.

27) The equivalence principle tells us that effects of these two things are indistinguishable A) space and time. B) gravity and acceleration. C) gravity and curvature of spacetime.

Answer: No Correct Answer Was Provided. 28) The surface of Earth has how many dimensions? A) one B) two C) three Answer: No Correct Answer Was Provided. 29) If two lines begin parallel but later diverge, the geometry is A) flat. B) spherical. C) saddle shaped.

Answer: No Correct Answer Was Provided. 30) You know that you are following the straightest possible path through spacetime if A) you are standing still. B) you are moving directly from one place to another. C) you are weightless. Answer: No Correct Answer Was Provided. 31) According to general relativity, Earth goes around the Sun rather than flying straight off into space because A) gravity creates an invisible bond holding Earth and the Sun together. B) Earth is going as straight as possible, but the shape of spacetime makes this path go round and round. C) in its own reference frame, Earth can consider itself to be stationary. Answer: No Correct Answer Was Provided.

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32) On the surface of which of the following objects would time run most slowly?

A) the Sun B) an object with the same mass as the Sun but twice the radius C) an object with the same mass as the Sun but only half the radius Answer: No Correct Answer Was Provided.

33) Gravitational lensing occurs because A) gravity causes light to slow down.

B) gravity curves space and light always follows the straightest possible path through space.

C) the effects of gravity are indistinguishable from the effects of glass lenses.

Answer: No Correct Answer Was Provided.

34) The first evidence that gravitational waves really exist came from A) seeing these waves with x-ray telescopes in space.

B) observing the orbital decay of two orbiting neutron stars.

C) Einstein's own calculations with his general theory of relativity.

Answer: No Correct Answer Was Provided.

35) If wormholes are real, which of the following best describes what one is?

A) a place where it is possible to travel faster than light B) a shortcut between distant parts of the universe C) a black hole with a wormlike shape Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

36) Your Permanent Record.

Relativity is often thought of as being "just for scientists."

However, many philosophers have concluded that it has important implications for all of us. For example, consider the fact that, according to our description of spacetime, you exist in spacetime as a "solid" object stretching through time. This can be taken to imply that you cannot erase anything you've ever said or done from spacetime; in a sense, everything you ever do becomes part of some kind of four-dimensional "permanent record." Should this idea affect the way we behave as human beings? If so, how do you think human interactions might change if everyone learned about relativity? Can you think of any other ways in which knowing about relativity might change your life and the lives of others around you?

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37) Group Discussion: International Gravitational Wave Collaboration. The LIGO detectors in the United States were the first to detect gravitational waves, but other nations have since joined in the effort. a) Working independently, find out which nations currently have or are building gravitational wave detectors, where those detectors are located, and how much they cost to build. Also, learn what you can about how scientists use multiple detections of a gravitational-wave signal to narrow down where it came from, and how separating detectors widely on Earth can aid in this process. b) Gather in groups to share what you've learned. Then make a list of reasons several nations working collaboratively can discover much more about gravitational waves than just one nation working alone. c) The international collaboration in gravitational wave detection raises issues of how the data should be shared for the purposes of publishing scientific papers. Consider three alternative sharing models: (1) Data are available only to the scientists directly involved in the data collection; (2)data are available to all scientists from the nations that paid for the detectors; (3) data are immediately made publicly available to any scientist from any nation. Make a list of the pros and cons of each approach, and try to reach a group consensus on the best way to share the data.

The Process of Science These questions may be answered individually in short-essay form or discussed in groups, except where identified as group-only.

38) Hallmarks of Science.

Chapter 3 discusses both the idealized scientific method and a set of hallmarks by which you can distinguish science from nonscience. Did Einstein follow the idealized scientific method in his work on general relativity? Does general relativity satisfy the hallmarks of science? Explain.

39) Why Keep Testing? General relativity has been extensively tested in many different ways and has passed every test with flying colors. Yet scientists continue to test it with sensitive measurements and attempts to detect gravitational waves. Why do scientists keep testing a theory that works so well? What would happen if it failed some future test? Explain.

40) Group Activity: Einstein's Quote. Discuss the quote from Einstein at the very end of the chapter, and debate whether the distinction among past, present, and future is real or an illusion.

Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39.

Investigate Further

41) Northward Journey. Suppose two people start out traveling due north from the same latitude, one near Denver and one near San Francisco. Are their paths parallel when they start? Will their paths stay parallel, or will they meet? If they will meet, where will they meet? Summarize your answers with a one- or two-paragraph description of the people's journey, and explain what the results tell us about the geometry of Earth.

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42) Alternative Geometries. Find three everyday objects that obey the rules of each of the three basic types of geometry–flat, spherical, and saddle shaped. Describe each object and explain how you know which geometry it has.

43) Funnels in Space? Many people have seen rubber sheet diagrams that show black holes looking like funnels, as in Figure S3.15, and therefore they assume that black holes really are funnel shaped. Imagine that you are talking to a person who believes this misconception to be true. Write one or two paragraphs in words the person would understand, explaining how the misconception arose and what a black hole would really look like if we could see one. (For example, what shape would it be?)

44) Black Hole Sun. Suppose the Sun were magically replaced by a black hole of precisely the same mass. a) How would the curvature of spacetime change in the region where the Sun used to be located? b) How would the curvature of spacetime change in the region of Earth's orbit? c) What effect would the change have on Earth's orbit?

45) Galileo and the Equivalence Principle. Galileo demonstrated that all objects near Earth's surface should fall with the same acceleration, regardless of their mass. According to general relativity, why shouldn't the mass of a falling object affect its rate of fall? Explain in one or two paragraphs.

46) Movie Science Fiction. Choose a popular science fiction movie that depicts interstellar travel and study it closely as you watch it. What aspects of the movie are consistent with relativity and other laws of physics? What aspects of the movie violate the laws of relativity or other laws of physics? Write a critical review of the movie and its treatment of science.

47) Person of the Century. At the end of the 20th century, Time magazine chose Einstein as the "Person of the Century." Find out why, and write a short essay in which you either defend or oppose the choice.

48) Relativity and Fate. Read the novel Slaughterhouse Five by Kurt Vonnegut. How does he connect relativity with ideas of fate? Do you agree with his assessment of fate? Why or why not?

49) Research: The Eötvös Experiment. Galileo's result that all objects fall to Earth with the same acceleration (neglecting air resistance) is very important to general relativity–if it were not true, general relativity would be in serious trouble. Describe the experiments of Baron Roland von Eötvös, who tested Galileo's conclusions in the late 19th century. How did the results of these experiments influence Einstein as he worked on general relativity? Write a short summary.

50) Research: Wormholes. Some scientists have thought seriously about wormholes and their consequences. Find and read a popular article or book about wormholes. Write a short summary of your reading, and discuss your opinion of the implications of wormholes raised in it.

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Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

51) Worldlines at Low Speed.Make a spacetime diagram and draw a worldline for each of the following situations. Explain your drawings. a) a person sitting still in a chair b) a person driving by at a constant velocity of 50 km/hr c) a person driving by at a constant velocity of 100 km/hr d) a person accelerating from rest to a speed of 50 km/hr e) a person decelerating from 50 km/hr to a stop 52) Worldlines at High Speed. Make a spacetime diagram on which the time axis is marked in seconds and the space axis is marked in light-seconds. Assume you are floating weightlessly and therefore consider yourself at rest. a) Draw your own worldline. b) Draw a worldline for Sebastian, who is moving to your right at 0.5c. c) Draw a worldline for Michaela, who is moving to your left at 0.7c. d) Draw a worldline for Angela, who is traveling away from you at a speed of 100,000 km/s.

53) Highly Sloped Worldline. Make a spacetime diagram on which the time axis is marked in seconds and the space axis is marked in light-seconds. Draw a worldline with a slope of 30° (from the horizontal). At what speed would an object have to be traveling to have this worldline?

Can any object have this worldline? Explain.

54) Triangle on Earth. Draw a simple sketch of Earth (a plain sphere will do), with a triangle on it that connects the following three points: (1) the equator at longitude 0°, (2) the equator at longitude 90°W, and (3) the North Pole. What is the sum of the angles in this triangle? Explain how you know the angles, and what the sum tells you about the geometry of Earth's surface.

55) Long Trips at Constant Acceleration: Earth Time. Suppose you stay on Earth and watch a spaceship leave on a long trip at a constant acceleration of 1g. a) At an acceleration of 1g, approximately how long will it take before you see the spaceship traveling away from Earth at half the speed of light? Explain. (Use g = 9.8 m/s2.) b) Describe how you will see its speed change as it continues to accelerate. Will it keep gaining speed at a rate of 9.8 m/s each second? Why or why not? c) Suppose the ship travels to a star that is 500 light-years away. From your perspective on Earth, approximately how long will this trip take? Explain.

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56) Long Trips at Constant Acceleration: Spaceship Time. Consider again the spaceship from Problem 55 on a long trip with a constant acceleration of 1g. As long as the ship is gone from Earth for many years, the amount of time that passes on the spaceship during the trip turns out to be approximately tship =

ln

where D is the distance to the destination and ln stands for the natural logarithm (which you can calculate with the "ln" key on most scientific calculators). If D is in meters, g = 9.8 m/s2, and c = 3 × 108 m/s, the answer will be in units of seconds. Use the formula to determine how much time will pass on the ship during its trip to a star that is 500 light-years away. Compare this to the amount of time that will pass on Earth. (Hint: Be sure you convert the distance from light-years to meters and your answer from seconds to years.)

57) Trip to the Center of the Galaxy. Use the same scenario as in Problem 56, but this time suppose the ship travels to the center of the Milky Way Galaxy, about 27,000 light-years away.

How much time will pass on the ship? Compare this to the amount of time that will pass on Earth.

58) Trip to Another Galaxy. The Andromeda Galaxy is about 2.5 million light-years away.

Suppose you had a spaceship that could constantly accelerate at 1g. Could you go to the Andromeda Galaxy and come back within your lifetime? Explain. What would you find when you returned to Earth? (Hint: You'll need the formula from Problem 56.)

59) Gravitational Time Dilation on Earth. For a relatively weak gravitational field, such as that of a planet or an ordinary star, the following formula tells us the fractional amount of gravitational time dilation at a distance r from the center of an object of mass Mobject: × [G = 6.67 × 10−11 m3/(kg × s2); c = 3 × 108 m/s.] For example, while 1 hour passes in deep space far from the object, the amount of time that passes at a distance r is 1 hour multiplied by the factor above. (This formula does not apply to strong gravitational fields, like those near black holes.) Calculate the amount of time that passes on Earth's surface while 1 hour passes in deep space.

60) Gravitational Time Dilation on the Sun. Use the formula given in Problem 59 to calculate the percentage by which time runs slower on the surface of the Sun than in deep space. Based on your answer, approximately how much of a gravitational redshift should you expect for a spectral line with a rest wavelength of 121.6 nm?

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The Cosmic Perspective, 9e (Bennett et al.)

Chapter S4 Building Blocks of the Universe Section S4.1 1) What is quantum mechanics?

A) a branch of physics that applies on very small scales B) the idea that mechanics can be understood only if we first understand quantum C) the idea that quantitative methods are needed to understand physics D) a branch of physics that deals with the curvature of spacetime E) a branch of physics that deals with the properties of gases Answer: A 2) If we had never discovered quantum mechanics, which of the following would be true?

A) We would not have electronic devices such as cell phones and laptops.

B) The physical world would behave much differently.

C) Nothing would be different because quantum mechanics has not led to any changes in the lives of people.

D) The Sun would not shine.

Answer: A Section S4.2 1) In what sense is our modern concept of an "atom" not accurate to its ancient meaning?

A) "Atom" means indivisible, but we now know that atoms are actually made of smaller particles.

B) "Atom" means particle, but we now know that particles also have the properties of waves.

C) "Atom" means identical, but we now know that not all atoms are the same.

D) "Atom means solid, but we now know that atoms are mostly empty space.

Answer: A 2) Which of the following statements best describes the quantum property spin?

A) Spin is a measure of the rotation rate of a subatomic particle.

B) Spin is a measure of the rate at which a particle orbits another particle.

C) Spin is a property that applies only to large objects, like baseballs.

D) Spin is not meant to be taken literally, but measures the inherent angular momentum of a subatomic particle.

E) Spin is not a fundamental property, but rather something that can change randomly at any time.

Answer: D

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3) Which of the following best describes why the standard model of particle physics is considered so successful? A) The standard model has been used to predict the existence of new particles that were later discovered in particle accelerators. B) The standard model combines quantum mechanics and general relativity, the two leading modern theories of physics. C) The standard model was selected as the best model in a vote of physicists around the world. D) The standard model is the newest, and therefore, the best theory physicists have. Answer: A 4) What is the characteristic that distinguishes fermions from bosons? A) their mass B) their electric charge C) their spin D) their size E) their speed of travel Answer: C 5) An up quark (u) has a charge of +2/3 and a down quark (d) has a charge of −1/3. Which of the following describes a neutron? A) uuu B) uud C) udd D) ddd Answer: C 6) What distinguishes quarks from leptons? A) Quarks interact with one another via the strong force, while leptons do not. B) Quarks are usually found in isolation, while leptons combine to make larger particles. C) Quarks have integer spins, but leptons have half-integer spins. D) Quarks have no charge, while leptons do. E) "Quark" and "lepton" are names for the same type of particle. Answer: A 7) How many and what types of quarks are there in the standard model? A) 6: up, down, top, bottom, strange, charmed B) 2: up, down C) 4: up, down, top, bottom D) 6: up, down, left, right, weird, enchanted E) 4: up, down, left, right Answer: A

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8) Which of the following is true about protons and neutrons?

A) They have exactly the same mass.

B) They have exactly the same charge.

C) They are composed of the same number of quarks.

D) They are fundamental particles that cannot be broken into smaller particles.

E) They are leptons.

Answer: C 9) Which of the following statements about neutrinos is true?

A) Neutrinos can be attracted or repelled by electrically charged particles.

B) Neutrinos have a mass that is much larger than the mass of an electron.

C) Neutrinos respond to the strong nuclear force.

D) There are three types of neutrinos, and these represent three of the six known types of lepton.

E) Neutrinos respond to the force of gravity.

Answer: E 10) How large is the estimated mass of a neutrino?

A) equal to the mass of an electron B) equal to the mass of a proton C) equal to the mass of a neutron D) less than the mass of an electron E) zero Answer: D 11) What are the two families of fundamental particles out of which all ordinary matter is made?

A) protons and neutrons B) leptons and quarks C) electrons and neutrinos D) quarks and neutrinos E) leptons and photons Answer: B 12) What happens when a particle of matter and its corresponding particle of antimatter meet?

A) No one knows, since antimatter is only theoretical and not known to really exist.

B) The particles join together to make an antimatter atom.

C) The particles collide and then bounce back apart.

D) The particle and the antiparticle are annihilated, turning all their mass into energy.

E) They live happily ever after.

Answer: D

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13) Which of the following is true regarding the masses of antimatter particles?

A) The mass of an antimatter particle is the same as that of its corresponding matter particle.

B) The mass of an antimatter particle is always larger than that of its corresponding matter particle.

C) The mass of an antimatter particle is always lower than that of its corresponding matter particle.

D) The mass of an antimatter particle is the negative value of that of its corresponding matter particle.

E) Antimatter has no mass.

Answer: A 14) What is pair production? Choose the best answer.

A) Energy turns into a particle and its corresponding antiparticle.

B) Energy turns into two copies of identical particles.

C) A particle splits into two completely different particles.

D) Two antimatter particles turn into ordinary matter.

E) A particle splits into two copies of itself.

Answer: A 15) Which force is the strongest within an atomic nucleus?

A) strong B) weak C) electromagnetic D) gravity E) They are all equal.

Answer: A 16) Which force, exerted between an electron and a proton, is the strongest on scales much larger than an atomic nucleus?

A) strong B) weak C) electromagnetic D) gravity E) They are all equal.

Answer: C 17) In the standard model, the role of exchange particles is to A) create the force of gravity.

B) change leptons into quarks.

C) transmit a force from one particle to another.

D) move a particle of matter from one place to another.

E) change matter into antimatter.

Answer: C

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Section S4.3 1) Which of the following best describes the meaning of the uncertainty principle as applied to an electron bound in an atom?

A) The electron follows a precise path around the nucleus, but it is impossible for us to actually measure this path.

B) The electron is actually a precisely defined sphere surrounding the nucleus rather than a point, which explains why we cannot locate it at a single position.

C) The electron always has a clearly defined position and velocity, but the laws of nature are set up so that, if we measure one, the other becomes instantly hidden from view.

D) The electron does not really exist, and what we perceive as an electron is really just an ill defined energy field.

E) The electron is both a particle and a wave and is therefore "smeared out" around the nucleus.

Answer: E 2) The uncertainty principle can be used to relate the uncertainties in which two quantities?

A) position and momentum B) the force of gravity and the force of electromagnetism C) spin and charge D) position and spin E) mass and energy Answer: A 3) Which of the following explains why electrons must arrange themselves in atoms with no more than two per energy level, thus making chemistry possible?

A) the exclusion principle B) the uncertainty principle C) the law of conservation of energy D) the law of conservation of angular momentum E) the electromagnetic force law Answer: A 4) What does it mean to say that the values of properties like spin are quantized?

A) The property can take on specific values, but not any values in between.

B) The property can only take values that are separated by integer amounts.

C) The property can only take values that are positive so they can be counted.

D) The property can take on any value, but our measurements will always rounded.

Answer: A 5) The exclusion principle applies to what kind of particles?

A) all fermions, but not bosons B) all bosons, but not fermions C) only leptons, but not other fermions D) both fermions and bosons E) only exchange bosons Answer: A

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Section S4.4 1) Which of the following will cause the degeneracy pressure within an object to increase?

A) raising its temperature B) lowering its temperature C) compressing it to higher density D) letting it expand to lower density E) shining a light on it, which adds radiative energy Answer: C 2) Which of the following statements about quantum tunneling is not true?

A) It is used in modern electronics and, in fact, our modern-day computers would not work without it.

B) It plays a crucial role in nuclear fusion in the Sun. C) It allows electrons and other subatomic particles to pass through wall-like energy barriers even when it seems they do not have enough energy to get through the barriers.

D) Although it has been observed to occur, it violates all other known laws of nature, and explaining it therefore represents a major challenge to physicists.

Answer: D 3) Which of the following statements best describes how the uncertainty principle affects conservation of energy?

A) The uncertainty principle allows very short time-scale violation of energy conservation, but energy must still be conserved on time scales that allow measurement.

B) The uncertainty principle invalidated the law of conservation of energy, and as a result, this law is no longer useful to physicists.

C) The uncertainty principle does not apply to energy, so conservation of energy is not affected by it.

D) The uncertainty principle tells us that the law of conservation of energy applies only to very large objects, not to atoms or molecules.

Answer: A 4) According to current understanding of physics, how empty is "empty" space?

A) Even empty space is teeming with "virtual particles" that we cannot directly detect.

B) It is completely empty; there is literally nothing in empty space.

C) Empty space will produce a black hole due to the uncertainty principle.

D) Empty space is constantly creating new fundamental particles that we can detect easily.

Answer: A

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5) Stephen Hawking predicted that black holes can "evaporate" by emitting what we now call Hawking radiation. What does it mean for a black hole to "evaporate"?

A) The black hole loses its electric charge.

B) The black hole's mass is gradually reduced.

C) The black hole itself remains unchanged in mass, but it is able to send radiation out into the universe.

D) Any matter that surrounds the black hole is heated to the point where it becomes a gas, if it wasn't already.

Answer: B Short Answer Questions 1) Briefly explain how antimatter can be produced on Earth.

Answer: Antimatter is produced in particle accelerators. When particles collide and release large amounts of energy in one place, some of the energy spontaneously turns into mass. This mass must contain particles and their antiparticles in equal quantities.

2) Briefly explain why antimatter is so difficult to store after it is produced in laboratories.

Answer: When matter and antimatter meet, the result is mutual annihilation. Because everything on Earth is made of ordinary matter, it is difficult to prevent antimatter produced in the lab from contacting matter and quickly becoming annihilated.

Reading Quiz Questions 1) What is the primary topic of study in quantum mechanics?

A) subatomic particles and the forces with which they interact B) the role played by atoms in astronomy C) the mechanical movements of the heavens D) the role of relativity in astronomy Answer: A 2) What do we mean by the spin of a subatomic particle?

A) It is one of the basic properties of any particle and is a measure of the particle's angular momentum.

B) It describes how fast the particle is rotating on its axis at a given time; if more torque is applied, the particle spins faster.

C) It is a measure of the particle's electrical charge.

D) The spin of a particle describes how the story of the particle is portrayed to journalists.

Answer: A 3) Based on their spin, all particles can be classified as either A) fermions or bosons.

B) quarks or leptons.

C) matter or antimatter.

D) color, flavor, or mass.

Answer: A

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4) According to modern understanding, protons and neutrons are each composed of A) three quarks.

B) three leptons.

C) two fermions.

D) four bosons.

Answer: A 5) Which of the following is not considered a fundamental particle?

A) a proton B) an electron C) an up quark D) a neutrino Answer: A 6) Which statement about antimatter is not true?

A) Antimatter has been hypothesized to exist but has never actually been detected.

B) An antielectron (positron) is identical to an ordinary electron except it has a positive charge.

C) Every ordinary particle has a corresponding antiparticle.

D) When a particle and its antiparticle meet, the result is mutual annihilation that turns all of the mass into energy.

Answer: A 7) The four fundamental forces are A) strong force, weak force, electromagnetic force, and gravity.

B) strong force, weak force, electric force, and magnetic force.

C) nuclear force, electromagnetic force, gravity, and tidal force.

D) nuclear force, gravity, electric force, and magnetic force.

Answer: A 8) What are the two fundamental laws that lie at the heart of quantum mechanics?

A) (1) the uncertainty principle and (2) the exclusion principle B) (1) the absoluteness of the speed of light and (2) the equivalence principle C) (1) the law of conservation of energy and (2) the electromagnetic force law D) (1) the law of conservation of angular momentum and (2) the law of conservation of energy Answer: A 9) According to the uncertainty principle, which of the following statements is true?

A) It is impossible to measure both the position and the momentum of a particle at the same time.

B) It is impossible to measure both the mass and the momentum of a particle at the same time.

C) It is impossible for science to make any meaningful predictions about nature whatsoever.

D) It is impossible to measure both the speed and direction of a particle at the same time.

Answer: A

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10) An alternative statement of the uncertainly principle tells us that we cannot know precisely both A) a particle's energy and the time during which it has this energy.

B) a particle's mass and energy.

C) whether a particle is a wave or whether it is truly a particle.

D) a particle's momentum and its energy.

Answer: A 11) What does the exclusion principle say?

A) Two fermions cannot occupy the same quantum state at the same time.

B) No two particles of any type can ever occupy the same quantum state at the same time.

C) If a particle has a precisely defined position it is excluded from having a precisely defined momentum.

D) The laws of quantum mechanics are excluded from our common sense.

Answer: A 12) How does degeneracy pressure differ from thermal pressure?

A) Degeneracy pressure continues to be felt even if an object becomes cold, but thermal pressure is drastically reduced as an object gets cold.

B) Degeneracy pressure cannot support an object against the crush of gravity, but thermal pressure can.

C) Degeneracy pressure can arise in a plasma, but thermal pressure cannot.

D) Degeneracy pressure affects stars, but thermal pressure does not.

Answer: A 13) How is quantum tunneling important to our existence here on Earth?

A) It plays a crucial role in nuclear fusion in the Sun, so that our Sun would not shine brightly without it.

B) It explains how neutron stars are able to stay stable rather than collapsing under their strong gravity.

C) It explains why electrons in atoms gradually fill different energy levels, thus making chemistry possible.

D) It is important to understanding the law of conservation of energy.

Answer: A 14) What is Hawking radiation?

A) a hypothesized way for black holes to gradually shrink in mass B) a type of high-energy radiation that comes from inside a black hole's event horizon C) a type of radiation coming from objects supported by degeneracy pressure D) a type of radiation that we have detected so far only with infrared telescopes in space Answer: A

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Concept Quiz Questions 1) Given that many of the ideas of quantum mechanics seem so strange to our intuition, why do scientists think these ideas are correct? A) Quantum ideas can be used to make precise predictions that have been tested and confirmed through observations and experiments. B) Although the ideas seem strange at first, they make perfect sense once they are studied in depth, which is why scientists accept them.

C) Quantum ideas were expected to be strange, so the fact that they are strange only lends them support. D) Actually, few scientists think the quantum ideas are correct, and most expect the theories of quantum mechanics to be completely replaced in the near future. Answer: A 2) Which of the following is not one of the basic properties by which we classify a subatomic particle? A) temperature B) mass C) charge (electrical) D) spin Answer: A 3) The primary scientific purpose of a particle accelerator is to A) create particles and study their behavior. B) make small particles achieve large accelerations. C) produce energy for commercial use. D) make small particles go fast. Answer: A 4) Which of the following statements about electrons is not true? A) They are composed of quarks.

B) In an atom, an electron can have two possible values for its spin, usually called spin up and spin down.

C) Electrons are one of the six known types of lepton. D) Electrons can behave both as particles and as waves. Answer: A 5) Recall that an up quark (u) has a charge of +2/3 and a down quark (d) has a charge of −1/3. Which of the following quark compositions describes a proton? A) uud B) uuu C) udd D) uuudd Answer: A

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6) Suppose that, through the malicious act of an eight-dimensional alien being, the strong force was suddenly turned off throughout the universe. What would happen almost immediately to atoms?

A) The nuclei of atoms would fall apart.

B) Electrons would fall into the nuclei of atoms.

C) Atoms would be immediately ionized.

D) Nothing; atoms would be unaffected by this change.

Answer: A 7) The force of gravity is the only one of the four forces felt on large size scales. Which of the following best explains why the other forces don't play a major role on large scales?

A) The strong and weak forces act only on small scales, and large objects tend to be electrically neutral and therefore don't feel an electromagnetic force.

B) The other three forces are all short in range, and cannot be felt over large distances.

C) Gravity is by far the strongest force, so it simply dominates on large size scales.

D) Effects of the strong and weak forces tend to cancel out effects of the electromagnetic force on large scales.

Answer: A 8) Does the uncertainty principle affect our ability to follow the path of a baseball? Why or why not?

A) no, because the uncertainties in the position and momentum of the baseball are so small in comparison to its size and total momentum that they are unnoticeable B) yes, because we cannot know both where the baseball is and which way it is going at the same time C) no, because the exclusion principle says that large objects are excluded from the consequences of the uncertainty principle D) no, because the uncertainty principle applies only to electrons Answer: A 9) All the following statements are true. Which one can be attributed to the exclusion principle?

A) Two electrons with the same spin cannot occupy the same energy level in an atom.

B) If we know the location of an electron quite well, we will have little idea where it is going next.

C) If an electron and antielectron meet, they will turn into energy through mutual annihilation.

D) An electron has energy equal to its mass times the speed of light squared.

Answer: A 10) What else always happens when an electron is produced from energy in a particle accelerator?

A) An antielectron (positron) is also produced.

B) The electron quickly becomes part of an atom.

C) The electron releases a tremendous amount of energy.

D) The electron begins moving at extremely high speed.

Answer: A

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11) Uh-oh, the malicious eight-dimensional alien beings are back. This time, imagine that they suddenly changed nature so that neither the uncertainty principle nor the exclusion principle applied to electrons. What would happen almost immediately to atoms?

A) Electrons would fall into the nuclei of atoms.

B) Atoms would annihilate each other, releasing huge amounts of energy.

C) Atoms would be immediately ionized.

D) Nothing; atoms would be unaffected by this change.

Answer: A 12) Why is there a limit to how much compression can be counterbalanced by electron degeneracy pressure?

A) At extreme compression, the electron speeds approach the speed of light and therefore cannot increase further.

B) At extreme compression, electrons are forced to stop moving, and once stopped there is nothing more they can do.

C) At extreme compression, atoms are fully ionized so that electrons go free and can no longer exert pressure.

D) Because electrons are small, they are simply incapable of exerting much pressure.

Answer: A 13) Which of the following statements about quantum tunneling is not true?

A) Although it has been observed to occur, it violates all other known laws of nature and explaining it therefore represents a major challenge to physicists.

B) It is used in modern electronics and, in fact, our modern-day computers would not work without it.

C) It plays a crucial role in nuclear fusion in the Sun. D) It allows electrons and other subatomic particles to pass through wall-like energy barriers even when it seems they do not have enough energy to get through the barriers.

Answer: A 14) Which of the following best explains the hypothesized phenomenon of black hole evaporation?

A) Particles (or anti-particles) are created by a quantum mechanical effect near, but outside, the event horizon of the black hole. The law of conservation of energy maintains that the black hole must lose energy to "pay" for the creation of this mass.

B) Particles (or anti-particles) are occasionally ejected from within the event horizon, causing the black hole to lose mass.

C) Virtual particles created near the black hole are constantly annihilating each other, causing a high temperature even if the black hole has no accretion disk. This high temperature provides escape velocity for the virtual particles, causing the entire "cloud" of virtual particles to expand away into space.

D) Black hole evaporation is a virtual process, meaning that it has been theorized by astrophysicists, but doesn't really occur.

Answer: A

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Visual Quiz Questions 1) What does the ring represent in this picture?

A) the path of the particle accelerator calledLarge the Hadron Collider B) a collision between high-speed subatomic particles C) the Sudbury neutrino observatory D) the Stanford linear accelerator Answer: A

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2) This diagram summarizes modern particle classification. Where do protons, neutrons, and electrons, the particles that make atoms, fit in?

A) Protons and neutrons are made of quarks, while electrons are leptons.

B) Protons and neutrons are quarks, while electrons are leptons.

C) Protons and neutrons are fermions, while electrons are bosons.

D) Protons, neutrons, and electrons have a separate classification scheme; they are made neither of fermions nor bosons.

Answer: A 3) The diagrams represent subatomic particles made up of three quarks; u stands for an up quark (charge +2/3) and d stand for a down quark (charge -1/3). Which one represents a proton?

A)

B)

C)

D) Answer: A

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4) The diagrams represent subatomic particles made up of three quarks; u stands for an up quark (charge +2/3) and d stand for a down quark (charge -1/3). Which one represents a neutron?

A)

B)

C)

D) Answer: A 5) What process is illustrated in this diagram?

A) the annihilation of a particle and its antiparticle to make two photons B) the creation of two photons from pure energy C) the formation of an atom with a positive nucleus and a negative electron D) the evaporation of a black hole Answer: A 6) What process is illustrated in this diagram?

A) the spontaneous production of a particle and its antiparticle from energy B) the deliberate production of a particle and its antiparticle by shining two flashlight beams at each other C) the creation of antimatter from matter D) quantum tunneling of two particles through an energy barrier Answer: A 15 Copyright © 2020 Pearson Education, Inc.

7) This photograph, taken with a blinking strobe light, shows the path of a bouncing tennis ball.

How could we make a similar photograph of the path of a bouncing electron?

A) We couldn't. An electron cannot have a well-defined path like a tennis ball.

B) We'd need to use a strobe that emitted short wavelength x-rays rather than visible light.

C) We'd need to view the electron through a powerful electron microscope.

D) We'd need a strobe that blinked much faster, at least a trillion times per second.

Answer: A 8) This diagram shows two energy levels in an atom. The lower level is "full" because it already has an electron with spin up and another with spin down. Therefore, a third electron must go into the higher energy level. What law or principle explains why?

A) the exclusion principle B) the uncertainty principle C) the law of conservation of angular momentum D) the law of conservation of energy Answer: A

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9) This diagram shows a probability map of electron positions in an atom. Blue represents a high probability of finding the electron, while black represents zero probability of finding the electron. Which statement is true as we understand it today?

A) The electron can "jump" from the inner blue region to the outer blue region without crossing the space in between.

B) An electron in the inner blue region can never move to the outer blue region.

C) We say that the black region is "excluded," in accord with the exclusion principle.

D) The electron can move from the inner blue region to the outer blue region only by moving through the black region at high speed.

Answer: A 10) This diagram shows an electron and an antielectron (positron) being created from nothing, then annihilating themselves back into nothing. According to our understanding of quantum physics, which statement about this process is true?

A) This creation of something from nothing is permitted by the uncertainty principle, as long as the particles return to nothingness in a short enough time.

B) The process shown is impossible because it would violate the law of conservation of energy.

C) The diagram must be missing something, such as photons coming in on the left and going out on the right.

D) This process can occur only near a black hole.

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Chapter Review Questions 1) What do we mean by the quantum realm? List five major ideas that come from the laws of quantum mechanics.

2) What is spin? What are the two basic categories of particles based on spin?

3) List the six quarks and six leptons in the standard model. Describe the quark composition of a proton and of a neutron.

4) What is antimatter? What happens when a particle and its antiparticle meet? Why is antimatter always produced along with matter in pair production?

5) List the four fundamental forces in nature, and name the exchange particles for each.

6) Describe the relative strengths of the four forces. Why does gravity dominate on large scales, even though it is by far the weakest of the four forces?

7) Why do many scientists believe that the standard model will eventually be replaced by an even simpler model of nature?

8) What is the uncertainty principle? How is it related to the idea of wave-particle duality?

9) Describe two ways of quantifying the uncertainty principle, and give an example showing the meaning of each.

10) What do we mean by the quantum state of a particle?

11) What is the exclusion principle? What types of particles obey it? Briefly explain how the exclusion principle determines how electrons fill energy levels in atoms.

12) What is degeneracy pressure? How does it differ from thermal pressure? How is it important in astronomy?

13) What is quantum tunneling? How is it important to modern electronics? How is it important to nuclear fusion in the Sun? 14) What do we mean by virtual particles? How might they help explain the observed acceleration of the expansion of the universe? How might they lead to gradual evaporation of black holes through Hawking radiation?

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Does It Make Sense?

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

15) Although there are six known types of quarks, ordinary atoms contain only two of these types.

16) If you put a quark and a lepton close together, they'll annihilate each other.

17) There's no such thing as antimatter, except in science fiction.

18) Some particle accelerators have been known to build up a huge electrical charge because of the electrons produced inside them.

19) According to the uncertainty principle, we can never be certain whether one theory is really better than another.

20) The exclusion principle describes the cases in which the uncertainty principle is excluded from being true.

21) No known astronomical objects exhibit any type of degeneracy pressure.

22) Although we speak of four fundamental forces–gravity, electromagnetism, strong force, and weak force—it is likely that these forces are different manifestations of a smaller number of truly fundamental forces.

23) Imagine that, somewhere in deep space, you met a person made entirely of antimatter.

Shaking that person's hand would be very dangerous.

24) Someday, we may detect radiation coming from an evaporating black hole.

Quick Quiz 25) The fundamental particles of matter are A) atoms and molecules.

B) quarks and leptons.

C) electrons, protons, and neutrons.

Answer: No Correct Answer Was Provided.

26) When an electron is produced from energy in a particle accelerator, the following also always happens.

A) An antielectron (positron) is produced.

B) A quark is produced.

C) The electron disintegrates into a bunch of quarks.

Answer: No Correct Answer Was Provided.

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27) Within an atomic nucleus, the strongest of the four forces is A) gravity.

B) electromagnetism.

C) the strong force.

Answer: No Correct Answer Was Provided.

28) Across a distance of 1 millimeter, the strongest force acting between two protons is A) gravity.

B) electromagnetism.

C) the strong force.

Answer: No Correct Answer Was Provided.

29) If we measure a subatomic particle's position very precisely, then A) its momentum will be highly uncertain.

B) its spin will be highly uncertain.

C) we violate the uncertainty principle.

Answer: No Correct Answer Was Provided.

30) In addition to photons, which of the following also behave sometimes as waves and sometimes as particles?

A) fermions B) bosons C) all other subatomic particles Answer: No Correct Answer Was Provided.

31) Which of the following is not allowed by the exclusion principle?

A) knowing both the precise position and the precise momentum of an electron B) having two electrons in the same quantum state C) having an electron with a spin that is neither up nor down Answer: No Correct Answer Was Provided.

32) The strength of degeneracy pressure in an object (such as a white dwarf) depends on A) its temperature.

B) its density.

C) both its temperature and its density.

Answer: No Correct Answer Was Provided.

33) In which of the following objects does degeneracy pressure play the most important role?

A) a neutron star B) the Sun C) a star 10 times as massive as the Sun Answer: No Correct Answer Was Provided.

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34) According to modern theories based on quantum mechanics, empty space (a vacuum) is A) truly empty.

B) bubbling with virtual particles.

C) filled with tiny black holes.

Answer: No Correct Answer Was Provided.

Inclusive Astronomy Use these questions to reflect on participation in science.

35) Group Discussion: Large Collaborations.

The world's largest particle accelerators are built and operated by teams of thousands of scientists. As a result, a single scientific paper describing research results from such a project may list hundreds of scientists as co-authors of the paper. a) Scientists' career prospects depend heavily on the number of papers they publish and the significance of their contributions to those papers. Discuss why scientists who participate in a large collaboration may face difficulty in getting recognized for their contributions. b) Often, the job prospects of junior scientists who are members of such collaborations depend on recommendation letters written by senior members of the collaboration. List some of the reasons those letters might be biased. c) List and discuss some ways in which the scientific community can account for the biases that might be present in those recommendation letters. d) Try to come to a group consensus on whether the potential biases you've discussed are likely to hurt the goal of making the scientific community more representative of the community at large. If you conclude that they are, decide what might be done to minimize the problem.

The Process of Science These questions may answered individually in short-essay form or discussed in groups, except where identified as group-only.

36) Hallmarks This chapter discussed many ideas that may seem quite bizarre, from of Science. the existence of quarks to the uncertainty principle. Choose one of these ideas and evaluate it in terms of the hallmarks of science from Chapter 3. Is the idea science? Defend your opinion.

37) Common Sense vs. Experiment. Even the most highly trained physicists find the results of quantum mechanics to be strange and counter to their everyday common sense, but the predictions of quantum mechanics have passed every experimental test yet posed for them. Does this difficulty in reconciling common sense with experiment or theory pose any problems for science? Defend your opinion.

38) The Meaning of the Uncertainty Principle. When first hearing about it, many people assume that the uncertainty principle means that we cannot measure the position and momentum of a particle precisely. According to current understanding, it really tells us that the particle does not have a precise position and momentum in the sense that we would expect from everyday life.

How do these two viewpoints differ? Discuss the different philosophical consequences of these two viewpoints.

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39) Funding Big Science. Large particle accelerators cost billions of dollars, more even than large telescopes in space. If you were a member of Congress and the government could afford either a new accelerator or a new large space observatory, which would you choose? Why? 40) Group Activity: New Particle Discovery. Form a small group, and imagine that your group is in charge of an important particle physics experiment. A junior scientist comes to your group reporting that she or he has discovered a new particle. Make a list of questions that, as responsible scientists, your group should ask and answer before announcing this claimed discovery to the world. Note: You may wish to do this activity using the four roles described in Chapter 1, Exercise 39.

Investigate Further 41) The Strong Force. The strong force is the force that holds the protons and neutrons in the nucleus together. Based on the fact that most atomic nuclei are stable, briefly explain how you can conclude that the strong force must be even stronger than the electromagnetic force, at least over very short distances.

42) Chemistry and Biology. All chemical and biological reactions involve the creation and breaking of chemical bonds, which are bonds created by interactions between the electrons of one atom and the electrons of others. Given this fact, explain why the electromagnetic force governs all chemical and biological reactions. Also explain why the strong force, the weak force, and gravity play no role in these reactions.

43) Gravity. In one or two paragraphs, explain both (1) what we mean when we say that gravity is the weakest of the four forces and (2) why it nevertheless dominates the universe on large scales.

44) Quantum Tunneling and Life. In one or two paragraphs, explain the role of quantum tunneling in creating the elements from which we are made. (Hint: Recall that we are star stuff in the sense that the elements of our bodies were produced by nuclear fusion inside stars.)

45) Antimatter Engines. In the Star Trek series, st are powered by matter-antimatter annihilation.

Explain why we should expect matter-antimatter annihilation to be the most efficient possible source of power. What practical problems would we face in developing matter-antimatter engines?

46) Exchange of Information. Suppose an electron is moving up and down at some place located far in the distance. How could we learn that the electron is moving? How long after the movement would we know that it had occurred? Does this idea have any practical importance in astronomy? Explain. (Hint: Remember that the photon is the exchange particle for the electromagnetic force.)

47) Nonquantum Baseball. In your own words, explain why a baseball player does not have to take into account the uncertainty principle when up at bat.

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48) The Electron. Based on what you have learned in this chapter, how would you explain an electron to a friend? Be sure that you explain why physicists don't think of electrons as tiny, negatively charged, spinning balls. Write your explanation in one or two paragraphs.

49) Movie Review. Several recent movies deal with concepts covered in this chapter, including the Oscar-winning Theory of Everything and the award-winning documentary Particle Fever, about the discovery of the Higgs boson. Choose one of these movies and write a one-page critical review, focusing on its entertainment and its educational value.

50) Research: Quantum Computing. Learn how computer scientists hope to harness quantum effects to build computers much more powerful than any existing today. Briefly summarize the ideas, and write an essay stating your opinion concerning the benefits and drawbacks of developing this technology.

51) Research: Beyond the Standard Model. Research some of the ideas that physicists are considering as possible improvements on the standard model. Choose one such idea, and write a short essay describing its potential effect on physics if it is correct, and how the idea may be tested.

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences.

52) Gravity and the EM Force.In this problem, we compare the strength of gravity to the strength of the electromagnetic (EM) force for two interacting electrons. Because both electrons are negatively charged, they repel each other through the EM force. Because electrons have mass, they attract each other through gravity. Let's see which effect will dominate. You will need the following information for this problem: • The force law for gravitation is Fg = G × where M1 and M2 are the masses of the two objects, d is the distance between them, and G is the gravitational constant. (N is the abbreviation for newton, the metric unit of force.)

• The force law for electromagnetism is FEM = k where q1 and q2 are the charges of the two objects (in coulombs, the standard unit of charge), d is the distance between them, and k is a constant. (Coul is an abbreviation for coulomb.)

• The mass of an electron is 9.10 × 10−31 kg.

• The charge of an electron is −1.6 × 10−19 Coul. a) Calculate the gravitational force, in newtons, that attracts the two electrons if a distance of 10−10 m (about the diameter of an atom) separates them. b) Calculate the electromagnetic force, in newtons, that repels the two electrons at the same distance. c) How many times stronger is the electromagnetic repulsion than the gravitational attraction between the two electrons?

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53) Large-Scale Gravity. Suppose Earth and the Sun each had an excess charge equivalent to the charge of just one electron. Using the data from Problem 52, compare the gravitational force between Earth and the Sun to the electromagnetic force between them in that case. What does your answer tell you about why gravity dominates on large scales? (Hint: See Appendix E for the data you'll need about Earth and the Sun.) 54) Evaporation of Black Holes. The time it takes for a black hole to evaporate through the process of Hawking radiation can be calculated using the following formula, in which M is the mass of the black hole in kilograms and t is the lifetime of the black hole in seconds: t = 10,240π2 =

Without doing any calculations, explain how this formula implies that lower-mass black holes have much shorter lifetimes than more massive ones and that the evaporation process accelerates as a black hole loses mass.

55) Solar Mass Black Holes. Use the formula from Problem 54 to calculate the lifetime of a black hole with the mass of the Sun (MSun = 2.0 × 1030 kg). How does your answer compare to the current age of the universe?

56) Long-Lived Black Holes. Some scientists speculate that in the far distant future, the universe will consist only of gigantic black holes and scattered subatomic particles. The largest black holes that conceivably might form would have a mass of about a trillion (1012) Suns. Using the formula from Problem 54, calculate the lifetime of such a giant black hole. How does your answer compare to the current age of the universe? (Hint: Your calculator may not be able to handle the large numbers involved in this problem, in which case you will need to rearrange the numbers so that you can calculate the powers of 10 without your calculator.)

57) Mini—Black Holes. Some scientists speculate that black holes of many different masses might have been formed during the early moments of the Big Bang. Some of these black holes might be mini—black holes, much smaller in mass than those that can be formed by the crush of gravity in today's universe. Use the formula from Problem 54 to calculate the lifetime of a mini—black hole with the mass of Earth (about 6 × 1024 kg). Compare this to the current age of the universe.

58) Black Holes Evaporating Today. Starting with the formula from Problem 54, calculate the mass of a mini—black hole that was formed in the Big Bang and would be completing its evaporation today. Compare your answer to the mass of Earth. Assume that the universe is 14 billion years old.

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59) Your Quantum Uncertainty. Suppose you are running at a speed of about 10 km/hr, but there is an uncertainty of 0.5 km/hr in your precise speed. Given your mass, you can calculate your momentum and the uncertainty of that momentum. What is the corresponding quantum limit to the measurement of your position? Is this significant? Why or why not? (Hint: You'll need the first form of the uncertainty principle given on page 453; you can use the value of Planck's constant, h, from Problem 54.)

60) An Electron's Quantum Uncertainty. You are conducting an experiment in which you can measure the location of individual electron collisions to within 10−10 m. What is the theoretical limit to which you can simultaneously measure the momentum of those collisions? What is the uncertainty in the electron's speed? (The electron has a rest mass of 9.1 × 10−31 kg.)

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