# The Cosmic Perspective: Exam 3 Flashcards

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The Cosmic Perspective
Chapters 15, 16, 18
updated 8 years ago by roxasiceblue
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1

2) Since all stars begin their lives with the same basic composition, what characteristic most determines how they will differ?
A) location where they are formed
B) time they are formed
C) luminosity they are formed with
D) mass they are formed with
E) color they are formed with

2

3) What are the standard units for luminosity?
A) watts
B) joules
C) Newtons
D) kilograms
E) watts per second

3

4) A star's luminosity is the
A) apparent brightness of the star in our sky.
B) surface temperature of the star.
D) total amount of light that the star will radiate over its entire lifetime.
E) total amount of light that the star radiates each second.

4

5) What are the standard units for apparent brightness?
A) watts
B) joules
C) Newtons
D) watts per second
E) watts per square meter

5

6) If the distance between us and a star is doubled, with everything else remaining the same, the luminosity
A) is decreased by a factor of four, and the apparent brightness is decreased by a factor of four.
B) is decreased by a factor of two, and the apparent brightness is decreased by a factor of two.
C) remains the same, but the apparent brightness is decreased by a factor of two.
D) remains the same, but the apparent brightness is decreased by a factor of four.
E) is decreased by a factor of four, but the apparent brightness remains the same.

6

7) Which of the following correctly states the luminosity-distance formula?
A) luminosity = (Apparent brightness)/4π × (distance)^2
B) apparent brightness = (luminosity)/4π × (distance)^2
C) apparent brightness = luminosity × 4π × (distance)^2
D) distance = (luminosity)/4π × (distance)^2

7

8) Why do astronomers often measure the visible-light apparent brightness instead of the total apparent brightness of a star?
A) All stars put out most of their light in the visible range of the spectrum.
B) In order to measure the total apparent brightness of a star, you must measure its brightness in all wavelengths, and this is difficult to do. The only wavelengths you can measure from the surface of Earth are visible and radio wavelengths.
C) Most stars do not put out light in other ranges of the spectrum.
D) They are identical for most stars.
E) Astronomers are lazy.

8

9) Suppose you measure the parallax angle for a particular star to be 0.1 arcsecond. The distance to this star is
A) 10 light-years.
B) 10 parsecs.
C) 0.1 light-year.
D) 0.1 parsec.
E) impossible to determine.

9

11) The most distant stars we can measure stellar parallax for are approximately
A) 50 parsecs away.
B) 500 parsecs away.
C) 5,000 parsecs away.
D) halfway across the Milky Way Galaxy.
E) in the Andromeda Galaxy.

10

2) Which of the following statements about apparent and absolute magnitudes is true?
A) The magnitude system that we use now is based on a system used by the ancient Greeks over 2,000 years ago that classified stars by how bright they appeared.
B) A star with apparent magnitude 1 is brighter than one with apparent magnitude 2.
C) The absolute magnitude of a star is another measure of its luminosity.
D) A star's absolute magnitude is the apparent magnitude it would have if it were at a distance of 10 parsecs from Earth.
E) All of the above are true.

11

13) The spectral sequence sorts stars according to
A) mass.
B) surface temperature.
C) luminosity.
D) core temperature.

12

14) The spectral sequence in order of decreasing temperature is
A) OFBAGKM.
B) OBAGFKM.
C) OBAFGKM.
D) ABFGKMO.
E) BAGFKMO.

13

15) Why is the spectral sequence of stars not alphabetical?
A) The letters refer to the initials of the original discovers.
B) The original alphabetical labeling did not correspond to surface temperature and thus had to be reordered.
C) They were chosen to fit a mnemonic.
D) Because there is still uncertainty over what generates the energy in stellar cores.
E) Because it refers to stellar masses and these were difficult to measure accurately.

14

16) Which of the following statements about spectral types of stars is true?
A) The spectral type of a star can be used to determine its surface temperature.
B) The spectral type of a star can be used to determine its color.
C) A star with spectral type A is cooler than a star with spectral type B.
D) A star with spectral type F2 is hotter than a star with spectral type F3.
E) All of the above are true.

15

17) Which of the following persons reorganized the spectral classification scheme into the one we use today and personally classified over 400,000 stars?
A) Annie Jump Cannon
B) Williamina Fleming
C) Cecilia Payne-Gaposchkin
D) Henry Draper
E) Edward Pickering

16

18) Which of the following persons used the ideas of quantum mechanics to describe why the spectral classification scheme is in order of decreasing temperature?
A) Annie Jump Cannon
B) Williamina Fleming
C) Cecilia Payne-Gaposchkin
D) Henry Draper
E) Edward Pickering

17

19) Suppose you see two main-sequence stars of the same spectral type. Star 1 is dimmer in apparent brightness than Star 2 by a factor of 100. What can you conclude? (Neglect any effects that might be caused by interstellar dust and gas.)
A) Without first knowing the distances to these stars, you cannot draw any conclusions about how their true luminosities compare to each other.
B) The luminosity of Star 1 is a factor of 100 less than the luminosity of Star 2.
C) Star 1 is 100 times more distant than Star 2.
D) Star 1 is 100 times nearer than Star 2.
E) Star 1 is 10 times more distant than Star 2.

18

20) Which of the following terms is given to a pair of stars that appear to change positions in the sky, indicating that they are orbiting one another?
A) visual binary
B) eclipsing binary
C) spectroscopic binary
D) double star
E) none of the above

19

22) Which of the following best describes the axes of a Hertzsprung-Russell (H-R) diagram?
A) surface temperature on the horizontal axis and luminosity on the vertical axis
B) mass on the horizontal axis and luminosity on the vertical axis
C) surface temperature on the horizontal axis and radius on the vertical axis
D) mass on the horizontal axis and stellar age on the vertical axis
E) interior temperature on the horizontal axis and mass on the vertical axis

20

23) On a Hertzsprung-Russell diagram, where would we find stars that are cool and dim?
A) upper right
B) lower right
C) upper left
D) lower left

21

24) On a Hertzsprung-Russell diagram, where would we find stars that are cool and luminous?
A) upper right
B) lower right
C) upper left
D) lower left

22

25) On a Hertzsprung-Russell diagram, where would we find stars that have the largest radii?
A) upper right
B) lower right
C) upper left
D) lower left

23

26) On a Hertzsprung-Russell diagram, where on the main sequence would we find stars that have the greatest mass?
A) upper right
B) lower right
C) upper left
D) lower left

24

27) On a Hertzsprung-Russell diagram, where would we find red giant stars?
A) upper right
B) lower right
C) upper left
D) lower left

25

28) On a Hertzsprung-Russell diagram, where would we find white dwarfs?
A) upper right
B) lower right
C) upper left
D) lower left

26

29) You observe a star in the disk of the Milky Way, and you want to plot the star on an H-R diagram. You will need to determine all of the following, except the
A) spectral type of the star.
B) distance to the star.
C) apparent brightness of the star in our sky.
D) rotation rate of the star.

27

30) On the main sequence, stars obtain their energy
A) from chemical reactions.
B) from gravitational contraction.
C) by converting hydrogen to helium.
D) by converting helium to carbon, nitrogen, and oxygen.
E) from nuclear fission.

28

31) The faintest star visible to the naked eye has an apparent visual magnitude of about
A) 10.
B) 6.
C) 1.
D) 0.
E) -6.

Answer: B the higher the number the fainter the star will appear

29

32) Which of the following is the most common type of main-sequence star?
A) an O star
B) an A star
C) an F star
D) an M star
E) a G star

30

33) Which of the following characteristics of stars has the greatest range in values?
A) mass
C) core temperature
D) surface temperature
E) luminosity

31

34) A star of spectral type O lives approximately how long on the main sequence?
A) 1,000 years
B) 10,000 years
C) 10 million years
D) 100 million years
E) 1 billion years

32

35) A star of spectral type G lives approximately how long on the main sequence?
A) 1,000 years
B) 10,000 years
C) 1 million years
D) 100 million years
E) 10 billion years

33

36) Which of the following is true about low-mass stars compared to high-mass stars?
A) Low-mass stars are cooler and less luminous than high-mass stars.
B) Low-mass stars are hotter and more luminous than high-mass stars.
C) Low-mass stars are cooler but more luminous than high-mass stars.
D) Low-mass stars are hotter but less luminous than high-mass stars.
E) Low-mass stars have the same temperature and luminosity as high-mass stars.

34

37) Which of the following luminosity classes refers to stars on the main sequence?
A) I
B) II
C) III
D) IV
E) V

35

38) In a pulsating variable star, which characteristic of the star changes dramatically with time?
A) mass
B) core temperature
C) luminosity
D) energy-generation process
E) rotation rate

36

39) Why are Cepheid variables so important for measuring distances in astronomy?
A) They all have the same luminosity.
B) They all have the same period.
C) Their luminosity can be inferred from their period.
D) They are close enough to have a detectable parallax.
E) They are circumpolar like Polaris, the North Star.

37

40) Which of the following statements about an open cluster is true?
A) All stars in the cluster are approximately the same color.
B) All stars in the cluster are approximately the same age.
C) All stars in the cluster have approximately the same mass.
D) All stars in the cluster will evolve similarly.
E) There is an approximately equal number of all types of stars in the cluster.

38

41) Which of the following statements about a globular cluster is true?
A) All stars in the cluster are approximately at the same stage in evolution.
B) Most of the stars in the cluster are younger than 10 billion years old.
C) Most stars in the cluster are yellow or reddish in color.
D) All stars in the cluster have approximately the same mass.
E) There is an approximately equal number of all types of stars in the cluster.

39

42) Cluster ages can be determined from
A) main sequence fitting.
B) main sequence turnoff.
C) pulsating variable stars.
D) spectroscopic binaries.
E) visual binaries.

40

43) In order to understand star clusters, we need to be able to estimate their ages. What technique do scientists use for this?
B) counting the planets that have formed around the largest stars
C) finding the main-sequence turnoff point of the stars
D) calculating orbital parameters using Kepler's Laws
E) measuring its parallax

41

1) The apparent brightness of a star depends only on its luminosity.

42

2) If the distance between us and a star is doubled, the apparent brightness is decreased by a factor of four.

43

3) The more distant a star, the smaller its parallax.

44

4) We can measure stellar parallax for most stars in our galaxy.

45

5) Spectral type, surface temperature, and color all describe the same basic characteristic of a star.

46

6) Some stars are cool enough to have molecules in their atmosphere.

47

7) We can measure the radii of stars in an eclipsing binary system, in addition to the masses.

48

8) Two stars have the same spectral type. Star X is in luminosity class III, while Star Y is in luminosity class V. Therefore, Star X is larger in radius than Star Y.

49

9) Two stars have the same luminosity. Star X is spectral type F, while Star Y is spectral type K. Therefore, Star X is larger in radius than Star Y.

50

10) Two stars both lie on the main sequence. Star X is spectral type A, while Star Y is spectral type G. Therefore, Star X is more massive than Star Y.

51

11) A 10-solar-mass star is about ten times more luminous than a 1-solar-mass star.

52

12) Most stars on the main sequence fuse hydrogen into helium in their cores, but some do not.

53

13) All stars spend approximately the same amount of time on the main sequence.

54

1) What is the approximate chemical composition (by mass) with which all stars are born?
A) three quarters hydrogen, one quarter helium, no more than 2% heavier elements
B) half hydrogen and half helium
C) 98% hydrogen, 2% helium
D) 95% hydrogen, 4% helium, no more than 1% heavier elements

55

2) The total amount of power (in watts, for example) that a star radiates into space is called its
A) apparent brightness.
B) absolute magnitude.
C) luminosity.
D) flux.

56

3) According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?
A) Its apparent brightness will increase by a factor of 9.
B) Its apparent brightness will decrease by a factor of 9.
C) Its apparent brightness will increase by a factor of 3.
D) Its apparent brightness will decrease by a factor of 3.

57

4) Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?
A) Determine both the star's distance and luminosity from its apparent brightness.
B) Determine the distance to the star from its apparent brightness.
C) Calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity.
D) Calculate the star's surface temperature if we know either its luminosity or its distance.

58

5) If Star A is closer to us than Star B, then Star A's parallax angle is
A) smaller than that of Star B.
B) larger than that of Star B.
C) hotter than that of Star B.
D) fewer parsecs than that of Star B.

59

A) 150 million kilometers.
B) 10,000 seconds.
C) 10 parallax seconds of angle.
D) 32.6 light-years.

60

7) Star A has an apparent magnitude of 3 and Star B has an apparent magnitude of 5. Which star is brighter in our sky?
A) Star A
B) Star B
C) The two stars have the same brightness in our sky, but Star A is closer to us than Star B.
D) There is not enough information to answer the question.

61

9) Our Sun is a star of spectral type
A) F.
B) M.
C) G.
D) S.

62

10) Astronomers can measure a star's mass in only certain cases. Which one of the following cases might allow astronomers to measure a star's mass?
A) The star is a member of a binary star system.
B) The star is of spectral type G.
C) The star is of spectral type A.
D) We know the star's luminosity and distance.

63

11) Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?
A) eclipsing binary
B) spectroscopic binary
C) visual binary
D) double star

64

12) The axes on a Hertzsprung-Russell (H-R) diagram represent
A) mass and luminosity.
B) luminosity and surface temperature.
C) luminosity and apparent brightness.

65

13) On an H-R diagram, stellar radii
A) are greatest in the lower left and least in the upper right.
B) decrease from left to right.
C) are impossible to determine.
D) increase diagonally from the lower left to the upper right.

66

14) On an H-R diagram, stellar masses
A) can be determined for main-sequence stars but not for other types of stars.
B) are greatest in the lower left and least in the upper right.
C) decrease from upper left to lower right.
D) are impossible to determine.

67

16) How is the lifetime of a star related to its mass?
A) More massive stars live slightly shorter lives than less massive stars.
B) More massive stars live much longer lives than less massive stars.
C) More massive stars live much shorter lives than less massive stars.
D) More massive stars live slightly longer lives than less massive stars.

68

17) Each choice below lists a spectral type and luminosity class for a star. Which one is a red supergiant?
A) spectral type M1, luminosity class V
B) spectral type O9, luminosity class I
C) spectral type G2, luminosity class V
D) spectral type M2, luminosity class I

69

18) What is the common trait of all main-sequence stars?
A) They are in the final stage of their lives.
B) They generate energy through hydrogen fusion in their core.
C) They are all spectral type G.
D) They all have approximately the same mass.

70

19) Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true?
A) Earth would be inside the supergiant.
B) The supergiant's surface temperature would be much hotter than the surface temperature of our Sun.
C) Earth would fly off into interstellar space.
D) The supergiant would appear as large as the full Moon in our sky.

71

20) What is a white dwarf?
A) It is a main-sequence star of spectral type F, which tends to look white in color.
B) It is the remains of a star that ran out of fuel for nuclear fusion.
C) It is a type of star that produces energy by gravitational contraction.
D) It is a star that follows a period-luminosity relation.

72

21) Which of the following statements comparing open and globular star clusters is not true?
A) Open clusters are found only in the disk of the galaxy while globular clusters may be found both in the disk and the halo of the galaxy.
B) Stars in open clusters are relatively young while stars in globular clusters are very old.
C) Open and globular clusters each typically contain a few hundred stars.
D) For both open and globular clusters, we can assume that all the stars in a particular cluster are about the same age.

73

22) What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?
A) It is the point in a star cluster beyond which main-sequence stars are not found, and it tells us the cluster's distance.
B) It is the spectral type of the hottest main-sequence star in a star cluster, and it tells us the cluster's age.
C) It is the luminosity class of the largest star in a star cluster, and it tells us the cluster's age.
D) It is the mass of the most massive star in the star cluster, and it tells us the cluster's size.

74

1) All stars are born with the same basic composition, yet stars can look quite different from one another. Which two factors primarily determine the characteristics of a star?
A) its mass and its stage of life
B) its apparent brightness and its distance
C) its age and its location in the galaxy
D) its mass and its surface temperature
E) its apparent brightness and its luminosity

75

3) Star A is identical to Star B, except that Star A is twice as far from us as Star B. Therefore
A) both stars have the same luminosity, but the apparent brightness of Star B is twice that of Star A.
B) both stars have the same apparent brightness, but the luminosity of Star B is four times that of Star A.
C) both stars have the same luminosity, but the apparent brightness of Star A is four times that of Star B.
D) both stars have the same luminosity, but the apparent brightness of Star B is four times that of Star A.

76

4) A star with a parallax angle of 1/20 arcsecond is
A) 20 light-years away.
B) 1/20 parsec away.
C) 20 parsecs away.
D) 10 parsecs away.

77

5) The star Vega has an absolute magnitude of about 4 and an apparent magnitude of about 0. Based on the definitions of absolute and apparent magnitude, we can conclude that
A) Vega is nearer than 10 parsecs from Earth.
B) Vega has a parallax angle of 1/10 arcsecond.
C) Vega's luminosity is less than that of our Sun.
D) Vega's surface temperature is cooler than the Sun.

78

6) Which of the following statements about spectral types of stars is not generally true?
A) The spectral type of a star can be used to determine its surface temperature.
B) The spectral type of a star can be used to determine its distance.
C) The spectral type of a star can be used to determine its color.
D) The spectral type of a star can be determined by identifying lines in its spectrum.

79

7) Sirius is a star with spectral type A star and Rigel is a star with spectral type B star. What can we conclude?
A) Rigel has a higher core temperature than Sirius.
B) Sirius has a higher core temperature than Rigel.
C) Rigel has a higher surface temperature than Sirius.
D) Sirius has a higher surface temperature than Rigel.

80

8) To calculate the masses of stars in a binary system, we must measure their
A) spectral types and distance from Earth.
B) absolute magnitudes and luminosities.
C) luminosities and distance from Earth.
D) orbital period and average orbital distance.

81

9) Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes dimmer for about 2 hours. The most likely explanation is that
A) the star is a Cepheid variable.
B) the star is a member of an eclipsing binary star system.
C) the star is periodically ejecting gas into space, every 73 hours.
D) the star is a white dwarf.

82

10) Which group represents stars that are cool and dim?
A) A
B) B
C) C
D) D
E) E

83

11) Which group represents stars of the largest radii?
A) A
B) B
C) C
D) D
E) E

84

12) Which group represents the most common type of stars?
A) A
B) B
C) C
D) D
E) E

85

13) Which group represents stars that are extremely bright and emit most of their radiation as ultraviolet light?
A) A
B) B
C) C
D) D
E) E

86

14) Which group represents stars with the longest main-sequence lifetimes?
A) A
B) B
C) C
D) D
E) E

87

15) Which group represents stars fusing hydrogen in their cores?
A) A
B) B
C) C
D) D
E) E

88

16) Which group represents stars that have no ongoing nuclear fusion?
A) A
B) B
C) C
D) D
E) E

89

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

90

18) The approximate main-sequence lifetime of a star of spectral type O is
A) 10,000 years.
B) 3 million years.
C) 300 million years.
D) 10 billion years.

91

19) How did astronomers discover the relationship between spectral type and mass for main-sequence stars?
A) by using computer models of hydrogen fusion and stellar structure
B) by measuring stellar radii with very powerful telescopes
C) by comparing stars with the same spectral type but different luminosities
D) by measuring the masses and spectral types of main-sequence stars in binary systems

92

20) The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is the youngest?
A) The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.
B) The diagram shows main-sequence stars of all the spectral types except O and B, along with a few giants and supergiants.
C) The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.
D) The diagram shows no main-sequence stars at all, but it has numerous supergiants and white dwarfs.

93

21) The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is most likely to be located in the halo of our galaxy?
A) The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.
B) The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.
C) The diagram shows main-sequence stars of all the spectral types except O and B, along with a few giants and supergiants.
D) The diagram shows no main-sequence stars at all, but it has numerous supergiants and white dwarfs.

94

1) Astronomers estimate that new stars form in our galaxy at the rate of about
A) one per year.
B) a few (2-3) per year.
C) ten per year.
D) 20-30 per year.
E) 100 per year.

95

2) By mass, the interstellar medium in our region of the Milky Way consists of
A) 70% Hydrogen, 30% Helium.
B) 70% Hydrogen, 28% Helium, 2% heavier elements.
C) 70% Hydrogen, 20% Helium, 10% heavier elements.
D) 50% Hydrogen, 50% Helium.
E) 50% Hydrogen, 30% Helium, 20% heavier elements.

96

3) What percentage of a molecular cloud's mass is interstellar dust?
A) 1%
B) 2%
C) 28%
D) 50%
E) 1-50%, depending on the mass of the molecular cloud

97

4) The typical density and temperature of molecular clouds are
A) 100 molecules per cubic centimeter, 10-30 Kelvin.
B) 300 molecules per cubic centimeter, 10-30 Kelvin.
C) 1000 molecules per cubic centimeter, 10-30 Kelvin.
D) 100 molecules per cubic centimeter, 100-300 Kelvin.
E) 300 molecules per cubic centimeter, 100-300 Kelvin.

98

6) The typical size of an interstellar dust grain is
A) 1 angstrom.
B) 1 nanometer.
C) 1 micrometer.
D) 1 millimeter.
E) 1 centimeter.

99

7) What is interstellar reddening?
A) Interstellar dust absorbs more red light than blue light, making stars appear redder than their true color.
B) Interstellar dust absorbs more red light than blue light, making stars appear bluer than their true color.
C) Interstellar dust absorbs more blue light than red light, making stars appear redder than their true color.
D) Interstellar dust absorbs more blue light than red light, making stars appear bluer than their true color.
E) The spectral line shift due to a star's motion through the interstellar medium.

100

8) If you wanted to observe stars behind a molecular cloud, in what wavelength of light would you most likely observe?
A) ultraviolet
B) visible
C) infrared
D) X-ray
E) gamma-ray

101

9) What happens to the visible radiation produced by new stars within a molecular cloud?
A) It escapes the cloud completely.
B) It is absorbed by dust grains and heats up the cloud.
C) It is reflected back onto the protostar, heating it up further.
D) The blue light is absorbed and the red light transmitted.
E) It shoots out in bright jets.

102

10) The thermal pressure of a gas depends on
A) density only.
B) temperature only.
C) density and temperature.
D) composition.
E) gravity.

103

11) The gravitational force in a molecular cloud depends on
A) density only.
B) temperature only.
C) density and temperature.
D) composition.
E) thermal pressure.

104

12) What prevents the pressure from increasing as a cloud contracts due to its gravity?
A) As the cloud becomes denser, gravity becomes stronger and overcomes the pressure buildup.
B) The pressure is transferred from the center of the cloud to its outer edges where it can dissipate.
C) Thermal energy is converted to radiative energy via molecular collisions and released as photons.
D) Excess pressure is released in jets of material from the young stars.
E) Once the cloud reaches a critical density, the pressure becomes degenerate and independent of temperature.

105

13) Calculations show that gravity begins to overcome thermal pressure in clouds that are
A) less massive than the Sun.
B) more massive than the Sun.
C) more massive than ten times the Sun.
D) more massive than a hundred times the Sun.
E) more massive than a thousand times the Sun.

106

14) What property of a molecular cloud does not counteract gravitational contraction?
A) thermal pressure
B) turbulent motions
C) magnetic fields
D) fragmentation

107

15) How do astronomers infer the presence of magnetic fields in molecular clouds?
A) by measuring the amount of interstellar reddening
B) by measuring the Doppler shifts of emission lines from gas clumps in the cloud
C) by measuring the infrared light emitted by the cloud
D) by measuring the polarization of starlight passing through the cloud
E) by measuring the amount by which gravity is reduced

108

16) What is the likely reason that we cannot find any examples of the first generation stars?
A) The first generation stars are too faint to be visible now.
B) The first generation stars formed such a long time ago that the light from them has not yet had time to reach us.
C) The first generation stars were all very massive and exploded as supernova.
D) The first generation stars formed with only H and He and therefore have no spectral features.
E) We do not know how the first generation stars were formed.

109

17) Why do we think the first generation of stars would be different from stars born today?
A) Without heavy elements, the clouds could not reach as low a temperature as today and had to be more massive to collapse.
B) Without heavy elements, the nuclear reactions at the center of the stars would be very different.
C) Without heavy elements, there was no dust in the clouds and they collapsed faster.
D) The Universe was much denser when the first stars were born.
E) There were no galaxies when the first stars were born.

110

18) What is the minimum temperature for a cloud to excite emission lines from H2?
A) 10 K
B) 30 K
C) 100 K
D) 300 K
E) 1000 K

111

19) When is thermal energy trapped in the dense center of a cloud?
A) when the gravity becomes so strong that photons cannot escape
B) when excited molecules collide with other molecules before they can release a photon
C) when the cloud becomes so hot and dense that nuclear fusion begins
D) when magnetic fields trap the radiation
E) when the cloud cools down so much that less light escapes than is produced by contraction

112

20) What happens to the rotation of a molecular cloud as it collapses to form a star?
A) The rotation rate remains the same and results in stellar rotation.
B) The rotation dissipates and any residual is left in small overall rotation of the star.
C) The rotation rate increases and results in fast rotation of the star.
D) The rotation rate increases and results in a disk of material around a protostar.
E) The rotation increases the speed of collapse and produces more massive stars.

113

21) Which of the following may be caused by a protostellar disk?
A) protostellar jets
B) protostellar winds
C) accretion of material onto the star
D) relatively slow protostellar rotation
E) all of the above

114

22) When does a protostar become a true star?
A) when the star is 1 million years old
B) when the central temperature reaches 1 million Kelvin
C) when nuclear fusion begins in the core
D) when the thermal energy becomes trapped in the center
E) when the stellar winds and jets blow away the surrounding material

115

23) How long does the protostellar stage last for a star like our Sun?
A) 1 million years
B) 3 million years
C) 10 million years
D) 30 million years
E) 100 million years

116

24) What is the range of timescales for star formation?
A) from 1 million years for the most massive stars up to 10 million years for the least massive stars
B) from 1 million years for the most massive stars up to 100 million years for the least massive stars
C) from 1 million years for the least massive stars up to 10 million years for the most massive stars
D) from 1 million years for the least massive stars up to 100 million years for the most massive stars
E) about 30 million years for all stars, whatever mass

117

25) What species absorbs photons in a protostar's outer layers?
A) H
B) H2
C) H+
D) H-
E) dust

118

26) When does a star become a main-sequence star?
A) when the protostar assembles from a molecular cloud
B) the instant when hydrogen fusion first begins in the star's core
C) when the rate of hydrogen fusion within the star's core is high enough to maintain gravitational equilibrium
D) when a star becomes luminous enough to emit thermal radiation
E) when hydrogen fusion is occurring throughout a star's interior

119

27) What happens to the surface temperature and luminosity when gravity first assembles a protostar from a collapsing cloud?
A) Its surface temperature and luminosity increase.
B) Its surface temperature remains the same and its luminosity decreases.
C) Its surface temperature and luminosity decrease.
D) Its surface temperature decreases and its luminosity increases.
E) Its surface temperature and luminosity remain the same.

120

28) What happens to the surface temperature and luminosity when a protostar undergoes convective contraction?
A) Its surface temperature and luminosity increase.
B) Its surface temperature remains the same and its luminosity decreases.
C) Its surface temperature and luminosity decrease.
D) Its surface temperature decreases and its luminosity increases.
E) Its surface temperature and luminosity remain the same.

121

29) What happens to the surface temperature and luminosity when a protostar radiatively contracts?
A) Its surface temperature and luminosity increase.
B) Its surface temperature remains the same and its luminosity decreases.
C) Its surface temperature and luminosity decrease.
D) Its surface temperature decreases and its luminosity increases.
E) Its surface temperature and luminosity remain the same.

122

30) When does hydrogen first begin to fuse into helium in the star formation process?
A) when the cloud first begins to contract
B) when the thermal pressure is trapped at the center of the cloud
C) when the protostars undergoes convective contraction
D) when the protostar undergoes radiative contraction
E) only when the star reaches the main-sequence

123

31) About how many times more luminous than our Sun is a young solar mass protostar just beginning convective contraction?
A) 2-5
B) 5-10
C) 10-100
D) 100-1000
E) a million

124

32) What is the smallest mass a newborn star can have?
A) 8 times the mass of Jupiter
B) 80 times the mass of Jupiter
C) 800 times the mass of Jupiter
D) about 1/80 the mass of our Sun
E) about 1/800 the mass of our Sun

125

33) What are the letters that follow the spectral sequence OBAFGKM?
A) NP
B) YZ
C) LT
D) CD
E) UV

126

34) What is the greatest mass a newborn star can have
A) 10 solar masses.
B) 20 solar masses.
C) 50 solar masses.
D) 150 solar masses.
E) 300 solar masses.

127

35) No stars have been found with masses greater than 300 times our Sun because
A) molecular clouds do not have enough material to form such massive stars.
B) they would fragment into binary stars because of their rapid rotation.
C) they would generate so much power that they would blow themselves apart.
D) they shine exclusively at X-ray wavelengths and become difficult to detect.
E) they are not bright enough to be seen nearby.

128

36) For every star with a mass greater than 10 solar masses, about how many stars are there with masses less than a solar mass?
A) 1
B) 3
C) 10
D) 30
E) 200

129

37) Which of the following discoveries, if they existed, would necessitate a reevaluation of our ideas of stellar formation?
A) a cluster of stars that appeared to be 13 billion years old
B) a 100-solar-mass star
C) a 0.01-solar-mass star
D) a molecular cloud without any stars
E) planetary systems around other stars than our own

130

38) What prevents a brown dwarf from undergoing nuclear fusion?
A) Degeneracy pressure halts the contraction of a protostar so the core never becomes hot or dense enough for nuclear fusion.
B) There is not enough mass to maintain nuclear reactions in a self-sustaining way.
C) The surface temperature never rises high enough for the radiation to be trapped and heat their interior to the temperatures required for nuclear fusion.
D) Radiation pressure halts the contraction of a protostar so the core never becomes hot or dense enough for nuclear fusion.
E) There are too many heavy elements and not enough hydrogen for fusion to occur in a self-sustaining way.

131

39) What is the eventual fate of a brown dwarf?
A) It remains the same forever.
B) It gradually cools down and becomes ever dimmer.
C) It gradually contracts and heats up until nuclear fusion ignites in its interior and it becomes a faint star.
D) It becomes ever denser and hotter until it becomes a white dwarf.
E) Gravity ultimately "wins" and it becomes a small black hole.

132

40) Where would a brown dwarf be located on an H-R diagram?
A) upper right
B) on the lower part of the main sequence
C) below and to the right of the lowest part of the main sequence
D) lower left
E) above and to the left of the main sequence

133

1) The most common constituent of molecular clouds, H2, is rarely detected within them.

134

2) Molecular clouds appear more transparent at longer wavelengths.

135

3) Clouds that appear dark in visible light often glow when observed at long infrared wavelengths.

136

4) Most stars are born in clusters containing thousands of stars.

137

5) Stars only form in molecular clouds that contain more than 100 times the mass of our Sun.

138

6) No stars have been found composed solely of Hydrogen and Helium (and no heavier elements).

139

7) Photographs of many young stars show long jets of material apparently being ejected from their poles.

140

9) Protostars start off more luminous than the main sequence stars they become.

141

10) In any star cluster, stars with lower masses greatly outnumber those with higher masses.

142

1) What do we mean by the interstellar medium?
A) The gas and dust that lies in between the stars in the Milky Way Galaxy.
B) The dust that fills the halo of the Milky Way Galaxy.
C) The middle section of the Milky Way Galaxy.
D) The name of an oracle who can channel messages from beings that live near the star called Vega.

143

2) The interstellar clouds called molecular clouds are
A) the clouds in which elements such as carbon, nitrogen, and oxygen are made.
B) clouds that are made mostly of complex molecules such as carbon dioxide and sulfur dioxide.
C) the hot clouds of gas expelled by dying stars.
D) the cool clouds in which stars form.

144

3) Which of the following types of molecule is the most abundant in an interstellar molecular cloud?
A) CO
B) H2O
C) H2
D) NH3

145

4) Interstellar dust consists mostly of
A) ozone "smog."
B) microscopic particles of carbon and silicon.
C) hydrogen and helium atoms.
D) tiny grains of water ice.
E) the same tiny particles found in household dust.

146

5) Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?
A) visible light
B) ultraviolet
C) infrared
D) blue light

147

6) Suppose you look by eye at a star near the edge of a dusty interstellar cloud. The star will look ________ than it would if it were outside the cloud.
A) dimmer and bluer
B) more redshifted
C) brighter and redder
D) dimmer and redder

148

7) Most interstellar clouds remain stable in size because the force of gravity is opposed by ________ within the cloud.
A) degeneracy pressure
C) stellar winds
D) thermal pressure

149

8) What kind of gas cloud is most likely to give birth to stars?
A) a hot, dense gas cloud
B) a cold, dense gas cloud
C) a cold, low-density gas cloud
D) a hot, low-density gas cloud

150

9) What effect are magnetic fields thought to have on star formation in molecular clouds?
A) They can help resist gravity, so that more total mass is needed before the cloud can collapse to form stars.
B) They accelerate the star formation process.
C) They allow small stars to form in isolation within gas clouds.
D) None—there are no magnetic fields in interstellar space.

151

10) Which of the following statements is probably true about the very first stars in the universe?
A) They were made only from hydrogen and helium.
B) They were made from pure energy.
C) They were probably orbited only by terrestrial planets, but no jovian planets.
D) They were made approximately of 98% hydrogen and helium, and 2% of heavier elements.

152

11) What is a protostar?
A) a star that has planets
B) an intermediate-mass star
C) a star that is still in the process of forming
D) a star in its final stage of life

153

12) Which of the following phenomena is not commonly associated with the star formation process?
A) the formation of a spinning disk of material around a protostar
B) powerful "jets" shooting out along the rotation axis of a protostar
C) strong winds of particles blowing out into space from a protostar
D) intense ultraviolet radiation coming from a protostar

154

13) What law explains why a collapsing cloud usually forms a protostellar disk around a protostar?
A) Kepler's third law
B) the universal law of gravitation
C) Wien's law
D) conservation of angular momentum

155

14) What can we learn about a star from a life track on an H-R diagram?
A) the star's age
B) the surface temperature and luminosity the star will have at each stage of its life
C) the star's current stage of life
D) how the star's distance from Earth varies at different times in its life

156

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

157

16) Approximately what core temperature is required before hydrogen fusion can begin in a star?
A) 10,000 K
B) 10 million K
C) 1 billion K
D) 10 billion K
E) 10 trillion K

158

17) Which star spends the longest time in the protostellar phase of life?
A) a 1-solar-mass star
B) a 2-solar-mass star
C) a 3-solar-mass star
D) a 4-solar-mass star
E) a 5-solar-mass star

159

18) What is the approximate range of masses that newborn main-sequence stars can have?
A) 0.001 to 150 solar masses
B) 0.1 to 1,000 solar masses
C) 0.1 to 150 solar masses
D) 0.001 to 10 solar masses
E) 0.1 to 10 solar masses

160

19) The vast majority of stars in a newly formed star cluster are
A) very high-mass, type O and B stars.
B) red giants.
C) about the same mass as our Sun.
D) less massive than the Sun.

161

20) Which of the following statements about brown dwarfs is not true?
A) Brown dwarfs eventually collapse to become white dwarfs.
B) Brown dwarfs are supported against gravity by degeneracy pressure, which does not depend on the object's temperature.
C) Brown dwarfs form like ordinary stars but are too small to sustain nuclear fusion in their cores.
D) All brown dwarfs have masses less than about 8% that of our Sun.

162

1) Which two processes can generate energy to help a star or gas cloud maintain its internal thermal pressure?
A) nuclear fusion and nuclear fission
B) nuclear fusion and supernovae
C) nuclear fission and supernovae
D) nuclear fusion and gravitational contraction

163

3) How do we learn the chemical composition of the interstellar medium?
A) We make an educated guess based on the Sun's composition.
B) By studying spectra of interstellar gas clouds.
C) We collect samples of gas and dust from interstellar space.
D) We use computer simulations of the interstellar medium.

164

4) What happens to the visible light radiated by stars located within a dusty gas cloud?
A) It is blocked by dust and its energy is thereby lost.
B) It is absorbed by dust, which heats the dust grains so that they emit the absorbed energy as infrared light.
C) It is reflected by dust back to the star from whence it came.
D) It passes through the cloud unaffected.

165

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

166

6) Which process is required to allow a gravitationally-collapsing gas cloud to continue to collapse?
A) The cloud must trap most of its thermal energy.
B) The cloud must collide with other clouds.
C) New dust particles must continually be made in the cloud.
D) The cloud must radiate away much of its thermal energy.

167

7) According to current understanding, how did the first generation of stars differ from stars born today?
A) They contained much more hydrogen and helium than stars born today.
B) They were much cooler in temperature than most stars born today.
C) They were much more likely to be members of binary star systems than stars are born today.
D) They were much more massive than most stars born today.

168

8) Angular momentum plays an important role in star formation. Which of the following characteristics of a protostellar system is probably not strongly affected by the star's angular momentum?
A) the existence of protostellar jets
B) the strength of protostellar winds
C) the onset of core hydrogen fusion

D) the formation of a protostellar disk

169

9) Close binary star systems are thought to form when
A) two interstellar gas clouds happen to contract so close together that there's no room for a disk or planets.
B) the protostellar disk around a protostar has enough material to form a second star.
C) gravity pulls two neighboring protostars quite close together, but angular momentum causes them to orbit each other rather than colliding.
D) a protostar emits two jets, each of which turns into a star.

170

10) Generally speaking, how does the surface temperature and luminosity of a protostar compare to the surface temperature and luminosity of the main-sequence star it becomes?
A) A main-sequence star is hotter and brighter than it was as a protostar.
B) A main-sequence star is cooler and dimmer than it was as a protostar.
C) A main-sequence star is cooler and brighter than it was as a protostar.
D) A main-sequence star is hotter and dimmer than it was as a protostar.

171

11) Where does a 1-solar-mass protostar appear on an H-R diagram?
A) to the right of the main sequence, and lower down than the Sun
B) to the right of the main sequence, and higher up than the Sun
C) to the left of the main sequence, and higher up than the Sun
D) Nowhere—only stars that have fusion in their cores can be shown on H-R diagrams.

172

12) Why does the rotation of a protostar slow down over time?
A) All rotating objects slow down over time.
B) Magnetic fields can transfer angular momentum to the protostellar disk and protostellar winds can carry angular momentum away.
C) The onset of fusion causes the rotation rate to slow dramatically.
D) Magnetic fields of other stars interact with the magnetic fields of the protostars, slowing its rotation.

173

13) The surface of a protostar radiates energy while its core
A) shrinks and cools.
B) shrinks and maintains a constant temperature.
C) shrinks and heats.

D) expands and cools.

174

14) The core of a protostar that will eventually become a brown dwarf shrinks until
A) the type of pressure called degeneracy pressure becomes important.
B) its central temperature is high enough to support fusion reactions.
C) it forms a rocky core.

175

15) If a star is extremely massive (well over 100 solar masses), why isn't it likely to survive for long?
A) It explodes as a supernova after just a few dozen years.
B) It may blow itself apart because of radiation pressure.
C) It eventually divides into two lower-mass stars.
D) Its great mass will cause it to suck itself into becoming a black hole.

176

16) Consider a large molecular cloud that will give birth to a cluster of stars. Which of the following would you expect to be true?
A) All the stars in the cluster will be of about the same mass.
B) A few massive stars will form, live, and die before the majority of the star's clusters even complete their protostar stage.
C) All the stars in the cluster will become main-sequence stars at about the same time.
D) All the stars in the cluster will have approximately the same luminosity and surface temperature.

177

17) We do not know for certain whether the general trends we observe in stellar birth masses also apply to brown dwarfs. But if they do, then which of the following would be true?
A) Brown dwarfs would outnumber all ordinary stars.
B) Brown dwarfs would be responsible for most of the overall luminosity of our Milky Way Galaxy.
C) Brown dwarfs would be extremely rare.
D) Most of the brown dwarfs in the Milky Way Galaxy would be quite young in age.

178

18) Where would a brown dwarf be located on an H-R diagram?
A) above and to the left of the highest part of the main sequence
B) in the upper right corner of the H-R diagram
C) in the lower left corner of the H-R diagram
D) below and to the right of the lowest part of the main sequence

179

1) What do astronomers mean when they say that we are all "star stuff"?
A) that life would be impossible without energy from the Sun
B) that Earth formed at the same time as the Sun
C) that the carbon, oxygen, and many elements essential to life were created by nucleosynthesis in stellar cores
D) that the Sun formed from the interstellar medium: the "stuff" between the stars
E) that the Universe contains billions of stars

180

3) What type of star is our Sun?
A) low-mass star
B) intermediate-mass star
C) high-mass star

181

4) What is the range of star masses for high-mass stars?
A) between 500 and about 1,000 solar masses
B) between 200 and about 500 solar masses
C) between 8 and about 100 solar masses
D) between 2 and about 10 solar masses
E) between 2 and about 5 solar masses

182

5) What can we learn about a star from a life track on an H-R diagram?
A) how long ago it was born
B) when it will die
C) where it is located
D) what surface temperature and luminosity it will have at each stage of its life
E) all of the above

183

6) Which of the following statements about degeneracy pressure is not true?
A) Degeneracy pressure varies with the temperature of the star.
B) Degeneracy pressure can halt gravitational contraction of a star even when no fusion is occurring in the core.
C) Degeneracy pressure keeps any protostar less than 0.08 solar mass from becoming a true, hydrogen-fusing star.
D) Degeneracy pressure arises out of the ideas of quantum mechanics.
E) Degeneracy pressure supports white dwarfs against gravity.

184

7) All of the following are involved in carrying energy outward from a star's core except
A) convection.
C) conduction.
D) neutrinos.

185

8) Which stars have convective cores?
A) low-mass stars
B) intermediate-mass stars
C) high-mass stars
D) all of the above
E) none of the above

186

9) Which of the following spectral types is more likely to be a flare star?
A) KIII
B) MV
C) GV
D) I
E) BII

187

10) Which of the following properties make flare stars so active?
A) fast rotation rates
B) deep convection zones
C) convecting cores
D) strong stellar winds
E) both A and B

188

11) What happens when a star exhausts its core hydrogen supply?
A) Its core contracts, but its outer layers expand and the star becomes bigger and brighter.
B) It contracts, becoming smaller and dimmer.
C) It contracts, becoming hotter and brighter.
D) It expands, becoming bigger but dimmer.
E) Its core contracts, but its outer layers expand and the star becomes bigger but cooler and therefore remains at the same brightness.

189

12) What is happening inside a star while it expands into a subgiant?
A) It is fusing hydrogen into helium in the core.
B) It is fusing hydrogen into helium in a shell outside the core.
C) It is fusing helium into carbon in the core.
D) It is fusing helium into carbon in a shell outside the core.
E) It is not fusing any element; it is contracting and heating up.

190

13) Compared to the star it evolved from, a red giant is
A) hotter and brighter.
B) hotter and dimmer.
C) cooler and brighter.
D) cooler and dimmer.
E) the same temperature and brightness.

191

14) At approximately what temperature can helium fusion occur?
A) 100,000 K
B) 1 million K
C) a few million K
D) 100 million K
E) 100 billion K

192

15) Why does a star grow larger after it exhausts its core hydrogen?
A) The outer layers of the star are no longer gravitationally attracted to the core.
B) Hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.
C) Helium fusion in the core generates enough thermal pressure to push the upper layers outward.
D) Helium fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.
E) The internal radiation generated by the hydrogen fusion in the core has heated the outer layers enough that they can expand after the star is no longer fusing hydrogen.

193

16) How many helium nuclei fuse together when making carbon?
A) 2
B) 3
C) 4
D) varies depending on the reaction
E) none of the above

194

17) The helium fusion process results in the production of
A) hydrogen.
B) oxygen.
C) carbon.
D) nitrogen.
E) iron.

195

18) What happens after a helium flash?
A) The core quickly heats up and expands.
B) The star breaks apart in a violent explosion.
C) The core suddenly contracts.
D) The core stops fusing helium.
E) The star starts to fuse helium in a shell outside the core.

196

19) What is a carbon star?
A) a red giant star whose atmosphere becomes carbon-rich through convection from the core
B) a star that fuses carbon in its core
C) another name for a white dwarf, a remnant of a star made mainly of carbon
D) a star that produces carbon by fusion in its atmosphere
E) a star that is made at least 50 percent of carbon

197

20) What is a planetary nebula?
A) a disk of gas surrounding a protostar that may form into planets
B) what is left of the planets around a star after a low-mass star has ended its life
C) the expanding shell of gas that is no longer gravitationally held to the remnant of a low-mass star
D) the molecular cloud from which protostars form
E) the expanding shell of gas that is left when a white dwarf explodes as a supernova

198

21) What happens to the core of a star after a planetary nebula occurs?
A) It contracts from a protostar to a main-sequence star.
B) It breaks apart in a violent explosion.
C) It becomes a white dwarf.
D) It becomes a neutron star.
E) none of the above

199

22) Which of the following sequences correctly describes the stages of life for a low-mass star?
A) red giant, protostar, main-sequence, white dwarf
B) white dwarf, main-sequence, red giant, protostar
C) protostar, red giant, main-sequence, white dwarf
D) protostar, main-sequence, white dwarf, red giant
E) protostar, main-sequence, red giant, white dwarf

200

23) Compared to the star it evolved from, a white dwarf is
A) hotter and brighter.
B) hotter and dimmer.
C) cooler and brighter.
D) cooler and dimmer.
E) the same temperature and brightness.

201

24) Most interstellar dust grains are produced in
A) the Big Bang.
B) the interstellar medium.
C) the atmospheres of red giant stars.
D) supernova explosions.
E) the solar nebula.

202

25) During which stage is the star's energy supplied by gravitational contraction?
A) ii
B) iii
C) v
D) vi
E) viii

203

26) During which stage does the star have an inert (nonburning) helium core?
A) iii
B) iv
C) vi
D) vii
E) viii

204

27) During which stage does the star have an inert (nonburning) carbon core?
A) ii
B) iii
C) iv
D) vi
E) viii

205

28) Which stage lasts the longest?
A) i
B) iii
C) iv
D) vi
E) viii

206

29) What will happen to the star after stage viii?
A) It will explode in a supernova.
B) It will begin burning carbon in its core.
C) It will eject a planetary nebula.
D) It will collapse to make a neutron star.
E) It will gain mass until it collapses under its own weight.

207

30) In the end, the remaining core of this star will be left behind as
A) a white dwarf made primarily of carbon and oxygen.
B) a white dwarf made primarily of silicon and iron.
C) a neutron star.
D) a black hole.
E) a supernova.

208

31) Based on its main-sequence turnoff point, the age of this cluster is
A) less than 1 billion years.
E) more than 15 billion years.

209

A) It is likely to be located in the halo of the galaxy.
B) It contains some stars that are burning helium in their cores.
C) It is the type of cluster known as an open cluster of stars.
D) It probably contains no young stars at all.
E) It is likely to be spherical in shape.

210

33) Consider the star to which the arrow points. How is it currently generating energy?
A) by gravitational contraction
B) by hydrogen shell burning around an inert helium core
C) by core hydrogen fusion
D) by core helium fusion combined with hydrogen shell burning
E) by both hydrogen and helium shell burning around an inert carbon core

211

34) Consider the star to which the arrow points. Which of the following statements about this star is not true?
A) It is significantly less massive than the Sun.
B) It is larger in radius than the Sun.
C) It is brighter than the Sun.
D) Its surface temperature is lower than the Sun's.
E) Its core temperature is higher than the Sun's.

212

35) What is the CNO cycle?
A) the process by which helium is fused into carbon, nitrogen, and oxygen
B) the process by which carbon is fused into nitrogen and oxygen
C) a type of hydrogen fusion that uses carbon, nitrogen, and oxygen atoms as catalysts
D) the period of a massive star's life when carbon, nitrogen, and oxygen are fusing in different shells outside the core
E) the period of a low-mass star's life when it can no longer fuse carbon, nitrogen, and oxygen in its core

213

37) What happens when the gravity of a massive star is able to overcome neutron degeneracy pressure?
A) The core contracts and becomes a white dwarf.
B) The core contracts and becomes a ball of neutrons.
C) The core contracts and becomes a black hole.
D) The star explodes violently, leaving nothing behind.
E) Gravity is not able to overcome neutron degeneracy pressure.

214

38) What types of stars end their lives with supernovae?
A) all stars that are red in color
B) all stars that are yellow in color
C) stars that are at least several times the mass of the Sun
D) stars that are similar in mass to the Sun
E) stars that have reached an age of 10 billion years

215

39) Which of the following statements about stages of nuclear burning (i.e., first-stage hydrogen burning, second-stage helium burning, etc.) in a massive star is not true?
A) Each successive stage of fusion requires higher temperatures than the previous stages.
B) As each stage ends, the core shrinks further.
C) Each successive stage creates an element with a higher atomic weight.
D) Each successive stage lasts for approximately the same amount of time.

216

40) Suppose the star Betelgeuse (the upper left shoulder of Orion) were to become a supernova tomorrow (as seen here on Earth). What would it look like to the naked eye?
A) Because the supernova event destroys the star, Betelgeuse would suddenly disappear from view.
B) We'd see a cloud of gas expanding away from the position where Betelgeuse used to be. Over a period of a few weeks, this cloud would fill our entire sky.
C) Betelgeuse would remain a dot of light but would suddenly become so bright that, for a few weeks, we'd be able to see this dot in the daytime.
D) Betelgeuse would suddenly appear to grow larger in size, soon reaching the size of the full moon. It would also be about as bright as the full moon.

217

43) Why is Supernova 1987A particularly important to astronomers?
A) It occurred only a few dozen light-years from Earth.
B) It provided the first evidence that supernovae really occur.
C) It provided the first evidence that neutron stars really exist.
D) It was the first supernova detected in nearly 400 years.
E) It was the nearest supernova detected in nearly 400 years.

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45) You discover a binary star system in which one member is a15MSun main-sequence star and the other star is a 10MSun giant. How do we believe that a star system such as this might have come to exist?
A) The giant must once have been the more massive star but transferred some of its mass to its companion.
B) Despite the low odds of finding a system with two such massive stars, there is nothing surprising about the fact that such systems exist.
C) The two stars probably were once separate but became a binary when a close encounter allowed their mutual gravity to pull them together.
D) The main-sequence star probably is a pulsating variable star and therefore appears to be less massive than it really is.
E) Although both stars probably formed from the same clump of gas, the more massive one must have had its birth slowed so that it became a main-sequence star millions of years later than its less massive companion.

219

46) Why do scientists think that our solar system must have formed sometime after nearby supernovae explosions?
A) Existence of heavy elements
B) Solar temperature too low
C) Our Sun is a G-type star
D) They don't—scientists believe our Sun is among the first generation of stars.

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1) Photographs of many young stars show long jets of material apparently being ejected from their poles.

221

2) Although some photographs show what looks like jets of material near many young stars, we now know that these "jets" actually represent gas from the surrounding nebula that is falling onto the stars.

222

3) In any star cluster, stars with lower masses greatly outnumber those with higher masses.

223

5) Stars with high masses live longer than stars with lower masses.

224

4) There is no limit to the mass with which a star can be born.

225

6) Stars of lower mass have deeper convection zones outside their cores than stars of higher mass.

226

7) Convection never occurs in the core of any type of star.

227

8) The helium fusion process works by fusing two helium nuclei into one beryllium nucleus.

228

) Our Sun will end its life in a planetary nebula and become a white dwarf.

229

10) The most massive stars generate energy at the end of their lives by fusing iron in their cores.

230

11) The heaviest element produced by stars or in supernovae is silicon.

231

12) All stars that become supernovae will leave behind a neutron star.

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A. H fusion by the proton-proton chain

B. H fusion by the CNO cycle

C. helium fusion
D. matter-antimatter annihilation
E. gravitational contraction

1) Which method of energy generation is used by the Sun today?

233

A. H fusion by the proton-proton chain
B. H fusion by the CNO cycle
C. helium fusion
D. matter-antimatter annihilation
E. gravitational contraction

2) Which one provided the energy that made the Sun hot in the first place?

234

A. H fusion by the proton-proton chain
B. H fusion by the CNO cycle
C. helium fusion
D. matter-antimatter annihilation
E. gravitational contraction

3) Which method of energy generation provides the source of energy for a protostar?

235

A. H fusion by the proton-proton chain
B. H fusion by the CNO cycle
C. helium fusion
D. matter-antimatter annihilation
E. gravitational contraction

4) Which process leads to the production of carbon?

236

A. H fusion by the proton-proton chain
B. H fusion by the CNO cycle
C. helium fusion
D. matter-antimatter annihilation
E. gravitational contraction

5) When a 1-solar-mass star stabilizes as a giant for about a billion years, which method of energy generation occurs in its central core?

237

A. H fusion by the proton-proton chain
B. H fusion by the CNO cycle
C. helium fusion
D. matter-antimatter annihilation
E. gravitational contraction

6) Which one is used by a main-sequence star of spectral type B2?

238

A. H fusion by the proton-proton chain
B. H fusion by the CNO cycle
C. helium fusion
D. matter-antimatter annihilation
E. gravitational contraction

7) Which method of energy generation provides the source of energy for a 10MSun main-sequence star?

239

1) Which of the following stars will live longest?
A) a 1-solar-mass star
B) a 2-solar-mass star
C) a 3-solar-mass star
D) a 4-solar-mass star
E) a 5-solar-mass star

240

2) In the context of understanding stellar lives, "high-mass" stars have masses
A) more than about 8 times the mass of our Sun.
B) more than about 3 times the mass of our Sun.
C) more than twice the mass of our Sun.
D) more than 20 times the mass of our Sun.

241

3) Which of the following lists the stages of life for a low-mass star in the correct order?
A) protostar, main-sequence star, red giant, planetary nebula, white dwarf
B) protostar, main-sequence star, red giant, supernova, neutron star
C) protostar, main-sequence star, planetary nebula, red giant
D) main-sequence star, white dwarf, red giant, planetary nebula, protostar

242

4) What happens when a main-sequence star exhausts its core hydrogen fuel supply?
A) The entire star shrinks in size.
B) The core shrinks while the rest of the star expands.
C) The core immediately begins to fuse its helium into carbon.
D) The star becomes a neutron star.

243

5) The main source of energy for a star as it grows in size to become a red giant is
A) hydrogen fusion in the central core.
B) helium fusion in the central core.
C) hydrogen fusion in a shell surrounding the central core.
D) gravitational contraction.

244

6) The overall helium fusion reaction is
A) three helium nuclei fuse to form one carbon nucleus.
B) two helium nuclei fuse to form one beryllium nucleus.
C) two hydrogen nuclei fuse to form one helium nucleus.
D) four helium nuclei fuse to form one oxygen nucleus.

245

7) What is a helium flash?
A) The ignition of helium shell burning in a high-mass star with a carbon core.
B) A sudden brightening of a low-mass star, detectable from Earth by observing spectral lines of helium.
C) It is another name for the helium fusion reaction.
D) The sudden onset of helium fusion in the core of a low-mass star.

246

8) An H-R diagram for a globular cluster will show a horizontal branch—a line of stars above the main-sequence but to the left of the subgiants and red giants.
Which of the following statements about these horizontal branch stars is true?
A) They have inert (non-burning) carbon cores.
B) Their sole source of energy is hydrogen shell burning.
C) They generate energy through both hydrogen fusion and helium fusion.
D) In a particular star cluster, all horizontal branch stars have the same spectral type.

247

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

248

10) The ultimate fate of our Sun is to
A) explode in a supernova.
B) become a white dwarf that will slowly cool with time.
C) become a rapidly spinning neutron star.
D) become a black hole.

249

11) Which low-mass star does not have fusion occurring in its central core?
A) a main-sequence star
B) a red giant
C) a helium-burning star

250

12) How are low-mass red giant stars important to our existence?
A) These stars manufactured virtually all the elements out of which we and our planet are made.
B) These stars generate the energy that makes life on Earth possible.
C) These stars manufactured most of the carbon atoms in our bodies.
D) These stars provide most of the light that reaches us from globular clusters.

251

13) Which of the following pairs of atomic nuclei would feel the strongest repulsive electromagnetic force if you tried to push them together?
A) helium and helium
B) hydrogen and hydrogen
C) hydrogen and helium
D) hydrogen and deuterium

252

14) Which of the following stars will certainly end its life in a supernova?
A) the Sun
B) a red giant star
C) a 10-solar-mass star
D) a neutron star

253

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

254

16) In order to predict whether a star will eventually fuse oxygen into a heavier element, what do you need to know about the star?
A) its luminosity
B) its overall abundance of elements heavier than helium
C) how much oxygen it now has in its core
D) its mass

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17) Why is iron significant to understanding how a supernova occurs?
A) Iron is the heaviest of all atomic nuclei, and thus no heavier elements can be made.
B) Supernovae often leave behind neutron stars, which are made mostly of iron.
C) The fusion of iron into uranium is the reaction that drives a supernova explosion.
D) Iron cannot release energy either by fission or fusion.

256

18) After a supernova explosion, the remains of the stellar core
A) will always be a neutron star.
B) be either a neutron star or a black hole.
C) will always be a black hole.
D) may be either a white dwarf, neutron star, or black hole

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19) Why is Supernova 1987A particularly important to astronomers?
A) It is the nearest supernova to have occurred at a time when we were capable of studying it carefully with telescopes.
B) It was the first supernova detected in nearly 400 years.
C) It provided the first evidence that supernovae really occur.
D) It occurred only a few light-years from Earth.

258

20) Algol consist of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant. Why does this seem surprising, at least at first?
A) The two stars in a binary system should both be at the same stage of life; that is, they should either both be main-sequence stars or both be subgiants.
B) It doesn't make sense to find a subgiant in a binary star system.
C) The two stars should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star.
D) A star with a mass of 3.7 MSun is too big to be a main-sequence star.

259

21) Where does gold (the element) come from?
A) It is produced by mass transfer in close binaries.
B) It is produced during the supernova explosions of high-mass stars.
C) It is produced during the late stages of fusion in low-mass stars.
D) It was produced during the Big Bang.

260

1) Sun is considered to be a
A) low-mass star.
B) intermediate-mass star
C) high-mass star.
D) brown dwarf.

261

2) Which of the following types of data provide evidence that helps us understand the life tracks of low-mass stars?
A) H-R diagrams of open clusters
B) observing a low-mass star over many years
C) H-R diagrams of globular clusters
D) spacecraft observations of the Sun

262

3) Why is a 1 solar-mass red giant more luminous than a 1 solar-mass main-sequence star?
A) The red giant has a hotter core.
B) The red giant's surface is hotter.
C) The red giant is more massive.
D) Fusion reactions are producing energy at a greater rate in the red giant.

263

4) Which of the following describes a star with a hydrogen-burning shell and an inert helium core?
A) It is a red giant that grows in luminosity until it dies in a planetary nebula.
B) It is a subgiant that gradually grows dimmer as its hydrogen-burning shell expands and cools.
C) It is a subgiant that grows in luminosity until helium fusion begins in the central core.
D) It is what is known as a helium-burning star, which has both helium fusion in its core and hydrogen fusion in a shell.

264

5) Which of the following observations would not be likely to provide information about the final, explosive stages of a star's life?
A) studying the light rings around Supernova 1987A in the Large Magellanic Cloud
B) decades of continuous monitoring of red giants in a globular cluster
C) observing the structures of planetary nebulae
D) neutrino detections from nearby supernovae

265

6) Which is more common: a star blows up as a supernova, or a star forms a planetary nebula/white dwarf system?
A) Supernovae are more common.
B) Planetary nebula formation is more common.
C) They both occur in about equal numbers.
D) It is impossible to say.

266

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

267

14) Which event marks the beginning of a supernova?
A) the sudden collapse of an iron core into a compact ball of neutrons
B) the onset of helium burning after a helium flash
C) the beginning of neon burning in an extremely massive star
D) the sudden initiation of the CNO cycle

268

16) Suppose that hydrogen, rather than iron, had the lowest mass per nuclear particle. Which of the following would be true?
A) Stars would be brighter.
B) Stars would be less massive.
C) All stars would be red giants.
D) Nuclear fusion could not power stars.

269

17) Observations show that elements with atomic mass numbers divisible by 4 (such as oxygen-16, neon-20, and magnesium-24) tend to be more abundant in the universe than elements with atomic mass numbers in between. Why do we think this is the case?
A) The apparent pattern is thought to be a random coincidence.
B) Elements with atomic mass numbers divisible by 4 tend to be more stable than elements in between.
C) At the end of a high-mass star's life, it produces new elements through a series of helium capture reactions.
D) This pattern in elemental abundances was apparently determined during the first few minutes after the Big Bang.

270

18) A spinning neutron star has been observed at the center of a
A) planetary nebula.
B) supernova remnant.
C) red supergiant.
D) protostar.

271

20) Tidal forces are very important to the Algol system today, but were not important when both stars were still on the main sequence. Why not?
A) Main-sequence stars in a system like the Algol system are small compared to their physical separation.
B) Main-sequence stars are too big to be affected by tidal forces.
C) Main-sequence stars are too massive to be affected by tidal forces.
D) Main-sequence stars are unaffected by tidally-induced mass transfer.

272

1) Degeneracy pressure is the source of the pressure that stops the crush of gravity in all the following except
A) a brown dwarf.
B) a white dwarf.
C) a neutron star.
D) a very massive main-sequence star.
E) the central core of the Sun after hydrogen fusion ceases but before helium fusion begins.

273

2) White dwarfs are so called because
A) they are both very hot and very small.
B) they are the end-products of small, low-mass stars.
C) they are the opposite of black holes.
D) it amplifies the contrast with red giants.
E) they are supported by electron degeneracy pressure.

274

3) A teaspoonful of white dwarf material on Earth would weigh
A) the same as a teaspoonful of Earth-like material.
B) about the same as Mt. Everest.
C) about the same as Earth.
D) a few tons.
E) a few million tons.

275

4) Which of the following is closest in mass to a white dwarf?
A) the Moon
B) Earth
C) Jupiter
D) Neptune
E) the Sun

276

6) What is the ultimate fate of an isolated white dwarf?
A) It will cool down and become a cold black dwarf.
B) As gravity overwhelms the electron degeneracy pressure, it will explode as a nova.
C) As gravity overwhelms the electron degeneracy pressure, it will explode as a supernova.
D) As gravity overwhelms the electron degeneracy pressure, it will become a neutron star.
E) The electron degeneracy pressure will eventually overwhelm gravity and the white dwarf will slowly evaporate.

277

8) Which of the following statements about novae is not true?
A) A star system that undergoes a nova may have another nova sometime in the future.
B) A nova involves fusion taking place on the surface of a white dwarf.
C) Our Sun will probably undergo at least one nova when it becomes a white dwarf about 5 billion years from now.
D) When a star system undergoes a nova, it brightens considerably, but not as much as a star system undergoing a supernova.
E) The word nova means "new star" and originally referred to stars that suddenly appeared in the sky, then disappeared again after a few weeks or months.

278

9) What kind of pressure supports a white dwarf?
A) neutron degeneracy pressure
B) electron degeneracy pressure
C) thermal pressure
E) all of the above

279

11) How does a 1.2-solar-mass white dwarf compare to a 1.0-solar-mass white dwarf?
A) It has a larger radius.
B) It has a smaller radius.
C) It has a higher surface temperature.
D) It has a lower surface temperature.
E) It is supported by neutron, rather than electron, degeneracy pressure.

280

12) Which of the following is closest in size (radius) to a white dwarf?
A) Earth
B) a small city
E) the Sun

281

13) What kind of star is most likely to become a white-dwarf supernova?
A) an O star
B) a star like our Sun
C) a binary M star
D) a white dwarf star with a red giant binary companion
E) a pulsar

282

14) Observationally, how can we tell the difference between a white-dwarf supernova and a massive-star supernova?
A) A massive-star supernova is brighter than a white-dwarf supernova.
B) A massive-star supernova happens only once, while a white-dwarf supernova can repeat periodically.
C) The spectrum of a massive-star supernova shows prominent hydrogen lines, while the spectrum of a white-dwarf supernova does not.
D) The light of a white-dwarf supernova fades steadily, while the light of a massive-star supernova brightens for many weeks.
E) We cannot yet tell the difference between a massive-star supernova and a white-dwarf supernova.

283

16) A teaspoonful of neutron star material on Earth would weigh
A) about the same as a teaspoonful of Earth-like material.
B) a few tons.
C) more than Mt. Everest.
D) more than the Moon.
E) more than Earth.

284

17) Which of the following is closest in size (radius) to a neutron star?
A) Earth
B) a city
E) the Sun

285

18) Which of the following best describes what would happen if a 1.5-solar-mass neutron star, with a diameter of a few kilometers, were suddenly (for unexplained reasons) to appear in your hometown?
A) The entire mass of Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star.
B) It would rapidly sink to the center of Earth.
C) The combined mass of Earth and the neutron star would cause the neutron star to collapse into a black hole.
D) It would crash through Earth, creating a large crater, and exit Earth on the other side.
E) It would crash into Earth, throwing vast amounts of dust into the atmosphere which in turn would cool Earth. Such a scenario is probably what caused the extinction of the dinosaurs.

286

19) From an observational standpoint, what is a pulsar?
A) a star that slowly changes its brightness, getting dimmer and then brighter with a period of anywhere from a few hours to a few weeks
B) an object that emits flashes of light several times per second or more, with near perfect regularity
C) an object that emits random "pulses" of light that sometimes occur only a fraction of a second apart and other times stop for several days at a time
D) a star that changes color rapidly, from blue to red and back again
E) a star that rapidly changes size as it moves off the main sequence

287

]20) From a theoretical standpoint, what is a pulsar?
A) a star that alternately expands and contracts in size
B) a rapidly rotating neutron star
C) a neutron star or black hole that happens to be in a binary system
D) a binary system that happens to be aligned so that one star periodically eclipses the other
E) a star that is burning iron in its core

288

21) What causes the radio pulses of a pulsar?
A) The star vibrates.
B) As the star spins, beams of radio radiation sweep through space. If one of the beams crosses Earth, we observe a pulse.
C) The star undergoes periodic explosions of nuclear fusion that generate radio emission.
D) The star's orbiting companion periodically eclipses the radio waves emitted by the main pulsar.
E) A black hole near the star absorbs energy and re-emits it as radio waves.

289

22) How do we know that pulsars are neutron stars?
A) We have observed massive-star supernovae produce pulsars.
B) Pulsars and neutron stars look exactly the same.
C) No massive object, other than a neutron star, could spin as fast as we observe pulsars spin.
D) Pulsars have the same upper mass limit as neutron stars do.
E) none of the above

290

23) What is the ultimate fate of an isolated pulsar?
A) It will spin ever faster, becoming a millisecond pulsar.
B) As gravity overwhelms the neutron degeneracy pressure, it will explode as a supernova.
C) As gravity overwhelms the neutron degeneracy pressure, it will become a white dwarf.
D) It will slow down, the magnetic field will weaken, and it will become invisible.
E) The neutron degeneracy pressure will eventually overwhelm gravity and the pulsar will slowly evaporate.

291

24) What is the basic definition of a black hole?
A) any compact mass that emits no light
C) any object from which the escape velocity exceeds the speed of light
D) any object made from dark matter
E) a dead galactic nucleus that can only be viewed in infrared

292

25) How does the gravity of an object affect light?
A) Light doesn't have mass; therefore, it is not affected by gravity.
B) Light coming from a compact massive object, such as a neutron star, will be redshifted.
C) Light coming from a compact massive object, such as a neutron star, will be blueshifted.
D) Visible light coming from a compact massive object, such as a neutron star, will be redshifted, but higher frequencies such as X rays and gamma rays will not be affected.
E) Less energetic light will not be able to escape from a compact massive object, such as a neutron star, but more energetic light will be able to.

293

26) How does a black hole form from a massive star?
A) During a supernova, if a star is massive enough for its gravity to overcome neutron degeneracy of the core, the core will be compressed until it becomes a black hole.
B) Any star that is more massive than 8 solar masses will undergo a supernova explosion and leave behind a black-hole remnant.
C) If enough mass is accreted by a white-dwarf star so that it exceeds the 1.4-solar-mass limit, it will undergo a supernova explosion and leave behind a black-hole remnant.
D) If enough mass is accreted by a neutron star, it will undergo a supernova explosion and leave behind a black-hole remnant.
E) A black hole forms when two massive main-sequence stars collide.

294

27) Which of the following statements about black holes is not true?
A) If you watch someone else fall into a black hole, you will never see him or her cross the event horizon. However, he or she will fade from view as the light he or she emits (or reflects) becomes more and more redshifted.
B) If we watch a clock fall toward a black hole, we will see it tick slower and slower as it falls nearer to the black hole.
C) A black hole is truly a hole in spacetime, through which we could leave the observable universe.
D) If the Sun magically disappeared and was replaced by a black hole of the same mass, Earth would soon be sucked into the black hole.
E) If you fell into a black hole, you would experience time to be running normally as you plunged rapidly across the event horizon.

295

28) In some cases, a supernova in a binary system may lead to the eventual formation of an accretion disk around the remains of the star that exploded. All of the following statements about such accretion disks are true except
A) X rays are emitted by the hot gas in the accretion disk.
B) the accretion disk consists of material that spills off the companion star.
C) the central object about which the accretion disk swirls may be either a neutron star or a black hole.
D) several examples of flattened accretion disks being "fed" by a large companion star can be seen clearly in photos from the Hubble Space Telescope.
E) the radiation from an accretion disk may vary rapidly in time.

296

29) When we see X rays from an accretion disk in a binary system, we can't immediately tell whether the accretion disk surrounds a neutron star or a black hole. Suppose we then observe each of the following phenomena in this system. Which one would force us to immediately rule out the possibility of a black hole?
A) bright X-ray emission that varies on a time scale of a few hours
B) spectral lines from the companion star that alternately shift to shorter and longer wavelengths
C) sudden, intense X-ray bursts
D) visible and ultraviolet light from the companion star

297

A) 3 km
B) 30 km
C) 3,000 km
D) 300 million km
E) 3 million km

298

32) What do we mean by the singularity of a black hole?
A) There are no binary black holes—each one is isolated.
B) An object can become a black hole only once, and a black hole cannot evolve into anything else.
C) It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.
D) It is the edge of the black hole, where one could leave the observable universe.
E) It is the "point of no return" of the black hole; anything closer than this point will not be able to escape the gravitational force of the black hole.

299

31) A 10-solar-mass main-sequence star will produce which of the following remnants?
A) white dwarf
B) neutron star
C) black hole
D) none of the above

300

33) How do we know what happens at the event horizon of a black hole?
A) Physicists have created miniature black holes in the lab.
B) Astronomers have sent spacecraft through the event horizon of a nearby black hole.
C) Astronomers have analyzed the light from matter within the event horizon of many black holes.
D) Astronomers have detected X rays from accretion disks around black holes.
E) We don't know for sure: we only know what to expect based on the predictions of general relativity.

301

34) Prior to the 1990s, most astronomers assumed that gamma-ray bursts came from neutron stars with accretion disks. How do we now know that this hypothesis was wrong?
A) We now know that gamma-ray bursts come not from neutron stars but from black holes.
B) Theoretical work has proven that gamma rays cannot be produced in accretion disks.
C) Observations from the Compton Gamma-Ray Observatory show that gamma-ray bursts come randomly from all directions in the sky.
D) Observations from the Compton Gamma-Ray Observatory show that gamma-ray bursts occur too frequently to be attributed to neutron stars.
E) Observations from the Compton Gamma-Ray Observatory have allowed us to trace gamma-ray bursts to pulsating variable stars in distant galaxies.

302

35) Why do astronomers consider gamma-ray bursts to be one of the greatest mysteries in astronomy?
A) because they are so rare
B) because we know they come from pulsating variable stars but don't know how they are created
C) because the current evidence suggests that they are the most powerful bursts of energy that ever occur anywhere in the universe, but we don't know how they are produced
D) because current evidence suggests that they come from our own Milky Way, but we have no idea where in the Milky Way they occur
E) because current evidence suggests that they come from massive black holes in the centers of distant galaxies, adding to the mystery of black holes themselves

303

39) If you were to come back to our Solar System in 6 billion years, what might you expect to find?
A) a red giant star
B) a white dwarf
C) a rapidly spinning pulsar
D) a black hole
E) Everything will be pretty much the same as it is now.

304

40) Black holes, by definition, cannot be observed directly. What observational evidence do scientists have of their existence?
A) Theoretical models predict their existence.
B) Gravitational interaction with other objects.
C) Space is, overall, very black.
D) We have sent spacecraft to nearby black holes.
E) We have detected neutrinos from them.

305

1) Brown dwarfs, white dwarfs, and neutrons stars are all kept from collapsing by degeneracy pressure.

306

2) The upper limit to the mass of a white dwarf is 1.4 solar masses.

307

3) More massive white dwarfs are smaller than less massive white dwarfs.

308

4) There is no upper limit to the mass of a neutron star.

309

The remnant left behind from a white-dwarf supernova is a neutron star.

310

Our Sun will likely undergo a nova event in about 5 billion years.

311

All pulsars are neutron stars, but not all neutron stars are pulsars.

312

8) Neutron stars are the densest objects that we can observe in the universe.

313

) No visible light can escape a black hole, but things such as gamma rays, X rays, and neutrinos can.

314

10) Light from white dwarfs shows a gravitational redshift.

315

All massive-star supernovae leave behind black holes as remnants.

316

Planets have been detected around a pulsar.

317

1) A white dwarf is
A) a precursor to a black hole.
B) an early stage of a neutron star.
C) what most stars become when they die.
D) a brown dwarf that has exhausted its fuel for nuclear fusion.

318

2) A typical white dwarf is
A) as large in diameter as the Sun but only about as massive as Earth.
B) as massive as the Sun but only about as large in size as Earth.
C) about the same size and mass as the Sun but much hotter.
D) as massive as the Sun but only about as large in size as Jupiter.

319

4) The maximum mass of a white dwarf is
A) about the mass of our Sun.
B) limitless; there is no theoretical limit to the maximum mass of a white dwarf.
C) about 3 times the mass of our Sun.
D) about 1.4 times the mass of our Sun.

320

5) What is an accretion disk?
A) any flattened disk in space, such as the disk of the Milky Way Galaxy
B) a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole
C) a stream of gas flowing from one star to its binary companion star
D) a disk of material found around every white dwarf in the Milky Way Galaxy

321

6) According to our modern understanding, what is a nova?
A) an explosion on the surface of a white dwarf in a close binary system
B) the explosion of a massive star at the end of its life
C) the sudden formation of a new star in the sky

D) a rapidly spinning neutron star

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7) Suppose that a white dwarf is gaining mass through accretion in a binary system. What happens if the mass someday reaches the 1.4 solar mass limit?
A) The white dwarf will collapse in size, becoming a neutron star.
B) The white dwarf will undergo a nova explosion.
C) The white dwarf will explode completely as a white dwarf supernova.
D) The white dwarf will collapse to become a black hole.

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8) A neutron star is
A) the remains of a star that died by expelling its outer layers in a planetary nebula.
B) a star made mostly of elements with high atomic mass numbers, so that they have lots of neutrons.
C) the remains of a star that died in a massive star supernova (if no black hole was created).
D) an object that will ultimately become a black hole.

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9) A typical neutron star is more massive than our Sun and about the size (radius) of
A) a small asteroid (10 km in diameter).
B) Earth.
C) the Moon.
D) Jupiter.

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11) Pulsars are thought to be
A) accreting white dwarfs.
B) rapidly rotating neutron stars.
C) unstable high-mass stars.
D) accreting black holes.

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12) How is an X-ray burst (in an X-ray binary system) similar to a nova?
A) Both involve explosions on the surface of stellar corpse.
B) Both typically recur every few hours to every few days.
C) Both are thought to involve fusion of hydrogen into helium.
D) Both result in the complete destruction of their host stars.

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13) What is the basic definition of a black hole?
B) any object made from dark matter
C) an object with gravity so strong that not even light can escape
D) a compact mass that emits no visible light

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14) Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly
A) 0.5 solar masses.
B) 1.4 solar masses.
C) 3 solar masses.
D) 10 solar masses.

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15) What do we mean by the event horizon of a black hole?
A) It is the very center of the black hole.
B) It is the distance from the black hole at which stable orbits are possible.
C) It is the place where X rays are emitted from black holes.
D) It is the point beyond which neither light nor anything else can escape.

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16) Imagine that our Sun were magically and suddenly replaced by a black hole of the same mass (1 solar mass). What would happen to Earth in its orbit?
A) Earth would almost instantly be sucked into oblivion in the black hole.
B) Earth would orbit faster, but at the same distance.
C) Earth would slowly spiral inward until it settled into an orbit about the size of Mercury's current orbit.
D) Nothing—Earth's orbit would remain the same.

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18) What makes us think that the star system Cygnus X-1 contains a black hole?
A) It emits X rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a neutron star.
B) No light is emitted from this star system, so it must contain a black hole.
C) The fact that we see strong X-ray emission tells us that the system must contain a black hole.
D) Cygnus X-1 is a powerful X-ray burster, so it must contain a black hole.

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19) The Schwarzschild radius of a black hole depends on
A) the observationally measured radius of the black hole.
B) the way in which the black hole formed.
C) only the mass of the black hole.
D) both the mass and chemical composition of the black hole.

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20) Scientists have detected thousands of gamma ray bursts. The evidence suggests that most or all of these bursts
A) have occurred in the central regions of the Milky Way.
B) have occurred in distant galaxies.
C) come from the same types of close binary systems that produce X-ray bursts.
D) come from the Oort cloud surrounding the Sun.

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21) Which of the following statements about electron degeneracy pressure and neutron degeneracy pressure is true?
A) Electron degeneracy pressure is the main source of pressure in white dwarfs, while neutron degeneracy pressure is the main source of pressure in neutron stars.
B) Both electron degeneracy pressure and neutron degeneracy pressure help govern the internal structure of a main-sequence star.
C) The life of a white dwarf is an ongoing battle between electron degeneracy pressure and neutron degeneracy pressure.
D) In a black hole, the pressure coming from neutron degeneracy pressure is slightly greater than that coming from electron degeneracy pressure.

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1) Which of the following statements about degeneracy pressure is not true?
A) Degeneracy pressure can continue to support an object against gravitational collapse even if the object becomes extremely cold.
B) Degeneracy pressure arises from a quantum mechanical effect that we don't notice in our daily lives.
C) Black holes form when gravity overcomes neutron degeneracy pressure.
D) Degeneracy pressure can arise only from interactions among electrons.

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2) The more massive a white dwarf, the
A) higher its temperature.
D) higher its luminosity.

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3) Which of the following best describes why a white dwarf cannot have a mass greater than the 1.4-solar-mass limit?
A) Electron degeneracy pressure depends on the speeds of electrons, which approach the speed of light as a white dwarf's mass approaches the 1.4-solar-mass limit.
B) White dwarfs get hotter with increasing mass, and above the 1.4-solar-mass limit they would be so hot that even their electrons would melt.
C) White dwarfs are made only from stars that have masses less than the 1.4-solar-mass limit.
D) The upper limit to a white dwarf's mass is something we have learned from observations, but no one knows why this limit exists.

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4) The white dwarf that remains when our Sun dies will be mostly made of
A) hydrogen.
B) helium.
C) carbon.
D) neutrons.

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5) Which statement about accretion disks is not true?
A) The gas in the inner parts of the disk travels faster than the gas in the outer parts of the disk.
B) The gas in the inner parts of the disk is hotter than the gas in the outer parts of the disk.
C) The primary factor determining whether a white dwarf has an accretion disk is the white dwarf's mass.
D) Accretion disks are made primarily of hydrogen and helium gas.

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6) According to present understanding, a nova is caused by
A) hydrogen fusion on the surface of a white dwarf.
B) carbon fusion in the core of a white dwarf.
C) hydrogen fusion on the surface of a neutron star.
D) a white dwarf that gains enough mass to exceed the 1.4-solar-mass limit.

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7) Which of the following is not true about differences between novae and supernovae?
A) Novae are much less luminous than supernovae.
B) Supernovae eject gas into space but novae do not.
C) Novae occur only in binary star systems, while supernovae can occur both among single stars and among binary star systems.
D) The same star can undergo novae explosions more than once, but can undergo only a single supernova.

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8) Will our Sun ever undergo a white dwarf supernova explosion? Why or why not?
A) Yes, right at the end of its double-shell burning stage of life.
B) Yes, about a million years after it becomes a white dwarf.
C) No, because it is not orbited by another star.
D) No, because the Sun's core will never be hot enough to fuse carbon and other heavier elements into iron.

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10) Each Voyager spacecraft carries a "postcard" designed to be understandable to any aliens that might someday encounter it. On the "postcard," scientists pinpointed the location of Earth by triangulating it between pulsars. Why did the scientists choose pulsars rather than some other type of star?
A) Pulsars are very bright and therefore easy to find.
B) Several pulsars are located within a dozen light-years of our solar system, making them useful for finding our solar system.
C) We're pretty sure that aliens will have only radio telescopes and not optical telescopes, so they'll have a better chance of seeing pulsars than ordinary stars.
D) Pulsars are easy to identify by their almost perfectly steady periods of pulsation.

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11) Which statement about pulsars is not thought to be true?
A) All pulsars are neutron stars, but not all neutron stars are pulsars.
B) Pulsars can form only in close binary systems.
C) A pulsar must have a very strong magnetic field and rotate quite rapidly.
D) Pulsars are kept from collapsing by neutron degeneracy pressure.

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12) How does an accretion disk around a neutron star differ from an accretion disk around a white dwarf?
A) The accretion disk around a neutron star is made mostly of helium while the accretion disk around a white dwarf is made mostly of hydrogen.
B) The accretion disk around a neutron star is more likely to give birth to planets.
C) The accretion disk around a neutron star is much hotter and emits higher-energy radiation.
D) The accretion disk around a neutron star always contains much more mass.

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13) Which statement concerning black hole masses and Schwarzschild radii is not true?
A) In a binary system with a black hole, the Schwarzschild radius depends on the distance from the black hole to the companion star.
B) The more massive the black hole, the larger the Schwarzschild radius.
C) Even an object as small as you could become a black hole if there were some way to compress you to a size smaller than your Schwarzschild radius.
D) For black holes produced in massive star supernovae, Schwarzschild radii are typically a few to a few tens of kilometers.

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14) Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice?
A) Time on the clock will run faster as it approaches the black hole, and light from the clock will be increasingly blueshifted.
B) The clock will fall toward the black hole at a steady rate, so that you'll see it plunge through the event horizon within just a few minutes.
C) The clock will fall faster and faster, reaching the speed of light as it crosses the event horizon.
D) Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.

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17) Which of the following observatories is most likely to discover a black hole in a binary system?
A) the Hubble Space Telescope
B) the Chandra X-Ray Observatory
C) the SOFIA airborne infrared observatory

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18) Which of the following statements about gamma ray bursts is not true?
A) Gamma ray bursts are among the most luminous events that ever occur in the universe.
B) The events responsible for gamma ray bursts apparently produce only gamma rays, and no other light that we can hope to detect.
C) Gamma ray bursts were originally discovered by satellites designed to look for signs of nuclear bomb tests on Earth.
D) Based on their distribution in the sky, we can rule out a connection between gamma ray bursts and X-ray binaries in the Milky Way Galaxy.

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19) Imagine an advanced civilization living on a planet orbiting at a distance of 10 AU (1.5 billion kilometers) from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole. The black hole is surrounded by an accretion disk. Sometime within the next million years or so, the civilization's planet is likely to be doomed because
A) the red giant will probably supernova within the next million years.
B) jets of material shot out of the accretion disk will shoot down their planet.
C) the red giant star, which provides most of energy the civilization needs to exist, will soon be destroyed in the accretion disk.
D) tidal forces from the black hole will rip the planet apart.

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20) Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.) One foolhardy day, a daring individual in their space force (let's call him Major Tom) decides to become the first of his species to cross the event horizon of the black hole. To add to the drama, he decides to go in wearing only a thin space suit, which offers no shielding against radiation, no cushioning against any forces, and so on. Which of the following is most likely to kill him first (or at least to start the process of killing him first)?
A) tidal forces due to the black hole
B) X rays from the accretion disk
C) the crush of gravity at the singularity embedded within the black hole
D) the sucking force from the black hole, which will cause his head to explode