21st century astronomy the solar system fifth edition test bank chapter 15 by asmacandy

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Chapter 15: Star Formation and the Interstellar Medium LEARNING OBJECTIVES. Define the bold-faced vocabulary terms within the chapter. 15.1 The Interstellar Medium Fills the Space between the Stars Describe the observational signatures of interstellar dust. Multiple Choice: 2, 13, 14, 15, 16, 17, 18 Short Answer: 3 Differentiate between interstellar extinction and reddening. Multiple Choice: 33 Short Answer: 7 Compare and contrast the densities and temperatures of the gas components of the interstellar medium. Multiple Choice: 1, 3, 4, 6, 7, 10, 11, 12, 20, 21, 28, 29, 31 Short Answer: 1, 2, 4, 10, 11, 13, 14 Describe the observational signatures of each gas component of the interstellar medium.

Multiple Choice: 5, 8, 22, 23, 24, 25, 26, 27, 30, 32 Short Answer: 5, 6, 8, 9 15.2 Molecular Clouds Are the Cradles of Star Formation Describe the process of fragmentation during the collapse of a cloud. Multiple Choice: 35, 36, 37, 39, 40, 41, 42, 43 Short Answer: 15, 16 Evaluate why molecular clouds are the cradles of star formation. Multiple Choice: 34, 38 Short Answer: 17, 18 15.3 Formation and Evolution of Protostars Explain why conversion of gravitational to thermal energy heats a collapsing gas. Multiple Choice: 47 Short Answer: 20, 21, 23 Describe how hydrostatic equilibrium supports a self-gravitating object. Illustrate the chain of events leading from molecular cloud to protostar. Multiple Choice: 9, 44, 45, 48, 53 Short Answer: 19 Distinguish between the conditions under which a protostar becomes a star and a brown dwarf. Multiple Choice: 49, 50, 51, 52 Short Answer: 22 15.4 Evolution before the Main Sequence Describe how the H− ion acts as a natural thermostat for a star. Multiple Choice: 54 Short Answer: 24 Explain why a protostar’s temperature rises but its luminosity drops as it settles onto the main sequence. Multiple Choice: 46, 55, 56, 57, 58, 59, 60, 62, 64 Short Answer: 25 Illustrate the observational features and possible origins of bipolar outflows. Multiple Choice: 67, 68 Short Answer: 26

Establish why a protostar’s mass influences the rate at which it collapses to become a star. Multiple Choice: 61, 63, 65, 66, 69 Short Answer: 28 Describe the conditions necessary for planets to form around protostars. Multiple Choice: 70 Short Answer: 30 Working It Out 15.1 Compute the peak wavelength of emission from dust grains. Multiple Choice: 19 Short Answer: 12 Working It Out 15.2 Use the blackbody luminosity, temperature, and size relationship to relate how changing a protostar’s size changes its luminosity. Short Answer: 27, 29

MULTIPLE CHOICE 1. The average density of the interstellar medium is a. much denser than the Earth’s atmosphere. b. much less dense than the best vacuum on Earth. c. about the same density as air on the peak of Mount Everest. d. zero. 2. The dust in the interstellar medium comes primarily from a. the stellar winds of main-sequence stars. b. the cooled material ejected from supernova explosions. c. cold cores of molecular clouds. d. all of the above 3. The lowest-density gas in the interstellar medium is also the a. coldest. b. least ionized. c. hottest. d. most localized, being found mostly around protostars. 4. The interstellar medium is divided up into three different kinds of gas clouds. These are a. cold gas at 100 K, warm gas at 8000 K, and hot gas at about 1 million K. b. warm gas at 8000 K, hot gas at 1 million K, and superhot gas at 10 million K. c. warm gas at 5000 K, warm-hot gas at 100,000 K, and hot gas at about 1 million K. d. cold gas at 100 K, cool gas at 1000 K, and warm gas at 8000 K. 5. We observe neutral hydrogen gas using a. X-ray radiation from highly ionized atoms. b. visible radiation at 656.3 nm from re-combined hydrogen. c. 21-cm emission. d. ultraviolet radiation from helium and oxygen. 6. Molecular hydrogen atoms are found a. everywhere throughout interstellar space.

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b. only in dense clouds where they are shielded from stellar radiation. c. in low density clouds of hot gas surrounding hot stars. d. only in the atmospheres of the giant planets, such as Jupiter. The coldest molecular clouds in our galaxy have temperatures of approximately a. 1000 K. b. 100 K. c. 10 K. d. 0 K. Electronic transitions from the H2 molecule are easily seen at a. X-ray wavelengths. b. visible wavelengths. c. radio wavelengths. d. infrared wavelengths. If you could watch stars forming out of a gas cloud, which stars would form first? a. low-mass stars b. medium-mass stars c. high-mass stars d. stars with low temperatures e. stars with more heavy elements When looking at the space between stars, what might you see? a. nothing; it is empty. b. spacecraft c. gas d. dark matter e. none of the above

11. The average density of the interstellar medium is a. 1 atom/cm3. b. 1,000 atom/cm3. c. 104 atom/cm3. d. 106 atom/cm3. e. 1012 atom/cm3. 12. If you wanted to observe heavy elements in the interstellar medium, where would be the best place to look? a. dust grains b. cold gas c. hot gas d. warm gas 13. When radiation from an object passes through the interstellar medium, a. the object appears dimmer. b. the object appears bluer. c. the object appears bluer and dimmer. d. the object appears redder and dimmer. e. the objectâ&#x20AC;&#x2122;s apparent velocity changes. 14. Dust in the ISM appears dark in _________ wavelengths and bright in _________ wavelengths. a. visible; ultraviolet b. infrared; radio c. infrared; visible d. radio; ultraviolet e. visible; infrared 15. Dust reddens starlight because it a. re-emits the light it absorbs at red wavelengths. b. emits mostly in the infrared due to its cold temperature.

c. is made mostly of hydrogen, which produces the red H-alpha emission line. d. preferentially affects light at visible and shorter wavelengths. e. primarily moves away from Earth. 16. What is the most likely explanation for the dark area in the figure shown below? a. It is a region where there are no stars. b. It is a region with lots of dark matter. c. It is a super-massive black hole. d. It is a region with thick dust blocking the starlight coming from behind. e. It is a dark star cluster. 17. The figure below shows the spectrum of a star, along with five other spectra labeled A through E. Which one of the labeled spectra shows what the spectrum of that star would look like if it were viewed through a significant amount of interstellar dust? a. A b. B c. C d. D e. E

18. The figure below shows three pictures of the disk of the Milky Way, taken in three different wavelength ranges. Put the three pictures in order from shortest to longest wavelength. a. I, II, III b. II, III, I c. I, III, II d. II, I, III e. III, I, II 19. Dust that is heated to 30 K will emit a blackbody spectrum that peaks at a. 1 µm. b. 30 µm. c. 50 µm.

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d. 100 µm. e. 500 µm. Sitting in a 100°F hot tub feels much hotter than standing outside on a 100°F day. This analogy illustrates why a. interstellar dust is dark at optical wavelengths but bright in the infrared. b. supernovae can heat their shells to such high temperatures. c. an astronaut would feel cold standing in the 106 K intercloud gas. d. the Solar System is immersed in a hot bubble of gas. e. fusion occurs only in the cores of stars. Which of the following is responsible for heating the bulk of the very hot intercloud gas? a. high-energy radiation from stars b. supernovae c. young O and B stars d. planetary nebulae e. The heating is an even mix of all of the sources above. Warm ionized gas in the interstellar medium appears _________ when imaged in the optical region of the electromagnetic spectrum. a. red b. yellow c. white d. blue e. dark The red emission in the figure shown below is due to

a. carbon monoxide (CO). b. warm, neutral hydrogen. c. molecular hydrogen (H2). d. ionized hydrogen (H II region). e. dust. 24. An H II region signals the presence of a. newly formed stars. b. young stars. c. ionized hydrogen gas. d. O- and B-type stars. e. all of the above 25. If you wanted to study regions where star formation is currently happening, you could use a. H-alpha emission to look for O and B stars. b. 21-cm radiation to find neutral hydrogen clouds. c. radio emission from carbon monoxide (CO) to find molecular cloud cores. d. infrared emission to identify T Tauri stars. e. all of the above 26. 21-cm radiation is important because it a. allows us to study the deep interiors of stars. b. allows us to image magnetic fields directly. c. allows us to study neutral hydrogen in the interstellar medium. d. is produced by every object in the universe. e. is the longest wavelength of light that can naturally be produced. 27. We detect neutral gas in the interstellar medium by looking for radiation at 21 cm that arises when a. an electron moves from the n = 1 to n = 2 state in a hydrogen atom. b. an electron is ionized from a hydrogen atom. c. carbon monoxide (CO) gas is excited by stellar radiation. d. the spin of an electron flips and aligns with the spin of a proton in a hydrogen atom. e. an electron combines with a proton to make a hydrogen atom. 28. In the interstellar medium, molecules survive only in regions with a. low temperatures. b. high densities. c. lots of dust. d. all of the above 29. Interstellar clouds are a. hydrogen gas, condensed out of the interstellar medium, like water clouds in the Earthâ&#x20AC;&#x2122;s atmosphere. b. regions where hydrogen tends to be denser than the surrounding gas. c. regions where water condenses out of the interstellar medium. d. oxygen gas, condensed out of the interstellar medium, like water clouds in the Earthâ&#x20AC;&#x2122;s atmosphere. e. regions where hydrogen combines with oxygen to create water molecules. 30. What primarily makes it difficult to observe the process of star formation? a. They occur in dusty regions. b. They have low luminosities. c. They do not shine at any wavelength until they become T Tauri stars. d. The star formation process happens so quickly. e. They are too small to be seen. 31. A typical molecular cloud has a temperature of approximately a. 0.3 K. b. 10 K. c. 80 K.

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d. 300 K. e. 1000 K. Molecular clouds, which have temperatures of around 10 K, are best observed at _________ wavelengths. a. X-ray b. ultraviolet c. optical d. infrared e. radio Interstellar extinction compared to interstellar reddening is like _______ as opposed to _______ a. viewing the Sun through a fog in Earth’s atmosphere; viewing the Sun through a cloud of haze from a forest fire. b. viewing the Sun through a cloud of haze from a forest fire; viewing the Sun looking outward from underwater. c. viewing the Sun through a cloud of haze from a forest fire; viewing the Sun through a fog in Earth’s atmosphere. d. viewing the Sun looking outward from underwater; viewing the Sun through a prism. Molecular cloud cores are places where you might find a. protostars b. Herbig-Haro objects. c. molecular hydrogen (H2). d. carbon monoxide (CO). e. all of the above For an object in hydrostatic equilibrium, if the temperature inside the object were to increase, the object would a. expand. b. contract. c. implode. d. remain the same size. e. explode. Because angular momentum must be conserved, as a gas cloud contracts due to gravity it will also a. spin slower. b. spin faster. c. increase in temperature. d. decrease in temperature. e. stay the same temperature. Star formation in a molecular cloud can be slowed by a. the presence of dust. b. the strength of its magnetic field. c. turbulence caused by supernovae and stellar winds from massive stars. d. all of the above Stars forming in molecular clouds tend to form first in a. the low-density periphery. b. the high-density core. c. random locations. d. any location where the temperature is highest. Of the following processes at work in molecular clouds, which is the one that inevitably dominates the clouds’ evolution? a. magnetic fields b. conservation of angular momentum c. pressure d. gravity

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e. turbulence Magnetic fields inside a molecular cloud act to a. inhibit gravitational collapse. b. fragment the cloud into numerous cores. c. modulate the temperature of the molecules. d. increase the formation of dust grains. e. increase the formation of protostars. The entire process of star formation is really just an evolving balance between a. heat and rotation. b. core temperature and surface temperature. c. pressure and gravity. d. radiation and heat. e. luminosity and distance. Which of the following traits does not help slow or prevent the collapse of a gas cloud? a. high mass b. high temperature c. turbulence d. magnetic fields e. angular momentum An accretion disk forms around a collapsing protostar because infalling material must conserve a. energy. b. centrifugal force. c. gravity. d. velocity. e. angular momentum. . As a protostar evolves, its temperature a. decreases because it is radiating. b. decreases because of gravitational contraction. c. decreases because of angular momentum. d. increases because of nuclear fusion. e. increases due to the kinetic energy of infalling material. A protostar is a. in hydrostatic equilibrium as it collapses. b. far out of hydrostatic equilibrium when it collapses. c. heated to millions of degrees as it collapses. d. flattened into a disk as it collapses. A young protostar is _________ than the Sun even though its surface temperature is _________ a. less luminous; hotter. b. larger; cooler. c. smaller; the same. d. more luminous; cooler. e. smaller; hotter. The source of energy for a contracting protostar comes from a. thermonuclear energy. b. kinetic energy. c. chemical energy. d. gravitational potential energy. e. radiation energy. What happens as a protostar contracts? a. Its density rises. b. Its temperature rises.

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c. Its radius decreases. d. Its pressure rises. e. All of the above are true. 49. What critical event transforms a protostar into a normal main-sequence star? a. Planets form in the accretion disk. b. The star grows suddenly larger in radius. c. Triple alpha reactions begin in the core. d. Nuclear fusion begins in the core. e. Convection begins throughout the star’s interior. 50. Stars with a mass from 0.01 M⊙ to 0.08 M⊙ are very different from the Sun because they a. do not have strong enough gravity to form planets. b. have much higher temperatures than the Sun. c. cannot successfully execute the proton-proton chain reactions. d. form much faster than the Sun did. e. do not exhibit sunspots.

51. A _________ is a failed star that shines primarily because of energy derived from its gravitational collapse rather than nuclear burning. a. black hole b. brown dwarf c. Herbig-Haro object d. protostar e. T Tauri star 52. Brown dwarfs are considered failed stars because a. they never reach masses larger than 50 Jupiter masses. b. hydrogen fusion never begins in their cores. c. convection never plays a role in their energy transport. d. they shine primarily at infrared wavelengths. e. they are never as luminous as the Sun. 53. The H− atom is important in protostars because it acts as a a. source of friction, stopping the cloud from collapsing too rapidly. b. source of infrared radiation, causing the cloud to cool off rapidly. c. temperature regulator. d. source of buoyancy, allowing the atmosphere of the protostar to expand. 54. The H− ion is very important in protostars because it a. reacts with oxygen to produce water. b. undergoes fusion and produces energy. c. helps make the protostars denser. d. acts as a temperature regulator. e. reduces angular momentum. 55. A protostar’s evolutionary “track” in the H-R diagram traces out a. only how the protostar’s radius changes with time. b. how the protostar’s luminosity, temperature, and radius change with time. c. only how the protostar’s luminosity changes with time. d. only how the protostar’s spectral type changes with time. e. the protostar’s location in the molecular cloud. 56. The Hayashi track of a low-mass protostar in the H-R diagram is a path of approximately constant a. density. b. luminosity. c. age. d. temperature. Copyright © 2015 Pearson Canada Inc.

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e. radius. Use the figure shown below to complete the following statement. A high-mass protostar remains roughly constant in _________ and increases in _________ as it follows its evolutionary track. a. temperature; luminosity b. radius; temperature c. luminosity; radius d. luminosity; temperature e. radius; luminosity Use the figure shown below to complete the following statement. A low-mass protostar remains roughly constant in _________ and decreases in _________ as it follows its evolutionary track. a. temperature; luminosity b. radius; temperature c. luminosity; radius d. luminosity; temperature e. radius; luminosity Use the figure shown below to complete the following statement. At the start of the evolution of a protostar, the radius of a 60 M⊙ protostar is roughly _________ that of a 1 M⊙ main-sequence star. a. 10 times bigger than b. 100 times bigger than c. 10 times smaller than d. 100 time smaller than e. the same as Use the figure shown below to complete the following statement. As a protostar contracts, a. the luminosity decreases. b. the luminosity increases. c. the temperature increases. d. the temperature decreases. e. either the luminosity decreases or the temperature increases. Star formation is an inefficient process, with only a few percent of the initial cloud fragment ending up as stars. This means the initial mass of a molecular cloud fragment that formed a 2 M⊙ star was probably close to a. 1 M⊙. b. 50 M⊙. c. 100 M⊙. d. 5000 M⊙. e. 1,000,000 M⊙.

62. If a 1 M⊙ protostar starts out on the Hayashi track with a temperature of 3300 K and a luminosity of 320 L⊙, what is its approximate radius? a. 5 R⊙ b. 50 R⊙ c. 100 R⊙ d. 200 R⊙ e. 500 R⊙ 63. Which of the following stars spend the longest time on their Hayashi tracks? a. 100 M⊙ stars b. 10 M⊙ stars c. 1 M⊙ stars d. 0.1 M⊙ stars

e. 0.08 M⊙ stars 64. A surprising fact about a 1 M⊙ protostar is that, even though nuclear reactions have not yet started in their cores, they are _________ than the Sun a. hotter b. rotating faster c. smaller d. denser e. more luminous 65. How long does it typically take for a protostar to form a 1 M⊙ star? a. 3 × 107 years

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b. 3 × 105 years c. 3,000 years d. 300 years e. 30 years The most common types of stars in our galaxy are a. high-mass stars. b. low-mass stars. c. an equal mix of high- and low-mass stars. d. low-mass stars near the Sun and high-mass stars far away. e. We do not yet know which types of stars are most common in our galaxy. Herbig-Haro objects are almost always found a. in pairs on either side of a young protostar. b. far away from molecular clouds where stars form. c. close to planets that are forming around protostars. d. deep inside molecular clouds. When winds blow the gas away from a forming protostar, the protostar a. expands rapidly to 100 times its original size. b. is revealed as a main-sequence star. c. becomes visible as a T Tauri star. d. is unable to reach the main sequence. When a molecular cloud fragments, a. the least massive stars are the first to form, while the most massive stars take longer. b. the most massive star are the first to form, while the least massive star take longer. c. the most massive stars promptly explode as supernovae, destroying all remaining gas. d. the stars form at the same rate, regardless of their mass. Where have astronomers observed the existence of planets? a. in our Solar System b. orbiting stars other than the Sun c. orbiting stars in binary systems d. traveling on their own through the Milky Way, not orbiting a star e. all of the above

SHORT ANSWER 1. Compare the volume of the Sun with the volume of interstellar space it occupies. Is the occupied percentage large or small? Consider the volume around the Sun to be a sphere whose radius is equal to the distance to the nearest star, which is equal to 5 light-years. (Note: the radius of the Sun is 7 × 105 km, and 1 light-year = 9.5 × 1012 km.) 2. What is the interstellar medium made of? Give rough percentages of each.

3. Why can we see dust in the interstellar medium better at far-infrared wavelengths than we can at optical wavelengths? 4. How are H II regions and the hot intercloud gas heated? 5. How are each of the following types of ISM detected by astronomers: hot intercloud gas, H II regions, neutral hydrogen gas, and molecular clouds. 6. At what wavelength are H II regions most clearly visible, and why do H II regions mark the regions where new stars are currently being formed? 7. What is the difference between interstellar extinction and interstellar reddening? 8. Suppose the 21-cm photon of neutral hydrogen were instead emitted at 500 nm (i.e., a visible blue photon). Would it still be a useful probe of the Milky Way’s structure? Why? 9. Why do H II regions mark the regions where new stars are currently being formed? 10. How are typical interstellar gas clouds different from the clouds that we see in the Earth’s sky? 11. Why do molecules readily exist in Earth’s atmosphere but not in most of interstellar space? 12. Suppose we observe two molecular clouds containing dust. The dust in Cloud 1 peaks in emission at 50 mm, while the dust in Cloud 2 peaks in emission at 78 mm. How much warmer is the dust in Cloud 1 compare to Cloud 2? 13. In the densest molecular clouds, the average density is approximately 300 atoms/cm3. If a cube of molecular cloud gas with this density contained 100 M⊙ of material (the amount needed to make a 1 M⊙ star), what would be the length of a side of the cube in units of AU? For reference, the mass of the Sun is 2 × 1030 kg, the mass of a hydrogen atom is 1.7 × 10−27 kg, and 1 AU = 1.5 × 1011 m. 14. Why is it possible for self-gravity to dominate pressure in molecular clouds but not in most interstellar clouds? 15. Some molecular clouds have so much internal pressure that it exceeds their self-gravity. What keeps them from expanding and dissipating? 16. Why do molecular clouds collapse from the inside out? 17. Why do many stars form from a single molecular cloud? 18. Why do stars form most often within molecular clouds? 19. Describe the general process of how the interstellar medium can create a star. 20. Why can’t very bright protostars be seen in visible light? Copyright © 2015 Pearson Canada Inc.

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21. Why does a protostar continue to collapse as it is forming? 22. What is the energy source that powers brown dwarf stars? 23. Explain why a star of higher mass must have a higher core temperature. 24. Why does the surface temperature of a low-mass protostar remain nearly constant as its core contracts? 25. In the figure shown below, the portion of the H-R diagram corresponding to the Hayashi track of a 1 M⊙ star is shown. Temperature increases toward the right, and luminosity increases toward the top of the diagram. Even though the temperature of the protostar is hardly changing as it approaches the main sequence, its luminosity is decreasing. Why? 26. How are Herbig-Haro objects related to T Tauri stars? 27. When a 3 M⊙ protostar forms, it starts out at the top of the Hayashi track with a luminosity of 4,000 L⊙ and a temperature of 3600 K. What is its radius at this point (give the answer in units of R⊙), and how many times larger is it at this stage compared to its radius as a mainsequence star, which is about 2.5 R⊙? For reference, the Sun’s temperature is 5800 K. 28. Astronomers cannot observe the entire process of star formation during a human lifetime. What property of star clusters allows them to circumvent much of this problem? 29. Suppose a protostar shrinks in size from 100 R⊙ down to 20 R⊙, while maintaining a constant temperature. By what factor does its luminosity decrease? 30. Name four conditions necessary for planets to exist with conditions suitable for life.

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