Astronomy Chapter 6

What do astronomers mean when they say that we are all “star stuff”?that the carbon, oxygen, and many elements essential to life were created by nucleosynthesis in stellar cores
Which two energy sources can help a star maintain its internal thermal pressure?nuclear fusion and gravitational contraction
What type of star our Sun?low-mass star
What is the range of star masses for high-mass stars?between 8 and 100 solar masses
What can we learn about a star from a life track on an H-R diagram?what surface temperature and luminosity it will have at each stage of its life
Which of the following statements about degeneracy pressure is not true?Degeneracy pressure varies with the temperature of the star
All of the following are involved in carrying energy outward from a star’s core exceptconduction
Which stars have convective cores?high-mass stars
Which of the following properties make flare stars so active?both A and B
When happens when a star exhausts its core hydrogen supply?Its core contracts, but its outer layers expand and the star becomes bigger and brighter
What is happening inside a star while it expands into a subgiant?It is fusing hydrogen into helium in a shell outside the core
Which of the following spectral types is more likely to be a flare star?MV
Compared to the star it evolved from, a red giant iscooler and brighter
At approximately what temperature can helium fusion occur?100 million K
Why does a star grow larger after it exhausts its core hydrogen?Hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward
How many helium nuclei fuse together when making carbon?3
The helium fusion process results in the productin ofcarbon
What happens after a helium flash?the core quickly heats up and expands
What is a carbon star?a red giant star whose atmosphere becomes carbon-rich through convection from the core
What is a planetary nebula?the expanding shell of gas that is no longer gravitationally held to the remnant of a low-mass star
What happens to the core of a star after a planetary nebula occurs?It becomes a white dwarf
Which of the following sequences correctly describes the stages of life for a low-mass star?protostar, main-sequence, red giant, white dwarf
Compared to the star it evolved from, a white dwarf ishotter and dimmer
Most interstellar dust grains are produced inthe atmospheres of red giant stars
What is the CNO cycle?a type of hydrogen fusion that uses carbon, nitrogen, and oxygen atoms as catalysts
Which element has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission?iron
What happens when the gravity of a massive star is able to overcome neutron degeneracy pressure?the core contracts and becomes a black hole
What types of stars end their lives with supernovae?stars that are at least several times the mass of the Sun
Which of the following statements about stages of nuclear burning in a massive star is not true?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
Which event marks the beginning of a supernova?the sudden collapse of an iron core into a compact ball of neutrons
After a supernova event, what is left behind?either a neutron star or a blackhole
Why is Supernova 1987A particularly important to astronomers?It was the nearest supernova detected in nearly 400 years
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?The two stars should be the same age, so the more massive one should have become a giant first
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?The giant must once have been the more massive star but transferred some of its mass to its companion
Why do scientists think that our solar system must have formed sometime after nearby supernovae explosions?Existence of heavy elements
Degeneracy pressure is the source of the pressure that stops the crush of gravity in all the following excepta very massive main-sequence star
White dwarfs are so called becausethey are both very hot and very small
A teaspoonful of white dwarf material on Earth would weigha few tons
Which of the following is closest in mass to a white dwarf?the sun
Why is there an upper limit to the mass of a white dwarf?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
What is the ultimate fate of an isolated white dwarf?It will cool down and become a cold black dwarf
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?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
Which of the following statements about novae is not true?Our Sun will probably undergo at least one nova when it becomes a white dwarf about 5 billion years from now
What kind of pressure supports a white dwarf?electron degeneracy pressure
What is the upper limit to the mass of a white dwarf?1.4 solar masses
How does a 1.2 solar-mass white dwarf compare to a 1.0 solar-mass white dwarf?It has a smaller radius
Which of the following is closest in size (radius) to a white dwarf?Earth
What kind of star is most likely to become a white-dwarf supernova?a white dwarf star with a red giant binary companion
Observationally, how can we tell the difference between a white-dwarf supernova and a massive star supernova?The spectrum of a massive-star supernova shows prominent hydrogen lines, white the spectrum of a white-dwarf supernova does not
After a massive-star supernova, what is left behind?either a neutron star or a black hole
What is the upper limit to the mass of a neutron star?There is an upper limit less than 3 solar masses, but we do not yet know precisely what it is
A teaspoonful of neutron star material on Earth would weighmore than Mt. Everest
Which of the following is closest in size (radius) to a neutron star?a city
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?The entire mass of Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star
From an observational standpoint, what is a pulsar?An object that emits flashes of light several times per second or more, with near perfect regularity
From a theoretical standpoint, what is a pulsar?a rapidly rotating neutron star
What causes the radio of a pulsar?As the star spins, beams of radio radiation sweep through space. If one of the beams crosses Earth, we observe a pulse
How do we know that the pulsars are neutron stars?No massive object, other than a neutron star, could spin as fast as we observe pulsars spin
What is the ultimate fate of an isolated pulsar?It will slow down, the magnetic field will weaken, and it will become invisible
What is the basic definition of a black hole?any object from which the escape velocity exceeds the speed of light
How does the gravity of an object affect light?Light coming from a compact massive object, such as a neutron star, will be redshifted
How does a blackhole form from a massive star?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
Which of the following statements about black holes is not true?If the sun magically disappeared and was replaced by a blackhole of the same mass, Earth would soon be sucked into the black hole
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:several examples of flattened accretion disks being “fed” by a large companion star can be seen clearly in photos from the Hubble Space Telescope
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?sudden, intense X-ray bursts
What is the Schwarzschild radius of a 100 million-solar-mass black hole?300 million km
A 10 solar mass main sequence will produce which of the following remnants?neutron star
What do we mean by the singularity of a black hole?It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions
How do we know what happens at the event horizon of a black hole?We don’t know for sure: we only know what to expect based on the predictions of general relativity
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?Observations from the Compton Gamma-Ray Observatory show that gamma-ray bursts come randomly from all directions in the sky
Why do astronomers consider gamma-ray bursts to be one of the greatest mysteries in astronomy?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