Choose the correct description of what accretion disks are, and why do we find them only in close binary systems.
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An accretion disk is a disk of orbiting material that is falling toward a central body, like a white dwarf. We see these only in close binary systems because they require material to be transferred from one star to another.
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Choose the correct explanation of how does the accretion disk provide a white dwarf with a new source of energy that we can detect from Earth.
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As the material falls onto a white dwarf, gravitational energy is turned into heat. The heat provides the white dwarf with a new energy source, allowing it to glow in the ultraviolet.
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Choose the correct description, of what happens to the electron speeds in a more massive white dwarf.
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As the mass of a white dwarf increases, the pressure must increase to resist gravity. To do this, the electrons must move faster.
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Choose the correct description, how does this behavior lead to the white dwarf limit for mass.
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If mass of the white dwarf becomes so great that the electrons would have to move faster than light to resist the gravity, the white dwarf must collapse into a neutron star. This limit is about 1.4
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Which of the following statements about electron degeneracy pressure and neutron degeneracy pressure is true?
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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.
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Which of these neutron stars must have had its angular momentum changed by a binary companion?
answer
a pulsar that pulses 600 times per second
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Based on current evidence, which of the following statements about gamma ray bursts is true?
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All those that we have detected occurred in distant galaxies.
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Where do gamma-ray bursts tend to come from?
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extremely distant galaxies
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What would happen if the Sun suddenly became a black hole without changing its mass?
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Earth would remain in the same orbit.
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The Schwarzschild radius of a black hole depends on ________.
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only the mass of the black hole
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What do we mean by the event horizon of a black hole?
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It is the point beyond which neither light nor anything else can escape.
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Degeneracy pressure is the source of the pressure that stops the crush of gravity in all the following except
1
answer
a very massive main-sequence star.
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A teaspoonful of white dwarf material on Earth would weigh
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a few tons
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Which of the following is closest in mass to a white dwarf?
answer
the sun
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What is the ultimate fate of an isolated white dwarf?
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It will cool down and become a cold black dwarf.
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What kind of star is most likely to become a white-dwarf supernova?
answer
a white dwarf star with a red giant binary companion
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Observationally, how can we tell the difference between a white-dwarf supernova and a massive-star supernova?
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The spectrum of a massive-star supernova shows prominent hydrogen lines, while the spectrum of a white-dwarf supernova does not.
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A typical white dwarf is _________.
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as massive as the Sun but only about as large in size as Earth
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According to our modern understanding, what is a nova?
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an explosion on the surface of a white dwarf in a close binary system
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Choose the correct description of processes, which cause a white dwarf supernova.
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A white dwarf supernova occurs when the white dwarf gains enough mass for the carbon interior of the star to begin carbon fusion. The fusion begins almost instantly throughout the star, so the entire star ignites and the white dwarf explodes completely.
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Choose the correct way to distinguish white dwarf and massive star supernovae observationally.
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These supernovae, unlike the massive star supernovae, lack hydrogen lines in their spectra, allowing astronomers to tell the two types of supernovae apart.
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Which of the following is closest in size (radius) to a neutron star?
answer
a city
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From an observational standpoint, what is a pulsar?
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an object that emits flashes of light several times per second or more, with near perfect regularity
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What causes the radio pulses of a pulsar?
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As the star spins, beams of radio radiation sweep through space. If one of the beams crosses Earth, we observe a pulse.
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All pulsars are neutron stars, but not all neutron stars are pulsars.
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True
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A typical neutron star is more massive than our Sun and about the size (radius) of _________.
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a small asteroid (10 km in diameter)
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How does the gravity of an object affect light?
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Light coming from a compact massive object, such as a neutron star, will be redshifted.
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Which of the following statements about black holes is not true?
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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.
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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?
answer
sudden, intense X-ray bursts
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A 10-solar-mass main-sequence star will produce which of the following remnants?
answer
neutron star
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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.
Sometime within the next million years or so, their planet is likely to be doomed because
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the red giant will probably undergo a supernova explosion within the next million years.
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No visible light can escape a black hole, but things such as gamma rays, X rays, and neutrinos can.
answer
False
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Which of these objects has the largest radius?
a 1.2MSunwhite dwarf
a 1.5MSun neutron star
a 3.0MSun black hole
answer
a 1.2MSunwhite dwarf
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Why do astronomers consider gamma-ray bursts to be one of the greatest mysteries in astronomy?
answer
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
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Which of the following statements about degeneracy pressure is not true?
Degeneracy pressure arises from a quantum mechanical effect that we don't notice in our daily lives.
Degeneracy pressure can continue to support an object against gravitational collapse even if the object becomes extremely cold.
Degeneracy pressure can arise only from interactions among electrons.
Black holes form when gravity overcomes neutron degeneracy pressure.
answer
Degeneracy pressure can arise only from interactions among electrons.
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The more massive a white dwarf, the _________.
answer
smaller its radius
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Which of the following best describes why a white dwarf cannot have a mass greater than the 1.4-solar-mass limit?
answer
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.
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The white dwarf that remains when our Sun dies will be mostly made of ______.
answer
carbon
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Which statement about accretion disks is not true?
Accretion disks are made primarily of hydrogen and helium gas.
The primary factor determining whether a white dwarf has an accretion disk is the white dwarf's mass.
The gas in the inner parts of the disk is hotter than the gas in the outer parts of the disk.
The gas in the inner parts of the disk travels faster than the gas in the outer parts of the disk.
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The primary factor determining whether a white dwarf has an accretion disk is the white dwarf's mass.
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According to present understanding, a nova is caused by _________.
answer
hydrogen fusion on the surface of a white dwarf
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Differences between nova and supernovae
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Novae are much less luminous than supernovae.
The same star can undergo novae explosions more than once but can undergo only a single supernova.
Novae occur only in binary star systems, whereas supernovae can occur both among single stars and among binary star systems.
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Will our Sun ever undergo a white dwarf supernova explosion? Why or why not?
answer
No, because it is not orbited by another star.
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Which statement about pulsars is not thought to be true?
All pulsars are neutron stars, but not all neutron stars are pulsars.
A pulsar must have a strong magnetic field and rotate quite rapidly.
Pulsars are kept from collapsing by neutron degeneracy pressure.
Pulsars can form only in close binary systems.
answer
Pulsars can form only in close binary systems.
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How does an accretion disk around a neutron star differ from an accretion disk around a white dwarf?
answer
The accretion disk around a neutron star is much hotter and emits higher-energy radiation.
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Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice?
answer
Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.
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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?
visible and ultraviolet light from the companion star
spectral lines from the companion star that alternately shift to shorter and longer wavelengths
intense X-ray bursts
bright X-ray emission that varies on a time scale of a few hours
answer
intense X-ray bursts
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Which of the following observatories is most likely to discover a black hole in a binary system?
answer
the Chandra X-Ray Observatory
question
Which of the following statements about gamma ray bursts is not true?
The events responsible for gamma ray bursts apparently produce only gamma rays, and no other light that we can hope to detect.
Many gamma ray bursts are associated with supernovas in distant galaxies.
Gamma ray bursts are among the most luminous events that ever occur in the universe.
Gamma ray bursts were originally discovered by satellites designed to look for signs of nuclear bomb tests on Earth.
answer
The events responsible for gamma ray bursts apparently produce only gamma rays, and no other light that we can hope to detect.
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