Chapter 14: The Death of Stars

Section: Introduction

1. The end states of low-mass stars differ dramatically from high-mass stars. In this context, what is the boundary between these two classifications?

A) 0.4 M_Sun

B) 2.0 M_Sun

C) 8.0 M_Sun

D) 16.0 M_Sun

2. Consider a group of stars with masses up to 8 times the mass of the Sun. Stars of which masses will end up as pure helium?

A) less than 0.4 M_Sun

B) 0.4 M_Sun to 2 M_Sun only

C) 2 M_Sun to 8 M_Sun only

D) all stars 0.4 M_Sun to 8 M_Sun

3. Consider a group of stars with masses up to 8 times the mass of the Sun. Stars of which masses undergo a helium flash?

A) less than 0.4 M_Sun

B) 0.4 M_Sun to 2 M_Sun only

C) 2 M_Sun to 8 M_Sun only

D) all stars 0.4 M_Sun to 8 M_Sun

4. Consider a group of stars with masses up to 8 times the mass of the Sun. Stars of which masses will pass along the horizontal branch?

A) less than 0.4 M_Sun

B) 0.4 M_Sun to 2 M_Sun only

C) 2 M_Sun to 8 M_Sun only

D) all stars 0.4 M_Sun to 8 M_Sun

5. What are the main products of helium nuclear fusion in red giant stars?

A) hydrogen nuclei by nuclear fission

B) energy from the complete transformation of the mass of helium to energy

C) iron nuclei

D) carbon and oxygen nuclei

6. Helium nuclear reactions (helium fusion) produce primarily

A) carbon and oxygen.

B) carbon and silicon.

C) iron.

D) oxygen and neon.

7. Nuclear fusion reactions of helium produce primarily

A) nitrogen and neon nuclei.

B) iron nuclei.

C) beryllium and lithium nuclei.

D) carbon and oxygen nuclei.

8. The nuclear process in which helium fusion occurs in the deep interiors of red giant stars produces

A) iron nuclei.

B) carbon and oxygen nuclei.

C) hydrogen nuclei by the splitting of helium nuclei.

D) pure energy from the nuclear mass.

Section: 14-1

9. Consider a group of stars with masses up to 8 times the mass of the Sun. Stars of which masses will pass along the asymptotic giant branch?

A) less than 0.4 M_Sun

B) 0.4 M_Sun to 2 M_Sun only

C) 2 M_Sun to 8 M_Sun only

D) all stars 0.4 M_Sun to 8 M_Sun

10. Low-mass stars can ascend the H–R diagram in two evolutionary phases called giant phases. What is the difference between them?

A) In the first, the primary production of energy is from hydrogen burning in the core. In the second, the primary production of energy is from helium burning in the core.

B) In the first, the primary production of energy is from hydrogen burning in a shell around the core. In the second, the primary production of energy is from helium burning in a shell around the core.

C) In the first, the star’s track on the Hertzsprung–Russell diagram lies along the red-giant branch. In the second, the track lies along the horizontal branch.

D) During the first red-giant phase, the star moves up and to the right along the red-giant branch. During the second red-giant phase the star’s track is down and to the left along the same red-giant branch.

11. The structure of the deep interior of a low-mass star near the end of its life is a(n)

A) carbon-oxygen core, a shell around the core where helium nuclei are undergoing fusion, and a surrounding shell of hydrogen.

B) inactive hydrogen core and a helium shell undergoing nuclear fusion surrounded by a carbon-oxygen shell.

C) turbulent mixture of hydrogen, helium, carbon, and oxygen in which only helium continues to undergo nuclear fusion.

D) helium core surrounded by a thin hydrogen shell undergoing nuclear fusion with very small concentrations of heavier nuclei.

12. A low-mass giant on the horizontal branch fuses hydrogen into helium in its core. The core temperature is insufficient to fuse helium into carbon. How does the star produce energy?

A) shell helium fusion

B) core oxygen fusion

C) shell beryllium fusion

D) core beryllium fusion

13. During which phase of a low-mass star’s life does helium shell fusion occur?

A) main sequence

B) asymptotic giant branch

C) first red-giant phase

D) horizontal branch

14. Helium nuclear reactions take place in a shell around the core of a low-mass star during its

A) horizontal-branch phase.

B) first red-giant phase.

C) main-sequence phase.

D) asymptotic giant branch phase.

15. A star at the tip of the asymptotic giant branch (AGB) is a

A) cool main-sequence star.

B) blue supergiant.

C) star in its first red-giant phase.

D) red supergiant.

16. In a star’s evolutionary life, the asymptotic giant branch (AGB) is the

A) helium core fusion phase.

B) pre–main-sequence core hydrogen fusion phase.

C) hydrogen shell fusion phase prior to helium ignition in the core.

D) helium shell fusion phase.

17. A star ascending the red-giant branch for the second time in the asymptotic giant branch (AGB) phase will have

A) no nuclear reactions in the core, but a helium-fusion shell outside the core, which itself is surrounded by a shell of hydrogen.

B) no fusion reactions; the star has used up all its nuclear fuel.

C) hydrogen-fusion reactions occurring in the core.

D) no nuclear reactions occurring in the core but hydrogen fusion in a shell outside the core.

18. How much brighter than its main-sequence luminosity will a Sunlike star become at the asymptotic giant branch (AGB) phase of its life?

A) 10 times brighter

B) 10⁴ times brighter

C) twice as bright

D) 10³ times brighter

19. During its time on the asymptotic giant branch, the Sun will eventually evolve into a red supergiant star with the brightness of _____ and a diameter of _____.

A) 10,000 Suns; about Earth’s orbit

B) the Sun; Mercury’s orbit

C) about 1 million Suns; the whole solar system

D) about 10,000 Suns; 1/10 that of the Sun

20. In the process of helium shell fusion in a low-mass star near the end of its life, the star moves upward and to the right on the asymptotic giant branch of the Hertzsprung–Russell diagram. In this process, the star is

A) contracting, cooling, and hence becoming less luminous.

B) expanding, heating up, and becoming more luminous.

C) contracting, becoming hotter, and becoming much less luminous.

D) expanding, cooling, and becoming more luminous.

21. In the asymptotic giant branch (AGB) phase of their lives, stars like the Sun lose mass to space through an outflowing “stellar wind.” Over a period of 1000 years, how much mass would such a star eject?

A) almost 1/10 solar mass

B) about 10^–5 solar mass

C) about 1/100 solar mass

D) almost none since most of the mass flows back in at the star’s poles

22. What will be the mass of the Sun at the end of its asymptotic giant branch (AGB) phase, due to mass loss to space by its stellar wind?

A) still almost 1 solar mass since mass loss is negligible for a low-mass star like the Sun

B) between 0.1 and 0.2 solar mass

C) about 0.8 solar mass

D) about 0.5 solar mass

23. What is the last nuclear fusion stage in the life of a low-mass star like the Sun?

A) fusion of silicon nuclei to form iron

B) fusion of oxygen nuclei to form sulfur

C) fusion of helium nuclei to form carbon and oxygen

D) fusion of hydrogen nuclei to form helium

24. What happens to the outer layers of a low-mass star after the helium core and shell fusion stages are completed?

A) The star stabilizes at the size of a red giant star, radiation pressure from below balancing gravity from the core, and slowly cools for the rest of its life.

B) The outer layers are spun off into space to make a spiral structure known as a spiral galaxy.

C) The outer layers are propelled slowly away from the core to form a planetary nebula.

D) The star contracts back onto the core and becomes hot enough to undergo further hydrogen fusion, leading to a very hot and active, white dwarf star.

25. In astronomical terms, planetary nebulae are

A) very long-lived objects, having been in existence since just after the Big Bang at the beginning of the universe.

B) relatively short-lived, existing around the central white dwarf star for millions of years before slowly spreading into space.

C) relatively long-lived since they form when the original stars form and remain as slowly rotating shells for the whole of their lifetimes of several billion years.

D) very short-lived, with lifetimes of about 20,000 years.

26. Which of these important components does a planetary nebula contribute to the interstellar medium?

A) molecules such as NH₃ and CH₄, which contribute to giant molecular clouds

B) UV light photoionizes hydrogen. The hydrogen, on recombination, produces the red Balmer- light by which interstellar emission nebulae are seen.

C) rotational motion from the original star, which serves to concentrate interstellar matter into new stars and planetary systems

D) nuclei of moderately heavy elements, major components of planets such as Earth.

27. A planetary nebula is a(n)

A) contracting spherical cloud of gas surrounding a newly formed star in which planets are forming.

B) expanding gas shell surrounding a hot, burned-out stellar core.

C) disk-shaped nebula of dust and gas around a relatively young star, from which planets will eventually form.

D) nebula caused by the supernova explosion of a massive star.

28. A planetary nebula is

A) a shell of ejected gases.

B) the formation stages of planets around stars.

C) a gas cloud surrounding a planet after its formation.

D) the spherical cloud of gas produced by a supernova explosion.

29. A planetary nebula is a

A) gas shell, the atmosphere of a red giant star, slowly expanding away from the core of the star.

B) contracting spherical cloud of gas surrounding a newly formed star in which planets are forming.

C) nebula caused by the supernova explosion of a massive star.

D) disk-shaped nebula of dust and gas rotating around a relatively young star in which planets will eventually form.

30. A planetary nebula is a

A) cloud of gas surrounding a very young star in which planets are expected to form.

B) spherical, rapidly expanding cloud of gas produced by a supernova explosion.

C) gas cloud surrounding a planet after its formation.

D) shell of gases ejected from the surface of red giant star.

31. The event that follows the asymptotic giant branch (AGB) phase in the life of a low-mass star is

A) the ejection of a planetary nebula.

B) core collapse and a supernova explosion.

C) helium flash and the start of helium fusion in the core.

D) the onset of hydrogen fusion in the core.

32. A planetary nebula is created

A) over several hundred years, during mass transfer in a close binary star system.

B) over a few thousand years or more, in a slow expansion away from a low-mass star, driven by a series of thermal pulses from helium fusion.

C) in hours or less, during the explosion of a massive star.

D) in seconds, during the helium flash in a low-mass star.

33. What physical process provides the energy for the ejection of a planetary nebula from a low-mass star?

A) transfer of hydrogen-rich material onto the surface of a white dwarf from its companion in a binary star system

B) helium shell flashes in the helium fusion shell

C) core collapse and the ensuing shock wave

D) collision with another star

34. What is the difference between a “helium flash” and a “helium shell flash”?

A) A helium flash occurs when the core becomes supported by electron-degeneracy pressure; a helium shell flash occurs when the helium shell becomes supported by electron-degeneracy pressure.

B) All stars with mass less than 8 times the mass of the Sun undergo a helium flash, but only those between 2 and 8 times the mass of the Sun undergo a helium shell flash.

C) A helium flash occurs just once; a helium shell flash can repeat many times.

D) A helium flash results in a supernova; a helium shell flash results on a planetary nebula.

35. The diameter of a typical planetary nebula, after 10,000 years of expansion, is

A) about 1000 ly.

B) about 1 au.

C) only about 3 to 5 stellar diameters.

D) a few light-years.

36. The fraction of the mass of a low-mass star that is ejected in its evolutionary phases, up to and including the planetary nebula phase, is

A) almost the entire star, more than 95%.

B) significant, up to 80%.

C) extremely small, less than 1 part in 10⁴, since it is only the star’s atmosphere that has been ejected.

D) very small, close to 10%.

37. The shell of a planetary nebula is measured by the Doppler shift of emission lines to be expanding outward at a speed of 10⁴ m/s, while its radius is measured to be 1 ly, or about 10¹⁶ m. Roughly how long has the shell been expanding? (Hint: 1 year = 3.15  10⁷ sec.)

A) 30 years

B) 30,000 years

C) 30 million years

D) 10¹² years

38. How much mass are planetary nebulae estimated to return to the interstellar medium each year over the Galaxy as a whole?

A) about 50,000 solar masses

B) about 5  10⁸ solar masses

C) about 5 solar masses

D) about 500 solar masses

39. What is the name given to the type of planetary nebula in which a doughnut-shaped cloud of gas and dust in the plane of the equator channels the outflow in opposite directions toward the poles?

A) dumbbell nebula

B) axial planetary nebula

C) doughnut planetary nebula

D) bipolar planetary nebula

Section: 14-2

40. The two longest stages in the lifetime of a solar mass star, each lasting billions of years, are

A) protostar and main sequence.

B) main sequence and red giant.

C) red giant and white dwarf.

D) main sequence and white dwarf.

41. How is it that white dwarfs, which are relatively small, can have surface temperatures of several tens of thousands of kelvins?

A) They are actually very similar in size and temperature to small main-sequence stars like red dwarfs.

B) The nuclear fusion in the core of a white dwarf involves carbon and oxygen, and this produces more energy and heat than either hydrogen or helium fusion.

C) The surface of the white dwarf is actually the core since the cooler outer layers have been blown off.

D) Because white dwarfs are fully convective, the nuclear fusion takes place on the surface.

42. Young white dwarfs radiate MOST strongly in the ultraviolet, with a peak wavelength of perhaps 300 nm. What would be the surface temperature of a white dwarf?

A) 12,600 K

B) 9700 K

C) 7800 K

D) 3500 K

43. In what manner does an isolated white dwarf generate energy?

A) hydrogen fusion

B) helium fusion

C) gravitational contraction

D) An isolated white dwarf does not generate energy.

44. What are “thermal pulses”?

A) These are the loops formed on the H–R diagram as the evolutionary tracks of low-mass stars move beyond the planetary nebula stage.

B) These are the pulses of radiation emitted during helium shell flashes.

C) This is another name for the high luminosity pulses of radiation emitted periodically by variable stars.

D) This is the burst of radiation given off during a nova.

45. Stars that have ejected a planetary nebula go on to become

A) red giants.

B) supernovae.

C) protostars.

D) white dwarfs.

46. The final remnant of the evolution of a red giant star that has ejected a planetary nebula is a

A) protostar.

B) blue supergiant.

C) white dwarf star.

D) supernova.

47. The “star” that is seen at the center of a planetary nebula is

A) a small, hot, and very dense white dwarf star.

B) composed almost entirely of neutrons and spinning rapidly.

C) the accretion disk around a black hole.

D) a planet in the process of formation.

48. A white dwarf star, the surviving core of a low-mass star toward the end of its life, can be found on the Hertzsprung–Russell diagram

A) at the upper-left end of the main sequence since its surface temperature is extremely high.

B) at the bottom end of the main sequence, along which it has evolved throughout its life.

C) below and to the left of the main sequence.

D) above and to the right of the main sequence since it evolved there after its hydrogen-fusion phase.

49. The interiors of white dwarf stars are characteristically

A) mainly carbon and oxygen nuclei supported by electron degeneracy pressure in a volume about the size of the Sun.

B) mostly hydrogen nuclei supported by normal gas pressure due to the very high gas temperature, in a volume about the size of Earth.

C) mainly carbon and oxygen nuclei supported by electron degeneracy pressure in a volume about the size of Earth.

D) mainly helium nuclei supported by electron degeneracy pressure in a volume with a radius about 11 times that of Earth, about the volume of Jupiter.

50. A white dwarf is a(n)

A) object like Jupiter that was not quite massive enough to become a star.

B) small low-mass star no longer undergoing nuclear fusion.

C) type of small protostar.

D) hot, main-sequence star.

51. In which order does a single star of about 1 solar mass progress through the various stages of evolution?

A) planetary nebula, main sequence, neutron star, black hole

B) T Tauri, red giant, white dwarf, neutron star

C) planetary nebula, main sequence, red giant, white dwarf

D) T Tauri, main sequence, planetary nebula, white dwarf

52. At which phase of its evolutionary life is a white dwarf star?

A) post-supernova phase, the central remnant of the explosion

B) just at main-sequence, or hydrogen-fusion, phase

C) very late for small-mass stars, in the dying phase

D) in its early phases, soon after formation

53. A white dwarf star is at what stage of its evolution?

A) protostar phase, just after formation, beginning to generate energy by nuclear fusion

B) main-sequence phase, “middle-aged,” generating energy by fusion of hydrogen to helium

C) post-supernova stage, after the explosion of a star

D) very late phase of evolution, no longer generating energy

54. The Sun will end its life by becoming a

A) molecular cloud.

B) black hole.

C) white dwarf.

D) pulsar.

55. A white dwarf star is about the same size as

A) the Sun.

B) Earth.

C) the total solar system.

D) a major city.

56. The energy-generation process inside a white dwarf star is

A) the combining of protons and electrons to form neutrons within its core.

B) hydrogen fusion.

C) nonexistent—a white dwarf star is simply cooling by radiating its original heat.

D) the helium flash—very efficient and rapid helium fusion.

57. How does a white dwarf generate its energy?

A) It no longer generates energy but is slowly cooling as it radiates away its heat.

B) Nuclear fusion of hydrogen into helium is producing energy in its core.

C) Nuclear fission of heavy elements in the central core is releasing energy.

D) Gravitational potential energy is released as the star slowly contracts.

58. Which type of dwarf is largest?

A) white dwarf

B) red dwarf

C) brown dwarf

D) All are about the same size.

59. The one characteristic shared by all solitary white dwarf stars is that they

A) have stopped generating thermonuclear energy but continue to shrink, thereby releasing gravitational energy as heat.

B) have never generated either thermonuclear or gravitational energy but are slowly cooling after their production in a supernova explosion.

C) are generating thermonuclear energy but are maintaining a constant radius and hence are not releasing gravitational energy.

D) have ceased to generate energy by thermonuclear processes or gravitational contraction and are slowly cooling down.

60. What is it that keeps a white dwarf star from collapsing inward on itself?

A) electron degeneracy, or “quantum crowding”

B) physical size of the neutrons of which this star is composed

C) convection currents or updrafts from the nuclear furnace

D) normal gas pressure

61. White dwarf stars are supported from gravitational collapse by

A) centrifugal force due to rapid rotation.

B) degenerate-electron pressure.

C) nuclear fusion reactions in their cores.

D) nuclear fusion reactions in a shell around the core.

62. A white dwarf star is supported from collapse under gravity by

A) pressure of the gas heated by nuclear fusion reactions in its core.

B) centrifugal force due to rapid rotation.

C) degenerate-electron pressure in the compact interior.

D) pressure of the gas heated by nuclear fusion reactions in a shell around its core.

63. One distinctive physical characteristic of matter inside a white dwarf star is that it is

A) composed only of protons, with electrostatic repulsion preventing stellar collapse.

B) of extremely high density compared with ordinary stellar matter.

C) composed only of neutrons.

D) composed only of electrons in a degenerate state.

64. As a white dwarf evolves, the direction of its motion on the Hertzsprung–Russell diagram is from upper left to lower right, which means that

A) it cools and becomes more luminous.

B) it cools and becomes less luminous.

C) it heats up and becomes more luminous.

D) it heats up and becomes less luminous.

65. A white dwarf star, as it evolves, undergoes which of these changes?

A) Its temperature remains constant, but its radius and therefore its luminosity decrease.

B) Luminosity and size decrease while its temperature remains constant.

C) It shrinks in size, the resulting release of gravitational energy keeping both luminosity and temperature constant.

D) Luminosity and temperature decrease.

66. Are crystalline stars or crystalline remnants of stars possible? Why or why not?

A) No. Stars and stellar remnants are too hot to crystallize.

B) Yes. They are the likely outcome of the cooling of a white dwarf.

C) No. The forces needed to crystallize a star would actually cause the star to collapse gravitationally into a black hole.

D) Yes. They are the likely outcome of the creation of iron in the core of a massive star.

67. The stars that eventually become white dwarfs start life with solar masses less than

A) 25.

B) 8.

C) 1.4.

D) 3.

68. Which of these is a method astronomers have recently used to study the structure of white dwarfs?

A) measuring how their light curves vary with time to study pulsations of the stars

B) measuring gravitational waves generated by pulsations of the stars

C) measuring neutrinos released when a cool white dwarf crystallizes

D) capturing material ejected by novae with spacecraft

Section: 14-3

69. Which of these types of stars or stellar remnants can have a mass no larger than about 1.4 times the mass of the Sun, or else they will collapse under their own gravity?

A) red giants

B) black holes

C) neutron stars

D) white dwarfs

70. The nova phenomenon, an occasional and sometimes repeated intense brightening of a star by a factor of about 10⁶, is caused by

A) a beam of radiation from a nearby pulsar illuminating the surface of a red giant star and inducing rapid and intense heating.

B) the capture and rapid compression of matter by a black hole.

C) the explosion of a single massive star at the end of its thermonuclear fusion phases.

D) explosive hydrogen fusion on the surface of a white dwarf star after mass transfer from a companion star in a binary system.

71. Which of these statements about white dwarfs is FALSE?

A) They generate energy by core fusion of carbon and hydrogen to produce oxygen.

B) Novae are generated by white dwarfs in close binary systems.

C) A white dwarf can be the source of more than one nova.

D) The Chandrasekhar limit restricts the maximum mass of a white dwarf.

72. Which of these statements about novae is FALSE?

A) Novae occur when two white dwarfs collide.

B) Novae eject gas in thousands of clumps, rather than in a smooth envelope.

C) A white dwarf can be the origin of more than one nova.

D) The light curve for a typical nova shows a rapid rise followed by a gradual decline lasting a few months.

73. When a typical nova explodes, it brightens in a few hours by a factor of

A) 10⁸ to 10¹⁰.

B) 2 to 5.

C) 10⁴ to 10⁶.

D) 10 to 100.

74. Recent observations reveal that the outer parts of novae are composed of

A) smooth shells of expanding gas.

B) shells that originate from bipolar jets.

C) thousands of clumps of gas.

D) warped rings of gas and dust.

75. The mechanism that gives rise to the phenomenon of the nova is

A) the impact and subsequent explosion of a large comet nucleus on a star’s surface.

B) material falling into a black hole and being condensed to the point where a thermonuclear explosion is produced.

C) the complete disintegration of a massive star due to a runaway thermonuclear explosion in the star’s interior.

D) matter from a companion star falling onto a white dwarf in a close binary system, eventually causing a nuclear explosion on the dwarf’s surface.

76. A nova is a sudden brightening of a star that occurs when

A) material is transferred onto the surface of a white dwarf from a companion star in a binary system, then subsequently blasted into space by a runaway thermonuclear explosion (leaving the white dwarf intact to repeat the process).

B) material from a companion star is transferred onto the surface of a white dwarf star in a binary system, after which runaway carbon-fusion reactions cause the entire white dwarf to be destroyed in an explosion.

C) the electron degenerate iron core of a massive star collapses after its mass becomes greater than the Chandrasekhar mass limit.

D) material is transferred onto a neutron star from a companion star in a binary system, causing the neutron star to collapse into a black hole.

77. A nova is an explosion involving a white dwarf. Can a white dwarf become a nova more than once? Why or why not?

A) No. The white dwarf’s magnetic field is eliminated in the explosion.

B) Yes. A white dwarf can become a nova more than once if its temperature is high enough for recurrent helium flashes in the core.

C) Yes. A white dwarf can become a nova more than once if it continues to receive matter from a companion star.

D) No. The white dwarf is destroyed in the explosion.

78. What is the Chandrasekhar limit?

A) time limit of the existence of a planetary nebula, beyond which the nebula dissipates and becomes too rarified to see

B) time limit for the transfer of mass to a white dwarf in a close binary system, beyond which the white dwarf erupts in a nova

C) mass limit to the total mass of a white dwarf, beyond which it will erupt in a nova

D) mass limit to the total mass of a white dwarf, beyond which the electron degeneracy pressure will be overcome and the core will collapse

79. There is a mass limit for a star in the white-dwarf phase, the Chandrasekhar limit, beyond which the electron degeneracy pressure can no longer support the star against its own gravity. This limit is

A) 0.2 solar mass.

B) 30 solar masses.

C) 1.4 solar masses.

D) 14 solar masses.

80. Suppose astronomers discover a nova in a distant galaxy. What do they immediately know about the star that gave rise to this nova?

A) The star is a white dwarf in a binary system in which the other star fills its Roche lobe.

B) The star is a neutron star in a binary system in which the other star is a white dwarf.

C) The star is a neutron star in a binary system in which the other star fills its Roche lobe.

D) The star is a black hole in a binary system in which the other star fills its Roche lobe.

Section: 14-4

81. The spectrum of a Type Ia supernova can be identified because it lacks evidence of

A) hydrogen.

B) carbon.

C) nickel.

D) cobalt.

82. A Type Ia supernova is the

A) collapse of a blue supergiant star to form a black hole.

B) explosion of a red giant star as a result of a helium flash in its core.

C) explosion of a white dwarf in a binary star system after mass has been transferred onto it from its companion.

D) explosion of a massive star after silicon fusion has produced a core of iron nuclei.

83. Can a white dwarf explode?

A) Yes, but only if another star collides with it; stars are so far apart in space that this is unlikely ever to have happened in the Galaxy.

B) Yes, but only if it is in a binary star system.

C) Yes, but only if nuclear reactions in the white dwarf core reach the stage of silicon fusion, producing iron.

D) No. White dwarfs are held up by electron degeneracy pressure, and this configuration is stable against collapse or explosion.

84. The center of the remnant of a Type Ia supernova MOST likely contains

A) a black hole or neutron star.

B) a white dwarf.

C) the binary companion of the supernova’s progenitor.

D) a planetary nebula.

85. Which of these is MOST likely to occur inside a binary system containing a white dwarf with a mass considerably below 1.4 M_Sun?

A) a nova

B) a Type Ia supernova

C) a Type II supernova

D) a supernova remnant

86. Which of these is MOST likely to occur in a binary system containing a white dwarf with a mass of about 1.4 M_Sun?

A) a nova

B) a Type Ia supernova

C) a Type II supernova

D) a planetary nebula

Section: 14-5

87. Which force induces the core to contract inward and get hotter in massive stars at the conclusion of each episode of nuclear fusion, such as the carbon-, oxygen-, and silicon- fusion cycles?

A) gravity

B) gas pressure produced by the very high gas temperatures

C) electron degeneracy pressure

D) nuclear attractive force between nuclei and between neutrons and protons

88. Which nuclear fusion cycle is the next one to begin after helium fusion ends in a massive star?

A) carbon fusion

B) iron fusion

C) silicon fusion

D) oxygen fusion

89. In which order do the stages of core nuclear fusion occur in the evolution of a massive star?

A) carbon, helium, oxygen, neon

B) helium, carbon, neon, oxygen

C) helium, oxygen, carbon, neon

D) helium, carbon, oxygen, neon

90. A star of 25 solar masses spends roughly what percentage of its life as a main-sequence star?

A) about 7%

B) very little, less than 1%

C) 93%

D) 58%

91. For a massive star (e.g., 25 solar masses), core hydrogen fusion lasts for several million years. In contrast, the final stage of fusion, core silicon fusion lasts for only about

A) 1 minute.

B) several thousand years.

C) 1 day.

D) 1 year.

92. Each successive stage of core nuclear reactions in a massive star lasts for considerably less time than the previous stage (e.g., for a 25-solar-mass star, carbon fusion lasts for 600 years, while neon fusion lasts for only 1 year). One reason is that the

A) later the stage of fusion, the less massive is the star.

B) later the stage of fusion, the more massive is the star.

C) more massive the nuclei, the fewer there are to react together.

D) more massive the nuclei, the less stable the end products.

93. The main product of silicon fusion reactions in the core of a massive star is

A) iron.

B) magnesium.

C) carbon.

D) helium.

94. Near the end of its life a high-mass star has six active fusion shells surrounding its core. How many such shells did it have when it was on the main sequence?

A) none

B) one

C) three

D) six

95. A high-mass star goes through many fusion cycles in its lifetime: hydrogen, helium, carbon, neon, oxygen, and silicon. How does the timescale for each cycle compare to the previous one?

A) Each new nucleus is heavier than the previous one, and thus it moves more slowly. Each successive stage therefore takes longer than the previous one.

B) The temperature is higher at each stage, and the number of nuclei to react is fewer. Thus, each stage takes a shorter time than the previous one.

C) 2 M_ to 8 M_ only

D) all stars 0.4 M_ to 8 M_

96. The duration of the oxygen-fusing stage in the core of a massive star is considerably shorter than the duration of the carbon-fusing stage. One main reason for this is that

A) carbon is a solid, while oxygen is a gas.

B) oxygen fusion does not begin until the contracting core has reached a higher temperature than the temperature at which carbon fusion is possible.

C) the electron degeneracy inhibits the carbon reaction, but it has been removed by the time oxygen fusion begins.

D) there are more fusion shells surrounding the oxygen fusion core than there are around the carbon fusion core. The shells prevent the escape of oxygen before it can fuse.

97. Which of these properties does NOT characterize a massive supergiant star?

A) high mass-loss rates caused by enormous stellar winds

B) longer and longer time periods required to complete each stage in nucleosynthesis because of the large amount of fuel to be consumed

C) circumstellar shells of ejected gas

D) fusion in concentric shells surrounding the core

Section: 14-6

98. A high-mass star near the end of its life undergoes successive cycles of energy generation in its core in which gravitational collapse increases the temperature to the point where a new nuclear fusion cycle generates sufficient energy to stop the collapse. This process does not work beyond the silicon-fusion cycle that produces iron. Explain.

A) The pressure from high-energy photons and neutrinos at the very high core temperatures reached at this stage of development is finally sufficient to halt the collapse.

B) Electrostatic forces between the highly charged iron nuclei are sufficient to overcome the collapse and stabilize the stellar core.

C) Fusion of iron nuclei into heavier nuclei requires energy rather than producing excess energy and therefore will not produce the additional gas pressure to halt the collapse.

D) Iron nuclei are so large that they occupy all remaining space, so the collapse cannot continue.

99. The sequence of thermonuclear fusion processes inside massive stars can transform elements such as carbon and oxygen into heavier elements and generate excess energy until iron has been produced. Why is it NOT possible for fusion reactions to release energy from iron nuclei?

A) The electrostatic charge of iron nuclei is so great that other nuclei cannot approach closely enough to react with them.

B) Iron has the largest nucleus of all elements, and fusing other nuclei with iron actually reduces the size of the nucleus.

C) Iron is the heaviest naturally occurring element.

D) The protons and neutrons in an iron nucleus are so tightly bound together that fusing other nuclei with iron absorbs energy rather than releases it.

100. A sequence of thermonuclear fusion processes inside massive stars can continue to transform the nuclei of elements such as carbon and oxygen into heavier nuclei and also generate excess energy up to a limit beyond which no further energy-producing reactions can occur. The element that is produced when this limit is reached is

A) oxygen.

B) iron.

C) silicon.

D) uranium.

101. In the collapsing core of a high-mass star just before a supernova explosion occurs, the density is about that

A) of degenerate gases in white dwarf stars, about 10⁹ kg/m³.

B) of nuclear matter in a normal nucleus, about 4 10¹⁷ kg/m³.

C) at the center of the Sun, about 1.5  10⁵ kg/m³.

D) of iron, 7.5  10³ kg/m³.

102. During the collapse of a massive star the core will rebound outward in a process called “core bounce”. What causes this?

A) A flood of neutrinos is released and pushes outward on the core.

B) A flood of high-energy gamma rays is released and pushes outward on the core.

C) The collapsing core reaches the density of nuclear matter and stiffens.

D) Radioactivity develops because of the tremendous amount of energy available, and the radiation reverses the movement of the core.

103. Which of these actions will a high-mass star (say, 25 times the mass of the Sun) NOT do at or near the end of its life?

A) eject its outer layers and become a neutron star

B) convert silicon into iron in its core

C) emit copious amounts of neutrinos

D) eject its outer layers and become a white dwarf

104. After the material in the core of a massive star has been converted to iron by thermonuclear reactions, further energy can be released to heat the core only by

A) gravitational contraction.

B) nuclear fission, or splitting of nuclei.

C) the absorption of neutrinos.

D) thermonuclear fusion of iron into heavier elements.

105. From the start of collapse to the attainment of nuclear density, the process of core collapse at the end of the life of a massive star takes about

A) 1/2000 second.

B) 8 minutes.

C) a few hours.

D) 1 second.

106. Which of these statements does NOT describe a consequence of core collapse at the end of the life of a massive star?

A) The silicon core is converted to iron by fusion reactions.

B) Electrons combine with protons to form neutrons.

C) Great numbers of neutrinos are produced.

D) The core density approaches the density of an atomic nucleus.

107. The very last nuclear process to occur at the center of a massive star (at the end of its life) is

A) the helium flash.

B) silicon fusion, resulting in the production of iron.

C) the photodisintegration of nuclei by gamma rays.

D) the capture of electrons by protons to produce neutrons.

108. What is photodisintegration?

A) heating and ejection of mass from the surface of a normal star by the radiation from an orbiting neutron star

B) destruction of a star by the pressure of the radiation inside it

C) splitting apart of atomic nuclei by gamma rays

D) ejection of a neutron or proton from an atomic nucleus, accompanied by the emission of a gamma ray

109. During its life, a massive star creates heavier and heavier elements in its core through thermonuclear fusion, leading up to silicon and iron. What is the fate of the iron that is created?

A) The nuclei are split apart by neutron bombardment, creating lighter elements such as carbon, oxygen, and neon.

B) The iron is locked up inside the star forever.

C) The iron is destroyed by later thermonuclear fusion reactions in the core that create even heavier elements such as lead, gold, and uranium.

D) The iron is torn apart by high-energy photons at the end of the star’s life.

110. A Type II supernova is the

A) collapse of a blue supergiant star to form a black hole.

B) explosion of a white dwarf in a binary star system after mass has been transferred onto it from its companion.

C) explosion of a red giant star as a result of a helium flash in the core.

D) explosion of a massive star after silicon fusion has produced a core of iron nuclei.

111. What is the main observational difference between a Type Ia and a Type II supernova?

A) Hydrogen lines are prominent in the spectrum of a Type Ia supernova but absent in that of a Type II.

B) The spectrum of a Type II supernova shows strong lines of both hydrogen and helium, whereas that of a Type Ia shows only hydrogen.

C) The spectrum of a Type Ia supernova shows strong lines of both hydrogen and helium, whereas that of a Type II shows only hydrogen.

D) Hydrogen lines are prominent in the spectrum of a Type II supernova but absent in that of a Type Ia.

112. Type II supernovae show prominent lines of hydrogen in their spectra, whereas hydrogen lines are absent in spectra of Type Ia supernovae. Explain. (Hint: Think about the type of star that gives rise to each of the two types of supernova.)

A) Massive stars contain large amounts of hydrogen, whereas white dwarfs are mostly carbon and oxygen.

B) White dwarfs have a thick surface layer of hydrogen, whereas neutron stars contain no hydrogen at all.

C) Massive stars have fused all their hydrogen into heavier elements, whereas low-mass stars still have large hydrogen-rich envelopes.

D) Massive stars contain large amounts of hydrogen, whereas neutron stars contain no hydrogen at all.

113. The estimated rate at which supernova explosions occur in a spiral galaxy, such as the Milky Way, is about once every _____ years.

A) 5

B) 300

C) 3000

D) 20

114. Based on observations of supernova explosions in distant galaxies, it is predicted that 5 supernovae per century should occur in the Milky Way Galaxy, but roughly 1 supernova every 300 years has been observed from Earth. Which statement BEST explains this?

A) The majority of supernovae produce no visible light, only radio and X-ray radiation, which have been observable from Earth for only the past three decades.

B) Most supernovae occur in the galactic plane, where interstellar dust has hidden them from the view from Earth.

C) Most supernovae occur in the Milky Way and can be seen only from the southern hemisphere, where there have been very few observers until recently.

D) The majority of stars in the Galaxy are old, well beyond the supernova stage of evolution.

115. Measurements from distant galaxies indicate that supernovae should occur at a rate of 5 per century in a spiral galaxy such as the Milky Way, but only 3 have been recorded in this Galaxy in the past 1000 years. Explain.

A) Most supernovae produce X-rays and radio waves, not visible light, and were hence invisible to earlier observers.

B) The majority of supernovae must have occurred in the plane of the Milky Way and hence were hidden from Earth by the dense gas and dust in the Milky Way plane.

C) The Milky Way Galaxy is somehow different, with much lower numbers of very massive stars in general, so many fewer stars have undergone supernova explosions.

D) Observers were not watching the sky carefully enough, particularly through the Dark Ages and over the past few centuries.

116. Which of these processes is NOT involved in the supernova explosion of a massive star?

A) photodisintegration of nuclei by gamma rays

B) helium flash in the star’s core

C) collapse of the star’s core

D) passage of a shock wave through the star’s envelope

117. Which of these phenomena is NEVER a consequence of a supernova explosion?

A) triggering of star formation by shock waves moving through interstellar space

B) formation of a planetary nebula

C) condensation of matter into a solid nuclear star composed entirely of neutrons

D) generation of a pulse of neutrino emission

118. What fraction of its mass does a 25-solar-mass main-sequence star eject into space during its lifetime?

A) almost all of it, greater than 80%

B) only a small fraction, about 1/100

C) between 1/4 and ½

D) only its outer atmosphere, less than 1 part in 10⁴

119. Relative to its luminosity as a supergiant, the luminosity of a typical supernova star during the initial phases of the explosion could increase by a factor of

A) 10⁸.

B) 10⁶.

C) 2 to 3, since the star is already very bright.

D) 10².

120. What is the source of MOST of the heavy elements on Earth and in human bodies?

A) thermonuclear fusion reactions in the cores of massive stars before the supernova phase

B) explosive nucleosynthesis during supernova explosions of massive stars

C) cosmic ray interactions with hydrogen and helium nuclei in interstellar clouds

D) nuclear reactions during the formation of the universe (the Big Bang)

121. The core collapse phase at the end of the life of a massive star is triggered when

A) the helium flash and thermal pulses have expelled the star’s envelope.

B) the density reaches the threshold for electron degeneracy pressure to become important.

C) nuclear fusion has produced a significant amount of iron in its core.

D) the core becomes as dense as an atomic nucleus.

122. What finally halts the core collapse in the initial stages of the formation of a Type II supernova?

A) The core becomes a singularity, with all matter concentrated at a single point.

B) The iron core stiffens when it reaches the density of nuclear matter, producing the core bounce.

C) The iron core becomes hot enough to begin fusion reactions, which produce a uranium core.

D) The temperature becomes great enough to produce radioactive nuclei.

123. The formation of an iron core is an important stage in the development of a supernova because

A) iron nuclei cannot participate in nuclear reactions.

B) when iron nuclei undergo nuclear reactions, they always absorb energy.

C) when iron nuclei undergo nuclear reactions, they always give out energy.

D) iron nuclei make the core magnetic.

124. An old high-mass star can have a number of shells (hydrogen, helium, carbon, neon, oxygen, silicon) plus an iron core. Fusion generally takes place in all these regions EXCEPT the

A) hydrogen shell.

B) helium shell.

C) silicon shell.

D) iron core.

125. Photodisintegration, the fissioning of iron nuclei into helium nuclei by high-energy gamma radiation, occurs only at the very end of the life of a massive star. Why can photodisintegration not occur earlier? Choose the statement that is NOT correct.

A) Iron nuclei do not exist in anything but trace amounts before this stage.

B) Gamma radiation of sufficient energy is not produced before this stage.

C) The fissioning of an iron nucleus requires energy, and the energy-rich environment that allows fissioning to happen does not exist before this stage.

D) Under the conditions of the previous stages of evolution, iron (²⁶Fe) disappears as quickly as it is formed by combining with helium (⁴He) to produce zinc (³⁰Zn).

Section: 14-7

126. Where should an astronomer look in order to find a core-collapse supernova?

A) in a globular cluster

B) near a star-forming region

C) in a binary star system

D) near a black hole

127. What is remarkable about the supernova remnant Cassiopeia A?

A) Cassiopeia A contains a binary neutron star system in which one neutron star is significantly more massive than the other.

B) Cassiopeia A is bright at all wavelengths from radio and visible light to X-rays.

C) Cassiopeia A is the nearest supernova remnant, located only about 300 ly from Earth.

D) Cassiopeia A shows that a supernova occurred in the Galaxy about 300 years ago, but there is no record of any supernova having been seen at that time.

128. Measurements suggest that light first arrived at Earth from the Cassiopeia A supernova about 300 years ago and that this supernova is about 10,000 ly distant from Earth. When did the explosion actually occur?

A) It is not possible to determine when the explosion occurred from the information given.

B) 9700 years ago, or about 7700 B.C.

C) 10,300 years ago, or about 8300 B.C.

D) 300 years ago, or about A.D. 1700.

129. MOST supernova remnants can be observed only at nonvisible wavelengths. Explain.

A) Supernova remnants are too cool to produce radiation at visible wavelengths.

B) Supernova remnants are too hot to produce radiation at visible wavelengths.

C) Visible wavelengths are produced by supernova remnants but they are blocked by the interstellar medium.

D) All the visible wavelengths are Doppler shifted to infrared because all the supernova remnants are moving away from Earth at high speeds.

130. The last supernova in the Milky Way Galaxy that was visible to the naked eye was observed when?

A) Astronomers have never seen a naked-eye supernova.

B) They are quite common, so one was observed within the last year.

C) There are reports of a supernova eruption in 1066.

D) Kepler observed one in 1604.

131. MOST supernova remnants have been discovered by observing in which wavelength region?

A) gamma ray

B) ultraviolet

C) visible

D) radio

132. Around 2 million years ago, astronomers believe, Earth’s ozone layer may have been significantly damaged, leading to radiation damage to life on Earth. What is believed to be the cause of the damage?

A) An OB association may have been close enough that a supernova flooded Earth with intense radiation.

B) The Sun went through its T Tauri expansion.

C) The Sun sent out a ring of material as it formed a planetary nebula.

D) The Gum supernova erupted.

133. The Scorpius-Centaurus OB association is predicted to have produced a supernova about 2 million years ago. What led to this prediction?

A) A supernova remnant with the appropriate location and age was found.

B) O and B stars are massive stars, many of them over 25 solar masses, and they would create a supernova. 

C) In a close association, collisions between stars are expected, and collisions would raise the mass to the supernova range.

D) Shock waves from the direction of Scorpius-Centaurus still pass by Earth at frequent intervals.

134. Where are supernova explosions MOST likely to occur?

A) only in other galaxies with active nuclei

B) in the globular clusters surrounding the Milky Way Galaxy

C) in the plane of the Milky Way Galaxy

D) only in irregular galaxies, like the Large Magellanic Cloud, where star formation remains an ongoing process

135. It was recently discovered that the Scorpius-Centaurus OB association passed within 150 light-years of the solar system perhaps 2 million years ago. What is the significance?

A) A supernova eruption in the association at that time may have sent shock waves that led to the collapse of a dark dust cloud to form the solar nebular disk.

B) A supernova eruption in the association at that time may have sent large amounts of radiation that overwhelmed Earth’s protective ozone layer and caused widespread biological extinctions.

C) The gravitational influence of the association probably caused the migration of the giant planets in the solar system to their present locations.

D) A supernova eruption in the association at that time may have been the source of the radioactive aluminum isotope that was once part of some meteorites, like the Allende meteorite.

136. The MOST recent supernova explosion known to have occurred in the Milky Way Galaxy

A) was seen in 1987 (supernova 1987A).

B) created the Gum Nebula.

C) gave rise to the Crab Nebula.

D) created the supernova remnant Cassiopeia A.

137. Supernovae have been detected

A) in both the Milky Way Galaxy and other galaxies.

B) only at X-ray wavelengths.

C) only in elliptical galaxies, never in spirals.

D) only in the Milky Way Galaxy.

Section: 14-8

138. The light from the most recent supernova that was visible to the unaided eye arrived at Earth in

A) A.D. 1604.

B) A.D. 1054.

C) A.D. 1987.

D) A.D. 1572.

139. The study of supernova 1987A and the study of the Sun have at least one aspect in common, namely

A) each of these objects was not as bright as it was predicted to be theoretically.

B) magnetic effects provided much of the information.

C) the information provided by nonvisible radiation contradicted that from visible observations.

D) the most direct information about the core was carried by neutrinos.

140. Just before it exploded, the star that became supernova 1987A was a(n)

A) pulsar.

B) white dwarf.

C) M2 supergiant.

D) B3 supergiant.

141. In units of the Sun’s mass, the original mass of the star that exploded to form supernova 1987A was probably about

A) 1.4.

B) 40.

C) less than 1.

D) 20.

142. Which observation has provided the MOST direct evidence that Type II supernovae are caused by the collapse of the core of a massive star?

A) detection of high-energy X-rays from the Crab supernova remnant

B) detection of iron in the Cassiopeia A supernova remnant

C) detection of neutrinos from supernova 1987A

D) discovery of cosmic rays

143. Which of these sentences about SN 1987A is NOT true?

A) Bursts of neutrinos were detected at multiple sites from the supernova.

B) The supernova did not become as intrinsically bright as originally expected.

C) The supernova was a white dwarf exploding after mass transfer from a companion star in a binary star system.

D) Observations of the star had been made before it blew up.

144. The rings of material surrounding the SN 1987A remnant glow. In what region of the spectrum do they emit most of their radiation?

A) The rings are mostly hydrogen shed from the star’s envelope, so the H_ line might be expected to be prominent in the resulting spectrum.

B) The rings glow because of natural radioactivity, which emits mostly gamma radiation.

C) The heavy elements created in the supernova have complex atomic structures that emit a continuous spectrum.

D) Only the longest radio waves can penetrate the dust clouds surrounding the supernova remnant.

145. What is the origin of the rings of gas that surround SN 1987A?

A) The rings were formed by stellar winds before the supernova erupted.

B) These were caused by the gamma rays, which also caused photodisintegration.

C) These rings were pushed out by the core bounce.

D) The rings were formed when the shock wave lifted the outer layers of the star.

146. Observing neutrinos from SN 1987A was a substantial part of the study of the supernova. Which of these statements about these neutrinos is NOT true?

A) The neutrinos arrived at Earth before the visible light.

B) Neutrino detectors using Cherenkov radiation need not be pointed at their targets, so the neutrinos could be detected even though they were unexpected.

C) Only neutrino detectors in the southern hemisphere were able to record the event since the Large Magellanic Cloud is only visible in the southern hemisphere.

D) The discovery of neutrinos from SN 1987A provided strong support for the theory that supernovae are caused by the collapse of a star’s core.

147. How did SN 1987A differ from most other observed supernovae?

A) SN 1987A occurred in an external galaxy, not the Milky Way Galaxy.

B) SN 1987A reached a maximum luminosity several times that of a normal supernova, because it exploded as a red supergiant.

C) SN 1987A declined in brightness much faster than most supernovae.

D) SN 1987A reached a maximum luminosity much later than a normal supernova, because it exploded as a blue supergiant.

148. SN 1987A in the Large Magellanic Cloud differed from most other Type II supernovae in being considerably fainter. Explain.

A) The star that exploded was much smaller than a normal Type II supernova.

B) The star was surrounded by interstellar gas and dust that partially hid it from view from Earth.

C) The star contained considerably more heavy elements than most Type II supernovae, and these elements are believed to have absorbed many of the neutrinos produced in the core collapse.

D) The star that exploded was considerably cooler than a normal Type II supernova.

149. The detection of neutrinos from SN 1987A occurred 3 hours before the detection of the burst of visible light. What caused this time lag?

A) Neutrinos travel faster in vacuum than light does.

B) The space between the supernova and Earth is not a perfect vacuum but is filled with very rarefied gas and dust, which impedes the passage of light (very slightly) but not of neutrinos.

C) The neutrinos and the light were produced at the same time, but the light bounced back and forth between the core and the outer layers several times before these layers expanded to the point that they became transparent.

D) The neutrinos were produced earlier (when the core collapsed), and the light was produced 3 hours later (when the shock wave reached the outer layers).

150. How many neutrinos from SN 1987A were detected on Earth?

A) none

B) about 400,000

C) 8  10¹⁷

D) 20

Section: 14-9

151. What are cosmic rays?

A) steady, low-energy flux of neutral atoms into the solar system due to the passage of the Sun through the interstellar medium

B) neutron beams emitted along the rotational axes of accretion disks around neutron stars

C) beams of photons produced by rotating, magnetic neutron stars

D) atomic nuclei and other subatomic particles traveling through space at more than 90% of the speed of light

152. The main component of primary cosmic rays is

A) protons.

B) gamma rays.

C) electrons.

D) nuclei of heavy elements such as carbon and iron.

153. Which one of these does NOT form a part of the flux of cosmic rays reaching Earth?

A) electrons

B) neutrons

C) protons

D) positrons

154. The main source of moderate-energy cosmic rays, the high-energy atomic nuclei that strike Earth’s atmosphere from interstellar space, is believed to be

A) rotating, magnetized neutron stars.

B) matter from accretion disks surrounding neutron stars and black holes.

C) acceleration of nuclei by neutrinos during supernova explosions.

D) supernova debris colliding with interstellar material.

155. What is believed to be the source of many ultrahigh-energy cosmic rays?

A) active supernovae

B) globular clusters

C) supermassive black holes

D) quasars

156. What is a cosmic ray shower?

A) shower of particles produced when a cosmic ray strikes atoms in Earth’s atmosphere

B) burst of high-energy atomic nuclei arriving at Earth from interstellar space

C) another name for a meteor shower

D) pulse of gamma rays arriving at Earth from a rotating, magnetized neutron star

Section: 14-10

157. What important event was recorded by ancient Chinese astronomers in A.D. 1054?

A) brilliant worldwide auroral display

B) appearance of a nearby nova explosion in the Milky Way Galaxy

C) bright binary star undergoing eclipse

D) appearance of a supernova in the Crab Nebula

158. Which major astronomical event was apparently recorded faithfully by Chinese astronomers in the Sung Dynasty in A.D. 1054?

A) supernova explosion in the Milky Way Galaxy

B) total eclipse of the Sun over China

C) rare passage of the planet Venus across the face of the Sun, a solar transit

D) discovery of the planet Mercury

159. The Crab Nebula is

A) a supernova remnant.

B) a planetary nebula surrounding a hot star.

C) a cool, gaseous nebula in which stars are forming.

D) the active nucleus of a nearby spiral galaxy.

160. In terms of the evolutionary life of a star, at what stage is the Crab Nebula?

A) middle-age: main-sequence star, relatively near the Sun

B) black hole: very late stage of evolution

C) beginning: nebula in which stars are forming

D) late: it is the remnant of a star explosion or supernova

161. The Crab Nebula is a nearby example of what type of physical phenomenon?

A) planetary nebula, a shell of gas leaving an old star

B) remnant of a supernova explosion

C) spiral galaxy, a collection of 100 billion stars

D) gas and dust cloud, the formation region for new stars

162. Where would one look for a pulsar among these locations in the universe?

A) at the precise center of the Milky Way Galaxy

B) in the Crab Nebula

C) in the Orion Nebula

D) in the middle of the Sun

163. The explosion of a supernova appears to leave behind

A) a rapidly expanding shell of gas and a central neutron star.

B) a rapidly rotating shell of gas, dust, and radiation, but no central object.

C) a rapidly expanding shell of gas and a compact white dwarf star at its center.

D) nothing; the explosion changes all the matter completely into energy, which then radiates into space at the speed of light.

164. The first pulsar was detected with

A) Galileo’s telescope.

B) the infrared satellite IRAS.

C) a radio telescope being used by Cambridge University.

D) the 200-inch telescope at Mount Palomar.

165. The first pulsar was discovered by

A) Albert Einstein in 1905.

B) Galileo Galilei in 1610.

C) the astronomer royal in Newton’s time, Sir Edmund Halley, in 1606.

D) an English graduate student, Jocelyn Bell, in 1967.

166. The first pulsar was discovered in which year?

A) 1967

B) 1930

C) 1978

D) 1054

167. Which of these astronomical objects is MOST closely associated with a pulsar?

A) red giant star

B) neutron star

C) black hole

D) white dwarf star

168. The great breakthrough in understanding the nature of pulsars was

A) the discovery of a pulsar in the middle of the Crab Nebula.

B) the realization that pulsars were found only in the galactic plane.

C) the connection between pulsars and variable stars.

D) the discovery that all pulsars have periods that are multiples of a universal pulsar period.

169. A stellar core of about 2 solar masses collapses inward in a supernova explosion. What is the result of this collapse?

A) neutron star supported from further collapse by neutron-degeneracy pressure

B) black hole with infinitely small radius because nothing can prevent such a mass from collapsing completely

C) star a little smaller than the size of the Sun supported from further collapse by gas pressure from the hot interior of the star

D) white dwarf star supported from further collapse by degenerate-electron pressure

170. The existence of stars composed almost entirely of neutrons was first predicted by

A) Jocelyn Bell in 1967.

B) Stephen Hawking in 1985.

C) Fritz Zwicky and Walter Baade in 1933.

D) Albert Einstein in 1908.

171. Neutron stars are believed to be created by

A) all types of supernovae.

B) Type Ia supernovae, i.e., exploding white dwarfs.

C) explosions of main-sequence stars.

D) Type II supernovae, i.e., explosions of high-mass stars.

172. When the core of a star becomes too massive to be held up by the pressure associated with electron degeneracy, it

A) is unable to collapse further because of the rules of quantum mechanics.

B) collapses further until it is held up by the pressure of neutron degeneracy.

C) collapses further until it is held up by quark degeneracy.

D) cannot be prevented from further collapse and thus forms a black hole.

173. The diameter of a typical neutron star of 2 solar masses is predicted to be approximately

A) 1 km.

B) that of a small city, about 20 kilometers.

C) that of the Sun.

D) that of Earth, 12,800 km.

174. What prevents a neutron star from collapsing and becoming a black hole?

A) Gravity in the neutron star is balanced by an outward force due to neutron degeneracy.

B) Gravity is balanced in neutron stars by the outward centrifugal force produced by their rapid rotation.

C) Gravity in the neutron star is balanced by an outward force due to gas pressure, as in the Sun.

D) Neutron stars are solid, and, like any solid sphere, they are held up by the repulsive forces between atoms in the solid matter.

175. Electron degeneracy occurs when the carbon core of a white dwarf becomes dense enough. Neutron degeneracy occurs when a neutron star becomes dense enough. Why is there no “proton degeneracy” in some density range between electron and neutron degeneracy?

A) These degeneracies can occur only for neutral particles, and protons carry an electric charge.

B) The pressure at which the electron degeneracy is overcome (so that the core collapses) is also sufficient to combine electrons and protons to form neutrons. Thus, most protons disappear when the electron degeneracy ends.

C) Protons are too large and heavy to exert degeneracy pressure. This concept applies only to smaller, lighter particles like electrons.

D) Protons are not governed by the Pauli exclusion principle, as are electrons and neutrons.

176. Many possible explanations were offered when pulsars were first discovered. Which one of these proved to be correct?

A) alien civilizations

B) expansion and contraction

C) small rotating object

D) large rotating object

Section: 14-11

177. What is a pulsar?

A) very hot material orbiting a black hole

B) Cepheid variable star with a period of a few days

C) pulsating white dwarf star, fluctuating rapidly in brightness

D) rapidly rotating neutron star, producing beams of radio energy and occasionally of X-rays and visible light

178. A pulsar is a(n)

A) rapidly rotating neutron star, emitting beams of radio energy and sometimes X-ray and visible energy.

B) binary star in which matter from one star is falling onto the second star.

C) object at the center of each galaxy, supplying energy from its rapid rotation.

D) pulsating star, in which size, temperature, and light intensity vary regularly.

179. A pulsar is a(n)

A) rapidly spinning neutron star.

B) type of variable star, pulsating rapidly in size and brightness.

C) very precise interstellar beacon perhaps operated by intelligent life-forms.

D) accretion disk around a black hole, emitting light as matter is accumulated on the disk.

180. Pulsars emitting very regular radio and sometimes visible light pulses are what type of object?

A) black holes, with material falling regularly into them

B) pulsating variable stars

C) rapidly rotating neutron stars

D) rapidly rotating binary star systems in which the stars undergo regular eclipses as seen from Earth

181. A pulsar is MOST probably formed

A) at the center of a supernova explosion.

B) in the high-temperature core of a star as it evolves through its main-sequence phase.

C) in a huge gas cloud by collisions between stars.

D) just after the formation of a protostar by gravitational condensation.

182. Which of these is NOT a typical property of young neutron stars?

A) Neutron stars rotate from 1 to 30 times per second.

B) Neutron stars emit relatively narrow beams of light and other radiation.

C) Neutron stars contain strong gravitational fields but weak magnetic fields.

D) Neutron stars are composed almost entirely of neutrons.

183. The very strong magnetic field on a neutron star is created by

A) the collapse of the star, which significantly intensifies the original weak magnetic field of the star.

B) differential rotation of the star, with the equator rotating faster than the poles, similar to sunspot formation.

C) a burst of neutrinos produced by the supernova explosion, the equivalent of a very large electrical current flowing for a short time.

D) turbulence in the electrical plasmas during the collapse of the star; the original star would have had no magnetic field.

184. The magnetic fields of planets demonstrate that rotation is a requirement for an object to produce a magnetic field. It is not surprising, then, that when a massive star collapses the rotation rate

A) and the magnetic field strength both increase.

B) increases but the magnetic field strength decreases.

C) decreases but the magnetic field strength increases.

D) and the magnetic field strength both decrease.

185. The pulsed nature of the radiation at all wavelengths that is seen to come from a pulsar is produced by

A) the rapid pulsation in size and brightness of a small white dwarf star.

B) the rapid rotation of a neutron star that is producing two oppositely directed beams of radiation.

C) the mutual eclipses of two very hot stars orbiting in a close binary system.

D) extremely hot matter that is rapidly orbiting a black hole just prior to descending into it.

186. The source of the beams of electromagnetic radiation (including light in some cases) emitted by pulsars is

A) charged particles traveling along the magnetic axes of rotating neutron stars; the particles emit light as they are accelerated.

B) electrons flowing out along the rotation axis of an accretion disk around a neutron star; the electrons are accelerated and hence emit light.

C) the surface of a normal star that has a white dwarf companion; the white dwarf creates a hot spot on the normal star that emits a beam of light as the stars rotate around each other.

D) jets of material flowing out along the rotation axis of the accretion disk around a black hole; collisions in the jets heat the material and produce light.

187. Pulsars are rotating neutron stars that emit beams of electromagnetic radiation that sweep around the universe like rotating lighthouse beams. The charged particles that emit these beams are believed to be

A) “charmed” mesons and other exotic particles that on Earth exist only in laboratories.

B) iron and other nuclei pulled from the neutron star’s crust by its intense magnetic fields.

C) plasma (ionized gas) spiraling onto the neutron star from a normal stellar companion.

D) electrons and protons accelerated by the neutron star’s intense magnetic fields.

188. A neutron star is detected from Earth as a pulsar by its regular radio pulses only if Earth lies

A) in the neutron star’s “equator,” the plane perpendicular to its spin axis.

B) within the path of the narrow beam of radiation being generated by the neutron star along its magnetic axis as the star rotates rapidly.

C) in the plane of the neutron star’s magnetic equator, halfway between its magnetic poles.

D) directly above the rotation axis of the rotating neutron star.

189. Visible pulses are seen to accompany radio pulses from a neutron star only

A) from neutron stars that are high above the galactic plane, where visible light is not obscured by dust and gas.

B) if the neutron star is relatively young.

C) if the neutron star is part of a binary star system.

D) if the neutron star is relatively old, since pulsars from recent supernova explosions produce only radio pulses.

190. The main reason for the observed slowdown of many pulsars is

A) friction between the stellar surface and the surrounding nebular material.

B) a slow buildup of the magnetic field; rotational energy is transferred to magnetic energy.

C) the loss of rotational energy to provide energy for the emission of radiation.

D) the slowing of rotation caused by slow expansion and redistribution of mass of the source, similar to the slowdown of a spinning skater.

191. As time progresses, the pulse rate for MOST solitary pulsars

A) varies periodically as the neutron star undergoes periodic expansions and contractions.

B) slows down since rotational energy is being used to generate the pulses.

C) speeds up as the neutron star slowly contracts under gravity.

D) remains absolutely constant; pulsars provide ideal frequency standards, or clocks.

192. The periods of MOST pulsars _____ with time.

A) increase

B) decrease

C) remain constant

D) alternately increase and decrease

Section: 14-12

193. Which one of these phenomena is NOT associated with a rotating neutron star?

A) magnetar

B) soft gamma-ray repeater

C) rotating radio transient

D) weakly interacting massive particles

194. How do magnetars form?

A) from strong jets near supermassive black holes

B) from the merger of two neutron stars

C) from the brightest supernovae

D) from acceleration of cosmic rays in supernova remnants

195. Some magnetars with particularly strong magnetic fields emit huge bursts of high- frequency radiation as well as output at other wavelengths. Such magnetars are called

A) soft gamma-ray repeaters.

B) hard gamma-ray repeaters.

C) neutron accretion disks.

D) ultrahigh-frequency sources.

196. One newly discovered type of neutron star is the “rotating radio transient,” which occasionally emits a very brief burst of intense radio radiation. What is the BEST explanation, at present, for this phenomenon?

A) Rotating radio transients are very young neutron stars, and they emit radio radiation in response to a “settling down” deep in their interiors.

B) Rotating radio transients are very old neutron stars. They have cooled off until their temperatures are low enough to emit radiation in the radio range.

C) Rotating radio transients are neutron stars in eclipsing binary systems, and the output of radio radiation is periodically eclipsed by the companion.

D) Rotating radio transients are pulsars whose rotation rate has slowed down to about one rotation every 0.4 to 10 seconds.

197. Why do astronomers believe a magnetar can have a much stronger magnetic field than that of an ordinary neutron star?

A) Magnetars were formed from supergiant stars many times larger than the stars that produced ordinary neutron stars.

B) When first formed as neutron stars, magnetars were spinning rapidly enough that the magnetic fields produced by convection amplified the magnetic field of the original star.

C) Magnetars are superconducting throughout.

D) Magnetars have a higher proportion of protons and thus have stronger electric currents.

Section: 14-13

198. “Starquakes” are the rearrangement of material in a neutron star, similar to an earthquake on Earth. Which one of these statements about starquakes is correct?

A) Starquakes are triggered by instabilities in the neutron star’s magnetic field.

B) Starquakes are caused by stresses in the crusts of neutron stars that form close binary systems and exert tremendous forces on each other.

C) Starquakes cause glitches in the pulsar period.

D) Starquakes cause rents in the crust of a neutron star and allow gamma rays and X-rays to escape from the interior, like the coronal holes in the Sun’s atmosphere.

199. What causes a glitch in the rotation rate of a pulsar?

A) Hot spots develop on the surface of a neutron star, and from these spots jets of material can suddenly shoot out causing a minor but abrupt chance in the rotation rate.

B) Because of their enormous density, the rotation rates of neutron stars are quantized and must change abruptly from one rotation rate to another.

C) As the star slows, its shape becomes more spherical. If its rotation rate adjusts to this suddenly, a glitch occurs.

D) When a pulsar occurs in a binary system, the binary companion can send clumps of material to land on its surface. This will change its rotation rate abruptly.

200. The interior of a neutron star is believed to contain

A) neutrons compressed into a crystalline lattice structure by very high pressure.

B) a dense gas consisting mostly of neutrons.

C) a metallic fluid of almost pure iron.

D) neutrons in a superfluid state.

201. The generation of magnetic fields requires both a rapid rotation and a fluid interior for the flow of currents of charged particles. But neutrons are electrically neutral, so how can a neutron star generate a magnetic field?

A) Neutrons in neutron degeneracy in a neutron star acquire an electric charge.

B) Because the neutrons in a neutron star flow through a superconducting medium, they do not need to carry an electric charge in order to constitute a current.

C) The enormous amounts of friction experienced by degenerate neutrons cause electric charges to build up on their surfaces, like static electricity.

D) Neutron stars actually contain some protons and electrons, which carry the electric currents.

202. Do neutron stars have atmospheres?

A) No. The gravitational pull is too great to allow anything to exist above the surface.

B) Yes, but the “atmosphere” consists of loosely bound neutrons circling the star.

C) Yes. Bright emission line spectra have been observed.

D) Yes. Dark absorption line spectra have been observed.

203. Aside from an atmosphere, the outermost layer of a neutron star is believed to consist of

A) a dense gas consisting mostly of neutrons.

B) a molten ocean of heavy elements such as iron, neon, and magnesium.

C) a solid crust.

D) superfluid neutrons.

204. In the study of neutron stars, what is a “glitch”?

A) sudden change in the rotation rate of the neutron star

B) change in the pulse period of a neutron star due to the gravitational effect of an orbiting planet

C) sudden pulse of light from the neutron star

D) collision between two neutron stars

205. What are the glitches occasionally observed by astronomers studying pulsars?

A) secondary pulses of radiation occasionally interspersed with the primary pulses

B) clumps of denser material in the jets of particles emitted along the magnetic poles

C) sudden flares caused by matter falling onto the surface of a neutron star

D) sudden changes in a pulsar’s rotation rate

Section: 14-14

206. The period of pulsation for the fastest pulsars is about

A) 1/1000 second.

B) 10^–6 second.

C) 1 second.

D) a few minutes.

207. The 2004 discovery of two pulsars in orbit around their common center of mass is significant because it gives a unique opportunity to study some of the predictions of

A) Kepler’s laws.

B) the theory of angular momentum.

C) the general theory of relativity.

D) the special theory of relativity.

208. In 2004 astronomers made a significant discovery that enabled them to test some of the predictions of general relativity. What was this discovery?

A) a neutron star with a mass more than 100 times the mass of the Sun

B) two neutron stars collided with each other

C) a pulsar that was not precessing and, in fact, that aimed its radiation beam directly at Earth

D) two pulsars in orbit around each other

209. What is the origin of millisecond pulsars?

A) As a pulsar cools, it collapses further. This causes its rotation rate to increase.

B) Pulsars in binary systems can draw mass from the binary companion. This mass, impacting the pulsar’s surface, causes its rotation rate to increase.

C) The jets that radiate energy away from a pulsar cause it to become smaller and to rotate faster.

D) The rotation period for most pulsars is actually less than a millisecond. So, the natural slowing as it loses energy moves it into the millisecond range.

210. A binary system contains a neutron star and a giant star that is slowly transferring mass to the neutron star. What is a likely result of this process?

A) millisecond pulsar

B) magnetar

C) soft gamma-ray repeater

D) white dwarf

211. A binary system contains a pulsar whose beam intercepts the atmosphere of its main-sequence companion. This system contains a

A) magnetar.

B) soft gamma-ray repeater.

C) black widow pulsar.

D) X-ray burster.

Section: 14-15

212. What is a hypernova?

A) a recurrent nova on the surface of a white dwarf star

B) a runaway nuclear reaction on the surface of a neutron star

C) a stellar explosion that is 10 times brighter and lasts several times longer than “ordinary” supernovae

D) the collision of two neutron stars

213. Hypernovae are associated with

A) binary neutron stars.

B) supermassive black holes.

C) magnetars.

D) explosions of extremely massive stars.

Section: 14-16

214. What is a supernova impostor?

A) a massive star that experiences recurrent brightening to levels comparable to a supernova explosion

B) an exceptionally strong nova, or runaway thermonuclear explosion on the surface of a white dwarf

C) the collision of two neutron stars

D) a gamma-ray burst with no afterglow

215. Eta Carina is a massive binary star that experiences occasional outbursts or explosions as powerful as Type II supernovae. This is an example of a

A) recurrent nova.

B) supernova impostor.

C) gamma-ray burst.

D) soft gamma-ray repeater.

216. Eta Carina is a

A) supernova remnant.

B) active supermassive black hole.

C) star-forming region with many O and B stars.

D) binary star system that periodically undergoes powerful outbursts.

Section: 14-17

217. Fast radio bursts originate from

A) Type Ia supernovae.

B) Type II supernovae

C) supermassive black holes.

D) sources outside the Galaxy that are otherwise unknown.

218. Fast radio bursts are thought to originate from outside the Milky Way. Why have astronomers reached this conclusion?

A) No source in the Galaxy could produce such prodigious energy and be invisible at other wavelengths.

B) The bursts experience time dilation.

C) The bursts are observed at a wide range of declinations and right ascensions.

D) The bursts are exceptionally luminous.

Section: 14-18

219. The enormous densities that develop during a supernova collapse do not appear to be sufficient to form the heaviest elements. Where do the conditions to produce these elements occur?

A) the Big Bang

B) collisions of neutron stars

C) magnetars

D) quasars

220. In 2017, astronomers observed the collision of two neutron stars

A) through its gravitational wave signature.

B) through its neutrino production.

C) as a Type Ia supernova.

D) as a hypernova.

221. During the collision of two neutron stars, ejected neutrons form

A) recurrent novae.

B) cosmic rays.

C) a neutron star ejected with a large velocity.

D) heavy elements.

222. Could the gold on Earth have any connection with neutron stars?

A) No. Neutron stars do not eject any significant amount of material into the Galaxy.

B) Yes. Some of the gold was created in collisions between neutron stars in binary systems.

C) No. Some of the hydrogen in Earth’s water may have been ejected from neutron stars as high-energy protons, but all heavy elements come from supernova explosions of normal stars.

D) Yes. The neutrons in the gold nuclei were very likely ejected from individual neutron stars.

223. Astronomers believe that elements heavier than carbon can be created in each of these EXCEPT

A) the cores of solar mass stars.

B) the cores of massive stars.

C) during supernova explosions.

D) when two neutron stars collide.

224. When a massive star collapses into a Type II supernova, elements heavier than iron are created. But the amounts of these elements that survive such a collapse are not sufficient to account for the amounts seen in the universe. Where are/were many of them likely produced?

A) Type Ia supernovae

B) the original Big Bang at the origin of the universe

C) collisions of neutron stars

D) nucleosynthesis in the cores of smaller but more numerous stars, like the Sun

225. What is the difference between a normal pulsar, which emits radiation in the radio range, and a pulsating X-ray source?

A) A pulsating X-ray source combines the characteristics of an ordinary pulsar and a variable star. The surface pulsates in and out creating a pulsating source.

B) A pulsating X-ray source is a binary system of two pulsars in very rapid rotation around one another. One sees the interference pattern of the two pulsar beams as a pulsating X-ray jet.

C) A pulsating X-ray source is an ordinary pulsar, but occasionally, when the neutron star experiences a glitch and the crust cracks, the hot interior is exposed and emits an intense beam of X-rays.

D) A pulsating X-ray source is a pulsar with a binary companion. Matter pulled off the companion spirals along the intense magnetic field of the pulsar, and its interaction with the surface produces X-ray jets.

226. Pulsating X-ray sources are believed to be

A) white dwarf stars with intense magnetic fields; the X-rays are generated by flares like those on the Sun but much stronger.

B) spinning neutron stars in binary systems, emitting X-rays because of mass transfer onto the neutron star from its normal companion.

C) the same as regular (optical) pulsars but observed in X-rays.

D) black holes in binary systems, with an accretion disk around the black hole emitting X-rays.

227. Pulsating X-ray sources with periods of a few seconds are caused by

A) the eclipsing of an X-ray-emitting star with a very hot surface by a cool companion in a close binary system.

B) the pulsation in radius, temperature, and hence luminosity of a hot Cepheid variable star with a surface temperature hot enough to emit X-rays.

C) matter falling violently onto the surface of a rotating neutron star from a close companion in a binary star system, causing an X-ray hot spot that disappears periodically behind the neutron star.

D) matter falling onto the surface of a very hot, rotating white dwarf star from an ordinary companion star in a binary system, producing an X-ray-emitting hot spot that disappears periodically behind the white dwarf.

228. A binary system is formed with a neutron star and another star that has overflowed its Roche lobe. How does the detection of this binary differ from the detection of an isolated neutron star?

A) The outer layers of the larger star envelop the neutron star, so only the spectrum of the larger star is detected.

B) This binary may emit intense X-rays while an isolated neutron star emits radio waves.

C) The interaction of the larger star with the neutron star may slow the neutron star’s rotation to the point where it no longer rotates. One then sees a steady radio beam rather than the series of pulses that the lighthouse model predicts for an isolated neutron star.

D) The spectrum of the larger star swamps the spectrum of the neutron star, thus showing a standard thermal blackbody spectrum. An isolated neutron star produces a nonthermal radiation spectrum.

229. In the binary pulsar Hercules X-1 the periodic Doppler shift, once every 1.7 days, was discovered rather quickly. Even though the binary pulsar Centaurus X-3 was discovered earlier, its Doppler shift was discovered only much later. What might be a reason for this?

A) The orbital plane of Centaurus X-3 is in the plane of the sky and therefore has no radial motion and no Doppler shift.

B) Centaurus X-3 is too far away to see the Doppler shift easily.

C) The orbital period of Centaurus X-3, 2087 days, suggests that it moves relatively slowly.

D) The binary companion of Centaurus X-3 is a black hole that distorts the pattern of its radiation output.

Section: 14-19

230. The temperature of the hot spots caused by the impact of transferred matter onto the surface of a pulsar can be 10⁸ K. What is the peak wavelength in the blackbody spectrum of such a spot, and in what range of the electromagnetic spectrum does it occur?

A) 2.9  10^–11 nm; gamma ray

B) 2.9  10^–2 nm; X-ray

C) 290 nm; ultraviolet

D) 2.9  10⁷ nm; radio

Section: 14-20

231. X-ray bursters emit occasional and intense bursts of X-rays on top of a steady low-level X-ray emission. These bursts of X-rays are believed to be caused by

A) material from a companion star pulled into an accretion disk around a black hole, and, consequently, periodic clumps of material falling from the disk into the black hole and being compressed produce the X-rays.

B) material transferred onto the surface of a neutron star, causing the neutron star to collapse suddenly into a black hole.

C) material transferred onto the surface of a neutron star in a binary system, then subsequently ignited in a thermonuclear explosion that leaves the neutron star intact to repeat the process.

D) material transferred onto the surface of a white dwarf in a binary star system, producing a thermonuclear explosion at the surface while leaving the white dwarf intact to repeat the process.

232. When mass transfer in a binary system involves mass transferred to a white dwarf, the result is a nova. When the mass is transferred to a neutron star, the result is a(n)

A) magnetar.

B) soft gamma-ray repeater.

C) black hole.

D) X-ray burster.

233. The difference between a nova and an X-ray burst is that a nova involves an explosion on the surface of a _____ whereas an X-ray burst involves a(n) _____.

A) white dwarf; explosion on the surface of a neutron star

B) neutron star; explosion on the surface of a white dwarf

C) white dwarf; collapse of a white dwarf to form a neutron star

D) neutron star; complete collapse of a neutron star to form a black hole

234. In a nova, the radiation outburst is caused by fusion of hydrogen to produce helium on the surface of a white dwarf. The radiation outburst from an X-ray burster is caused by fusion of

A) hydrogen to produce helium on the surface of a neutron star.

B) hydrogen to produce helium in the interior of a neutron star.

C) helium to produce carbon on the surface of a neutron star.

D) carbon to produce oxygen in the interior of a neutron star.

235. The X-ray bursts from an X-ray burster are caused by

A) hot spots caused by material falling onto the poles of a rotating neutron star.

B) explosive photodisintegration of iron nuclei on the surface of a neutron star.

C) explosive helium fusion on the surface of a neutron star.

D) explosive hydrogen fusion on the surface of a neutron star.