18

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

: 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

: A

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.

: A

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.

: A

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.

: A

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.

: 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.

: A

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.

: B

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

: 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

: 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.

: B

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.

: B

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

: B

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

: B

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

: B

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.

: B

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

: B

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.

: C

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.

: C

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

: C

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

: C

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

: C

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.

: C

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.

: 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

: C

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.

: C

An advanced civilization lives on a planet orbiting a close binary star system that consists of a 15MSun red giant and a 10MSun black hole. Assume that the two stars are quite close together, so that an accretion disk surrounds the black hole. The planet on which the civilization lives orbits the binary star at a distance of 10 AU. 36) Sometime within the next million years or so, their planet is likely to be doomed because A) jets of material shot out of the accretion disk will shoot down their planet. B) their planet receives most of its energy from the red giant. However, this star will soon be completely devoured in the accretion disk and thus will no longer exist. C) the red giant will probably undergo a supernova explosion within the next million years. D) tidal forces from the black hole will rip the planet apart. E) the planet's orbit gradually will decay as it is sucked in by the black hole.

: C

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.

: D

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

: D

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

: D

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.

: D

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.

: 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.

: D

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.

: D

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

: D

37) One foolhardy day, a daring major (let's call him Tom) in the space force decides to become the first of his race to cross the event horizon of the black hole. To add to the drama, he decides to go in wearing only a thin space suit, which offers no shielding against radiation, no cushioning against any forces, and so on. Which of the following is most likely to kill him first (or at least cause significant damage)? (Hint: The key word here is first. Be sure to consider the distances from the black hole at which each of the noted effects is likely to become damaging.) A) the crush of gravity at the singularity embedded within the black hole B) the tidal forces due to the black hole C) the strong acceleration as he descends towards the black hole D) the X rays from the accretion disk E) the sucking force from the black hole, which will cause his head to explode

: D

38) Through a bizarre (and scientifically unexplainable) fluctuation in the spacetime continuum, a copy of a book titled Iguoonos: How We Evolved appears on your desk. As you begin to read, you learn that the book describes the evolution of the people living in the star system described above. In the first chapter, you learn that these people evolved from organisms that lived 5 billion years ago. Which of the following statements should you expect to find as you continue to read this book? A) As a result of traumatic experiences of their evolutionary ancestors, they dislike television. B) Their immediate ancestors were chimpanzees. C) They found that the presence of two stars in their system was critical to their evolution. D) They evolved on a different planet in a different star system and moved to their current location. E) They evolved from primitive wormlike creatures that had 13 legs, 4 eyes, and bald heads, thus explaining why such critters are now considered a spectacular delicacy.

: D

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

: E

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.

: E

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

: E