ASTR 113: Module 18

lesson 3: 11. 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) Both involve explosions on the surface of stellar corpse.

lesson 4: 2. How does a black hole form from a massive star? a) 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. b) Any star that is more massive than 8 solar masses will undergo a supernova explosion and leave behind a black-hole remnant. c) If enough mass is accreted by a white-dwarf star so that it exceeds the 1.4-solar-mass limit, it will undergo a supernova explosion and leave behind a black-hole remnant. d) If enough mass is accreted by a neutron star, it will undergo a supernova explosion and leave behind a black-hole remnant. e) A black hole forms when two massive main-sequence stars collide.

a) 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.

lesson 1: 10. 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) 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.

lesson 3: 4. 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) 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.

lesson 4: 10. 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) In a binary system with a black hole, the Schwarzschild radius depends on the distance from the black hole to the companion star.

lesson 4: 12. 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) It emits X rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a neutron star.

lesson 1: 8. What is the ultimate fate of an isolated white dwarf? a) It will cool down and become a cold black dwarf. b) As gravity overwhelms the electron degeneracy pressure, it will explode as a nova. c) As gravity overwhelms the electron degeneracy pressure, it will explode as a supernova. d) As gravity overwhelms the electron degeneracy pressure, it will become a neutron star. e) The electron degeneracy pressure will eventually overwhelm gravity and the white dwarf will slowly evaporate.

a) It will cool down and become a cold black dwarf.

lesson 2: 8. Suppose a white dwarf is gaining mass because of accretion in a binary system. What happens if the mass someday reaches the 1.38 MSun? a) The white dwarf undergoes a catastrophic "carbon bomb" explosion, and blows itself apart in a Type Ia supernova. b) The white dwarf, which is made mostly of carbon, suddenly becomes much hotter in temperature and therefore is able to begin fusing the carbon. This turns the white dwarf back into a star supported against gravity by ordinary pressure. c) The white dwarf immediately collapses into a black hole, disappearing from view. d) A white dwarf can never gain enough mass to reach the limit because a strong stellar wind prevents the material from reaching it in the first place.

a) The white dwarf undergoes a catastrophic "carbon bomb" explosion, and blows itself apart in a Type Ia supernova.

lesson 2: 3. According to present understanding, a nova is caused by a) an explosive hydrogen fusion on the surface of a white dwarf in a close binary system. b) an explosive carbon fusion in the core of a white dwarf. c) an explosive hydrogen fusion on the surface of a neutron star. d) a white dwarf that gaining enough mass to exceed the Chandrasekhar limit. e) an explosion of a massive star at the end of its life f) a sudden formation of a new star in the sky

a) an explosive hydrogen fusion on the surface of a white dwarf in a close binary system.

lesson 2: 5. Fill in the blanks in the statements below: Nova occurs when a white dwarf in a close [a] system accretes mass from its companion star. This mass is composed mostly of [b] since it comes from the companion's outer layers. An [c] disk forms, in which matter gradually spirals onto the white dwarf's surface as it loses [d] momentum and [e] energy. A lot of [f] energy gets converted into heat as the gas is compressed into a dense, thin surface layer atop the white dwarf. Eventually this thin surface layer composed of [g] becomes hot enough to abruptly start explosive fusion - a [h].

a) binary b) hydrogen c) accretion d) angular e) orbital f) orbital g) hydrogen h) nova

lesson 5: 4. Fill the blanks in the statements below: Gamma ray bursts are associated with extremely [a] cosmic explosions. Gamma ray bursts were first discovered by satellites deployed to monitor compliance with the [b] Test Ban Treaty of 1963. Up to the best of our current knowledge the best candidates for progenitors of the short gamma ray bursts are [c] of compact objects such as neutron stars and black holes. Up to the best of our current knowledge the best candidates for progenitors of the long gamma ray bursts are [d] collapse events.

a) energetic b) nuclear c) mergers d) core

lesson 2: 9. Fill in the blanks in the statements below: Classic scenario for a Type Ia supernova involves a [a] dwarf that gains mass from a bloated [b] giant companion and ultimately reaches the temperatures sufficient for [c] to ignite. This happens when white dwarf reaches the mass of about 1.38 MSun.. Based on this scenario, it has been assumed that since the mass is the same and the entire object-made of degenerate matter-explodes at once, the maximum [d] is the same for all Type Ia supernovae. Thus Type Ia supernovae have been used as standard candles to estimate cosmic [e]. New observational evidence supports that the classic scenario does in fact happen, but it also indicates that there is a more common scenario for a Type Ia supernova: a collision and [f] of two white dwarfs in a close binary. In the latter case the total mass can easily exceed the [g] limit and result in supernova of greater luminosity. Such supernova may easily be mistaken for a classic one occurring at a smaller [h]. New studies also suggest a great deal of variety in [i] of the progenitors of the Type Ia supernovae and, thus, potentially in their maximum [j], which creates a need to revisit the "standard" in this standard candle.

a) white b) red c) carbon d) luminosity e) distances f) merger g) Chandrasekhar/white dwarf h) distance i) masses/types j) luminosities

lesson 3: 7. What causes the radio pulses of a pulsar? a) The star vibrates. b) As the star spins, beams of radio radiation sweep through space. If one of the beams crosses Earth, we observe a pulse. c) The star undergoes periodic explosions of nuclear fusion that generate radio emission. d) The star's orbiting companion periodically eclipses the radio waves emitted by the main pulsar. e) A black hole near the star absorbs energy and re-emits it as radio waves.

b) As the star spins, beams of radio radiation sweep through space. If one of the beams crosses Earth, we observe a pulse.

lesson 4: 11. Black holes, by definition, cannot be observed directly. What observational evidence do scientists have of their existence? a) Theoretical models predict their existence. b) Gravitational interaction with other objects. c) Space is, overall, very black. d) We have sent spacecraft to nearby black holes. e) We have detected neutrinos from them.

b) Gravitational interaction with other objects.

lesson 3: 8. 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) Pulsars can form only in close binary systems.

lesson 2: 6. 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) Supernovae eject gas into space but novae do not.

lesson 5: 2. 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.

b) The events responsible for gamma ray bursts apparently produce only gamma rays, and no other light that we can hope to detect.

lesson 3: 3. A typical neutron star is more massive than our Sun and about the size of a) Earth b) a city c) a football stadium d) a basketball e) the Sun

b) a city

lesson 2: 1. 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) a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole

lesson 3: 6. From a theoretical standpoint, what is a pulsar? a) a unstable high-mass star that alternately expands and contracts in size b) a rapidly rotating neutron star or white dwarf c) a neutron star or black hole that happens to be in a binary system d) a binary system that happens to be aligned so that one star periodically eclipses the other e) a star that is burning iron in its core f) accreting white dwarfs. g) accreting black holes.

b) a rapidly rotating neutron star or white dwarf

lesson 3: 5. From an observational standpoint, what is a pulsar? a) a star that slowly changes its brightness, getting dimmer and then brighter with a period of anywhere from a few hours to a few weeks b) an object that emits flashes of light several times per second or more, with near perfect regularity c) an object that emits random "pulses" of light that sometimes occur only a fraction of a second apart and other times stop for several days at a time d) a star that changes color rapidly, from blue to red and back again e) a star that rapidly changes size as it moves off the main sequence

b) an object that emits flashes of light several times per second or more, with near perfect regularity

lesson 1: 2. What kind of pressure supports a white dwarf? a) neutron degeneracy pressure b) electron degeneracy pressure c) thermal pressure d) radiation pressure e) all of the above

b) electron degeneracy pressure

lesson 5: 3. 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) have occurred in distant galaxies.

lesson 1: 7. The more massive a white dwarf, the a) higher its temperature. b) smaller its radius. c) larger its radius. d) higher its luminosity.

b) smaller its radius

lesson 4: 3. 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) 3 solar masses.

lesson 4: 5. What do we mean by the singularity of a black hole? a) There are no binary black holes—each one is single (isolated). b) An object can become a black hole only once, and a black hole cannot evolve into anything else. c) It is the center of the black hole, a place of infinite density where our current laws of physics break down. d) It is the edge of the black hole, where one could leave the observable universe. e) 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 center of the black hole, a place of infinite density where our current laws of physics break down.

lesson 3: 9. Why do we think that pulsars, at least those that spin the fastest (i.e. the millisecond pulsars), are neutron stars? a) We have observed massive-star supernovae produce pulsars. b) Pulsars and neutron stars look exactly the same. c) No massive object, other than a neutron star, could spin hundreds of time per second as we observe pulsars spin, and not fall apart. d) Pulsars have the same upper mass limit as neutron stars do. e) none of the above

c) No massive object, other than a neutron star, could spin hundreds of time per second as we observe pulsars spin, and not fall apart.

lesson 2: 11. 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 and it's mass is too low. d) No, because the Sun's core will never be hot enough to fuse carbon and other heavier elements into iron.

c) No, because it is not orbited by another star and it's mass is too low.

lesson 2: 4. Which of the following statements about novae is not true? a) A star system that undergoes a nova may have another nova sometime in the future. b) A nova involves fusion taking place on the surface of a white dwarf. c) Our Sun will probably undergo at least one nova when it becomes a white dwarf about 5 billion years from now. d) When a star system undergoes a nova, it brightens considerably, but not as much as a star system undergoing a supernova. e) The word nova means "new star" and originally referred to stars that suddenly appeared in the sky, then disappeared again after a few weeks or months.

c) Our Sun will probably undergo at least one nova when it becomes a white dwarf about 5 billion years from now.

lesson 3: 10. 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) The accretion disk around a neutron star is much hotter and emits higher-energy radiation.

lesson 2: 2. 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) The primary factor determining whether a white dwarf has an accretion disk is the white dwarf's mass.

lesson 2: 10. Observationally, how can we tell the difference between Type Ia supernova and a core collapse supernova? a) A core collapse supernova is brighter than Type Ia supernova. b) A core collapse supernova happens only once, while Type Ia supernova can repeat periodically. c) The spectrum of a core collapse supernova and type Ia supernova have different spectra and light curves. d) The light of Type Ia supernova fades steadily, while the light of a core collapse supernova brightens for many weeks. e) We cannot yet tell the difference between a core collapse supernova and Type Ia supernova.

c) The spectrum of a core collapse supernova and type Ia supernova have different spectra and light curves.

lesson 3: 2. A teaspoonful of neutron star material on Earth would weigh a) about the same as a teaspoonful of Earth-like material. b) a few tons - about as much as a truck. c) about as much as Mt. Everest. d) about as much as the Moon. e) about as much as the Earth.

c) about as much as Mt. Everest.

lesson 4: 1. What is the basic definition of a black hole? a) any object that emits no visible light b) a dead star that has faded from view c) any object from which the escape velocity exceeds the speed of light d) any object made from dark matter e) a dead galactic nucleus that can only be viewed in infrared

c) any object from which the escape velocity exceeds the speed of light

lesson 5: 1. Why do astronomers consider gamma-ray bursts to be one of the greatest mysteries in astronomy? a) because they are so rare b) because we know they come from pulsating variable stars but don't know how they are created c) 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 exact mechanism or mechanisms that produce them d) because current evidence suggests that they come from our own Milky Way, but we have no idea where in the Milky Way they occur e) because current evidence suggests that they come from massive black holes in the centers of distant galaxies, adding to the mystery of black holes themselves

c) 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 exact mechanism or mechanisms that produce them

lesson 4: 4. A 40-solar-mass main-sequence star will produce which of the following remnants? a) white dwarf b) neutron star c) black hole d) none of the above

c) black hole

lesson 1: 3. The white dwarf that remains when our Sun dies will be mostly made of a) hydrogen. b) helium. c) carbon and oxygen. d) oxygen, neon, and magnesium e) neutrons.

c) carbon and oxygen.

lesson 4: 8. 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) only the mass of the black hole.

lesson 1: 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) what most stars become when they die.

lesson 1: 9. What is the upper limit to the mass of a white dwarf? a) There is no upper limit. b) There is an upper limit, but we do not yet know what it is. c) 2 solar masses d) 1.4 solar masses e) 1 solar mass

d) 1.4 solar masses

lesson 4: 6. 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) It is the point beyond which neither light nor anything else can escape.

lesson 4: 9. 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) Nothing—Earth's orbit would remain the same.

lesson 2: 7. What kind of system can become a Type Ia supernova? a) a binary consisting of an OV star and a red dwarf b) a star like our Sun c) a binary M star d) a white dwarf star with a red giant binary companion e) a pulsar f) a binary consisting of two white dwarfs

d) a white dwarf star with a red giant binary companion f) a binary consisting of two white dwarfs

lesson 1: 6. A teaspoonful of white dwarf material on Earth would weigh a) the same as a teaspoonful of Earth-like material. b) about 5 pounds c) as much as an average person. d) as much as a truck (a few tons). e) about the same as Mt. Everest. f) about the same as Earth.

d) as much as a truck (a few tons).

lesson 1: 5. Which of the following is closest in size to a white dwarf? a) a basketball b) a football stadium c) a small city d) the Moon e) Earth f) Jupiter g) Neptune h) the Sun

e) Earth

lesson 4: 13. Which of the following observatories has recently discovered black holes in binary systems? a) the Hubble Space Telescope b) the Chandra X-Ray Observatory c) the SOFIA airborne infrared observatory d) the Arecibo Radio Observatory e) The Laser Interferometer Gravitational-Wave Observatory LIGO

e) The Laser Interferometer Gravitational-Wave Observatory LIGO

lesson 4: 7. How do we know what happens at the event horizon of a black hole? a) Physicists have created miniature black holes in the lab. b) Astronomers have sent spacecraft through the event horizon of a nearby black hole. c) Astronomers have analyzed the light from matter within the event horizon of many black holes. d) Astronomers have detected X rays from accretion disks around black holes. e) We don't know for sure: we only know what to expect based on the predictions of general relativity.

e) We don't know for sure: we only know what to expect based on the predictions of general relativity.

lesson 3: 1. After a massive-star supernova, what is left behind? a) always a white dwarf b) always a neutron star c) always a black hole d) either a white dwarf or a neutron star e) either a neutron star or a black hole

e) either a neutron star or a black hole

lesson 1: 4. Which of the following is closest in mass to a white dwarf? a) a basketball b) a football stadium c) a small city d) the Moon e) Earth f) Jupiter g) Neptune h) the Sun

h) the Sun