Astronomy Chapter 6 Answers – Flashcards
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that the carbon, oxygen, and many elements essential to life were created by nucleosynthesis in stellar cores
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What do astronomers mean when they say that we are all "star stuff"?
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nuclear fusion and gravitational contraction
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Which two energy sources can help a star maintain its internal thermal pressure?
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low-mass star
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What type of star our Sun?
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between 8 and 100 solar masses
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What is the range of star masses for high-mass stars?
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what surface temperature and luminosity it will have at each stage of its life
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What can we learn about a star from a life track on an H-R diagram?
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Degeneracy pressure varies with the temperature of the star
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Which of the following statements about degeneracy pressure is not true?
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conduction
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All of the following are involved in carrying energy outward from a star's core except
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high-mass stars
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Which stars have convective cores?
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both A and B
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Which of the following properties make flare stars so active?
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Its core contracts, but its outer layers expand and the star becomes bigger and brighter
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When happens when a star exhausts its core hydrogen supply?
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It is fusing hydrogen into helium in a shell outside the core
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What is happening inside a star while it expands into a subgiant?
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MV
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Which of the following spectral types is more likely to be a flare star?
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cooler and brighter
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Compared to the star it evolved from, a red giant is
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100 million K
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At approximately what temperature can helium fusion occur?
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Hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward
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Why does a star grow larger after it exhausts its core hydrogen?
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3
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How many helium nuclei fuse together when making carbon?
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carbon
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The helium fusion process results in the productin of
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the core quickly heats up and expands
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What happens after a helium flash?
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a red giant star whose atmosphere becomes carbon-rich through convection from the core
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What is a carbon star?
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the expanding shell of gas that is no longer gravitationally held to the remnant of a low-mass star
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What is a planetary nebula?
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It becomes a white dwarf
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What happens to the core of a star after a planetary nebula occurs?
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protostar, main-sequence, red giant, white dwarf
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Which of the following sequences correctly describes the stages of life for a low-mass star?
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hotter and dimmer
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Compared to the star it evolved from, a white dwarf is
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the atmospheres of red giant stars
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Most interstellar dust grains are produced in
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a type of hydrogen fusion that uses carbon, nitrogen, and oxygen atoms as catalysts
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What is the CNO cycle?
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iron
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Which element has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission?
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the core contracts and becomes a black hole
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What happens when the gravity of a massive star is able to overcome neutron degeneracy pressure?
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stars that are at least several times the mass of the Sun
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What types of stars end their lives with supernovae?
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Betelgeuse would remain a dot of light but would suddenly become so bright that, for a few weeks, we'd be able to see this dot in the daytime
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Which of the following statements about stages of nuclear burning in a massive star is not true?
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the sudden collapse of an iron core into a compact ball of neutrons
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Which event marks the beginning of a supernova?
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either a neutron star or a blackhole
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After a supernova event, what is left behind?
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It was the nearest supernova detected in nearly 400 years
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Why is Supernova 1987A particularly important to astronomers?
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The two stars should be the same age, so the more massive one should have become a giant first
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You discover a binary star system in which one member is 15M Sun main-sequence star and the other star is a 10M Sun giant. Why would you be surprised, at least at first?
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The giant must once have been the more massive star but transferred some of its mass to its companion
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You discover a binary star system in which one member is a 15M Sun main-sequence star and the other star is a 10M Sun giant. How do we believe that a star system such as this might have come to exist?
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Existence of heavy elements
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Why do scientists think that our solar system must have formed sometime after nearby supernovae explosions?
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a very massive main-sequence star
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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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they are both very hot and very small
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White dwarfs are so called because
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a few tons
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A teaspoonful of white dwarf material on Earth would weigh
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the sun
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Which of the following is closest in mass to 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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Why is there an upper limit to the mass of a white dwarf?
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It will cool down and become a cold black dwarf
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What is the ultimate fate of an isolated white dwarf?
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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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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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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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Which of the following statements about novae is not true?
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electron degeneracy pressure
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What kind of pressure supports a white dwarf?
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1.4 solar masses
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What is the upper limit to the mass of a white dwarf?
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It has a smaller radius
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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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Earth
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Which of the following is closest in size (radius) to a white dwarf?
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a white dwarf star with a red giant binary companion
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What kind of star is most likely to become a white-dwarf supernova?
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The spectrum of a massive-star supernova shows prominent hydrogen lines, white the spectrum of a white-dwarf supernova does not
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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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either a neutron star or a black hole
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After a massive-star supernova, what is left behind?
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There is an upper limit less than 3 solar masses, but we do not yet know precisely what it is
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What is the upper limit to the mass of a neutron star?
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more than Mt. Everest
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A teaspoonful of neutron star material on Earth would weigh
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a city
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Which of the following is closest in size (radius) to a neutron star?
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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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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?
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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 an observational standpoint, what is a pulsar?
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a rapidly rotating neutron star
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From a theoretical standpoint, what is 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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What causes the radio of a pulsar?
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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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How do we know that the pulsars are neutron stars?
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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 ultimate fate of an isolated pulsar?
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any object from which the escape velocity exceeds the speed of light
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What is the basic definition of a black hole?
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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 the gravity of an object affect light?
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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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How does a blackhole form from a massive star?
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If the sun magically disappeared and was replaced by a blackhole of the same mass, Earth would soon be sucked into the black hole
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Which of the following statements about black holes is not true?
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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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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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sudden, intense X-ray bursts
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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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300 million km
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What is the Schwarzschild radius of a 100 million-solar-mass black hole?
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neutron star
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A 10 solar mass main sequence will produce which of the following remnants?
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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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What do we mean by the singularity 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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How do we know what happens at the event horizon of a black hole?
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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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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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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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Why do astronomers consider gamma-ray bursts to be one of the greatest mysteries in astronomy?
