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Astronomy Chapter 18 Test Questions

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Degeneracy pressure is the source of the pressure that stops the crush of gravity in all the following except
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a very massive main-sequence star.
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White dwarfs are so called because
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they are both very hot and very small.
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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?
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the Sun
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Why is there an upper limit to the mass of a white dwarf?
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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.
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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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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?
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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.
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Which of the following statements about novae is not true?
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Our Sun will probably undergo at least one nova when it becomes a white dwarf about 5 billion years from now.
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What kind of pressure supports a white dwarf?
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electron degeneracy pressure
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What is the upper limit to the mass of a white dwarf?
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1.4 solar masses
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How does a 1.2-solar-mass white dwarf compare to a 1.0-solar-mass white dwarf?
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It has a smaller radius.
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Which of the following is closest in size (radius) to a white dwarf?
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Earth
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What kind of star is most likely to become a white-dwarf supernova?
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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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After a massive-star supernova, what is left behind?
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either a neutron star or a black hole
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A teaspoonful of neutron star material on Earth would weigh
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more than Mt. Everest
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Which of the following is closest in size (radius) to a neutron star?
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a city
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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?
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The entire mass of Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star.
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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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From a theoretical standpoint, what is a pulsar?
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a rapidly rotating neutron star
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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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How do we know that pulsars are neutron stars?
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No massive object, other than a neutron star, could spin as fast as we observe pulsars spin.
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What is the ultimate fate of an isolated pulsar?
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It will slow down, the magnetic field will weaken, and it will become invisible.
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What is the basic definition of a black hole?
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any object from which the escape velocity exceeds the speed of light
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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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How does a black hole form from a massive star?
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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.
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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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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
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several examples of flattened accretion disks being “fed” by a large companion star can be seen clearly in photos from the Hubble Space Telescope.
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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?
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sudden, intense X-ray bursts
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What is the Schwarzschild radius of a 100 million-solar-mass black hole? The mass of theSun 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)
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300 million km
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A 10-solar-mass main-sequence star will produce which of the following remnants?
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neutron star
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What do we mean by the singularity of a black hole?
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It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.
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How do we know what happens at the event horizon of a black hole?
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We don’t know for sure: we only know what to expect based on the predictions of general relativity.
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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?
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Observations from the Compton Gamma-Ray Observatory show that gamma-ray bursts come randomly from all directions in the sky.
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Why do astronomers consider gamma-ray bursts to be one of the greatest mysteries in astronomy?
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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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An advanced civilization lives on a planet orbiting a close binary star system that consists of a 15 M Sun red giant and a 10 M Sun 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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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. 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.)
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the X rays from the accretion disk
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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?
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They evolved on a different planet in a different star system and moved to their current location.
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If you were to come back to our Solar System in 6 billion years, what might you expect to find?
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a white dwarf
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Black holes, by definition, cannot be observed directly. What observational evidence do scientists have of their existence?
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Gravitational interaction with other objects.