Deck 14: The Death of Stars

Question 1

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.

Answer 1

red supergiant.

Question 2

Helium nuclear reactions (helium fusion) produce primarily

A) carbon and oxygen.
B) carbon and silicon.
C) iron.
D) oxygen and neon.

Answer 2

carbon and oxygen.

Question 3

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.

Answer 3

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

Question 4

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

Question 5

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.

Question 6

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.

Question 7

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

Question 8

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

Question 9

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.

Question 10

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

Question 11

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.

Question 12

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.

Question 13

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

Question 14

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

Question 15

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.

Question 16

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

Question 17

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.

Question 18

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

Question 19

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

Question 20

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

Question 21

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

Question 22

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

Question 23

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.

Question 24

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- $\alpha$ 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.

Question 25

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

Question 26

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

Question 27

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.

Question 28

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.

Question 29

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

Question 30

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.

Question 31

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.

Question 32

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.

Question 33

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.

Question 34

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%.

Question 35

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

Question 36

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

Question 37

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.

Question 38

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.

Question 39

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.

Question 40

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.

Question 41

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.

Question 42

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.

Question 43

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.

Question 44

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.

Question 45

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.

Question 46

Stars that have ejected a planetary nebula go on to become

A) red giants.
B) supernovae.
C) protostars.
D) white dwarfs.

Question 47

A white dwarf star is about the same size as

A) the Sun.
B) Earth.
C) the total solar system.
D) a major city.

Question 48

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.

Question 49

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

Question 50

The Sun will end its life by becoming a

A) molecular cloud.
B) black hole.
C) white dwarf.
D) pulsar.

Question 51

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

Question 52

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

Question 53

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.

Question 54

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.

Question 55

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.

Question 56

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

Question 57

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.

Question 58

Which type of dwarf is largest?

A) white dwarf
B) red dwarf
C) brown dwarf
D) All are about the same size.

Question 59

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

Question 60

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.

Question 61

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.

Question 62

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.

Question 63

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.

Question 64

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.

Question 65

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.

Question 66

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.

Question 67

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.

Question 68

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.

Question 69

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.

Question 70

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

Question 71

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.

Question 72

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.

Question 73

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.

Question 74

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

A) 25.
B) 8.
C) 1.4.
D) 3.

Question 75

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.

Question 76

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.

Question 77

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

Question 78

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

Question 79

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.

Question 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.