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16) Briefly summarize the stages of life for a low-mass star.

Answer: The protostar assembles from the molecular clouds, heats up from gravitational contraction, and begins hydrogen fusion in the core. The star settles onto the main sequence, where it will fuse hydrogen in its core for 10 billion years. When the core hydrogen is used up, the core contracts until it is degenerate, hydrogen fusion continues in a shell outside the core, and the outer layers expand and cool the star becomes a red giant. Helium fusion begins in the core, but since the core is degenerate a helium flash takes place and rapidly spreads throughout the core. Helium fusion stabilizes, and the star moves left on the H-R diagram. Core helium is used up and helium begins fusing in a shell outside the core, with hydrogen still fusing in a shell above it. The outer layers expand, and the star again becomes a red giant.

The star undergoes thermal pulses and loses its outer layers through a stellar wind. The core shrinks and heats up but is not able to fuse any more elements. The star becomes a planetary nebula as heat from the core blows away and heats up the gas left over from the red giant phase. Only the naked degenerate core is left, a white dwarf.

17) Briefly summarize the stages of life for a high-mass star.

Answer: The first stages are similar to those of a low-mass star, except that they happen over much shorter time periods. While on the main sequence, the star fuses hydrogen by the CNO cycle and remains at this stage only for several million years. In addition to helium fusion, high-mass stars also undergo alpha-capture, which creates heavier elements by fusing a helium nucleus with an existing atom. After helium is used up in the core, the core contracts while helium and hydrogen fusion continue in outer shells. The core contracts until carbon ignition occurs, and the star moves left again on the H-R diagram while carbon fusion occurs in the core. The process continues for stars of still higher mass, zigzagging across the H-R diagram as heavier elements are fused in the core and used up as fuel. Each fusion stage requires less time until iron is finally produced in the core. Iron cannot be fused to produce energy, so the core collapses and pressures increase so that electrons and protons are converted to neutrons. A high quantity of neutrinos is released, which may help force the outer layers violently outward in an explosion called a supernova. Elements heavier than iron are created, the outer layers move away from the core at great velocities, and only a neutron star or black hole is left as a remnant.

18) Briefly explain why high-mass stars have shorter lifetimes than low-mass stars.

Answer: High-mass stars have 10 to 100 times more mass (fuel) than a typical low-mass star. This greater mass produces a much higher downward gravitational pressure, leading to much higher core temperatures and higher rates of fusion. The luminosity of such stars is therefore 1,000 to 1 million times greater than in low-mass stars. So, although high-mass stars have more fuel to burn, they burn it at a much higher rate and therefore run out of fuel much more quickly.

19) Process of Science: Based on what you learned in this chapter, would you expect life to be able to evolve around first-generation stars in our universe? Why or why not?

Answer: Most would say no, as the first-generation stars should not have yet been enriched with the heavy elements we believe are necessary for life. A student could also argue that life could form without these and our current understanding of life is incomplete.

20) Process of Science: Explain how patterns in cosmic abundances (Figure 17.15 in the textbook) fit theoretical predictions for the origin of the elements.

Answer: Heavier elements are rarer as they are produced in shorter-lived phases of rare, massive stars. Iron is relatively abundant because its production is energetically favored as the end step of fusion and there is only a short time during the supernova phase when it can be destroyed by fission. The abundances of nuclei with even numbers of protons is greater than neighboring nuclei with odd numbers of protons as expected for nuclear reactions through the addition of helium nuclei.

21) Process of Science: How do observations of stars help us understand the theory of atomic nuclei?

Answer: Many nuclear reactions occur during the late stages of stellar evolution. By observing how stars evolve and the production of different elements, we can learn about how nuclei react.

17.4 Mastering Astronomy Reading Quiz

1) Which of the following stars will live longest?

A) a 1-solar-mass star

B) a 2-solar-mass star

C) a 3-solar-mass star

D) a 4-solar-mass star

E) a 5-solar-mass star

Answer: A

2) In the context of understanding stellar lives, "high-mass" stars have masses

A) more than about 8 times the mass of our Sun.

B) more than about 3 times the mass of our Sun.

C) more than twice the mass of our Sun.

D) more than 20 times the mass of our Sun.

Answer: A

3) Which of the following lists the stages of life for a low-mass star in the correct order?

A) protostar, main-sequence star, red giant, planetary nebula, white dwarf

B) protostar, main-sequence star, red giant, supernova, neutron star

C) protostar, main-sequence star, planetary nebula, red giant

D) main-sequence star, white dwarf, red giant, planetary nebula, protostar

Answer: A

4) What happens when a main-sequence star exhausts its core hydrogen fuel supply?

A) The entire star shrinks in size.

B) The core shrinks while the rest of the star expands.

C) The core immediately begins to fuse its helium into carbon.

D) The star becomes a neutron star.

Answer: B

5) The main source of energy for a star as it grows in size to become a red giant is

A) hydrogen fusion in the central core.

B) helium fusion in the central core.

C) hydrogen fusion in a shell surrounding the central core.

D) gravitational contraction.

Answer: C

6) The overall helium fusion reaction is

A) three helium nuclei fuse to form one carbon nucleus.

B) two helium nuclei fuse to form one beryllium nucleus.

C) two hydrogen nuclei fuse to form one helium nucleus.

D) four helium nuclei fuse to form one oxygen nucleus.

Answer: A

7) What is a helium flash?

A) The ignition of helium shell burning in a high-mass star with a carbon core.

B) A sudden brightening of a low-mass star, detectable from Earth by observing spectral lines of helium.

C) It is another name for the helium fusion reaction.

D) The sudden onset of helium fusion in the core of a low-mass star.

Answer: D

8) An H-R diagram for a globular cluster will show a horizontal branch—a line of stars above the main-sequence but to the left of the subgiants and red giants. Which of the following statements about these horizontal branch stars is true?

A) They have inert (non-burning) carbon cores.

B) Their sole source of energy is hydrogen shell burning.

C) They generate energy through both hydrogen fusion and helium fusion.

D) In a particular star cluster, all horizontal branch stars have the same spectral type.

Answer: C

9) What is a planetary nebula?

A) gas created from the remains of planets that once orbited a dead star

B) interstellar gas from which planets are likely to form in the not-too-distant future

C) the remains of a high-mass star that has exploded

D) gas ejected from a low-mass star in the final stage of its life

Answer: D

10) The ultimate fate of our Sun is to

A) explode in a supernova.

B) become a white dwarf that will slowly cool with time.

C) become a rapidly spinning neutron star.

D) become a black hole.

Answer: B

11) Which low-mass star does not have fusion occurring in its central core?

A) a main-sequence star

B) a red giant

C) a helium-burning star

Answer: B

12) How are low-mass red giant stars important to our existence?

A) These stars manufactured virtually all the elements out of which we and our planet are made.

B) These stars generate the energy that makes life on Earth possible.

C) These stars manufactured most of the carbon atoms in our bodies.

D) These stars provide most of the light that reaches us from globular clusters.

Answer: C

13) Which of the following pairs of atomic nuclei would feel the strongest repulsive electromagnetic force if you tried to push them together?

A) helium and helium

B) hydrogen and hydrogen

C) hydrogen and helium

D) hydrogen and deuterium

Answer: A

14) Which of the following stars will certainly end its life in a supernova?

A) the Sun

B) a red giant star

C) a 10-solar-mass star

D) a neutron star

Answer: C

15) What is the CNO cycle?

A) a set of steps by which four hydrogen nuclei fuse into one helium nucleus

B) the process by which helium is fused into carbon, nitrogen, and oxygen

C) the process by which carbon is fused into nitrogen and oxygen

D) the set of fusion reactions that have produced all the carbon, nitrogen, and oxygen in the universe

Answer: A

16) In order to predict whether a star will eventually fuse oxygen into a heavier element, what do you need to know about the star?

A) its luminosity

B) its overall abundance of elements heavier than helium

C) how much oxygen it now has in its core

D) its mass

Answer: D

17) Why is iron significant to understanding how a supernova occurs?

A) Iron is the heaviest of all atomic nuclei, and thus no heavier elements can be made.

B) Supernovae often leave behind neutron stars, which are made mostly of iron.

C) The fusion of iron into uranium is the reaction that drives a supernova explosion.

D) Iron cannot release energy either by fission or fusion.

Answer: D

18) After a supernova explosion, the remains of the stellar core

A) will always be a neutron star.

B) be either a neutron star or a black hole.

C) will always be a black hole.

D) may be either a white dwarf, neutron star, or black hole.

Answer: B

19) Why is Supernova 1987A particularly important to astronomers?

A) It is the nearest supernova to have occurred at a time when we were capable of studying it carefully with telescopes.

B) It was the first supernova detected in nearly 400 years.

C) It provided the first evidence that supernovae really occur.

D) It occurred only a few light-years from Earth.

Answer: A

20) Algol consist of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant. Why does this seem surprising, at least at first?

A) The two stars in a binary system should both be at the same stage of life; that is, they should either both be main-sequence stars or both be subgiants.

B) It doesn't make sense to find a subgiant in a binary star system.

C) The two stars should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star.

D) A star with a mass of 3.7 MSun is too big to be a main-sequence star.

Answer: C

21) Where does gold (the element) come from?

A) It is produced by mass transfer in close binaries.

B) It is produced during the supernova explosions of high-mass stars.

C) It is produced during the late stages of fusion in low-mass stars.

D) It was produced during the Big Bang.

Answer: B

17.5 Mastering Astronomy Concept Quiz

1) Sun is considered to be a

A) low-mass star.

B) intermediate-mass star.

C) high-mass star.

D) brown dwarf.

Answer: A

2) Which of the following types of data provide evidence that helps us understand the life tracks of low-mass stars?

A) H-R diagrams of open clusters

B) observing a low-mass star over many years

C) H-R diagrams of globular clusters

D) spacecraft observations of the Sun

Answer: C

3) Why is a 1 solar-mass red giant more luminous than a 1 solar-mass main-sequence star?

A) The red giant has a hotter core.

B) The red giant's surface is hotter.

C) The red giant is more massive.

D) Fusion reactions are producing energy at a greater rate in the red giant.

Answer: D

4) Which of the following describes a star with a hydrogen-burning shell and an inert helium core?

A) It is a red giant that grows in luminosity until it dies in a planetary nebula.

B) It is a subgiant that gradually grows dimmer as its hydrogen-burning shell expands and cools.

C) It is a subgiant that grows in luminosity until helium fusion begins in the central core.

D) It is what is known as a helium-burning star, which has both helium fusion in its core and hydrogen fusion in a shell.

Answer: C

5) Which of the following observations would not be likely to provide information about the final, explosive stages of a star's life?

A) studying the light rings around Supernova 1987A in the Large Magellanic Cloud

B) decades of continuous monitoring of red giants in a globular cluster

C) observing the structures of planetary nebulae

D) neutrino detections from nearby supernovae

Answer: B

6) Which is more common: a star blows up as a supernova, or a star forms a planetary nebula/white dwarf system?

A) Supernovae are more common.

B) Planetary nebula formation is more common.

C) They both occur in about equal numbers.

D) It is impossible to say.

Answer: B

This diagram represents the life track of a 1-solar-mass star. Refer to the life stages labeled with Roman numerals.

7) During which stage is the star's energy supplied by primarily by gravitational contraction?



C) V



Answer: A

8) During which stage does the star have an inert (non-burning) helium core?






Answer: B

9) Which stage lasts the longest?

A) I




Answer: C

10) During which stage does the star have an inert (non-burning) carbon core surrounded by shells of helium and hydrogen burning?






Answer: E

11) What will happen to the star after stage VIII?

A) Its outer layers will be ejected as a planetary nebula and its core will become a white dwarf.

B) It will continue to expand gradually until carbon fusion begins in its core.

C) It will explode as a supernova and leave a neutron star or black hole behind.

D) It will remain in stage VIII for about 10 billion years, after which its outer layers will shrink back and cool.

Answer: A

12) Carbon fusion occur in high-mass stars but not in low-mass stars because

A) the cores of low-mass stars never contain significant amounts of carbon.

B) the cores of low-mass stars never get hot enough for carbon fusion.

C) only high-mass stars do fusion by the CNO cycle.

D) carbon fusion can occur only in the stars known as carbon stars.

Answer: B

13) Which of the following statements about various stages of core nuclear burning (hydrogen, helium, carbon, etc.) in a high-mass star is not true?

A) As each stage ends, the core shrinks and heats further.

B) Each successive stage creates an element with a higher atomic number and atomic mass number.

C) As each stage ends, the reactions that occurred in previous stages continue in shells around the core.

D) Each successive stage lasts for approximately the same amount of time.

Answer: D

14) Which event marks the beginning of a supernova?

A) the sudden collapse of an iron core into a compact ball of neutrons

B) the onset of helium burning after a helium flash

C) the beginning of neon burning in an extremely massive star

D) the sudden initiation of the CNO cycle

Answer: A

15) Suppose that the star Betelgeuse (the upper left shoulder of Orion) were to supernova tomorrow (as seen here on Earth). What would it look like to the naked eye?

A) Betelgeuse would remain a dot of light, but would suddenly become so bright that, for a few weeks, we'd be able to see this dot in the daytime.

B) We'd see a cloud of gas expanding away from the position where Betelgeuse used to be. Over a period of a few weeks, this cloud would fill our entire sky.

C) Because the supernova destroys the star, Betelgeuse would suddenly disappear from view.

D) Betelgeuse would suddenly appear to grow larger in size, soon reaching the size of the full Moon. It would also be about as bright as the full Moon.

Answer: A

16) Suppose that hydrogen, rather than iron, had the lowest mass per nuclear particle. Which of the following would be true?

A) Stars would be brighter.

B) Stars would be less massive.

C) All stars would be red giants.

D) Nuclear fusion could not power stars.

Answer: D

17) Observations show that elements with atomic mass numbers divisible by 4 (such as oxygen-16, neon-20, and magnesium-24) tend to be more abundant in the universe than elements with atomic mass numbers in between. Why do we think this is the case?

A) The apparent pattern is thought to be a random coincidence.

B) Elements with atomic mass numbers divisible by 4 tend to be more stable than elements in between.

C) At the end of a high-mass star's life, it produces new elements through a series of helium capture reactions.

D) This pattern in elemental abundances was apparently determined during the first few minutes after the Big Bang.

Answer: C

18) A spinning neutron star has been observed at the center of a

A) planetary nebula.

B) supernova remnant.

C) red supergiant.

D) protostar.

Answer: B

19) You discover a binary star system in which one star is a 15 MSun main-sequence star and the other is a 10 MSun giant. How do we think that a star system such as this might have come to exist?

A) The giant must once have been the more massive star, but is now less massive because it transferred some of its mass to its companion.

B) Although both stars probably formed from the same clump of gas, the more massive one must have had its birth slowed so that it became a main-sequence stars millions of years later than its less massive companion.

C) The two stars probably were once separate, but became a binary when a close encounter allowed their mutual gravity to pull them together.

D) The two stars are simply evolving normally and independently, and one has become a giant before the other.

Answer: A

20) Tidal forces are very important to the Algol system today, but were not important when both stars were still on the main sequence. Why not?

A) Main-sequence stars in a system like the Algol system are small compared to their physical separation.

B) Main-sequence stars are too big to be affected by tidal forces.

C) Main-sequence stars are too massive to be affected by tidal forces.

D) Main-sequence stars are unaffected by tidally-induced mass transfer.

Answer: A

21) Will our Sun contribute heavy elements to the cosmos at the end of its life?

A) Yes, all stars do.

B) No, only high mass stars make heavy elements, and our Sun is a low mass star.

C) It is possible, but there is no way for us to know.

D) Yes, but only because it is a low mass star and has plenty of time to produce those heavy elements.

Answer: B

The Cosmic Perspective, 8e (Bennett)

Chapter 18 The Bizarre Stellar Graveyard

18.1 Multiple-Choice Questions

1) Degeneracy pressure is the source of the pressure that stops the crush of gravity in all the following except

A) a brown dwarf.

B) a white dwarf.

C) a neutron star.

D) a very massive main-sequence star.

E) the central core of the Sun after hydrogen fusion ceases but before helium fusion begins.

Answer: D

2) White dwarfs are so called because

A) they are both very hot and very small.

B) they are the end-products of small, low-mass stars.

C) they are the opposite of black holes.

D) it amplifies the contrast with red giants.

E) they are supported by electron degeneracy pressure.

Answer: A

3) A teaspoonful of white dwarf material on Earth would weigh

A) the same as a teaspoonful of Earth-like material.

B) about the same as Mt. Everest.

C) about the same as Earth.

D) a few tons.

E) a few million tons.

Answer: D

4) Which of the following is closest in mass to a white dwarf?

A) the Moon

B) Earth

C) Jupiter

D) Neptune

E) the Sun

Answer: E

5) Why is there an upper limit to the mass of a white dwarf?

A) White dwarfs come only from stars smaller than 1.4 solar masses.

B) The more massive the white dwarf, the greater the degeneracy pressure and the faster the speeds of its electrons. Near 1.4 solar masses, the speeds of the electrons approach the speed of light, so more mass cannot be added without breaking the degeneracy pressure.

C) The more massive the white dwarf, the higher its temperature and hence the greater its degeneracy pressure. At about 1.4 solar masses, the temperature becomes so high that all matter effectively melts, even individual subatomic particles.

D) The upper limit to the masses of white dwarfs was determined through observations of white dwarfs, but no one knows why the limit exists.

E) Above this mass, the electrons would be pushed together so closely they would turn into neutrons and the star would become a neutron star.

Answer: B

6) What is the ultimate fate of an isolated white dwarf?

A) It will cool down and become a cold black dwarf.

B) As gravity overwhelms the electron degeneracy pressure, it will explode as a nova.

C) As gravity overwhelms the electron degeneracy pressure, it will explode as a supernova.

D) As gravity overwhelms the electron degeneracy pressure, it will become a neutron star.

E) The electron degeneracy pressure will eventually overwhelm gravity and the white dwarf will slowly evaporate.

Answer: A

7) Suppose a white dwarf is gaining mass because of accretion in a binary system. What happens if the mass someday reaches the 1.4-solar-mass limit?

A) The white dwarf undergoes a catastrophic collapse, leading to a type of supernova that is somewhat different from that which occurs in a massive star but is comparable in energy.

B) The white dwarf, which is made mostly of carbon, suddenly becomes much hotter in temperature and therefore is able to begin fusing the carbon. This turns the white dwarf back into a star supported against gravity by ordinary pressure.

C) The white dwarf immediately collapses into a black hole, disappearing from view.

D) A white dwarf can never gain enough mass to reach the limit because a strong stellar wind prevents the material from reaching it in the first place.

Answer: A

8) Which of the following statements about novae is not true?

A) A star system that undergoes a nova may have another nova sometime in the future.

B) A nova involves fusion taking place on the surface of a white dwarf.

C) Our Sun will probably undergo at least one nova when it becomes a white dwarf about 5 billion years from now.

D) When a star system undergoes a nova, it brightens considerably, but not as much as a star system undergoing a supernova.

E) The word nova means "new star" and originally referred to stars that suddenly appeared in the sky, then disappeared again after a few weeks or months.

Answer: C

9) What kind of pressure supports a white dwarf?

A) neutron degeneracy pressure

B) electron degeneracy pressure

C) thermal pressure

D) radiation pressure

E) all of the above

Answer: B

10) What is the upper limit to the mass of a white dwarf?

A) There is no upper limit.

B) There is an upper limit, but we do not yet know what it is.

C) 2 solar masses

D) 1.4 solar masses

E) 1 solar mass

Answer: D

11) How does a 1.2-solar-mass white dwarf compare to a 1.0-solar-mass white dwarf?

A) It has a larger radius.

B) It has a smaller radius.

C) It has a higher surface temperature.

D) It has a lower surface temperature.

E) It is supported by neutron, rather than electron, degeneracy pressure.

Answer: B

12) Which of the following is closest in size (radius) to a white dwarf?

A) Earth

B) a small city

C) a football stadium

D) a basketball

E) the Sun

Answer: A

13) What kind of star is most likely to become a white-dwarf supernova?

A) an O star

B) a star like our Sun

C) a binary M star

D) a white dwarf star with a red giant binary companion

E) a pulsar

Answer: D

14) Observationally, how can we tell the difference between a white-dwarf supernova and a massive-star supernova?

A) A massive-star supernova is brighter than a white-dwarf supernova.

B) A massive-star supernova happens only once, while a white-dwarf supernova can repeat periodically.

C) The spectrum of a massive-star supernova shows prominent hydrogen lines, while the spectrum of a white-dwarf supernova does not.

D) The light of a white-dwarf supernova fades steadily, while the light of a massive-star supernova brightens for many weeks.

E) We cannot yet tell the difference between a massive-star supernova and a white-dwarf supernova.

Answer: C

15) After a massive-star supernova, what is left behind?

A) always a white dwarf

B) always a neutron star

C) always a black hole

D) either a white dwarf or a neutron star

E) either a neutron star or a black hole

Answer: E

16) A teaspoonful of neutron star material on Earth would weigh

A) about the same as a teaspoonful of Earth-like material.

B) a few tons.

C) more than Mt. Everest.

D) more than the Moon.

E) more than Earth.

Answer: C

17) Which of the following is closest in size (radius) to a neutron star?

A) Earth

B) a city

C) a football stadium

D) a basketball

E) the Sun

Answer: B

18) Which of the following best describes what would happen if a 1.5-solar-mass neutron star, with a diameter of a few kilometers, were suddenly (for unexplained reasons) to appear in your hometown?

A) The entire mass of Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star.

B) It would rapidly sink to the center of Earth.

C) The combined mass of Earth and the neutron star would cause the neutron star to collapse into a black hole.

D) It would crash through Earth, creating a large crater, and exit Earth on the other side.

E) It would crash into Earth, throwing vast amounts of dust into the atmosphere which in turn would cool Earth. Such a scenario is probably what caused the extinction of the dinosaurs.

Answer: A

19) From an observational standpoint, what is a pulsar?

A) a star that slowly changes its brightness, getting dimmer and then brighter with a period of anywhere from a few hours to a few weeks

B) an object that emits flashes of light several times per second or more, with near perfect regularity

C) an object that emits random "pulses" of light that sometimes occur only a fraction of a second apart and other times stop for several days at a time

D) a star that changes color rapidly, from blue to red and back again

E) a star that rapidly changes size as it moves off the main sequence

Answer: B

20) From a theoretical standpoint, what is a pulsar?

A) a star that alternately expands and contracts in size

B) a rapidly rotating neutron star

C) a neutron star or black hole that happens to be in a binary system

D) a binary system that happens to be aligned so that one star periodically eclipses the other

E) a star that is burning iron in its core

Answer: B

21) What causes the radio pulses of a pulsar?

A) The star vibrates.

B) As the star spins, beams of radio radiation sweep through space. If one of the beams crosses Earth, we observe a pulse.

C) The star undergoes periodic explosions of nuclear fusion that generate radio emission.

D) The star's orbiting companion periodically eclipses the radio waves emitted by the main pulsar.

E) A black hole near the star absorbs energy and re-emits it as radio waves.

Answer: B

22) How do we know that pulsars are neutron stars?

A) We have observed massive-star supernovae produce pulsars.

B) Pulsars and neutron stars look exactly the same.

C) No massive object, other than a neutron star, could spin as fast as we observe pulsars spin.

D) Pulsars have the same upper mass limit as neutron stars do.

E) none of the above

Answer: C

23) What is the ultimate fate of an isolated pulsar?

A) It will spin ever faster, becoming a millisecond pulsar.

B) As gravity overwhelms the neutron degeneracy pressure, it will explode as a supernova.

C) As gravity overwhelms the neutron degeneracy pressure, it will become a white dwarf.

D) It will slow down, the magnetic field will weaken, and it will become invisible.

E) The neutron degeneracy pressure will eventually overwhelm gravity and the pulsar will slowly evaporate.

Answer: D

24) What is the basic definition of a black hole?

A) any compact mass that emits no light

B) a dead star that has faded from view

C) any object from which the escape velocity exceeds the speed of light

D) any object made from dark matter

E) a dead galactic nucleus that can only be viewed in infrared

Answer: C

25) How does the gravity of an object affect light?

A) Light doesn't have mass; therefore, it is not affected by gravity.

B) Light coming from a compact massive object, such as a neutron star, will be redshifted.

C) Light coming from a compact massive object, such as a neutron star, will be blueshifted.

D) Visible light coming from a compact massive object, such as a neutron star, will be redshifted, but higher frequencies such as X-rays and gamma rays will not be affected.

E) Less energetic light will not be able to escape from a compact massive object, such as a neutron star, but more energetic light will be able to.

Answer: B

26) How does a black hole form from a massive star?

A) During a supernova, if a star is massive enough for its gravity to overcome neutron degeneracy of the core, the core will be compressed until it becomes a black hole.

B) Any star that is more massive than 8 solar masses will undergo a supernova explosion and leave behind a black-hole remnant.

C) If enough mass is accreted by a white-dwarf star so that it exceeds the 1.4-solar-mass limit, it will undergo a supernova explosion and leave behind a black-hole remnant.

D) If enough mass is accreted by a neutron star, it will undergo a supernova explosion and leave behind a black-hole remnant.

E) A black hole forms when two massive main-sequence stars collide.

Answer: A

27) Which of the following statements about black holes is not true?

A) If you watch someone else fall into a black hole, you will never see him or her cross the event horizon. However, he or she will fade from view as the light he or she emits (or reflects) becomes more and more redshifted.

B) If we watch a clock fall toward a black hole, we will see it tick slower and slower as it falls nearer to the black hole.

C) A black hole is truly a hole in spacetime, through which we could leave the observable universe.

D) If the Sun magically disappeared and was replaced by a black hole of the same mass, Earth would soon be sucked into the black hole.

E) If you fell into a black hole, you would experience time to be running normally as you plunged rapidly across the event horizon.

Answer: D

28) In some cases, a supernova in a binary system may lead to the eventual formation of an accretion disk around the remains of the star that exploded. All of the following statements about such accretion disks are true except

A) X-rays are emitted by the hot gas in the accretion disk.

B) the accretion disk consists of material that spills off the companion star.

C) the central object about which the accretion disk swirls may be either a neutron star or a black hole.

D) several examples of flattened accretion disks being "fed" by a large companion star can be seen clearly in photos from the Hubble Space Telescope.

E) the radiation from an accretion disk may vary rapidly in time.

Answer: D

29) When we see X-rays from an accretion disk in a binary system, we can't immediately tell whether the accretion disk surrounds a neutron star or a black hole. Suppose we then observe each of the following phenomena in this system. Which one would force us to immediately rule out the possibility of a black hole?

A) bright X-ray emission that varies on a time scale of a few hours

B) spectral lines from the companion star that alternately shift to shorter and longer wavelengths

C) sudden, intense X-ray bursts

D) visible and ultraviolet light from the companion star

Answer: C

30) What is the Schwarzschild radius of a 100 million-solar-mass black hole? The mass of the Sun is about 2 × 1030 kg, and the formula for the Schwarzschild radius of a black hole of mass M is:

Rs = (G = 6.67 × 10-11 ; c = 3 × 108 m/s)

A) 3 km

B) 30 km

C) 3,000 km

D) 300 million km

E) 3 million km

Answer: D

31) A 10-solar-mass main-sequence star will produce which of the following remnants?

A) white dwarf

B) neutron star

C) black hole

D) none of the above

Answer: B

32) What do we mean by the singularity of a black hole?

A) There are no binary black holes—each one is isolated.

B) An object can become a black hole only once, and a black hole cannot evolve into anything else.

C) It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.

D) It is the edge of the black hole, where one could leave the observable universe.

E) It is the "point of no return" of the black hole; anything closer than this point will not be able to escape the gravitational force of the black hole.

Answer: C

33) How do we know what happens at the event horizon of a black hole?

A) Physicists have created miniature black holes in the lab.

B) Astronomers have sent spacecraft through the event horizon of a nearby black hole.

C) Astronomers have analyzed the light from matter within the event horizon of many black holes.

D) Astronomers have detected X-rays from accretion disks around black holes.

E) We don't know for sure: we only know what to expect based on the predictions of general relativity.

Answer: E

34) Prior to the 1990s, most astronomers assumed that gamma-ray bursts came from neutron stars with accretion disks. How do we now know that this hypothesis was wrong?

A) We now know that gamma-ray bursts come not from neutron stars but from black holes.

B) Theoretical work has proven that gamma rays cannot be produced in accretion disks.

C) Observations from the Compton Gamma-Ray Observatory show that gamma-ray bursts come randomly from all directions in the sky.

D) Observations from the Compton Gamma-Ray Observatory show that gamma-ray bursts occur too frequently to be attributed to neutron stars.

E) Observations from the Compton Gamma-Ray Observatory have allowed us to trace gamma-ray bursts to pulsating variable stars in distant galaxies.

Answer: C

35) Why do astronomers consider gamma-ray bursts to be one of the greatest mysteries in astronomy?

A) because they are so rare

B) because we know they come from pulsating variable stars but don't know how they are created

C) because the current evidence suggests that they are the most powerful bursts of energy that ever occur anywhere in the universe, but we don't know how they are produced

D) because current evidence suggests that they come from our own Milky Way, but we have no idea where in the Milky Way they occur

E) because current evidence suggests that they come from massive black holes in the centers of distant galaxies, adding to the mystery of black holes themselves