Chapter 14

1. Degeneracy pressure 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.

2. A white dwarf is A) the exposed core of a dead star, supported by electron degeneracy pressure. B) the exposed core of a dead star, supported by neutron degeneracy pressure. C) a hot but very small main sequence star with a mass of less than 1.4 solar masses. D) a cool and very small main sequence star with a mass of less than 1.4 of a solar masses. E) the name for the singularity at the center of a black hole.

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

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

5. Why is there an upper limit to the mass of a white dwarf? A) White dwarfs come only from stars with masses less 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, and no more mass can be supported. C) The more massive the white dwarf, the higher its temperature and hence the greater its degeneracy pressure. Near 1.4 solar masses, the temperature becomes so high that all matter effectively melts into subatomic particles. D) The upper limit to the masses of white dwarfs was determined through observations of white dwarfs in binary systems, but no one knows why the limit exists.

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 slowly overwhelms gravity and the white dwarf evaporates.

7. Suppose a white dwarf is gaining mass because of accretion from a binary companion. What happens if its mass reaches the 1.4 solar mass limit? A) The white dwarf undergoes a collapse and expels the excess mass in a nova eruption. B) The white dwarf (which is made mostly of carbon) suddenly detonates carbon fusion and this creates a white dwarf supernova explosion. 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 accreting material from reaching it in the first place.

8. Which of the following hypothetical observations would contradict our theories about the formation and evolution of white dwarfs? A) discovery of a white dwarf with a mass 1.5 times that of the Sun (1.5 Msun) B) discovery of a white dwarf with a surface temperature of 6000 K C) discovery of a white dwarf at the center of a planetary nebula D) discovery of a white dwarf with a 1.5 Msun mass main-sequence companion

9. In which wavelength region(s) would we need to carry out observations in order to study the accretion disk around a white dwarf in a binary system? A) a. visible light B) b. ultraviolet light C) c. X-ray light D) a and b E) b and c

10. 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.

11. 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 these answers

12. 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

13. Imagine comparing a 1.2 solar mass white dwarf to a 1.0 solar mass white dwarf. Which of the following must be true? A) The 1.2 solar mass white dwarf has a larger radius. B) The 1.2 solar mass white dwarf has a smaller radius. C) The 1.2 solar mass white dwarf has a higher surface temperature. D) The 1.2 solar mass white dwarf has a lower surface temperature. E) The 1.2 solar mass white dwarf is supported by neutron degeneracy pressure; the 1 solar mass white dwarf is supported by electron degeneracy pressure.

14. Which of the following is closest in size (radius) to a white dwarf? A) the Earth B) a small city C) a football stadium D) a basketball E) the Sun

15. 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

16. 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 continues to brighten for many weeks. E) We cannot yet tell the difference between a massive-star supernova and a white-dwarf supernova.

17. 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

18. A paperclip with the density of a neutron star would weigh (on the Earth) A) about the same as a regular paperclip. B) a few tons. C) more than Mt. Everest. D) more than the Moon. E) more than the Earth.

19. Which of the following is closest in size (radius) to a neutron star? A) the Earth B) a city C) a football stadium D) a basketball E) the Sun

20. 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 to appear in your hometown? A) The entire mass of the 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 the Earth. C) The combined mass of the Earth and the neutron star would cause the neutron star to collapse into a black hole. D) It would crash through the Earth, creating a large crater, and exit the Earth on the other side. E) It would crash into the Earth, throwing vast amounts of dust into the atmosphere which in turn would cool the Earth. Such a scenario is probably what caused the extinction of the dinosaurs.

21. 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 even faster), with near perfect regularity C) an object that emits random "pulses" of light, sometimes with only a fraction of a second between pulses and other times with several days between pulses D) a star that changes color rapidly, from blue to red and back again

22. 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

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

24. How do we know that pulsars must be neutron stars? A) We have observed massive-star supernovae produce pulsars. B) Telescopic images of 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 to spin and remain intact. D) Pulsars have the same upper mass limit as neutron stars do. E) This is only a theory that has not yet been confirmed by observations.

25. 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 spin ever slower, 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.

26. What causes X-ray bursters? A) Helium fusion, which occurs when the thin layer of accreted material on a neutron star reaches 100 million K B) The mass of an accreting neutron star passes a critical threshold between 2 and 3 Msun and explodes. C) The binary companion of a neutron star spirals in and combines with the neutron star. D) The rapid rotation of the neutron star causes it to fly apart.

27. Which of the following correctly describes how light will be affected as it tries to escape from a massive object? A) Light doesn't have mass; therefore, it is not affected by gravity. B) Light escaping from a compact massive object, such as a neutron star, will be redshifted. C) Light escaping from a compact massive object, such as a neutron star, will be blueshifted. D) Visible light escaping 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.

28. 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 pressure in the core, the core will collapse to 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 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.

29. 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 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 towards the black hole. C) The event horizon of a black hole represents a boundary from which nothing can escape. D) If the Sun magically disappeared and was replaced by a black hole of the same mass, the Earth would soon be sucked into the black hole. E) If you fell into a supermassive black hole (so that you could survive the tidal forces), you would experience time to be running normally as you plunged across the event horizon.

30. Consider an X-ray binary system in which a compact object, surrounded by an accretion disk, is in a binary orbit with another star. 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 compact object 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.

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 these answers

32. 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.

33. Prior to 1991, most astronomers assumed that gamma-ray bursts came from neutron stars (with accretion disks) within the Milky Way Galaxy. 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 showed that gamma-ray bursts come randomly from all directions in the sky. D) Observations from the Compton Gamma-Ray Observatory showed that gamma-ray bursts occur too frequently to be attributed to neutron stars. E) Observations from the Compton Gamma-Ray Observatory allowed us to trace gamma-ray bursts to pulsating variable stars in distant galaxies.

34. What is the origin of short gamma ray bursts? A) new stars forming in the Milky Way B) supernovae in the Milky Way C) very powerful supernovae occurring in distant galaxies D) the collision of stars in the dense nuclei of distant galaxies E) It is not known, but it may be the collision of a neutron star with a black hole.

35. What evidence suggests that long gamma ray bursts originate from supernovae of stars massive enough to form black holes? A) a. Rapid observations in other wavelengths show that some gamma ray bursts coincide with points showing typical supernova light curves. B) b. Some gamma ray bursts have been found to originate with galaxies that are actively forming stars, and would thus have a few very massive (but short lived) stars. C) c. The locations of gamma ray bursts suddenly begin blocking light from more distant stars. D) a and b E) b and c

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