Deck 14: The Bizarre Stellar Graveyard

Question 1

Which of the following statements about gamma ray bursts is not true?

A)The events responsible for gamma ray bursts apparently produce only gamma rays, and no other light that we can hope to detect.
B)Gamma ray bursts are among the most luminous events that ever occur in the universe.
C)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.
D)Gamma ray bursts were originally discovered by satellites designed to look for signs of nuclear bomb tests on Earth.

Answer 1

A

Question 2

You are studying a mystery companion to an evolved star, with mass transfer happening. Which of the following properties suggests that the companion is definitely a black hole?

A)The mystery companion has a mass of over 3 solar masses.
B)The mystery companion gives off periodic X- ray bursts.
C)The mystery companion has an X- ray emitting accretion disk.
D)The mystery companion has a mass of over 1.4 solar masses.

Answer 2

A

Question 3

Which stars are more common?

A)White dwarfs and neutron stars are about equally common.
B)black holes
C)Neutron stars and black holes are about equally common.
D)neutron stars
E)white dwarfs

Answer 3

E

Question 4

According to our modern understanding, what is a nova?

A)a rapidly spinning neutron star
B)the sudden formation of a new star in the sky
C)an explosion on the surface of a white dwarf in a close binary system
D)the explosion of a massive star at the end of its life

Question 5

What causes X- ray bursters?

A)The binary companion of a neutron star spirals in and combines with the neutron star.
B)The rapid rotation of the neutron star causes it to fly apart.
C)Helium fusion, which occurs when the thin layer of accreted material on a neutron star reaches 100 million K
D)The mass of an accreting neutron star passes a critical threshold between 2 and 3 Msun and explodes.

Question 6

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)1)4 solar masses
D)2 solar masses
E)1 solar mass

Question 7

Imagine an advanced civilization living on a planet orbiting at a distance of 10 AU (1.5 billion kilometers)from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole. The black hole is surrounded by an accretion disk. Sometime within the next million years or so, the civilization's planet is likely to be doomed because .

A)jets of material shot out of the accretion disk will shoot down their planet
B)tidal forces from the black hole will rip the planet apart
C)the red giant star, which provides most of energy the civilization needs to exist, will soon be destroyed in the accretion disk
D)the planet's orbit gradually will decay, as it is sucked in by the black hole
E)the red giant will probably supernova within the next million years

Question 8

A white dwarf supernova event is very important to calibrating Hubble's Law. Why? Choose the best answer for the question.

A)It is a very luminous standard candle.
B)It is a very rare event.
C)The white dwarf supernova in a galaxy tells us how fast a galaxy is expanding away from us.
D)It can only happen to white dwarfs.

Question 9

Which of the following correctly describes how light will be affected as it tries to escape from a massive object?

A)Light escaping from a compact massive object, such as a neutron star, will be redshifted.
B)Visible light escaping from a compact massive object, such as a neutron star, will be redshifted, but higher frequencies, such as X- rays and gamma rays, will not be affected.
C)Light escaping from a compact massive object, such as a neutron star, will be blueshifted.
D)Less energetic light will not be able to escape from a compact massive object, such as a neutron star, but more energetic light will be able to.
E)Light doesn't have mass; therefore, it is not affected by gravity.

Question 10

Degeneracy pressure stops the crush of gravity in all the following except

A)a brown dwarf.
B)a very massive main- sequence star.
C)the central core of the Sun after hydrogen fusion ceases but before helium fusion begins.
D)a white dwarf.
E)a neutron star.

Question 11

Order the following objects in increasing size (radius):

A)neutron star, white dwarf, Jupiter, Sun
B)Jupiter, white dwarf, Sun, neutron star
C)neutron star, Jupiter, white dwarf, Sun
D)Jupiter, white dwarf, neutron star, Sun

Question 12

What is an accretion disk?

A)any flattened disk in space, such as the disk of the Milky Way Galaxy
B)a disk of material found around every white dwarf in the Milky Way Galaxy
C)a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole
D)a stream of gas flowing from one star to its binary companion star

Question 13

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 always contains much more mass.
D)The accretion disk around a neutron star is much hotter and emits higher- energy radiation.

Question 14

What kind of star is most likely to become a white- dwarf supernova?

A)a star like our Sun
B)a white dwarf star with a red giant binary companion
C)a binary M star
D)an O star
E)a pulsar

Question 15

A white dwarf is .

A)an early stage of a neutron star
B)a brown dwarf that has exhausted its fuel for nuclear fusion
C)a precursor to a black hole
D)what most stars become when they die

Question 16

Which of the following observatories is most likely to discover a black hole in a binary system?

A)the Arecibo Radio Observatory
B)the Hubble Space Telescope
C)the SOFIA airborne infrared observatory
D)the Chandra X- Ray Observatory

Question 17

You want to determine whether a mystery object is a neutron star or a white dwarf. Which of the following properties would demonstrate that it is definitely a neutron star?

A)It emits most strongly in visible and ultraviolet light.
B)Every decade or so, it erupts in a nova explosion.
C)It is surrounded by a planetary nebula.
D)It dims and brightens more than twice per second.

Question 18

What do we mean by the singularity of a black hole?

A)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.
B)The term is intended to emphasize the fact that an object can become a black hole only once, and a black hole cannot evolve into anything else.
C)It is the edge of the black hole, where one could leave the observable universe.
D)It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.

Question 19

Will our Sun ever undergo a white dwarf supernova explosion? Why or why not?

A)No, because the Sun's core will never be hot enough to fuse carbon and other heavier elements into iron.
B)Yes, right at the end of its double- shell burning stage of life.
C)Yes, about a million years after it becomes a white dwarf.
D)No, because it is not orbited by another star.

Question 20

Prior to 1991, most astronomers assumed that gamma- ray bursts came from neutron stars (with accretion disks)within the Milky Way Galaxy. How do we now know that this hypothesis was wrong?

A)Theoretical work has proven that gamma rays cannot be produced in accretion disks.
B)Observations from the Compton Gamma- Ray Observatory showed that gamma- ray bursts occur too frequently to be attributed to neutron stars.
C)Observations from the Compton Gamma- Ray Observatory showed that gamma- ray bursts come randomly from all directions in the sky.
D)We now know that gamma- ray bursts come not from neutron stars but from black holes.
E)Observations from the Compton Gamma- Ray Observatory allowed us to trace gamma- ray bursts to pulsating variable stars in distant galaxies.

Question 21

Which of statement below about black holes is not true?

A)If you watch someone else fall into a black hole, you will never see him (or her)cross the event horizon; you'll only see him fade from view as the light he emits or reflects becomes more and more redshifted.
B)Although we are not 100% certain that black holes exist, we have strong observational evidence in favor of their existence.
C)A spaceship passing near a 10 solar mass black hole is much more likely to be destroyed than a spaceship passing at the same distance from the center of a 10 solar mass main- sequence star.
D)If you fell into a black hole, you would experience time to be running normally as you plunged rapidly across the event horizon.

Question 22

Which of the following best describes why a white dwarf cannot have a mass greater than the 1.4- solar- mass limit?

A)White dwarfs are made only from stars that have masses less than the 1.4- solar- mass limit.
B)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.
C)The upper limit to a white dwarf's mass is something we have learned from observations, but no one knows why this limit exists.
D)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.

Question 23

The more massive a white dwarf, the _.

A)larger its radius
B)higher its luminosity
C)smaller its radius
D)higher its temperature

Question 24

Suppose that a white dwarf is gaining mass through accretion in a binary system. What happens if the mass someday reaches the 1.4 solar mass limit?

A)The white dwarf will collapse to become a black hole.
B)The white dwarf will explode completely as a white dwarf supernova.
C)The white dwarf will collapse in size, becoming a neutron star.
D)The white dwarf will undergo a nova explosion.

Question 25

The maximum mass of a white dwarf is .

A)limitless; there is no theoretical limit to the maximum mass of a white dwarf
B)about the mass of our Sun
C)about 1.4 times the mass of our Sun
D)about 3 times the mass of our Sun

Question 26

If you had something the size of a sugar cube that was made of neutron star matter, it would weigh .

A)about 50 pounds
B)as much as the entire Earth
C)about as much as a truck
D)about as much as a large mountain

Question 27

Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly _ .

A)10 solar masses
B)3 solar masses
C)1)4 solar masses
D)0)5 solar masses

Question 28

What is the ultimate fate of an isolated white dwarf?

A)As gravity overwhelms the electron degeneracy pressure, it will explode as a nova.
B)It will cool down and become a cold black dwarf.
C)As gravity overwhelms the electron degeneracy pressure, it will explode as a supernova.
D)The electron degeneracy pressure slowly overwhelms gravity and the white dwarf evaporates.
E)As gravity overwhelms the electron degeneracy pressure, it will become a neutron star.

Question 29

What evidence suggests that long gamma ray bursts originate from supernovae of stars massive enough to form black holes?

A)Rapid observations in other wavelengths show that some gamma ray bursts coincide with points showing typical supernova light curves.
B)Some gamma ray bursts have been found to originate with galaxies that are actively forming stars, and would thus have a few very massive (but short lived)stars.
C)The locations of gamma ray bursts suddenly begin blocking light from more distant stars.
D)A and B
E)B and C

Question 30

How does a black hole form from a massive star?

A)A black hole forms when two massive main- sequence stars collide.
B)If enough mass is accreted by a white dwarf star that it exceeds the 1.4 solar mass limit, it will undergo a supernova explosion and leave behind a black- hole remnant.
C)If enough mass is accreted by a neutron star, it will undergo a supernova explosion and leave behind a black- hole remnant.
D)Any star that is more massive than 8 solar masses will undergo a supernova explosion and leave behind a black hole remnant.
E)During a supernova, if a star is massive enough for its gravity to overcome neutron degeneracy pressure in the core, the core will collapse to a black hole.

Question 31

What causes the radio pulses of a pulsar?

A)A black hole near the neutron star absorbs energy and re- emits it as radio waves.
B)The neutron star undergoes periodic explosions of nuclear fusion that generate radio pulses.
C)As the neutron star spins, beams of radio radiation sweep through space. If one of the beams crosses the Earth, we observe a pulse.
D)The neutron star's orbiting companion periodically eclipses the radio waves that the neutron star emits.
E)The vibration of the neutron star

Question 32

This figure shows how the luminosity of supernovae change over time. How long does it take a white dwarf supernova to decrease in luminosity by a factor of 100 from its peak?

A)about 200 days
B)about 100 days
C)about 300 days
D)about 25 days

Question 33

If you had something the size of a sugar cube that was made of white dwarf matter, it would weigh .

A)as much as an average person
B)as much as the entire Earth
C)as much as a truck
D)about 5 pounds

Question 34

If you were to come back to our Solar System in 6 billion years, what might you expect to find?

A)a rapidly spinning pulsar
B)a white dwarf
C)a black hole
D)a red giant star
E)Everything will be essentially the same as it is now.

Question 35

Suppose a white dwarf is gaining mass because of accretion from a binary companion. What happens if its mass reaches the 1.4 solar mass limit?

A)The white dwarf immediately collapses into a black hole, disappearing from view.
B)The white dwarf undergoes a collapse and expels the excess mass in a nova eruption.
C)A white dwarf can never gain enough mass to reach the limit because a strong stellar wind prevents the accreting material from reaching it in the first place.
D)The white dwarf (which is made mostly of carbon)suddenly detonates carbon fusion and this creates a white dwarf supernova explosion.

Question 36

Which of the following is closest in size (radius)to a white dwarf?

A)the Sun
B)a small city
C)a basketball
D)the Earth
E)a football stadium

Question 37

Which of the following hypothetical observations would contradict our theories about the formation and evolution of white dwarfs?

A)discovery of a white dwarf with a 1.5 Msun mass main- sequence companion
B)discovery of a white dwarf at the center of a planetary nebula
C)discovery of a white dwarf with a mass 1.5 times that of the Sun (1.5 Msun)
D)discovery of a white dwarf with a surface temperature of 6000 K

Question 38

A white dwarf supernova leaves behind

A)a newborn star.
B)a white dwarf.
C)nothing.
D)a neutron star or black hole.

Question 39

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)In a black hole, the pressure coming from neutron degeneracy pressure is slightly greater than that coming from electron degeneracy pressure.
C)Both electron degeneracy pressure and neutron degeneracy pressure help govern the internal structure of a main- sequence star.
D)The life of a white dwarf is an ongoing battle between electron degeneracy pressure and neutron degeneracy pressure.

Question 40

Based on current evidence, which of the following statements about gamma ray bursts is true?

A)All those that we have detected occurred in distant galaxies.
B)All gamma ray bursts are produced by supernovae.
C)They come primarily from the Milky Way's central black hole.
D)They occur in the same types of close binary systems that produce X- ray bursts.

Question 41

Imagine comparing a 1.2 solar mass white dwarf to a 1.0 solar mass white dwarf. Which of the following must be true?

A)The 1.2 solar mass white dwarf has a higher surface temperature.
B)The 1.2 solar mass white dwarf is supported by neutron degeneracy pressure; the 1 solar mass white dwarf is supported by electron degeneracy pressure.
C)The 1.2 solar mass white dwarf has a smaller radius.
D)The 1.2 solar mass white dwarf has a lower surface temperature.
E)The 1.2 solar mass white dwarf has a larger radius.

Question 42

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 reason)to appear in your home town?

A)It would crash into Earth, throwing vast amounts of dust into the atmosphere that, in turn, would cool the Earth; this is probably what caused the extinction of the dinosaurs.
B)The combined mass of Earth and the neutron star would cause the neutron star to collapse into a black hole.
C)The entire Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star.
D)It would rapidly sink to the center of Earth.

Question 43

Which statement about pulsars is not thought to be true?

A)Pulsars can form only in close binary systems.
B)All pulsars are neutron stars, but not all neutron stars are pulsars.
C)Pulsars are kept from collapsing by neutron degeneracy pressure.
D)A pulsar must have a very strong magnetic field and rotate quite rapidly.

Question 44

How do we know what happens at the event horizon of a black hole?

A)Astronomers have sent spacecraft through the event horizon of a nearby black hole.
B)Astronomers have detected X- rays from accretion disks around black holes.
C)Astronomers have analyzed the light from matter within the event horizon of many black holes.
D)Physicists have created miniature black holes in the lab.
E)We don't know for sure; we only know what to expect based on the predictions of general relativity.

Question 45

Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.)One foolhardy day, a daring individual in their space force (let's call him Major Tom)decides to become the first of his species 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 to start the process of killing him first)?

A)the sucking force from the black hole, which will cause his head to explode
B)tidal forces due to the black hole
C)X- rays from the accretion disk
D)the crush of gravity at the singularity embedded within the black hole

Question 46

A paperclip with the density of a neutron star would weigh (on the Earth)

A)a few tons.
B)about the same as a regular paperclip.
C)more than the Moon.
D)more than the Earth.
E)more than Mt. Everest.

Question 47

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)Nothing; Earth's orbit would remain the same.
C)Earth would slowly spiral inward until it settled into an orbit about the size of Mercury's current orbit.
D)Earth would orbit faster, but at the same distance.

Question 48

What is the ultimate fate of an isolated pulsar?

A)As gravity overwhelms the neutron degeneracy pressure, it will explode as a supernova.
B)It will spin ever slower, the magnetic field will weaken, and it will become invisible.
C)It will spin ever faster, becoming a millisecond pulsar.
D)As gravity overwhelms the neutron degeneracy pressure, it will become a white dwarf.
E)The neutron degeneracy pressure will eventually overwhelm gravity and the pulsar will slowly evaporate.

Question 49

Consider an object in orbit around a star. The star suddenly shrinks in size, but does not change its mass. What happens to the object's orbit?

A)It is instantly ejected from its orbit.
B)It gradually spirals into the star.
C)It remains in the same orbit.
D)It settles into a smaller orbit, with similar distance from the surface as the original orbit.

Question 50

Imagine what would happen if Jupiter were suddenly replaced by a black hole with the same mass as Jupiter.

A)The entire solar system would instantly be sucked into the black hole.
B)The other planets and the Sun would slowly be pulled into Jupiter.
C)The other planets would slowly be pulled into Jupiter, but the Sun would be unaffected.
D)The orbits of the solar system would be unaffected (including Jupiter's).

Question 51

From a theoretical standpoint, what is a pulsar?

A)a star that is burning iron in its core
B)a binary system that happens to be aligned so that one star periodically eclipses the other
C)a neutron star or black hole that happens to be in a binary system
D)a rapidly rotating neutron star
E)a star that alternately expands and contracts in size

Question 52

Which of the following is closest in size (radius)to a neutron star?

A)a basketball
B)the Earth
C)a city
D)the Sun
E)a football stadium

Question 53

What makes us think that the star system Cygnus X- 1 contains a black hole?

A)Cygnus X- 1 is a powerful X- ray burster, so it must contain a black hole.
B)The fact that we see strong X- ray emission tells us that the system must contain a black hole.
C)No light is emitted from this star system, so it must contain a black hole.
D)It emits X- rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a neutron star.

Question 54

Each Voyager spacecraft carries a "postcard" designed to be understandable to any aliens that might someday encounter it. On the "postcard," scientists pinpointed the location of Earth by triangulating it between pulsars. Why did the scientists choose pulsars rather than some other type of star?

A)Pulsars are very bright and therefore easy to find.
B)We're pretty sure that aliens will have only radio telescopes and not optical telescopes, so they'll have a better chance of seeing pulsars than ordinary stars.
C)Several pulsars are located within a dozen light- years of our solar system, making them useful for finding our solar system.
D)Pulsars are easy to identify by their almost perfectly steady periods of pulsation.

Question 55

A typical white dwarf is .

A)as massive as the Sun but only about as large in size as Earth
B)as massive as the Sun but only about as large in size as Jupiter
C)about the same size and mass as the Sun but much hotter
D)as large in diameter as the Sun but only about as massive as Earth

Question 56

A 10 solar mass main sequence star will produce which of the following remnants?

A)neutron star
B)black hole
C)white dwarf
D)none of the above

Question 57

Which statement about accretion disks is not true?

A)The gas in the inner parts of the disk is hotter than the gas in the outer parts of the disk.
B)Accretion disks are made primarily of hydrogen and helium gas.
C)The gas in the inner parts of the disk travels faster than the gas in the outer parts of the disk.
D)The primary factor determining whether a white dwarf has an accretion disk is the white dwarf's mass.

Question 58

Pulsars are thought to be _ .

A)accreting white dwarfs
B)unstable high- mass stars
C)accreting black holes
D)rapidly rotating neutron stars

Question 59

The white dwarf that remains when our Sun dies will be mostly made of .

A)hydrogen
B)carbon
C)neutrons
D)helium

Question 60

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?

A)bright X- ray emission that varies on a time scale of a few hours
B)intense X- ray bursts
C)visible and ultraviolet light from the companion star
D)spectral lines from the companion star that alternately shift to shorter and longer wavelengths

Question 61

Rank the following 4 items in order of increasing density (low to high).

A)main- sequence star, white dwarf, neutron star, black hole singularity
B)main- sequence star, neutron star, white dwarf, black hole singularity
C)main- sequence star, black hole singularity, neutron star, white dwarf
D)black hole singularity, main- sequence star, white dwarf, neutron star

Question 62

Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.)Through a bizarre (and scientifically unexplainable)fluctuation in the space- time continuum, a copy of a book from that civilization arrives on your desk; it is entitled Iguoonos: How We Evolved. In the first chapter, you learn that these beings 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?

A)Their immediate ancestors were chimpanzees.
B)They evolved on a different planet in a different star system, and moved to their current location.
C)They evolved from primitive wormlike creatures that had 13 legs, 4 eyes, and bald heads, thus explaining why such critters are now considered a spectacular delicacy.
D)As a result of traumatic experiences to their evolutionary ancestors, they dislike television.
E)They believe that the presence of two stars in their system was critical to their evolution.

Question 63

How do we know that pulsars must be neutron stars?

A)Pulsars have the same upper mass limit as neutron stars do.
B)Telescopic images of pulsars and neutron stars look exactly the same.
C)No massive object, other than a neutron star, could spin as fast as we observe pulsars to spin and remain intact.
D)We have observed massive- star supernovae produce pulsars.
E)This is only a theory that has not yet been confirmed by observations.

Question 64

Which of the following is closest in mass to a white dwarf?

A)the Earth
B)the Sun
C)the Moon
D)Jupiter

Question 65

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?

A)It would rapidly sink to the center of the Earth.
B)The entire mass of the Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star.
C)The combined mass of the Earth and the neutron star would cause the neutron star to collapse into a black hole.
D)It would crash into the Earth, throwing vast amounts of dust into the atmosphere which in turn would cool the Earth. Such a scenario is probably what caused the extinction of the dinosaurs.
E)It would crash through the Earth, creating a large crater, and exit the Earth on the other side.

Question 66

Why is there an upper limit to the mass of a white dwarf?

A)The upper limit to the masses of white dwarfs was determined through observations of white dwarfs in binary systems, but no one knows why the limit exists.
B)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, and no more mass can be supported.
C)White dwarfs come only from stars with masses less than 1.4 solar masses.
D)The more massive the white dwarf, the higher its temperature and hence the greater its degeneracy pressure. Near 1.4 solar masses, the temperature becomes so high that all matter effectively melts into subatomic particles.

Question 67

The Crab Pulsar is pulsing in visible light 30 times per second. Why?

A)It rotates 30 times per second.
B)It's a mystery; no one really knows.
C)It is eclipsed by a companion 30 times per second.
D)A jet ejects energy and particles from a hot spot 30 times per second.

Question 68

Which of the following statements about black holes is not true?

A)If we watch a clock fall toward a black hole, we will see it tick slower and slower as it falls towards the black hole.
B)If you fell into a supermassive black hole (so that you could survive the tidal forces), you would experience time to be running normally as you plunged across the event horizon.
C)If the Sun magically disappeared and was replaced by a black hole of the same mass, the Earth would soon be sucked into the black hole.
D)If you watch someone else fall into a black hole, you will never see him or her cross the event horizon. However, he or she will fade from view as the light he or she emits becomes more and more redshifted.
E)The event horizon of a black hole represents a boundary from which nothing can escape.

Question 69

A white dwarf is

A)the exposed core of a dead star, supported by neutron degeneracy pressure.
B)a cool and very small main sequence star with a mass of less than 1.4 of a solar masses.
C)a hot but very small main sequence star with a mass of less than 1.4 solar masses.
D)the name for the singularity at the center of a black hole.
E)the exposed core of a dead star, supported by electron degeneracy pressure.

Question 70

After a massive- star supernova, what is left behind?

A)always a black hole
B)always a white dwarf
C)always a neutron star
D)either a white dwarf or a neutron star
E)either a neutron star or a black hole

Question 71

Which of the following is not true about differences between novae and supernovae?

A)Supernovae eject gas into space, but novae do not.
B)The same star can undergo novae explosions more than once, but can undergo only a single supernova.
C)Novae are much less luminous than supernovae.
D)Novae occur only in binary star systems, while supernovae can occur both among single stars and among binary star systems.

Question 72

What is the origin of short gamma ray bursts?

A)supernovae in the Milky Way
B)new stars forming in the Milky Way
C)the collision of stars in the dense nuclei of distant galaxies
D)very powerful supernovae occurring in distant galaxies
E)It is not known, but it may be the collision of a neutron star with a black hole.

Question 73

The Schwarzschild radius of a black hole depends on .

A)only the mass of the black hole
B)both the mass and chemical composition of the black hole
C)the way in which the black hole formed
D)the observationally measured radius of the black hole

Question 74

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 even faster), with near perfect regularity
C)a star that changes color rapidly, from blue to red and back again
D)an object that emits random "pulses" of light, sometimes with only a fraction of a second between pulses and other times with several days between pulses

Question 75

A teaspoonful of white dwarf material on Earth would weigh

A)about the same as the Earth.
B)about the same as Mt. Everest.
C)a few grams.
D)a few pounds.
E)a few tons.

Question 76

How is an X- ray burst (in an X- ray binary system)similar to a nova?

A)Both involve explosions on the surface of a stellar corpse.
B)Both typically recur every few hours to every few days.
C)Both result in the complete destruction of their host stars.
D)Both are thought to involve fusion of hydrogen into helium.

Question 77

The surface of the neutron star RXJ2015 has a temperature of 10 million K. This neutron star glows most strongly in

A)infrared light.
B)X- ray light.
C)emission lines.
D)radio light.
E)visible light.

Question 78

Which of the following statements about novae is not true?

A)When a star system undergoes a nova, it brightens considerably, but not as much as a star system undergoing a supernova.
B)Our Sun will probably undergo at least one nova when it becomes a white dwarf about 5 billion years from now.
C)A star system that undergoes a nova may have another nova sometime in the future.
D)A nova involves fusion taking place on the surface of a white dwarf.
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.

Question 79

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)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.
C)The more massive the black hole, the larger the Schwarzschild radius.
D)For black holes produced in massive star supernovae, Schwarzschild radii are typically a few to a few tens of kilometers.

Question 80

What is the basic definition of a black hole?

A)any object made from dark matter
B)a compact mass that emits no visible light
C)an object with gravity so strong that not even light can escape
D)a dead star that has faded from view