Ch.13 The Lives of Stars from Birth Through + Full Test Bank - Discovering the Universe 14e Test Bank + Answers by Neil F. Comins. DOCX document preview.
Chapter 13: The Lives of Stars from Birth Through Middle Age
Section: 13-1
1. Most interstellar matter is too cold to be observed optically. In which part of the electromagnetic spectrum might its radiation be detected?
A) gamma ray
B) ultraviolet
C) infrared
D) X-ray
2. To detect most interstellar material requires
A) optical telescopes.
B) gamma-ray observatories.
C) radio telescopes.
D) ultraviolet detectors.
3. Most interstellar matter is too cold to be observed optically. Its radiation is predominantly emitted at longer wavelengths, such as infrared and radio. This behavior is an example of
A) Bode’s law.
B) Wien’s law.
C) the Stefan–Boltzmann law.
D) Kirchhoff’s law.
4. The space between stars is known to contain
A) large quantities of dust that absorb and scatter light but no gas, either atomic or molecular.
B) variable amounts of gas but no dust, which forms only in planetary systems near stars.
C) a perfect vacuum.
D) gas, both atomic and molecular, and dust.
5. Which of these objects is NOT an abundant physical component of the interstellar medium?
A) atoms and ions
B) molecules
C) dust
D) radioactive elements
6. Which of these methods has NOT yet been used to study the material of the interstellar medium?
A) scattering of starlight
B) collection of dust and gas by spacecraft
C) emission of electromagnetic radiation by atoms and molecules
D) absorption of light from more distant stars
7. A star cluster at a great distance from the Sun, but inside the Milky Way, appears to be fainter than a similar star cluster at a closer distance. Part of this is the expected result from the inverse square law, but the distant cluster will also appear fainter than expected by distance alone because
A) the photons of light become “tired” and appear less bright as they travel.
B) the cosmological redshift has moved some of the light into the infrared spectral region.
C) star clusters are systematically smaller and hence less bright the farther they are from the galactic center and hence from the Sun.
D) light is scattered and absorbed by interstellar dust and gas between distant clusters and Earth.
8. Interstellar extinction is the
A) assimilation of interstellar matter by stars after gravitational attraction and capture.
B) reduction of the apparent brightness of stars by scattering and absorption of their light by intervening interstellar clouds.
C) wipeout of species on Earth by intense radiation from a nearby supernova.
D) deaths of high-mass stars in the space between other long-lived stars.
9. What do a reflection nebula surrounding a star and Earth’s atmosphere have in common?
A) The reflection nebula and Earth’s atmosphere both appear blue because of preferred scattering of this color of light.
B) The reflection nebula and Earth’s atmosphere both have about the same temperature.
C) The reflection nebula and Earth’s atmosphere have almost the same average density of gas.
D) The reflection nebula and Earth’s atmosphere both contain the same types of molecules.
10. The apparent reddening of light from stars after its passage through the interstellar medium is caused by
A) the additional contribution to this starlight by emission from hydrogen gas in the interstellar medium.
B) preferential scattering of blue starlight by dust grains.
C) Zeeman shift of the light by the powerful magnetic fields existing within the interstellar medium.
D) scattering of this light from rapidly moving material; the light is Doppler-shifted toward the red end of the spectrum.
11. The light from a distant cloud of gas and dust looks distinctly red to the unaided eye. When a spectrum is taken, the red color is found to come from a single, bright spectral line. Thus, the red color in this situation is due to
A) interstellar reddening.
B) the Balmer spectrum of hydrogen.
C) the Doppler effect.
D) interstellar reddening, the Balmer spectrum of hydrogen, or the Doppler effect.
12. The light from a distant cloud of gas and dust looks distinctly red to the unaided eye. When a spectrum is taken, the short wavelengths are all found to be dimmed in intensity compared with the longer wavelengths, which are all more intense. Thus, the red color in this situation is due to
A) interstellar reddening.
B) the Balmer spectrum of hydrogen.
C) the Doppler effect.
D) any one of these three phenomena.
13. The light from a distant cloud of gas and dust looks distinctly red to the naked eye. When a spectrum is taken, it is found that many wavelengths that would normally be expected to be in the middle of the spectrum have been shifted into the long-wavelength end of the spectrum. Thus, the red color in this situation is due to
A) interstellar reddening.
B) the Balmer spectrum of hydrogen.
C) the Doppler effect.
D) interstellar reddening, the Balmer spectrum of hydrogen, or the Doppler effect.
14. The Pleiades cluster consists of a number of bright stars wrapped in a cloud of gas and dust that appears blue. This cluster is an example of
A) a giant molecular cloud.
B) a dark nebula.
C) a reflection nebula.
D) a stellar “nursery.”
15. What is the MOST abundant element in the interstellar medium?
A) carbon
B) helium
C) oxygen
D) hydrogen
16. What is the second MOST abundant element in the interstellar medium (after hydrogen)?
A) helium
B) carbon
C) nitrogen
D) iron
17. What are the two MOST abundant elements in the interstellar medium?
A) hydrogen and carbon
B) hydrogen and oxygen
C) hydrogen and helium
D) nitrogen and oxygen
18. Which of these molecules is likely to be the MOST common in interstellar space?
A) OH, hydroxyl
B) H2, molecular hydrogen
C) CO, carbon monoxide
D) H2O, water
19. Which of these common molecules found in interstellar space contains nitrogen atoms but no oxygen?
A) water vapor (H2O)
B) formaldehyde (H2CO)
C) methane (CH4)
D) ammonia (NH3)
20. How have complex molecules such as formaldehyde (H2CO) been detected in interstellar clouds?
A) by direct sampling by space probes
B) by observing the chemical reactions in which they are created
C) only by theoretical modeling since it is known that the component elements (H, C, O) are present in space
D) by molecular emission lines
21. In which part of the electromagnetic spectrum are molecules in the interstellar medium MOST easily detected?
A) visible light
B) radio waves
C) X-rays
D) ultraviolet light
22. What wavelengths have astronomers used to map and study the distribution of the giant molecular clouds in space?
A) X-ray
B) visible
C) ultraviolet
D) radio
23. Which of these important atomic or molecular species is particularly difficult to detect in the interstellar medium?
A) molecular hydrogen (H2)
B) atomic hydrogen (H)
C) carbon monoxide (CO)
D) water vapor (H2O)
24. Which of these easily observed molecular species is used as a tracer for the fundamental but difficult to observe H2 molecules in giant molecular clouds?
A) hydroxyl (OH)
B) water vapor (H2O)
C) carbon dioxide (CO2)
D) carbon monoxide (CO)
25. Hydrogen in molecular form, H2, is thought to be very abundant in gas clouds in space, but these molecules emit radiation relatively inefficiently since they are symmetrical. Which other molecular element occurs in close association with H2 and is used as a probe for molecular clouds?
A) carbon monoxide (CO)
B) methane (CH4)
C) water vapor (H2O)
D) carbon dioxide (CO2)
26. In star-forming regions in interstellar space, which molecule is the easiest to detect?
A) ammonia (NH3)
B) hydrogen (H2)
C) formaldehyde (H2CO)
D) carbon monoxide (CO)
27. The density of carbon monoxide is observed in a particular interstellar cloud to be 500 CO molecules per cubic meter. What will be the expected density of molecular hydrogen gas, H2, in this cloud?
A) 500 million molecules of H2 per cubic meter
B) 500 molecules of H2 per cubic meter
C) 5 million molecules of H2 per cubic meter
D) 50,000 molecules of H2 per cubic meter
28. How is gas distributed in interstellar space?
A) in clumps, concentrated in interstellar clouds
B) concentrated in narrow riverlike streams of gas that circle the Galaxy
C) uniformly distributed through space
D) concentrated around existing stars because of the stars’ gravitational pull
29. When looking for molecular hydrogen, astronomers actually look for
A) atomic hydrogen.
B) carbon monoxide.
C) carbon dioxide.
D) polycyclic aromatic hydrocarbons (PAHs).
30. What is the typical mass of a giant molecular cloud?
A) 10 to 100 solar masses
B) 100 to 1000 solar masses
C) 1000 to 1 million solar masses
D) 1 million to 1 billion solar masses
31. What is a typical size for a giant molecular cloud?
A) 100 ly across
B) 1000 ly across
C) anything up to about 1 ly across
D) 5 ly across
32. Which of these fall within the typical range of dimensions of a giant molecular cloud in interstellar space?
A) mass of about 1000 solar masses in volume with a diameter of about 1000 ly
B) mass of about 1 million solar masses in volume with a diameter of about 100 ly
C) mass of about 1 million solar masses in volume with a diameter of about 100 au, about the size of the solar system
D) mass of about 1000 solar masses in volume with a diameter of about 100 ly
33. Giant molecular clouds, which are major sites of star formation, can be up to about
A) 10 times the size of the solar system and contain 2 to 3 solar masses of material.
B) 10 pc across and contain a few thousand solar masses of material.
C) 100 pc across and contain 2 million solar masses of material.
D) 1000 pc across and contain 100 million solar masses of material.
34. What fraction of the mass of a typical interstellar cloud is hydrogen?
A) 2 percent
B) almost 50 percent
C) 74 percent
D) 98 percent
35. What fraction of the mass of a typical interstellar cloud is helium?
A) 50 percent
B) 2 percent
C) 10 percent
D) 25 percent
36. The mass of a particular interstellar giant molecular cloud is 2 million solar masses. Approximately what is the mass of the hydrogen in this cloud?
A) 40,000 solar masses
B) 1 million solar masses
C) 1.5 million solar masses
D) 1.96 million solar masses
37. The mass of a particular giant molecular cloud in interstellar space is 2 million solar masses. What is the mass of the helium in this cloud?
A) 40,000 solar masses
B) 1 million solar masses
C) 200,000 solar masses
D) 500,000 solar masses
38. What is the characteristic color of a reflection nebula?
A) red
B) yellow
C) green
D) blue
39. What causes the characteristic blue color of a reflection nebula?
A) electrons dropping between energy levels n = 3 and n = 2 in hydrogen atoms
B) electrons dropping between energy levels n = 2 and n = 1 in hydrogen atoms
C) thermal blackbody radiation emitted by the hot gas
D) scattering of starlight from dust grains in the nebula
40. A reflection nebula is made visible by
A) thermal energy emitted as a continuous spectrum by the very hot gas, much like that emitted by a hot body on Earth.
B) light from embedded stars reflected over a wide range of wavelengths toward Earth by crystals of water, methane, and ammonia ices.
C) emission lines from hydrogen, which itself has been ionized by UV light from embedded stars.
D) blue light preferentially scattered by dust grains.
41. The distinctive color of a reflection nebula is
A) light of all colors predominantly in the red part of the spectrum, emitted by cool stars and reflected by crystals of water ice surrounding the stars.
B) several specific colors, resulting from fluorescence of atoms excited by ultraviolet radiation emitted by hot stars.
C) blue, caused by the scattering of light from dust grains.
D) red, resulting from the emission of light from hydrogen gas.
42. The distinct blue color of the nebulosity around stars in young clusters such as the Pleiades (see Figure 13-4 in the text) is caused by
A) atoms of gas emitting light by fluorescence, having been excited by ultraviolet radiation from hot stars.
B) light emitted by interstellar gases but Doppler-shifted by motion toward the observer.
C) starlight reflected by blue-colored interstellar material.
D) starlight scattered and reflected by small dust grains in the interstellar material.
43. In photographs, the Pleiades open star cluster is surrounded by a bluish haze (see Figure 13-4 in the text). What causes this blue light?
A) starlight scattered from interstellar dust in the star cluster
B) starlight scattered by the light-sensitive grains in the photographic plate when the picture was taken
C) shock waves losing energy to interstellar gas in the star cluster, causing the atoms to emit light
D) starlight absorbed and reemitted by interstellar gas in the star cluster
44. Dark nebulae are extreme examples of
A) interstellar extinction.
B) interstellar reddening.
C) emission nebulae.
D) polycyclic aromatic hydrocarbons (PAHs).
45. The Horsehead Nebula in Orion (see Figure 13-5 in the text) is a distinct dark region surrounded by brighter regions. The Horsehead is an example of
A) an emission nebula.
B) a black hole.
C) a Bok gobule.
D) a dark nebula.
Section: 13-2
46. There are several mechanisms that can trigger star formation in a cold, dark nebula. In each mechanism, the key to star formation is
A) bathing the cold, dark nebula in ultraviolet radiation and sweeping away some of the colder material.
B) compressing the gas and dust so that gravitation will overcome the gas pressure.
C) heating the gas so that gas pressure will overcome gravitation.
D) subjecting the dark nebula to an intense magnetic field so that supersonic jets will form.
47. What is the ultimate fate of an open star cluster?
A) The shape of the cluster will remain more or less as it is at the present time as the stars in it age and die.
B) The stars in the cluster escape one by one until the cluster no longer exists.
C) Over time the stars collide and merge, eventually creating a black hole.
D) The stars gradually sink toward the center, creating a globular cluster.
48. Which of these statements about open star clusters is TRUE?
A) Open star clusters slowly condense into globular clusters as the stars drive off the remaining interstellar dust and gas.
B) One star in an open cluster eventually undergoes a supernova explosion that quickly disperses the other stars.
C) As open star clusters slowly condense, their residual rotation spins them into a flat pancake shape and they become spiral galaxies.
D) The motions of individual stars are such that all open clusters eventually disperse.
49. Open clusters are NOT
A) gravitationally bound.
B) the result of star formation within a single giant molecular cloud.
C) loose groupings of stars of a wide range of ages.
D) known to contain more than about 100 stars.
50. How do massive stars normally end their lives?
A) Massive stars gradually shrink to the size of Earth.
B) Astronomers don’t know how massive stars normally end their lives since their lifetimes are longer than the age of the universe.
C) Massive stars collapse and become black holes.
D) Massive stars explode.
51. The MOST likely places in which stars and planetary systems are forming in the universe are
A) the centers of galaxies.
B) nebulae composed of dense gas and dust.
C) regions surrounding quasars.
D) the rarified space between galaxies.
52. New stars are formed from
A) hot supernova remnants.
B) activity around black holes in the centers of galaxies.
C) huge, cool dust and gas clouds.
D) pure energy in free space.
53. What determines whether a particular region of an interstellar cloud can collapse and form a star?
A) temperature, since higher temperatures act to prevent collapse
B) amount of gravity pulling inward compared with gas pressure pushing outward
C) gas density (the ratio of the mass of the cloud over its volume), since density determines how gravity will act on the cloud material
D) amount of mass in the cloud, since mass determines the strength of gravity
54. What condition is considered sufficient for an interstellar cloud to collapse and form a star or stars (i.e., if this condition holds then the cloud has to collapse)?
A) The cloud must be alone in space (far from stars and other interstellar clouds).
B) Gravity must dominate gas pressure inside the cloud.
C) Gravity must be strong enough to reach all parts of the cloud.
D) The cloud must be cooler than 100 K.
55. Which of these mechanisms is NOT a common way in which star formation is triggered or started?
A) collisions between interstellar clouds
B) heating of an interstellar cloud by radiation from embedded young stars
C) compression of an interstellar cloud by the shock waves from a supernova explosion
D) compression of an interstellar cloud by the pressure of light from nearby stars
56. Which of these mechanisms is NOT thought to be significant in the formation of new stars in the interstellar medium?
A) heating of an interstellar cloud by concentrated beams of neutrinos from nearby stars
B) compression of cold interstellar gas by radiation pressure from light from very bright stars
C) condensation of matter by the shock wave from a nearby supernova
D) collision of two cold interstellar clouds
57. Which of these mechanisms is thought to be ineffective and inefficient in the triggering of star birth in molecular clouds?
A) gravitational contraction of a hot gas cloud
B) collisions between two interstellar clouds
C) supernova explosions and the resultant shock waves
D) radiation pressure from the intense UV radiation from hot stars
58. The Jeans instability describes the
A) explosion of a star at the end of its life, the supernova phenomenon.
B) overcoming of gas pressure by self-gravity in a cold and dense interstellar cloud, to form a star.
C) conditions under which sufficient numbers of neutrinos can trigger the collapse of an interstellar cloud.
D) expansion of a gas cloud after gravitational contraction because of buildup of great heat within the cloud from gravitational potential energy.
59. The group of stars that is formed when a giant molecular cloud collapses is called a(n)
A) open cluster.
B) galaxy.
C) constellation.
D) gravitational lens.
60. Which wavelength region is MOST useful for investigating the dense cores inside giant molecular clouds?
A) infrared
B) X-ray
C) ultraviolet
D) optical
61. The Cygnus Loop Nebula is characterized by having an arched, shell-like appearance. The Cygnus Loop is a
A) dark nebula.
B) reflection nebula.
C) supernova remnant.
D) giant molecular cloud.
62. Parts of a supernova remnant become visible
A) because of radioactive elements that were created in the supernova and carried along with the remnant.
B) when large hot stars form within the gas and dust of the remnant and emit radiation, which excites the remaining gas.
C) when they collide with other clouds of gas and dust.
D) when they interact with the galaxy’s strong magnetic field.
63. How does a dense core collapse to become a star?
A) The innermost part collapses first; then the outer part is drawn in by the gravity of the inner part.
B) The collapse is turbulent and chaotic, with no overall pattern.
C) The outer, less dense part falls in first and its weight accelerates the collapse of the inner part.
D) All parts of the cloud accelerate inward more or less smoothly and evenly.
64. An open cluster has
A) a few hundred members, often very young and still embedded in the gas and dust from which they were formed.
B) hundreds of thousands of members, all very old, surrounded by very little interstellar gas and dust.
C) a few dozen members, the remnant of a globular cluster of stars from which most of the members have escaped.
D) many thousands of members of different ages.
65. In which one of these locations are clumps of gas MOST likely to be collapsing to form stars?
A) outer part of the solar system
B) globular cluster
C) reflection nebulae
D) giant molecular clouds
66. What is a protostar?
A) sphere of gas after collapse from an interstellar cloud but before nuclear reactions have begun
B) small interstellar cloud before it collapses to become a star
C) star near the end of its life before it explodes as a supernova
D) shell of gas left behind from the explosion of a star as a supernova
67. At what stage in its life does a star pass through the protostar phase?
A) while it is converting hydrogen into helium in its core
B) when it is expanding in size as a red giant or supergiant
C) after nuclear reactions end in its core but before the red giant phase
D) after dense core collapse but before nuclear reactions begin in its core
68. Where in the universe would one look for a protostar?
A) near black holes
B) in dense dust and gas clouds
C) in the empty space between galaxies
D) in globular clusters of stars
69. Where are protostars MOST likely to form?
A) emission nebulae
B) reflection nebulae
C) dark nebulae
D) planetary nebulae
70. Protostars are
A) very young objects still contracting before becoming true stars.
B) old stars contracting after using up all of their available hydrogen fuel.
C) objects with less than about 0.08 solar mass that do not have enough mass to become true stars.
D) stars made almost entirely out of protons.
71. Star formation takes place in
A) giant molecular clouds.
B) globular clusters.
C) blue reflection nebulae.
D) hot, turbulent gas thrown out in a supernova explosion.
72. Protostars, when they first form from the interstellar medium, are usually
A) very bright in ultraviolet light due to numerous flares that are hotter and brighter than solar flares.
B) detected by emission lines in their visible spectra, emitted by gas being blown off their surfaces into space.
C) easily detected because their light ionizes the surrounding interstellar gas, or reflection nebulae.
D) hidden from sight by dust clouds that emit infrared radiation.
73. Which range of electromagnetic radiation is MOST useful for observing newborn protostars in their gas and dust nebulae?
A) infrared
B) visible
C) highly penetrating X-ray
D) radio
74. The source of MOST of a protostar’s heat is
A) gravitational energy released as the protostar expands.
B) nuclear reactions in its core converting helium to carbon and oxygen.
C) gravitational energy released as the star contracts.
D) nuclear reactions in its core converting hydrogen into helium.
75. The major source of energy in the pre–main-sequence life of the Sun was
A) nuclear fusion.
B) burning of carbon atoms.
C) gravitation.
D) nuclear fission.
76. Protostars are slowly
A) expanding at the surface while the core contracts.
B) contracting and heating up.
C) heating up and expanding.
D) contracting and cooling.
77. What is believed to be the MOST important factor determining whether a collapsing region (dense core) in an interstellar cloud becomes a single-star or a multiple-star system?
A) fraction of heavy elements in the cloud
B) temperature
C) mass of the collapsing region
D) amount of rotation (spin)
78. Accretion of matter in an interstellar cloud leads to
A) a supernova explosion, since accretion is a nuclear process.
B) explosion of this matter when it is attracted to and falls onto the surfaces of stars.
C) the formation of molecules from atomic gases.
D) a protostar.
79. In order to produce protoplanets around a new star within a condensing interstellar cloud, a necessary condition seems to be that the cloud must
A) be rotating.
B) have a relatively high temperature.
C) have no rotational motion at all.
D) contain a high fraction of its mass as dust.
80. The solar system could have formed from a
A) nonrotating Bok globule with a relatively low density.
B) rotating Bok globule with a relatively low density.
C) nonrotating Bok globule with a relatively high density.
D) rotating Bok globule with a relatively high density.
81. Can astronomers observe nebulae, the regions where stars form?
A) Yes. A number of them can be seen easily with low-power binoculars.
B) Yes, but a medium-size telescope (say, 16) is required to see any of them.
C) Yes, but only a powerful research instrument can see them.
D) No. Nebulae are very short-lived phenomena. They either collapse to form star systems, or they disperse. Theoretically, it is known they exist for short periods of time, but astronomers have never observed any of them directly.
82. For an amateur astronomer to observe some of the more visible nebulae, it is suggested that the instrument to be used is
A) a telescope because very high magnification is needed to resolve even the most visible nebulae.
B) binoculars because they have a larger field of view and can allow the whole nebula to be seen.
C) binoculars because telescopes tend to wash out the beautiful colors of the nebulae.
D) the naked eye because these objects are easily seen when one knows where to look.
83. If one were to look for a protostar in its early stages, one would observe in which wavelength region?
A) ultraviolet
B) visible
C) infrared
D) radio
Section: 13-3
84. When attempting to observe a protostar, which region of the spectrum do astronomers usually use?
A) visible, because protostars do emit visible light
B) infrared, because protostars emit only infrared light
C) infrared, because these longer wavelengths can penetrate the dust surrounding the protostar
D) ultraviolet, because their higher energy photons can penetrate the dust surrounding the protostar
85. How large was the Sun when it first formed as a protostar?
A) about 5 times its present diameter
B) about 15 times its present diameter
C) about 50 times its present diameter
D) same diameter it has now
86. Most of the energy emitted by a protostar comes from
A) radioactive elements.
B) gravitational potential energy being converted to heat as the protostar collapses.
C) infalling material colliding with the protostar’s surface.
D) nuclear fusion.
87. What is the MOST important process that causes a protostar to stop accreting mass?
A) Radiation and particles from the hot protostar push infalling matter away from the protostar.
B) Other protostars formed in the vicinity pass randomly through the infalling material and eventually disperse it.
C) The dense core spins up as it collapses, and eventually the infalling matter is held away from the protostar by the centrifugal force.
D) All of the infalling matter has been used up in the accretion.
88. What is a protostar called in the stage after it has finished accreting mass?
A) white dwarf
B) red giant (or supergiant)
C) pre–main-sequence star
D) main-sequence star
89. A protostar eventually becomes a pre–main-sequence star. Which of these statements does NOT characterize the transition?
A) In the pre–main-sequence star, the rate of collapse slows.
B) In the pre–main-sequence star, the core temperature is greater.
C) The pre–main-sequence star is larger.
D) The pre–main-sequence star immediately ejects its outer shell of gas and dust.
90. At what point in its evolution does a protostar stop shrinking and stabilize into a star?
A) when nuclear processes generate enough energy and internal pressure to resist gravitational contraction
B) when gravitational contraction has heated up the gas to the point where radiation pressure opposes gravity for the first time
C) when the protostar has spun off enough of its matter and is spinning fast enough that centrifugal force opposes the gravitational contraction
D) when nuclear reactions end in the star’s core
91. What event occurs at the end of the pre–main-sequence stage of a star’s life?
A) The star explodes, forming a supernova remnant.
B) Gas is spun off from the star’s equator, from which planets may form.
C) Nuclear reactions begin in the star’s core, converting hydrogen into helium.
D) The star begins a long period of contraction in which gravitational energy is converted into heat.
92. If a protostar were able to contract (get smaller) without any change in its surface temperature, what would happen to its luminosity?
A) It is not possible to predict the change in the protostar’s luminosity since other factors are involved.
B) The star’s luminosity would increase due to the compression of the gas.
C) The star’s luminosity would decrease due to the reduced surface area of the protostar.
D) The star’s luminosity would remain the same since the temperature does not change.
93. What point defines the end of the pre–main-sequence phase of a star’s life and the start of the main-sequence phase?
A) The star begins to expand and become a red giant.
B) Convection begins in its interior.
C) The star stops accreting mass from the interstellar cloud.
D) Nuclear reactions begin in its core.
94. At what temperature do nuclear reactions begin in the core of a pre–main-sequence star?
A) 100 million K
B) 1 million K
C) 10 million K
D) 100,000 K
95. Which of these conditions does NOT characterize the transition from a pre–main-sequence star to a main-sequence star?
A) The star begins fusing hydrogen into helium in its core.
B) The star stops contracting.
C) The gas and dust around the pre–main-sequence star largely dissipates.
D) Helium burning becomes the principal energy-generating mechanism in the star’s core.
96. If one were to look for a star at the end of its pre–main-sequence stage, one might observe in which wavelength region?
A) ultraviolet
B) visible
C) infrared
D) radio
Section: 13-4
97. In which region of the Hertzsprung–Russell diagram does a newly formed protostar first appear when it begins to shine at visible wavelengths?
A) bottom right corner; very low luminosity because it is both small and cool
B) center of the main sequence since all protostars begin their lives at this position and move up or down the diagram, depending on their mass as time passes
C) right side; relatively large luminosity because of its size but cool temperature
D) top left corner; at the top of the main sequence, down which it will progress with time
98. Which part of the Hertzsprung–Russell diagram is occupied by protostars?
A) left of the main sequence
B) band running from upper right to lower left
C) band running from upper left to lower right
D) right of the main sequence
99. Astronomers studying stellar evolution draw a curve on a Hertzsprung–Russell diagram called a “birth line”. What does the birth line represent?
A) evolutionary track of a 1-solar-mass star (i.e., the Sun)
B) coordinates of the initial collapse of dense cores of various initial masses
C) coordinates of the transformation of protostars of various masses into pre–main-sequence stars
D) coordinates of the transformation of pre–main-sequence stars of various masses into main-sequence stars
100. How much time does it take for a 1-solar-mass protostar to reach the main sequence?
A) less than 10,000 years
B) 100,000 to 1 million years
C) 1–10 billion years
D) 10–100 million years
101. A protostar of about 1 solar mass is gradually contracting and becoming hotter. This activity causes its position in the Hertzsprung–Russell diagram to shift slowly
A) upward and toward the right.
B) downward and toward the left.
C) upward and toward the left.
D) downward and toward the right.
102. When any star on the main sequence was a protostar, it was
A) larger and hotter.
B) hotter but not necessarily larger.
C) cooler but not necessarily larger.
D) larger and cooler.
103. The “evolutionary track” of a star represents
A) its orbit around the center of the galaxy in which it resides.
B) changes in its luminosity and temperature, as the star ages, on a graph of these two parameters.
C) its path across Earth’s sky as a consequence of its true motion in space.
D) changes in its size and mass, as the star ages, on a graph of these two parameters.
104. The lowest mass a protostar can have and still become a star (i.e., start thermonuclear reactions of normal hydrogen in its core) is
A) slightly less than 1/10 of a solar mass.
B) slightly less than 1/100 of a solar mass.
C) 8/10 of a solar mass.
D) about 1/2 a solar mass.
105. A main-sequence star can be no smaller than 0.08 solar mass. The reason is that
A) thermonuclear reactions begin so suddenly in stars of less than 0.08 solar mass that the star is disrupted by an explosion.
B) protostars of less than 0.08 solar mass cannot form.
C) protostars of less than 0.08 solar mass are not massive enough to contract.
D) the temperature in a contracting protostar of less than 0.08 solar mass does not become high enough for nuclear reactions in normal hydrogen to start.
106. What is the lowest mass an object can have and still be a star?
A) 0.002 solar mass (twice Jupiter’s mass)
B) 0.80 solar mass
C) 0.08 solar mass
D) 0.02 solar mass
107. Which one of these objects is the LEAST massive?
A) a brown dwarf
B) the Sun
C) a giant molecular cloud
D) a T Tauri star
108. What is the relationship between the mass of a protostar and the time needed for it to reach the main sequence after it forms inside an interstellar cloud?
A) More massive protostars reach the main sequence in a shorter time than less massive protostars.
B) Less massive protostars reach the main sequence in a shorter time than more massive protostars.
C) The time needed is independent of the mass of the protostar.
D) The time needed is least for a protostar of approximately 4 solar masses and longer for protostars of either greater or lesser mass.
109. About how long did the Sun spend in the protostar phase of its life?
A) 4.6 billion years
B) 400,000 years
C) 50,000 years
D) 20 million years
110. Brown dwarfs and Jovian planets have many similarities. In which of these aspects are they NOT similar?
A) None produce energy by hydrogen fusion.
B) They are primarily composed of hydrogen and helium.
C) Storms occur in their atmospheres.
D) Their cores are not hot enough to fuse deuterium into helium.
111. Suppose that an astronomy news item announces the discovery of a brown dwarf. What is it that has been discovered?
A) prestellar object undergoing gravitational collapse
B) Plutolike object in the Kuiper belt, beyond the edge of the planetary system
C) object too large to be a planet but too small to be a star
D) protostar still embedded in the cloud of gas and dust from which it formed
112. A brown dwarf is a(n)
A) object intermediate between a planet and a star, with not enough mass to begin normal nuclear reactions in its core.
B) general name for an object similar to the planet Jupiter.
C) star whose blackbody spectrum peaks in the brown region of the visible spectrum.
D) star of less than about 1.5 solar masses at the very end of its life, after it has cooled to near invisibility.
113. A brown dwarf is a
A) high-mass star surrounded by an extensive and cool atmosphere of dust and gas.
B) star of mass less than about 0.08 solar mass whose core temperature is too low to initiate hydrogen fusion.
C) type of asteroid, so named because of its color, which indicates oxidized iron, or rust.
D) star in the late phases of evolution following the red giant phase whose temperature is very low—hence the brown color.
114. Suppose that an astronomical observatory announces the discovery of an object with about 50 times the mass of Jupiter. Since this mass is not enough for the object to be a star, what name would the observatory apply to the object?
A) red dwarf
B) infrared dwarf
C) brown dwarf
D) white dwarf
115. An object that is too massive to be a planet but NOT massive enough to be a star is called a
A) red dwarf.
B) brown dwarf.
C) white dwarf.
D) T Tauri star.
116. The T Tauri phase of a protostar is characterized by
A) the formation of a planetary nebula.
B) strong spectral absorption lines of metals.
C) the ejection of substantial amounts of gas from the protostar.
D) the helium flash.
117. What is believed to prevent stars from being larger than a few hundred solar masses?
A) Stars of larger mass would collapse under their own gravity and become black holes.
B) No interstellar clouds have masses of more than 100 solar masses.
C) The cores of larger-mass stars would run through their lives and explode before the stars finish contracting as protostars.
D) The temperature becomes so high that the excess mass is pushed back into space by radiation from the star.
118. What appears to be the limit to the amount of mass that can accumulate as a star before energy from intense nuclear fusion, emitted in the form of electromagnetic radiation, ejects further mass away from the star?
A) at least 150 solar masses, but not more than several hundred solar masses
B) only about 3 solar masses, hence the similarity of all observed stars
C) 10 solar masses
D) 106 solar masses
119. Main-sequence stars apparently are not larger than a few hundred solar masses. The reason stars of larger mass do NOT exist is that
A) their temperatures becomes so high that their outer envelopes are expelled by the pressure of radiation inside them.
B) the thermonuclear reactions in such stars proceed so rapidly that the stars explode.
C) such stars contract directly to become planetlike objects.
D) interstellar clouds of greater mass break up to become binary or multiple-star systems, not single stars.
120. The Pistol in Sagittarius is one of the most massive known stars. But it has lost much of its original 200 solar masses. How long is the entire mass-loss process estimated to take?
A) an explosion lasting a few seconds
B) hundreds of millions of years
C) a few million years
D) a billion years
121. What is the minimum mass necessary in a protostar for it to begin hydrogen fusion reactions and become a main-sequence star?
A) 0.08 solar masses
B) 0.4 solar masses
C) 1.0 solar masses
D) 1.4 solar masses
122. Objects with masses between 13 and 75 times the mass of Jupiter do not fuse hydrogen but do fuse deuterium and lithium into helium. What are such objects called?
A) extrasolar planets
B) sub-brown dwarfs
C) brown dwarfs
D) red dwarfs
123. Which one (if any) of these objects is a star in the sense that its main source of energy is nuclear fusion of normal hydrogen?
A) T Tauri star
B) sub-brown dwarf
C) brown dwarf
D) None of these has nuclear fusion as its main energy source.
124. What is the difference between an extrasolar planet and a sub-brown dwarf?
A) There is no difference. These are two different names for the same object.
B) In its core a sub-brown dwarf generates a limited amount of energy through fusion of deuterium into helium. An extrasolar planet generates no fusion energy.
C) In its core a sub-brown dwarf generates a limited amount of energy through fusion of lithium into helium. An extrasolar planet generates no fusion energy.
D) An extrasolar planet orbits a star; a sub-brown dwarf does not. Otherwise, they are the same.
125. Which of these features do astronomers NOT have direct evidence for brown dwarfs?
A) sunspots
B) flares
C) rotation
D) magnetic fields
126. How many brown dwarfs and sub-brown dwarfs are estimated to exist in the Milky Way Galaxy?
A) about 450
B) at least a million, but less than 100 million
C) at least 100 billion
D) at least 100 million, but less than 100 billion
127. What is a T Tauri star?
A) young G, K, or M star that is ejecting gas
B) giant or supergiant star that varies regularly in brightness
C) giant or supergiant star that varies randomly in brightness
D) massing O or B star that ionizes hydrogen in interstellar space
128. What name is given to a young, cool star (spectral class G, K, or M) that is ejecting gas into the interstellar medium?
A) Cepheid variable
B) flare star
C) RR Lyrae star
D) T Tauri star
129. A T Tauri star is at what stage of its stellar evolution?
A) just before red giant phase, when variability begins
B) well-established on the main sequence
C) the end of its life, decaying away and cooling
D) pre–main-sequence phase
130. T Tauri stars are at what stage of stellar evolution?
A) main sequence, or “middle age”
B) horizontal-branch phase
C) early phase: after protostar but before main sequence
D) post-main sequence before helium shell flash
131. A T Tauri star is a
A) protostar that is ejecting mass near the end of its pre–main-sequence lifetime.
B) young, massive O or B star.
C) young protostar embedded in a cocoon of dust clouds, visible only by infrared radiation.
D) high-mass yellow giant star that pulsates regularly in size and brightness.
132. Which of these properties is NOT a characteristic of T Tauri stars?
A) variations in brightness
B) ejection of mass into space
C) nuclear reactions in the core
D) emission lines in the spectrum
133. Which of these properties is a known characteristic of a T Tauri star?
A) ejection of mass into space
B) great age, near the end of its life as a star
C) high mass, greater than about 3 solar masses
D) nuclear reactions in the core
Section: 13-5
134. The Orion Nebula is a
A) spiral galaxy in the constellation Orion.
B) red supergiant star.
C) large interstellar gas and dust cloud containing young stars.
D) supernova remnant, the material thrown out by an exploding star.
135. Place these objects in order, from smallest to largest.
A) giant molecular cloud, H II region, OB association
B) OB association, H II region, giant molecular cloud
C) OB association, giant molecular cloud, H II region
D) H II region, giant molecular cloud, OB association
136. What is the relationship between a giant molecular cloud and an H II region?
A) They are two names for the same entity.
B) In giant molecular clouds, H II regions surround ultraviolet-emitting stars (types O and B), which have ionized the hydrogen around them.
C) In H II regions, giant molecular clouds are concentrations of other molecules like CO and H2O.
D) Giant molecular clouds evolve into H II regions as the molecules other than hydrogen are used up in star formation.
137. How is an H II region formed?
A) In a supernova explosion large amounts of hydrogen are ionized, and this forms the H II region.
B) They are remnants of the earliest period in the universe when only hydrogen existed. They were ionized when the cosmic microwave background radiation was released.
C) They are hydrogen clouds ionized by the radiation from hot O- and B- type stars.
D) They are hydrogen clouds ionized in the eruption of the T Tauri phase of the star that they surround.
138. Why do H II regions glow?
A) The hydrogen cannot emit radiation because it is fully ionized, so the radiation comes from neighboring molecules of carbon monoxide.
B) The H II region collides with a giant molecular cloud exciting the molecules in the cloud to radiate.
C) Some H II ions unite with electrons to re-form neutral hydrogen. In this process, the electron passes from level to level in the atom and emits a cascade of photons.
D) The protons, which make up H II, collide with each other, resulting in gamma-ray emission from the nuclei.
139. Which of these statements about the capability of an O-type star to trigger the formation of other stars is NOT correct?
A) O-type stars are massive. When they form stellar nebulae, as the Sun did, small stars form instead of planets.
B) The H II region surrounding an O-type star expands into the giant molecular cloud, producing a shock wave that can compress the un-ionized material and cause it to collapse.
C) O-type stars are massive and thus have short lifetimes. After an O-type star erupts in a supernova, the material it ejects into space can eventually condense to form a new generation of stars.
D) The shock wave produced when an O-type star forms a supernova can compress the interstellar medium and cause it to collapse.
140. What is an emission nebula, also known as an H II region?
A) region of ionized hydrogen around one or more hot O- and B-type stars
B) region of molecular hydrogen inside a giant molecular cloud
C) region of neutral, atomic hydrogen in interstellar space
D) region of gas and dust formed by the explosion of a massive star
141. Why do many H II regions glow with a prominent red color?
A) Ionized hydrogen, like neutral hydrogen, has the Hα line as one of its electron transitions.
B) Ionized hydrogen does not emit a red line, but it has a yellow line and a magenta line, and the combination of these produces the sensation of red.
C) Ionized hydrogen cannot be detected directly, so CO is used as a tracer, and CO produces a red line.
D) Neutral hydrogen atoms are formed temporarily in the plasma of an H II region, and they emit the red Hα line.
142. Where are emission nebulae (also known as H II regions) found?
A) in or near old open clusters
B) around hot stars
C) in globular star clusters
D) around low-mass stars
143. The bright stars at the center of an emission nebula (H II region) are
A) young O stars and B stars.
B) red supergiants.
C) hot white dwarfs.
D) T Tauri stars.
144. What radiation ionizes the hydrogen in an emission nebula (H II region)?
A) ultraviolet radiation from O and B stars
B) X-rays from the coronas of solar-type stars
C) infrared radiation from pre–main-sequence stars
D) gamma rays from neutron stars
145. The energy required to ionize the hydrogen gas in an emission nebula (H II region) comes from
A) UV emission from hot O and B stars.
B) supernovae (exploding stars).
C) collisions between gas clouds in interstellar space.
D) T Tauri stars.
146. Evidence of massive amounts of hydrogen gas surrounding some young, hot stars comes from
A) observation of the reddening of the spectra of these stars because of absorption of blue light by hydrogen.
B) observation of emission characteristic of red Balmer light from nebulosity around them.
C) theoretical calculations that correctly describe stellar formation by the gravitational contraction of hydrogen gas.
D) observation of the blue glow from scattered light in their reflection nebulae.
147. Long-exposure color photographs of the night sky often show regions that glow red, such as the Rosette Nebula in Figure 13-8 in the text. This distinctive red color is caused by the
A) emission of red and infrared light by warm dust grains.
B) collective glow of many red giant stars in the region.
C) scattering of starlight by dust grains in the nebula.
D) ionization and subsequent recombination of hydrogen atoms.
148. In an open cluster, the sizes of some of the lower-mass stars may have been limited when the surrounding material the stars were accreting was blown away by ultraviolet radiation. What is the source of this UV radiation?
A) The radiation sweeps generally through the Galaxy.
B) The radiation comes from supernova explosions outside the cluster.
C) The more massive O and B stars in the cluster evolve more quickly and produce ultraviolet radiation, while the less massive stars are still evolving toward the main sequence.
D) The ultraviolet radiation is emitted by the low-mass stars themselves when they begin hydrogen burning.
Section: 13-6
149. On a Hertzsprung–Russell diagram describing the stars in a young cluster, in which position would one expect to find the T Tauri stars?
A) well above the main sequence and to the left
B) just above and slightly to the right of the main sequence
C) well below the main sequence
D) upper right, well above the main sequence
150. The H-R diagram for an open cluster gives special information because all stars in an open cluster are
A) the same age.
B) the same mass.
C) moving in the same direction.
D) the same spectral type.
151. Which of these facts referring to stars in a cluster is NOT particularly useful for interpreting the evolution of these stars?
A) The stars are all at the same distance from Earth, so one can easily relate their apparent magnitudes to their true luminosities.
B) The stars all formed at about the same time.
C) The stars formed from the same mix of chemical elements but with a mix of original masses.
D) The majority of the material in these stars is hydrogen.
152. The stars in an open cluster are useful for studying the early stages of stellar evolution because all the stars in a cluster have the same
A) age.
B) spectral type.
C) luminosity class.
D) radius.
153. Why are the stars in an open cluster useful for studying just the EARLY stages of stellar evolution?
A) These stars are all massive and thus erupt in a supernova explosion before the later stages of evolution are reached.
B) There are many different paths a star can take after the main sequence, and open clusters contain so few stars that each of these paths is followed by only a few stars, making them statistically unreliable for study.
C) The stars in an open cluster tend to dissipate after a few tens of millions of years and cease to be a part of the group.
D) These stars all tend to be very small, and after the early stages they begin to experience mass loss and disappear from view.
154. Because all the stars in a cluster begin forming at the same time, the stars arrive on the main sequence at
A) the same time.
B) different times; more massive stars arrive first.
C) different times; less massive stars arrive first.
D) different times; later spectral types (G, K, M) arrive first.
155. If astronomers plotted the stars in a young star cluster on a Hertzsprung–Russell diagram, they would expect to see
A) all the stars on the main sequence.
B) the more massive stars above the main sequence and the less massive stars on the main sequence.
C) some stars on the main sequence and others above the main sequence, in random fashion, depending on when each star condensed from the interstellar cloud.
D) the more massive stars on the main sequence and the less massive stars above the main sequence.
156. A relatively fast star escapes an open cluster. What does this do to the stability of the remaining cluster?
A) The remaining cluster becomes MORE stable because only the slower stars are left.
B) The gravitational attraction of the escaping star tends to pull other stars after it, thus leading to the dissipation of the entire cluster.
C) The remaining cluster becomes LESS stable because the mass and thus the escape speed are reduced.
D) Because of the conservation of momentum, a second star must be expelled from the cluster in the opposite direction. The remaining cluster is then as stable as it was originally.
157. The stars of a cluster are plotted on an H-R diagram. It is seen that the main sequence is well populated at the upper end and that there are many stars above (and to the right of) the main sequence at its lower end. What can be concluded about the age of this cluster?
A) This cluster is very young.
B) This cluster is very old.
C) These characteristics could represent either a very young or a very old cluster.
D) This description is not sufficient to make any conclusions about the age of the cluster.
158. An astronomer plots the H-R diagram of a star cluster and finds that it contains hot B-type stars on the main sequence and cooler G- and K-type stars noticeably above the main sequence. This cluster is
A) of indeterminate age since the age of the cluster cannot be estimated from the information given.
B) impossible because cool stars cannot be above the main sequence when hot stars are on the main sequence.
C) old because the G and K stars are already evolving off (away from) the main sequence.
D) very young because the G and K stars are still evolving toward the main sequence.
159. When plotted on an H-R diagram, the more massive stars of a particular star cluster are on the main sequence and the less massive stars are above the main sequence. Can the age of this cluster be determined? (In other words, how long is it since these stars condensed from the interstellar medium?)
A) Yes, by finding the most massive star that is on the main sequence. The age of the cluster equals the time this star spent as a protostar.
B) No.
C) Yes, by finding the least massive protostar. The age of the cluster equals the time this star took to reach the main sequence.
D) Yes, by finding the least massive star on the main sequence. The age of the cluster equals the time this star spent as a protostar.
Section: 13-7
160. In a group of stars with a range of masses, stars with different mass
A) leave the main sequence of the Hertzsprung–Russell diagram at different times after their birth.
B) have about the same age, as massive stars burn their fuel more rapidly than low-mass stars.
C) all go through the supernova phase of evolution at varying times after their births.
D) all pass, at some stage in their evolution, through one particular point on the Hertzsprung–Russell diagram, occupied at present by the Sun.
161. In the Hertzsprung–Russell diagram, how does the position of a typical star change while it is at the main-sequence phase of its evolution?
A) Stars move from upper right to lower left while they are on the main sequence.
B) Stars move from upper left to lower right while they are on the main sequence.
C) Massive stars (4 solar masses) move toward the upper left as their luminosity increases, while lower-mass stars move toward the lower right as their temperature decreases.
D) A star’s position on the main sequence is determined only by its mass, not its age, so stars do not move significantly along the main sequence during evolution.
162. The definition of a main-sequence star is a star
A) with a surface temperature equal to that of the Sun.
B) in which nuclear fusion reactions generate sufficient energy to oppose further compression of the star.
C) whose age after birth is about 1 million years.
D) with a luminosity precisely equal to that of the Sun.
163. At what stage of its evolutionary life is the Sun?
A) main sequence, or “middle age”
B) pre–main sequence, variable star
C) just before supernova stage (perhaps 5 years), late evolutionary stage
D) post–main sequence, red giant (cool) phase
164. What major physical process is taking place inside stars that are on the main sequence in the Hertzsprung–Russell diagram?
A) Hydrogen is being converted to helium in their cores.
B) Hydrogen is being converted to helium in a shell around the helium-rich core.
C) Helium is being converted to carbon in their cores.
D) The gas is contracting gravitationally without nuclear reactions taking place.
165. Thermonuclear reactions convert hydrogen into helium in the core of a star during which phase of a star’s life?
A) main-sequence phase
B) protostar phase
C) horizontal-branch phase
D) as the star moves up the red giant branch for the first time
166. All stars on the main sequence
A) generate energy by hydrogen fusion in their centers.
B) have approximately the same age, to within a few million years.
C) are changing slowly in size, by gravitational contraction.
D) are at a late stage of evolution, after the red giant stage.
167. What is happening in a star that is on the main sequence on the Hertzsprung–Russell diagram?
A) The star is generating energy by helium fusion, having stopped hydrogen “burning.”
B) The star is slowly shrinking as it slides down the main sequence from top left to bottom right across the H-R diagram.
C) Stars that have reached the main sequence have ceased nuclear “burning” and are simply cooling down by emitting radiation.
D) The star is generating internal energy by hydrogen fusion.
168. The “zero-age main sequence” describes the sequence of stars in the Hertzsprung–Russell diagram that
A) contain no processed elements (elements heavier than hydrogen or helium).
B) are just beginning to convert helium into carbon in their cores.
C) have just collapsed from the interstellar medium.
D) have just become stable objects.
169. Why is it that the majority of stars in the sky are in the main-sequence phase of their lives?
A) Most stars die at the end of the main-sequence phase.
B) The main-sequence phase is the longest-lasting phase in each star’s life.
C) The main-sequence phase is the only phase that is common to all stars.
D) Most stars in the sky were created at about the same time, so these are all in the same phase of their lives.
170. How is the length of a star’s lifetime related to the mass of the star?
A) Lower-mass stars run through their lives faster and have shorter lifetimes.
B) The lifetimes of stars are too long to measure, so it is not known how (or if) their lifetimes depend on mass.
C) A star’s lifetime does not depend on its mass.
D) Higher-mass stars run through their lives faster and have shorter lifetimes.
171. The stars that last longest are the stars
A) with the largest luminosity and highest temperature, since they take the longest to cool down to invisibility.
B) with the largest mass, that is, the largest amount of fuel.
C) with the smallest mass.
D) of intermediate mass; small-mass stars have little fuel and burn out quickly, while very massive stars burn their fuel very rapidly.
172. In terms of the mass and lifetime of a star, which of these statements is true?
A) The mass of a star has no bearing on the length of a star’s life or the speed of its evolution.
B) Stars of about 1 solar mass have the shortest lives; less massive stars evolve slowly and live a longer time, while more massive stars have long lives because of the large amount of fuel they contain.
C) The less massive the star, the shorter is its life because it has less hydrogen “fuel” to burn.
D) The more massive the star, the faster it evolves through its life.
173. If the universe is only 14 billion years old, which of these groups of stars have never moved beyond the main sequence?
A) stars with masses less than 0.08 solar mass
B) stars with masses less than 0.75 solar mass
C) stars with masses greater than 100 solar masses
D) stars with masses greater than 10 solar masses
174. What is the MOST important property of a star that governs its evolution and lifetime?
A) mass
B) surface temperature
C) abundance of heavy elements
D) speed of rotation
175. How long will the Sun have spent as a main-sequence star when it finally begins to evolve toward the red giant phase?
A) 1 billion years
B) 1 million years
C) 1011 years
D) 1010 years
176. The total time the Sun will spend as a main-sequence star is
A) at least 200 billion years (2 1011) years.
B) about 1 million years.
C) about 4.5 million years.
D) about 10 billion years (1010 years).
177. Approximately what fraction of its main-sequence lifetime has the Sun completed at the present time?
A) about 1/4
B) less than 10 percent
C) about 1/2
D) about 3/4
178. What does the term “hydrostatic equilibrium” mean in reference to stars?
A) The pressure throughout a star is constant.
B) The pressure within a star is sufficient to cause the star to expand at a constant rate.
C) The gravitational force within a star is sufficient to cause the star to collapse inward at a steady rate.
D) Each layer within a star is in balance with respect to pressure and gravity.
179. When does a star achieve hydrostatic equilibrium?
A) when it becomes a main-sequence star
B) when it leaves the main sequence
C) when it becomes a protostar
D) This question has different answers for stars of different masses. Massive stars never achieve equilibrium; small stars achieve hydrostatic equilibrium as soon as they reach the main sequence.
180. Which of these statements about the rate of stellar evolution is true?
A) The more massive the original star, the faster is the evolution.
B) The chemical makeup of the original nebula is the major factor in deciding the rate of evolution, whatever the mass of the star.
C) Star mass has no bearing on stellar evolution since all stars evolve at the same rate, controlled by nuclear fusion.
D) The more massive the original star, the slower is the evolution since there is more material for thermonuclear burning.
181. A star is observed to be on or close to the main-sequence region of the Hertzsprung–Russell diagram. What particular feature of stellar behavior can be inferred by this observation?
A) The star is generating internal energy by hydrogen fusion.
B) The star is generating energy by helium fusion, having stopped hydrogen “burning.”
C) The star is slowly shrinking and thereby releasing gravitational potential energy as it slides down the main sequence from top left to bottom right across the H-R diagram.
D) Stars that have reached the main sequence have ceased nuclear “burning” and are simply cooling down by emitting radiation.
182. The main-sequence lifetime of a star with half the mass of the Sun
A) could be longer or shorter than that of the Sun; more information is required to determine the length of its lifetime.
B) is longer than that of the Sun.
C) is the same as that of the Sun because mass does not affect the lifetime of a star.
D) is shorter than that of the Sun.
183. How long does a star of 3 times the Sun’s mass live compared with the lifetime of the Sun?
A) A star of 3 times the Sun’s mass lives about 1/3 as long as the Sun.
B) A star of 3 times the Sun’s mass lives about 1/20 as long as the Sun.
C) A star of 3 times the Sun’s mass lives about 3 times as long as the Sun.
D) A star of 3 times the Sun’s mass lives about 1/500 as long as the Sun.
184. The present estimate of the age of the universe is about 14 billion years. How long will a half-solar-mass star spend on the main sequence, compared with the present age of the universe?
A) about 3 times the age of the universe
B) more than 10 times the age of the universe
C) less than 1/10 of the age of the universe
D) about 1/2 of the age of the universe
185. The star Zeta Pegasi (in the constellation Pegasus, the Flying Horse) has a spectral luminosity class of B8 V, which gives it a surface temperature of about 11,000 K. According to Table 13-2 in the text, the expected main-sequence lifetime of Zeta Pegasi is
A) much greater than 500 million years.
B) about 500 million years.
C) not defined by this information.
D) about 500,000 years.
186. The spiral galaxy of which Earth is a part is roughly 14 billion years old. Which of these statements about the stars that formed when the Milky Way Galaxy was very young (say, during the first billion years) is true?
A) No stars are still on the main sequence—in this time all stars have finished their lives.
B) All stars of less than 3/4 of the mass of the Sun are still on the main sequence.
C) All stars of more than 15 times the mass of the Sun are still on the main sequence.
D) All stars of 3 times the mass of the Sun are still on the main sequence.
187. If one were able to visit Earth one million years into the future, which of these views of the sky would be MOST likely?
A) A few red stars would be missing since they would have evolved, but otherwise the stars would be the same and be in the same positions as they are today.
B) Nearby stars would have moved in position, and no blue stars would be visible.
C) Nearby stars would have moved in position and some stars that are presently blue would have changed color, but otherwise the sky would be very much as it is now since 1 million years is a very short time in astronomical terms.
D) All the present stars, both blue and red, would be visible, but nearby stars would have moved in position.
188. When the Sun entered the main sequence, its core was approximately 75 percent hydrogen and 25 percent helium, by mass. What was the ratio of hydrogen nuclei to helium nuclei?
A) 3:1
B) 4:1
C) 12:1
D) 16:1
189. How long ago was the Sun a ZAMS star?
A) 10 billion years
B) 5 billion years
C) 60 million years
D) 187 million years
190. Main-sequence stars can be divided into those with more than 75 percent of the mass of the Sun and those with less than this amount of mass. What is different about these two groups?
A) The less massive group does not have enough mass to undergo nuclear fusion.
B) Stars in the less massive group are all fully convective.
C) Stars in the less massive group are all free-floating bodies that have never been part of a binary system.
D) Some of the stars in the more massive group have evolved off the main sequence while none of the stars in the less massive group have done so.
191. What is unusual about main-sequence stars that are about 75 percent the mass of the Sun?
A) These stars fuse deuterium and lithium instead of hydrogen.
B) These stars are too small to develop any nuclear reactions.
C) None of these stars has yet evolved off the main sequence.
D) These stars develop fission reactions rather than fusion reactions in their cores.
192. The total lifespan of the Sun is believed to be
A) a few million years.
B) half a billion years.
C) 10 billion years.
D) infinite.
Section: 13-8
193. The final stage in the evolution of a red dwarf star is
A) a star of pure helium that generates energy by fusing helium into carbon.
B) a star of pure helium that no longer generates energy by nuclear fusion.
C) a red giant.
D) unknown because no red dwarf has ever evolved off the main sequence.
194. The end of the life of a red dwarf star is predicted to be a sphere of almost pure helium. But no such spheres have been detected. What do astronomers believe is the reason?
A) These helium stars are very dim and consequently hard to detect.
B) There are very few red dwarfs, so their end products are expected to be rare.
C) The evolution rate for red dwarfs is so slow that none has yet evolved to its end stage.
D) Helium is a relatively light material, so these helium spheres are expected to dissipate in a short time.
195. What is unique about a low-mass red dwarf star compared with stars at other stages of evolution, such as main-sequence stars?
A) A low-mass red dwarf is the smallest in size of any star.
B) Nuclear fusion occurs not in the core of a red dwarf but in a region just beneath its surface.
C) A red dwarf has the lowest surface temperature of any star.
D) Convection occurs throughout the interior of a red dwarf such that the star does not develop separate core and outer layer regions.
196. Some protostars, when they evolve onto the main sequence, are fully convective and use convection to transport material all the way from the core to the surface. Which stars show this characteristic?
A) only the smallest, of less than about 0.4 solar mass
B) intermediate stars, of about 0.4 solar mass up to about 4 solar masses
C) the largest, of more than about 4 solar masses
D) all protostars
197. It is now believed that MOST stars are
A) isolated and not in binary systems.
B) in binary systems.
C) more massive than the Sun.
D) red giants.
198. Red dwarfs are fully convective. In this, they are like
A) white dwarfs.
B) brown dwarfs.
C) the Sun.
D) variable stars.
199. Of all the low-mass red dwarfs (0.08 M < M < 0.4 M) that have formed since the Galaxy came into being 15 billion years ago, how many are presently on the main sequence?
A) none
B) a few, less than 20%
C) most of them, more than 80%
D) all
200. A low-mass red dwarf (0.08 M < M < 0.4 M) experiences which of these fusion reactions as the last fusion reaction in its core?
A) hydrogen fusion
B) helium fusion
C) carbon fusion
D) silicon fusion
201. A low-mass red dwarf (0.08 M < M < 0.4 M) will eventually NOT become
A) a red giant.
B) a brown dwarf.
C) a white dwarf.
D) a red giant, a brown dwarf, or a white dwarf.
202. It is now believed that MOST stars are
A) members of binary systems.
B) members of triple systems.
C) solitary stars.
D) variable stars.
203. Which of these objects is NOT fully convective?
A) brown dwarf
B) low-mass main-sequence star
C) Sun
D) All objects undergoing fusion are fully convective.
Section: 13-9
204. If an astronomer were to look at 1 kilogram of material taken from the surface of the Sun and 1 kilogram taken from the center, which of these statements would be true of the two kilograms?
A) Both kilograms have the same amount of hydrogen and are in fact mostly hydrogen.
B) The kilogram from the surface contains more hydrogen than the one from the center.
C) Neither kilogram contains any hydrogen.
D) The kilogram from the surface contains less hydrogen than the one from the center.
205. Why does the core of the Sun contain more helium and less hydrogen than the surface of the Sun?
A) The hydrogen has been lifted out of the core by the Sun’s magnetic field.
B) Helium is heavier than hydrogen and has sunk toward the center in a process of chemical differentiation.
C) Helium condenses more easily, so when the Sun was forming the core became helium-rich; vast quantities of hydrogen were added only after the core became massive enough.
D) Thermonuclear reactions have converted much of the original hydrogen in the core into helium.
206. When a star leaves the main sequence and expands toward the red giant region, what is happening inside the star?
A) Helium burning is taking place in a spherical shell just outside the core; the core itself is almost pure carbon and oxygen.
B) Hydrogen burning is taking place in a spherical shell just outside the core; the core itself is almost pure helium.
C) Hydrogen burning is taking place in a spherical shell just outside the core; the core has not yet started thermonuclear reactions and is still mostly hydrogen.
D) Helium is being converted into carbon and oxygen in the core.
207. In about 5 billion years the core of the Sun will be composed of
A) hydrogen.
B) helium.
C) carbon.
D) silicon.
208. In terms of nuclear reactions, what is the next stage of a star’s life after the end of hydrogen burning in the core?
A) hydrogen burning in a thin shell around the core
B) helium burning in the core
C) carbon burning
D) death (it becomes either a supernova or a white dwarf)
209. The next stage in the life of a star like the Sun, after the main-sequence phase, is
A) the red giant phase.
B) the horizontal-branch phase.
C) a protostar.
D) death (i.e., either a supernova or a white dwarf).
210. The energy emitted by a star increases as the fourth power of its surface temperature, and a red giant star has a lower temperature than the Sun. Why then is the luminosity of a red giant about 1000 times greater than that of the Sun?
A) A red giant is bigger and therefore has a much larger surface area.
B) The energy transfer from a red giant’s interior is much more efficient than the transfer is in the Sun.
C) More of a red giant’s total output of energy is in the visible range than is the energy of the Sun, and hence the red giant appears brighter.
D) A red giant’s nuclear furnace produces much more energy than does that of the Sun.
211. When a star consumes all of the energy it can from hydrogen fusion in its core,
the next long-term energy-producing phase is
A) helium fusion in the core.
B) helium fusion in a shell around the core.
C) hydrogen fusion in a shell around the core.
D) gravitational energy released in a Kelvin–Helmholtz condensation.
212. Which of these objects are NOT very young stars or prestellar objects?
A) red giants
B) infrared-emitting stars in gas and dust clouds
C) protostars
D) T Tauri stars
213. Compared with the composition of the early Sun, the composition of the gas at the core of a star that has just become a red giant is
A) very different, since thermonuclear fusion has transformed all the hydrogen and helium into heavier elements.
B) very different; there is lots of hydrogen but almost no helium left after thermonuclear fusion.
C) very different, since thermonuclear fusion has transformed all the hydrogen into helium.
D) the same, with a higher fraction of hydrogen than helium since these stars were produced with the same initial material.
214. What makes a red giant star so large?
A) The helium-rich core has expanded, pushing the outer layers of the star outward.
B) The hydrogen-burning shell is heating the envelope and making it expand.
C) Red giants are rapid rotators, and centrifugal force pushes the surface of the star outward.
D) The star has many times more mass than the Sun.
215. Over which of these stages of stellar evolution does the radius of a star remain approximately constant?
A) main-sequence phase
B) red giant phase
C) birth and initial formation
D) The radius of a star is approximately constant in ALL of these phases.
216. In the first stage after main-sequence hydrogen fusion, the envelope of the star is pushed outward to expand. The nuclear reactions that produce the outward pressure for this expansion are
A) hydrogen fusion in the core.
B) hydrogen fusion in the shell.
C) helium fusion in the core.
D) helium fusion in the shell.
217. What rates of mass loss from red giant stars are typical?
A) 10–14 solar mass per year
B) 10–3 solar mass per year
C) Red giant stars do not suffer mass loss.
D) 10–7 solar mass per year
218. Which of these statements characterizes the early stages of a red giant?
A) large, red star that is burning hydrogen into helium in its core
B) protostar in the upper right of the Hertzsprung–Russell diagram
C) large-emission nebula
D) star that is burning hydrogen into helium in a shell around the core
219. When a star like the Sun expands to become a red giant, its surface will move outward at about 10 km/s. For an observer looking directly at the Sun, what will be the Doppler-shifted wavelength of the H line (in nm)? From a source at rest, the H wavelength is 656.28 nm.
A) 646.28
B) 656.26
C) 656.30
D) 666.28
220. How large will the Sun be as a red giant?
A) about 1 au radius (out to Earth’s orbit)
B) about 1/10 au radius (1/4 of Mercury’s orbit)
C) about 1.5 au radius (out to Mars’s orbit)
D) about 1/2 au radius (beyond Mercury’s orbit)
221. When core helium burning begins in the Sun, the Sun will have a size (radius) roughly equal to
A) half the size of Earth’s orbit.
B) the size of Earth.
C) half its present size.
D) twice its present size.
Section: 13-10
222. What happens to the helium-rich core of a star after the core runs out of hydrogen?
A) The core cools down and contracts.
B) The core expands and cools down.
C) The core heats up and expands.
D) The core contracts and heats up.
223. After the helium flash in a low-mass star,
A) all nuclear reactions cease.
B) helium burning ceases in the core, but hydrogen burning continues in the layers around the core.
C) helium burning begins in the core, and hydrogen burning ceases in the layers around the core.
D) helium burning begins in the core, and hydrogen burning continues in the layers around the core.
224. Will the Sun undergo a helium flash?
A) Yes.
B) No. Only stars more massive than the Sun experience a helium flash.
C) No. Only stars less massive than the Sun experience a helium flash.
D) No. The Sun is now in the correct mass range to experience a helium flash, but by the time it ends its life on the main sequence it will have lost enough mass that it will no longer be in the correct range.
225. Consider stars of these masses (in solar masses). Which one would be expected to experience a helium flash?
A) 0.2
B) 1.2
C) 2.6
D) 4.2
226. Consider stars of these masses (in solar masses). Which one would NOT be expected to experience a helium flash?
A) 0.2
B) 0.8
C) 1.2
D) 1.8
227. How hot does the core of a star have to be for helium burning to begin?
A) 100 million K
B) 1 billion K
C) 1 million K
D) 10 million K
228. The temperature at which thermonuclear reactions begin to convert helium into carbon (helium burning) is
A) 15 million K.
B) 100 million K.
C) 1 billion K.
D) 1 million K.
229. What is the Pauli exclusion principle?
A) Photons of less than a certain wavelength cannot eject electrons from a metal.
B) Two identical particles cannot occupy the same place at the same time.
C) Two different atoms cannot have the same energy levels.
D) Two identical photons cannot be absorbed by the same atom.
230. The Pauli exclusion principle states that two identical electrons (or other particles) cannot have the same
A) electrical charge at the same time.
B) speed at the same time.
C) position, speed, and direction at the same time.
D) speed and direction at the same time.
231. When is electron degeneracy pressure important in a star like the Sun?
A) just before the start of helium burning in the core
B) during core hydrogen burning
C) in a Cepheid variable that is burning helium in its core
D) in a protostar evolving toward the main sequence
232. The Pauli exclusion principle
A) prevents high-mass stars from forming in low-mass interstellar clouds.
B) sets a limit to the crowding together of electrons in any given small volume of space.
C) limits the number of atoms that can become ionized in a star.
D) prevents two stars of the same spectral class from occupying the same binary star system.
233. What is the “safety valve” that prevents normal (nondegenerate) stars from self-destructing?
A) If the pressure rises, the volume occupied by the matter decreases, reducing the nuclear reaction rate.
B) If a part of a star where thermonuclear reactions are occurring is suddenly heated, it expands and cools.
C) If thermonuclear reactions proceed too quickly, the star runs out of fuel before anything drastic happens.
D) If the temperature rises, the thermonuclear reaction rate increases to match it.
234. What is the “safety valve” that operates in normal (nondegenerate) stars?
A) If thermonuclear reactions proceed too quickly, the star runs out of fuel before anything drastic happens.
B) If the stellar gas is suddenly heated, it expands and cools.
C) If the star gets too big, it collapses into a black hole.
D) If the pressure becomes too high, electrons combine with protons to relieve the pressure.
235. A degenerate-electron gas, which occurs in the core of a red giant star, lacks the “safety valve” of a normal gas because a rise in
A) pressure releases more electrons, thus increasing the pressure further.
B) pressure reduces the number of electrons, causing the core to collapse.
C) temperature does not change the pressure, so the gas does not expand and cool.
D) temperature lowers the pressure, causing the star to contract.
236. Under what conditions does electron degeneracy occur?
A) when electrons become crowded too closely together
B) when thermonuclear reactions release more electrons than protons
C) when electrons and positrons annihilate, releasing energy
D) when ultraviolet light from hot, young O and B stars ionizes the interstellar medium
237. Degeneracy occurs when
A) electrons inside a star resist being pushed closer together than a certain limit.
B) solar wind particles become trapped in Earth’s magnetic field.
C) magnetic fields inhibit the motion of charged particles in sunspots.
D) thermonuclear reactions halt the contraction of a protostar.
238. If electrons are collectively compressed into a very small volume where the Pauli exclusion principle become important in preventing one electron from occupying space near a second electron, what is the result?
A) The electrons generate a very large pressure to oppose further compression.
B) The electrons fall into orbit around one another in mutual pairs that reduce the restricted quantum space, allowing further shrinkage of the star.
C) Nuclear fusion occurs between electrons, producing energy and heating the star’s core.
D) Half the electrons are transformed into antimatter (positrons), which then annihilates electrons, producing a burst of energy and the explosion of the star.
239. Electron degeneracy, which is a result of the Pauli exclusion principle, is important in
A) a Cepheid variable that is burning helium in its core.
B) the core of a star during core hydrogen burning.
C) a protostar evolving toward the main sequence.
D) the core of a star just before the start of core helium burning.
240. The helium flash results from the
A) sudden release of energy in strong magnetic fields near a sunspot.
B) electron degeneracy or quantum crowding in the core of a low-mass red giant star.
C) sudden onset of nuclear reactions at the end of the protostar.
D) high temperature in the helium core of a blue (spectral class O or B) supergiant star.
241. Which component in the core of a star undergoing helium fusion leads to the behavior known as the “helium flash?”
A) neutrinos, since it is this component that, in interacting with helium nuclei, triggers the helium flash
B) helium, since its density has to be above a certain limit to trigger this explosive event
C) electrons, since they control the pressure, and their pressure does not depend on temperature. Under these conditions, buildup of heat can occur without leading to the normal gas expansion and the resulting thermal equilibrium.
D) remnant hydrogen since loss of this component allows uncontrolled helium burning rather than a uniform rate of energy production
242. The helium flash is another name for a sudden
A) release of energy at the end of helium burning due to core contraction.
B) onset of helium fusion reactions in red giant and supergiant stars of any mass.
C) onset of helium fusion reactions in the core of a low-mass red giant star.
D) appearance of helium during a supernova explosion (explosion of a star at the end of its life).
243. Which stars undergo a helium flash in their cores?
A) stars of less than 2 solar masses
B) stars that contain helium
C) stars of more than 2 solar masses
D) stars that have become red giants
244. The helium flash occurs in a 2-solar-mass star when
A) the core heats up from helium fusion and the gas does not expand to cool down and stabilize the temperature because the gas properties are controlled by degenerate electrons.
B) convection within the star carries helium to the surface, where it shows its own characteristic spectrum for the first time.
C) hydrogen fusion ceases abruptly, leaving only the fusion of helium to generate energy.
D) the core temperature becomes sufficiently high for helium fusion for the first time.
245. What process is involved in the helium flash?
A) An increase in temperature causes an increase in the nuclear reaction rate but has no effect on pressure.
B) An increase in the pressure causes a decrease in the temperature and the nuclear reaction rate.
C) An increase in temperature causes an increase in the pressure with no increase in the nuclear reaction rate.
D) An increase in the nuclear reaction rate causes an increase in the temperature and the pressure.
246. What happens to a star after the start of helium nuclear reactions in its core compared with what it was like before these reactions began?
A) The star is larger and hotter.
B) The star is larger and cooler.
C) The star is smaller and hotter.
D) The star is smaller and cooler.
247. What happens to the luminosity of a low-mass star as it passes through a helium flash?
A) The luminosity, which was increasing slowly before the helium flash, suddenly brightens by several orders of magnitude.
B) The luminosity, which was decreasing slowly before the helium flash, suddenly brightens by several orders of magnitude.
C) The helium flash affects only the core. The luminosity of the star remains unchanged for a long time after the flash.
D) The luminosity, which was rising before the helium flash, drops quickly after the helium flash.
248. After hydrogen burning ends in a star’s core, its position on the Hertzsprung–Russell diagram moves toward the
A) lower right.
B) upper left.
C) lower left.
D) upper right.
249. Which of these events does NOT happen in a star in the course of its evolution?
A) buildup of heavy elements from fusion of hydrogen
B) loss of mass as radiation
C) loss of mass through winds
D) buildup of hydrogen from breakdown of heavier elements by fission
250. After it leaves the main sequence, a star of more than 1 solar mass moves generally to the right on the H-R diagram because the star’s
A) surface temperature is increasing while its radius is decreasing.
B) surface temperature is decreasing while its radius is increasing.
C) core is expanding while its outer layers are contracting.
D) core is expanding, causing its outer layers to expand as well.
251. On the Hertzsprung–Russell diagram, in which direction does the position occupied by a star move after helium burning begins in the star’s core?
A) toward the upper right
B) toward the lower left
C) toward the lower right
D) toward the upper left
Section: 13-11
252. Nuclear fusion reactions that convert helium into carbon and oxygen in the central core of a star occur during which phase of a star’s life?
A) in the red giant stage, before the helium flash
B) during and immediately after the (first) red giant or supergiant stage
C) during the protostar stage
D) after the main-sequence phase, before the star becomes a red giant
253. What is the dominant nuclear reaction during helium burning in the core of a star?
A) 1H + 4He combine to form C.
B) 4He fuses to form C.
C) 2He fuses to form C.
D) 3He fuses to form C.
254. During helium burning in a star’s later life, the chemical element produced by the combination of helium nuclei is
A) the light isotope of helium, 3He.
B) heavy hydrogen, 2H.
C) beryllium, 8Be.
D) carbon, 12C.
255. What are the products of helium burning in a star?
A) magnesium and silicon
B) nitrogen and oxygen
C) carbon and oxygen
D) hydrogen and lithium
256. The next lengthy period in the evolution of a star after hydrogen fusion is helium fusion. What is the main product of this helium fusion?
A) deuterium
B) beryllium
C) carbon
D) oxygen
257. At what stage of evolution are the stars with similar luminosities between 50 and 100 times that of the Sun but spread over a range of temperatures?
A) main-sequence stage because the horizontal-branch region is part of the main sequence
B) pre–main-sequence stage, evolving toward the main sequence
C) white dwarf stage
D) post–main-sequence stage
258. How long does core helium burning last in a star compared with core hydrogen burning?
A) about 1/2 of the time spent burning hydrogen
B) about 1/10 of the time spent burning hydrogen
C) about 1/100 of the time spent burning hydrogen
D) about 3 times as long as the time spent burning hydrogen
259. In a list of the 100 brightest stars in the sky, MOST of them are
A) white dwarfs.
B) main-sequence stars.
C) giants and supergiants.
D) brown dwarfs.
260. What is the luminosity class of most of the stars visible to the naked eye? Are these actually the MOST numerous stars?
A) Most visible stars are main-sequence stars; yes, they are the majority of all stars.
B) Most visible stars are main-sequence stars; but most stars are actually giants.
C) Most visible stars are giants; but most stars are actually main-sequence stars.
D) Most visible stars are giants; yes, most stars are actually giants.
261. In a list of the 100 nearest stars in the sky, MOST of them are
A) white dwarfs.
B) main-sequence stars.
C) giants and supergiants.
D) brown dwarfs.
262. About 3000 stars are visible to the unaided eye on a clear night, and the majority of these are giants and supergiants. However, theories predict the most numerous stars should be red dwarfs. Is there an explanation for this discrepancy? If so, what is it?
A) The Sun’s location in the galactic plane is particularly rich in gas and dust, so an unusually large number of giants and supergiants formed here.
B) Early in the history of the Milky Way Galaxy, a shock wave passed through the Sun’s part of the spiral arm creating a large number of stars all at one time. These stars are now mostly in their giant phases.
C) The bright giants and supergiants are the most likely to be seen, even though they are not in the majority numerically.
D) The explanation for this discrepancy is not well understood.
263. When a star’s evolutionary track in the Hertzsprung–Russell diagram carries it into the instability strip, what happens to the star?
A) The star pulsates randomly in brightness.
B) The star collapses and forms a black hole.
C) The star explodes.
D) The star pulsates regularly in brightness.
264. Which of these do NOT characterize Cepheids and RR Lyrae stars?
A) giants
B) unstable
C) supernova remnants
D) variable
265. What are the stars in the upper part of the instability strip called?
A) protostars
B) T Tauri stars
C) RR Lyrae variables
D) Cepheid variables
266. What are the stars in the lower part of the instability strip called?
A) T Tauri stars
B) protostars
C) Cepheid variables
D) RR Lyrae variables
267. The instability strip on the H-R diagram
A) includes part of the main sequence.
B) is passed through by stars on their way to the helium flash.
C) involves only giant stars.
D) is where stars erupt as novas and supernovas.
268. RR Lyrae stars are
A) pulsating stars that vary regularly, all with periods of less than 1 day.
B) rotating neutron stars with very rapid and regular brightness fluctuations.
C) eclipsing binary stars.
D) pulsating stars that vary irregularly, with periods of several hundred days.
Section: 13-12
269. What is a Cepheid variable star?
A) one of several classes of stars that pulsate randomly in brightness
B) low-mass, horizontal-branch star that pulsates regularly in brightness
C) star that normally remains constant in brightness but occasionally flares up in brightness by several magnitudes
D) high-mass star that pulsates regularly in brightness
270. Cepheid stars are
A) giant stars that pulsate in brightness, size, and temperature.
B) white dwarf stars late in their evolutionary life.
C) members of binary systems in which one star periodically eclipses the other.
D) stars at an early stage in stellar evolution, pre–main-sequence.
271. A Cepheid variable is a
A) type of eclipsing binary star.
B) high-mass giant or supergiant star that pulsates regularly in size and brightness.
C) low-mass red giant that varies in size and brightness in an irregular way.
D) variable emission nebula near a T Tauri star.
272. Suppose a Cepheid oscillates from an initial state 1 to a state 2 where its radius has decreased 10 percent, and its temperature has increased 10 percent compared with the values at state 1. How do the luminosities at states 1 and 2 compare?
A) They are the same.
B) The luminosity is greater at position 1.
C) The luminosity is greater at position 2.
D) It is not possible to compare the luminosities with the information given.
273. What physical mechanism is responsible for the oscillations in brightness that a Cepheid variable star undergoes?
A) the fusion rate in the star’s core varies periodically, changing the energy generation rate
B) the amount of ionized helium in the star’s outer layers vary, changing the opacity
C) the fusion rate in the shell burning hydrogen varies, changing the energy generation rate
D) the star’s outer layers cycle between convection and radiative transport, changing the temperature
274. What scientific method is used to observe the pulsation in size of a Cepheid variable star?
A) This behavior has only been predicted theoretically; it has never been detected.
B) observed perturbations in the orbits of planets around the star
C) observed increase and decrease in the size of the star’s image
D) Doppler shift of absorption lines in the star’s spectrum
275. When a variable star periodically changes its luminosity, many of its properties also change. Which of these does NOT change?
A) temperature
B) radial surface speed
C) size
D) rotation rate
276. Cepheid variables pulsate because they contain a layer that alternately blocks photons while heating and then becomes transparent to photons as it expands. What substance composes this layer?
A) hydrogen
B) deuterium
C) ionized helium
D) twice ionized carbon
277. In a Cepheid variable, what happens to make its temperature rise?
A) A layer of ionized helium absorbs photons.
B) The fusion reaction switches from hydrogen fusion to helium fusion.
C) The hydrogen fusion in the shell increases as it receives a periodic burst of convection current-carried fuel.
D) The electron degeneracy in the core refuses to allow the core to expand in response to energy generation.
278. In a Cepheid variable, what happens to make the temperature decrease?
A) Massive solar flares release great amounts of energy, after which the star goes through a partial collapse.
B) Hydrogen fusion in the shell begins to shut down because it is running low on fuel.
C) A periodic eruption of sunspots blocks the radiation from below.
D) The ionized helium layer expands and cools.
Section: 13-13
279. What characteristic of Cepheid variables makes them extremely useful to astronomers?
A) The absolute magnitude of Cepheid variables is related directly to their period of pulsation.
B) The absolute magnitude of Cepheid variables is directly related to their diameter.
C) The absolute magnitude of Cepheid variables is related directly to their metal content (heavy element abundance).
D) The absolute magnitude of Cepheid variables is related directly to their surface temperature.
280. What is the difference between a Type I Cepheid ( Cepheid star) and a Type II Cepheid?
A) Type I Cepheids are main-sequence stars, while Type II Cepheids are giant stars.
B) Type I Cepheids are short-period variables, while Type II Cepheids are long-period variables.
C) Only Type I Cepheids have a simple relationship between luminosity and period.
D) Type I Cepheids are brighter metal-rich stars, while Type II Cepheids are dimmer metal-poor stars.
281. The periods of Type I Cepheids and Type II Cepheids are different for stars of the same luminosity. What is the physical difference between these stars that causes the periods to be different?
A) Type II Cepheids are older, having evolved from Type I Cepheids.
B) The two types have different metallicities.
C) Type I Cepheids are members of binary systems, whereas Type II Cepheids are solitary stars.
D) Type II Cepheids have gone through the helium flash, whereas Type I Cepheids have yet to do so.
282. The period of variability of a Cepheid variable star, which is easily measured, is directly related to which stellar parameter, thereby providing a reliable method for the measurement of distance to stars?
A) luminosity
B) velocity away from Earth
C) surface temperature
D) surface magnetic field
283. What is the MOST important use of Cepheid variables for astronomers?
A) The distance to a Cepheid variable can be found very easily.
B) The characteristics of the pulsation of a Cepheid variable can be used to investigate conditions in the core of the star.
C) The diameter of a Cepheid variable can be found very easily.
D) The metal content of a Cepheid variable can be found very easily.
284. Once astronomers obtain the apparent magnitude for a Cepheid variable star by direct observation and the absolute magnitude from the period-luminosity relation, what do they need to use to find its distance?
A) the H-R diagram (spectroscopic parallax)
B) the distance-magnitude relationship
C) the Stefan–Boltzmann relationship
D) parallax
285. Why should the period-luminosity relationship be different for Type I Cepheids and Type II Cepheids?
A) Type II Cepheids are dwarf stars and thus much less luminous than Type II giants.
B) Type I Cepheids are metal-rich and thus have more heavy elements in their atmospheres.
C) Type II Cepheids are metal-rich and thus have more heavy elements in their atmospheres.
D) Type I Cepheids get their energy primarily from hydrogen fusion while Type II Cepheids get their energy primarily from helium fusion.
Section: 13-14
286. The study of stars in clusters has especially helped astronomers to understand
A) stellar evolution, the development of stars with time.
B) the mechanism of mass loss in stars.
C) the reason for differences in surface temperatures of stars.
D) the action of nuclear fusion in stars.
287. What are the main general features that make clusters of stars useful to astronomers?
A) The stars all have the same apparent magnitude, the same surface temperatures, and the same sizes.
B) The stars are all at the same distance from Earth, have the same surface temperature, and joined the cluster at various times.
C) The stars all have the same intrinsic brightness but differ in size and surface temperature.
D) The stars are at the same distance from Earth, were formed at approximately the same time, and were made from the same chemical mix.
288. How many stars are there in a large globular cluster?
A) about 1 million
B) about 1000
C) up to 100 billion
D) only a few dozen
289. The characteristic of a globular cluster of stars is
A) hundreds of thousands of members, all very old and generally metal-poor.
B) a few dozen members, the remnant of a globular cluster of stars from which most of the members have escaped.
C) many thousands of members of different ages.
D) a few hundred members, often very young and still embedded in the gas and dust from which they were formed.
290. What would one expect to find in the population of stars in a globular cluster?
A) mainly white dwarf stars and dying stars but no faint red stars, red giants, or bright blue stars
B) mostly blue giants and supergiants and a few red giant stars, white dwarfs, and dim red stars
C) many red giants, white dwarfs, and dim red stars
D) stars along the entire main sequence from bright blue to dim red, with no bright red giant stars but significant amounts of dust and gas
291. Why does the Hertzsprung–Russell diagram of a globular cluster NOT contain any stars with high luminosity and high temperature on the main sequence?
A) These high-mass stars evolved away from the main sequence long ago.
B) Stars that will occupy this position on the main sequence have not yet evolved there from the protostar stage.
C) The stars that were in this position on the main sequence have undergone splitting into binary stars and hence appear lower down on the diagram.
D) This type of cluster contains only low-mass stars and has never had high-luminosity stars on its main sequence.
292. Which of these statements about a globular cluster is NOT true?
A) A globular cluster has a round shape.
B) A globular cluster can contain up to 1 million stars.
C) A globular cluster contains many main-sequence stars.
D) A globular cluster contains both high- and low-mass stars on the main sequence.
293. Which of these statements about a globular cluster is NOT true?
A) A globular cluster can contain up to 1 million stars.
B) A globular cluster contains only low-mass stars on the main sequence.
C) A globular cluster does not contain main-sequence stars.
D) A globular cluster has a round shape.
294. How do astronomers know that globular clusters are very old?
A) Globular clusters do not contain any red giant stars.
B) There are no main-sequence stars in globular clusters.
C) The stars in a globular cluster have a high abundance of heavy elements.
D) There are no massive main-sequence stars in globular clusters.
295. Horizontal-branch stars have a wide range of temperatures and luminosities between 50 and 100 times that of the Sun, so they are in what stage of their lives?
A) gravitational contraction before the start of core hydrogen burning
B) hydrogen shell burning, with a degenerate helium core
C) core helium burning and shell hydrogen burning
D) core helium burning only
296. Where do horizontal-branch stars get their energy?
A) They have run out of nuclear fuel and must rely upon the Kelvin–Helmholtz mechanism.
B) They have hydrogen fusion in their cores.
C) They have helium fusion in their cores only.
D) They have both core helium fusion and shell hydrogen fusion.
297. Which of these stars are metal-rich?
A) Population I stars
B) very old stars
C) Population II stars
D) globular cluster stars
298. How do the stars in a star cluster change with time?
A) All stars in a cluster evolve at the same rate.
B) The stars with the greatest heavy element content evolve the most quickly.
C) The lowest-mass stars evolve the most quickly.
D) The highest-mass stars evolve the most quickly.
299. What is the best way to estimate the age of a star cluster?
A) Plot the stars on a Hertzsprung–Russell diagram, and see which stars are still on the main sequence.
B) Measure the motions of the stars in the cluster, and compare the measurements with theoretical models.
C) Measure the heavy element abundance for the stars in the cluster.
D) Count the number of T Tauri stars in the cluster.
300. Which of these astronomical objects or systems is likely to be the oldest?
A) T Tauri star
B) Pleiades
C) Sun
D) globular cluster
301. The Sun is a member of which one of these stellar groups?
A) Cepheid variables
B) Population II
C) horizontal-branch stars
D) Population I
302. An astronomer plots the H-R diagram of a star cluster and finds that it contains hot B-type stars on the main sequence and cooler G- and K-type stars noticeably above the main sequence. This cluster is
A) very young because the G and K stars are still evolving toward the main sequence.
B) of indeterminate age since the age of the cluster cannot be estimated from the information given.
C) impossible because cool stars cannot exist above the main sequence when hot stars are on the main sequence.
D) old because the G and K stars are already evolving off (away from) the main sequence.
303. In the H-R diagram shown in Figure 13-31b in the text, the brightest stars in the Pleiades cluster are not on the main sequence but away from it toward the upper right. Explain.
A) The brightest stars have not yet reached the main sequence and are in the T Tauri phase.
B) The brightest stars have already evolved through the red giant phase and have now returned to the blue giant phase on their way to the white dwarf phase.
C) The brightest stars have already become white dwarf stars, as shown by their position.
D) These blue supergiant stars have already begun to evolve toward the red supergiant phase.
304. What is the turnoff point for a star cluster?
A) point in the Hertzsprung–Russell diagram occupied by stars undergoing (or about to undergo) the helium flash
B) point in the Hertzsprung–Russell diagram occupied by the highest-mass main-sequence stars in the cluster
C) point in the Hertzsprung–Russell diagram occupied by the lowest-mass main-sequence stars in the cluster
D) point in the Hertzsprung–Russell diagram occupied by the highest-mass stars that have not yet reached the main sequence
305. For an astronomer, what is the significance of the turnoff point in the Hertzsprung–Russell diagram of a star cluster?
A) The turnoff point tells the astronomer the age of the star cluster.
B) The turnoff point tells the astronomer which stars might be about to explode as supernovae.
C) The turnoff point tells the astronomer which stars pulsate in brightness (variable stars).
D) The turnoff point tells the astronomer the metal content of the star cluster.
306. The process of evolution of stars in the universe has been interpreted by
A) matching theoretical models to the properties of similar stars at different distances from Earth, since more distant stars from an earlier time are observed, when the light left them.
B) matching theoretical models to the collective properties, such as luminosity, temperature, and size, of millions of stars as observed.
C) designing theoretical models solely on the basis of the known properties of matter, without reference to observational data on real stars, since there is not have a sufficiently long time base for stellar observations.
D) matching theoretical models to the detailed observation of a few stars as astronomers watch them change in luminosity, temperature, and size during their evolution.
307. The age of a cluster can be determined by
A) carrying out a number count of the stars in the cluster.
B) measuring the cluster’s speed of motion relative to the Sun.
C) determining the turnoff point on the main sequence of the cluster’s Hertzsprung–Russell diagram.
D) observing the cluster’s position in the sky with respect to the Sun.
308. The age of a cluster of stars can be judged by the
A) amount of radioactive elements detected on the surfaces of its stars.
B) turnoff point on the main sequence of its Hertzsprung–Russell diagram.
C) total number of stars within the cluster.
D) number of novae per year occurring within the cluster.
309. The stars at the turnoff point in the Hertzsprung–Russell diagram of the Hyades star cluster have a luminosity of approximately 10 times that of the Sun, while those at the turnoff point in the cluster M41 have a luminosity of about 80 times that of the Sun. From this information, it can be said with certainty that the Hyades cluster
A) has more stars in it than the M41 cluster.
B) is younger than the M41 cluster.
C) is farther away than the M41 cluster.
D) is older than the M41 cluster.
310. Which of these stars are metal-poor?
A) open cluster stars
B) very young stars
C) Population II stars
D) Population I stars
311. Stars are formed from interstellar matter. Why then are stars in open clusters metal-rich, while stars in globular clusters are metal-poor?
A) Open clusters contain very hot stars that produce metals by nuclear reactions in their outer layers.
B) Globular clusters have “burned” their heavy elements over their longer lifetime.
C) Globular clusters are too young for their stars to have produced any significant amount of metals.
D) Open clusters are young, and stars have formed from material that has been enriched in metals by supernova explosions of previous stars.
312. The spectrum of stars in an open cluster is found to contain spectral lines from many heavy elements; the stars are known as “metal-rich.” What is the source of these heavy elements?
A) fusion reactions on the very hot surfaces of these stars
B) The heavy element content of these stars is not enhanced but the hydrogen and helium content has been depleted by intense stellar winds made up preferentially of these light elements.
C) fusion reactions in the star cores, carried to the surface by convection
D) interstellar medium, which originated in explosions of stars earlier in history that had manufactured heavy elements by nuclear fusion
313. Which of these stars could be classified as a Population II star?
A) star with very low abundance of heavy elements
B) star in an open star cluster
C) star with approximately the same abundance of heavy elements found in the Sun
D) star with much higher abundance of heavy elements than found in the Sun
314. In connection with a star, what does the phrase “metal-poor” mean?
A) The star has a low abundance of all elements heavier than hydrogen in its spectrum.
B) The star has a low abundance of all elements heavier than hydrogen and helium in its spectrum.
C) The star has a low abundance of all elements in its spectrum.
D) The star might or might not have a low abundance of carbon in its spectrum but is definitely weak in iron.
Section: 13-15
315. Which of these major perturbations can occur in a close binary system and radically alter the evolution and behavior of the two individual stars?
A) heating of the localized areas of the atmosphere of one star by its companion
B) transfer of matter from one star to its companion
C) gravitational disturbance of one star’s motion by its companion, forcing it to move in an orbit
D) eclipse of the light from one star by the other when viewed from Earth
316. What particular and very important phenomenon frequently occurs in binary star systems where the stars are very close together?
A) The radiation from the hotter star slowly heats and evaporates away the cooler star.
B) Mass lost from one star is deposited on its companion.
C) The less massive star, in its elliptical orbit, repeatedly passes through the thin, extended atmosphere of the second star, producing periodic rises and falls in light output from the star system.
D) The less massive star spirals slowly into its more massive companion because of tidal interactions.
317. The shape of the cross-section of the Roche lobes around a close binary star system, taken through the centers of both stars, is
A) two ellipses that touch at the center of the lobes.
B) a sphere centered on the center of mass of the star system.
C) an ellipse with a star at each focus.
D) a figure eight.
318. The transfer of mass from the surface of one star onto the surface of another is MOST often observed in
A) detached binaries.
B) overcontact binaries.
C) contact binaries.
D) semidetached binaries.
319. In what type of binary system can the two stars share the same outer atmosphere?
A) detached binary
B) semidetached binary
C) contact binary
D) overcontact binary
320. In some binary star systems, such as Algol, the less massive star is an old red giant and the more massive star is on the main sequence, evidence that
A) the more massive star captured the other one into orbit sometime after the two stars had formed.
B) the more massive star formed later, from a disk of gas surrounding the less massive star.
C) mass transfer from one of the stars to the other has occurred.
D) stars evolve differently in binary star systems, with less massive stars evolving faster than more massive stars.
321. In some binary star systems, such as Lyrae, essentially no light is seen from the more massive star because the
A) more massive star is hidden by an accretion disk of material from the less massive star.
B) orbit of the more massive star keeps it hidden behind the larger but less massive star as seen from Earth.
C) more massive star is a black hole from which light cannot escape.
D) more massive star is still hidden in the dust clouds from which it formed.
322. Two close binary stars exchange mass. Which of these does NOT describe a possible outcome of such a mass exchange?
A) The star that was originally smaller can become the larger of the two.
B) An accretion disk can be formed around one of the stars, which can almost block it from view.
C) One of the stars can begin to spin so rapidly that its shape will be distorted.
D) The two stars can merge to form a single star and then fly apart to form two wholly new stars.
323. Which factor, more than any other, modifies the evolutionary tracks of stars in binary combinations compared with their single-star counterparts?
A) tidal distortion of the shapes of the stars
B) reduction of the quantum mechanical limitation on continued shrinking of one star by the gravitational field of the second star
C) mass exchange between the stars
D) radiation from one star heating the surface of the second star
324. A Roche lobe is an imaginary boundary around each star in a close binary system
A) encompassing the mass gravitationally bound to that star.
B) that marks the limit outside which planets cannot form.
C) that marks the limit inside which planets cannot form because they would be torn apart by the star’s gravity.
D) that marks the limit of each star’s gravitational pull: As long as each star is within its own Roche lobe, the stars will have no effect on each other.
325. The components of a binary star, particularly if they are close, can influence each other in various ways. Which of these is NOT a likely effect of one star on its companion?
A) One star can be enshrouded in an accretion disk.
B) The gravitational force of one star will make its companion move in an orbit rather than remaining stationary.
C) Intense radiation from a hot star can produce nuclear reactions on the surface of a cooler companion and initiate a nova explosion.
D) Mass can be transferred from one star to its companion.
326. Stars in a binary system are useful in studying mass transfers because the two stars have the same
A) mass.
B) age.
C) spectral type.
D) radius.