Ch 18 Reading/Concept
12) How is an X-ray burst (in an X-ray binary system) similar to a nova? A) Both involve explosions on the surface of stellar corpse. B) Both typically recur every few hours to every few days. C) Both are thought to involve fusion of hydrogen into helium. D) Both result in the complete destruction of their host stars.
A
13) Which statement concerning black hole masses and Schwarzschild radii is not true? A) In a binary system with a black hole, the Schwarzschild radius depends on the distance from the black hole to the companion star. B) The more massive the black hole, the larger the Schwarzschild radius. C) Even an object as small as you could become a black hole if there were some way to compress you to a size smaller than your Schwarzschild radius. D) For black holes produced in massive star supernovae, Schwarzschild radii are typically a few to a few tens of kilometers.
A
16) 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 rule out the possibility of a black hole? A) intense X-ray bursts B) spectral lines from the companion star that alternately shift to shorter and longer wavelengths C) visible and ultraviolet light from the companion star D) bright X-ray emission that varies on a time scale of a few hours
A
17) What do we mean by the singularity of a black hole? A) It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions. B) 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) It is the edge of the black hole, where one could leave the observable universe. D) The term is intended to emphasize the fact that an object can become a black hole only once, and a black hole cannot evolve into anything else.
A
18) What makes us think that the star system Cygnus X-1 contains a black hole? A) It emits X rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a neutron star. B) No light is emitted from this star system, so it must contain a black hole. C) The fact that we see strong X-ray emission tells us that the system must contain a black hole. D) Cygnus X-1 is a powerful X-ray burster, so it must contain a black hole.
A
19) Imagine an advanced civilization living on a planet orbiting at a distance of 10 AU (1.5 billion kilometers) from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole. The black hole is surrounded by an accretion disk. Sometime within the next million years or so, the civilization's planet is likely to be doomed because A) the red giant will probably supernova within the next million years. B) jets of material shot out of the accretion disk will shoot down their planet. C) the red giant star, which provides most of energy the civilization needs to exist, will soon be destroyed in the accretion disk. D) tidal forces from the black hole will rip the planet apart.
A
21) Which of the following statements about electron degeneracy pressure and neutron degeneracy pressure is true? A) Electron degeneracy pressure is the main source of pressure in white dwarfs, while neutron degeneracy pressure is the main source of pressure in neutron stars. B) Both electron degeneracy pressure and neutron degeneracy pressure help govern the internal structure of a main-sequence star. C) The life of a white dwarf is an ongoing battle between electron degeneracy pressure and neutron degeneracy pressure. D) In a black hole, the pressure coming from neutron degeneracy pressure is slightly greater than that coming from electron degeneracy pressure.
A
3) If you had something the size of a sugar cube that was made of white dwarf matter, it would weigh A) as much as a truck. B) about 5 pounds. C) as much as the entire Earth. D) as much as an average person.
A
3) Which of the following best describes why a white dwarf cannot have a mass greater than the 1.4-solar-mass limit? A) Electron degeneracy pressure depends on the speeds of electrons, which approach the speed of light as a white dwarf's mass approaches the 1.4-solar-mass limit. B) White dwarfs get hotter with increasing mass, and above the 1.4-solar-mass limit they would be so hot that even their electrons would melt. C) White dwarfs are made only from stars that have masses less than the 1.4-solar-mass limit. D) The upper limit to a white dwarf's mass is something we have learned from observations, but no one knows why this limit exists.
A
6) According to our modern understanding, what is a nova? A) an explosion on the surface of a white dwarf in a close binary system B) the explosion of a massive star at the end of its life C) the sudden formation of a new star in the sky D) a rapidly spinning neutron star
A
6) According to present understanding, a nova is caused by A) hydrogen fusion on the surface of a white dwarf. B) carbon fusion in the core of a white dwarf. C) hydrogen fusion on the surface of a neutron star. D) a white dwarf that gains enough mass to exceed the 1.4-solar-mass limit.
A
9) A typical neutron star is more massive than our Sun and about the size (radius) of A) a small asteroid (10 km in diameter). B) Earth. C) the Moon. D) Jupiter.
A
9) 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 home town? A) The entire 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 into Earth, throwing vast amounts of dust into the atmosphere that, in turn, would cool Earth; this is probably what caused the extinction of the dinosaurs.
A
11) Pulsars are thought to be A) accreting white dwarfs. B) rapidly rotating neutron stars. C) unstable high-mass stars. D) accreting black holes.
B
11) Which statement about pulsars is not thought to be true? A) All pulsars are neutron stars, but not all neutron stars are pulsars. B) Pulsars can form only in close binary systems. C) A pulsar must have a very strong magnetic field and rotate quite rapidly. D) Pulsars are kept from collapsing by neutron degeneracy pressure.
B
17) Which of the following observatories is most likely to discover a black hole in a binary system? A) the Hubble Space Telescope B) the Chandra X-Ray Observatory C) the SOFIA airborne infrared observatory D) the Arecibo Radio Observatory
B
2) A typical white dwarf is A) as large in diameter as the Sun but only about as massive as Earth. B) as massive as the Sun but only about as large in size as Earth. C) about the same size and mass as the Sun but much hotter. D) as massive as the Sun but only about as large in size as Jupiter.
B
2) The more massive a white dwarf, the A) higher its temperature. B) smaller its radius. C) larger its radius. D) higher its luminosity.
B
20) Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.) One foolhardy day, a daring individual in their space force (let's call him Major Tom) decides to become the first of his species 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 to start the process of killing him first)? A) tidal forces due to the black hole B) X rays from the accretion disk C) the crush of gravity at the singularity embedded within the black hole D) the sucking force from the black hole, which will cause his head to explode
B
20) Scientists have detected thousands of gamma ray bursts. The evidence suggests that most or all of these bursts A) have occurred in the central regions of the Milky Way. B) have occurred in distant galaxies. C) come from the same types of close binary systems that produce X-ray bursts. D) come from the Oort cloud surrounding the Sun.
B
5) What is an accretion disk? A) any flattened disk in space, such as the disk of the Milky Way Galaxy B) a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole C) a stream of gas flowing from one star to its binary companion star D) a disk of material found around every white dwarf in the Milky Way Galaxy
B
7) Which of the following is not true about differences between novae and supernovae? A) Novae are much less luminous than supernovae. B) Supernovae eject gas into space but novae do not. C) Novae occur only in binary star systems, while supernovae can occur both among single stars and among binary star systems. D) The same star can undergo novae explosions more than once, but can undergo only a single supernova.
B
1) A white dwarf is A) a precursor to a black hole. B) an early stage of a neutron star. C) what most stars become when they die. D) a brown dwarf that has exhausted its fuel for nuclear fusion.
C
12) How does an accretion disk around a neutron star differ from an accretion disk around a white dwarf? A) The accretion disk around a neutron star is made mostly of helium while the accretion disk around a white dwarf is made mostly of hydrogen. B) The accretion disk around a neutron star is more likely to give birth to planets. C) The accretion disk around a neutron star is much hotter and emits higher-energy radiation. D) The accretion disk around a neutron star always contains much more mass.
C
13) What is the basic definition of a black hole? A) a dead star that has faded from view B) any object made from dark matter C) an object with gravity so strong that not even light can escape D) a compact mass that emits no visible light
C
14) Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly A) 0.5 solar masses. B) 1.4 solar masses. C) 3 solar masses. D) 10 solar masses.
C
15) Which of statement below about black holes is not true? A) Although we are not 100% certain that black holes exist, we have strong observational evidence in favor of their existence. B) If you watch someone else fall into a black hole, you will never see him (or her) cross the event horizon; you'll only see him fade from view as the light he emits or reflects becomes more and more redshifted. C) A spaceship passing near a 10-solar-mass black hole is much more likely to be destroyed than a spaceship passing at the same distance from the center of a 10-solar-mass main-sequence star. D) If you fell into a black hole, you would experience time to be running normally as you plunged rapidly across the event horizon.
C
19) The Schwarzschild radius of a black hole depends on A) the observationally measured radius of the black hole. B) the way in which the black hole formed. C) only the mass of the black hole. D) both the mass and chemical composition of the black hole.
C
4) The white dwarf that remains when our Sun dies will be mostly made of A) hydrogen. B) helium. C) carbon. D) neutrons.
C
5) Which statement about accretion disks is not true? A) The gas in the inner parts of the disk travels faster than the gas in the outer parts of the disk. B) The gas in the inner parts of the disk is hotter than the gas in the outer parts of the disk. C) The primary factor determining whether a white dwarf has an accretion disk is the white dwarf's mass. D) Accretion disks are made primarily of hydrogen and helium gas.
C
7) Suppose that a white dwarf is gaining mass through accretion in a binary system. What happens if the mass someday reaches the 1.4 solar mass limit? A) The white dwarf will collapse in size, becoming a neutron star. B) The white dwarf will undergo a nova explosion. C) The white dwarf will explode completely as a white dwarf supernova. D) The white dwarf will collapse to become a black hole.
C
8) A neutron star is A) the remains of a star that died by expelling its outer layers in a planetary nebula. B) a star made mostly of elements with high atomic mass numbers, so that they have lots of neutrons. C) the remains of a star that died in a massive star supernova (if no black hole was created). D) an object that will ultimately become a black hole.
C
8) Will our Sun ever undergo a white dwarf supernova explosion? Why or why not? A) Yes, right at the end of its double-shell burning stage of life. B) Yes, about a million years after it becomes a white dwarf. C) No, because it is not orbited by another star. D) No, because the Sun's core will never be hot enough to fuse carbon and other heavier elements into iron.
C
1) Which of the following statements about degeneracy pressure is not true? A) Degeneracy pressure can continue to support an object against gravitational collapse even if the object becomes extremely cold. B) Degeneracy pressure arises from a quantum mechanical effect that we don't notice in our daily lives. C) Black holes form when gravity overcomes neutron degeneracy pressure. D) Degeneracy pressure can arise only from interactions among electrons.
D
10) Each Voyager spacecraft carries a "postcard" designed to be understandable to any aliens that might someday encounter it. On the "postcard," scientists pinpointed the location of Earth by triangulating it between pulsars. Why did the scientists choose pulsars rather than some other type of star? A) Pulsars are very bright and therefore easy to find. B) Several pulsars are located within a dozen light-years of our solar system, making them useful for finding our solar system. C) We're pretty sure that aliens will have only radio telescopes and not optical telescopes, so they'll have a better chance of seeing pulsars than ordinary stars. D) Pulsars are easy to identify by their almost perfectly steady periods of pulsation.
D
10) If you had something the size of a sugar cube that was made of neutron star matter, it would weigh A) about 50 pounds. B) as much as the entire Earth. C) about as much as a truck. D) about as much as a large mountain.
D
14) Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice? A) Time on the clock will run faster as it approaches the black hole, and light from the clock will be increasingly blueshifted. B) The clock will fall toward the black hole at a steady rate, so that you'll see it plunge through the event horizon within just a few minutes. C) The clock will fall faster and faster, reaching the speed of light as it crosses the event horizon. D) Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.
D
15) What do we mean by the event horizon of a black hole? A) It is the very center of the black hole. B) It is the distance from the black hole at which stable orbits are possible. C) It is the place where X rays are emitted from black holes. D) It is the point beyond which neither light nor anything else can escape.
D
16) Imagine that our Sun were magically and suddenly replaced by a black hole of the same mass (1 solar mass). What would happen to Earth in its orbit? A) Earth would almost instantly be sucked into oblivion in the black hole. B) Earth would orbit faster, but at the same distance. C) Earth would slowly spiral inward until it settled into an orbit about the size of Mercury's current orbit. D) Nothing—Earth's orbit would remain the same.
D
18) Which of the following statements about gamma ray bursts is not true? A) Gamma ray bursts are among the most luminous events that ever occur in the universe. B) The events responsible for gamma ray bursts apparently produce only gamma rays, and no other light that we can hope to detect. C) Gamma ray bursts were originally discovered by satellites designed to look for signs of nuclear bomb tests on Earth. D) Based on their distribution in the sky, we can rule out a connection between gamma ray bursts and X-ray binaries in the Milky Way Galaxy.
D
4) The maximum mass of a white dwarf is A) about the mass of our Sun. B) limitless; there is no theoretical limit to the maximum mass of a white dwarf. C) about 3 times the mass of our Sun. D) about 1.4 times the mass of our Sun.
D
21) Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.) Through a bizarre (and scientifically unexplainable) fluctuation in the space-time continuum, a copy of a book from that civilization arrives on your desk; it is entitled Iguoonos: How We Evolved. In the first chapter, you learn that these beings 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 to their evolutionary ancestors, they dislike television. B) Their immediate ancestors were chimpanzees. C) They believe that the presence of two stars in their system was critical to their evolution. D) 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. E) They evolved on a different planet in a different star system, and moved to their current location.
E
