Deck 13: Bizarre Stellar Graveyard

Question 1

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 slowly overwhelms gravity and the white dwarf evaporates.

Answer 1

A

Question 2

A teaspoonful of white dwarf material on Earth would weigh

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

Answer 2

C

Question 3

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

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

Answer 3

C

Question 4

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

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

Question 5

A white dwarf is

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

Question 6

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

Question 7

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

Question 8

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

Question 9

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

A) an O star
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

Question 10

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) 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.
D) A star that changes color rapidly, from blue to red and back again.

Question 11

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) The entire mass of the Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star.
B) It would rapidly sink to the center of the Earth.
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 through the Earth, creating a large crater, and exit the Earth on the other side.
E) 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.

Question 12

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

A) White dwarfs come only from stars with masses less than 1.4 solar masses.
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) 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.
D) 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.

Question 13

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 larger radius.
B) The 1.2 solar mass white dwarf has a smaller radius.
C) The 1.2 solar mass white dwarf has a higher surface temperature.
D) The 1.2 solar mass white dwarf has a lower surface temperature.
E) 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.

Question 14

From a theoretical standpoint, what is a pulsar?

A) a star that alternately expands and contracts in size
B) a rapidly rotating neutron star
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

Question 15

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.

Question 16

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

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

Question 17

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

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

Question 18

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

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

Question 19

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 undergoes a collapse and expels the excess mass in a nova eruption.
B) The white dwarf (which is made mostly of carbon) suddenly detonates carbon fusion and this creates a white dwarf supernova explosion.
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 accreting material from reaching it in the first place.

Question 20

Observationally, how can we tell the difference between a white-dwarf supernova and a massive-star supernova?

A) A massive-star supernova is brighter than a white-dwarf supernova.
B) A massive-star supernova happens only once, while a white-dwarf supernova can repeat periodically.
C) The spectrum of a massive-star supernova shows prominent hydrogen lines, while the spectrum of a white-dwarf supernova does not.
D) The light of a white-dwarf supernova fades steadily, while the light of a massive-star supernova continues to brighten for many weeks.
E) We cannot yet tell the difference between a massive-star supernova and a white-dwarf supernova.

Question 21

All pulsars are neutron stars, but not all neutron stars are pulsars.

Question 22

What causes the radio pulses of a pulsar?

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

Question 23

How do we know that pulsars must be neutron stars?

A) We have observed massive-star supernovae produce pulsars.
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) Pulsars have the same upper mass limit as neutron stars do.
E) This is only a theory that has not yet been confirmed by observations.

Question 24

More massive white dwarfs are smaller than less massive white dwarfs

Question 25

Brown dwarfs, white dwarfs, and neutrons stars are all kept from collapsing by degeneracy pressure.

Question 26

The maximum mass for a white dwarf is 1.4 solar masses.

Question 27

What is the ultimate fate of an isolated pulsar?

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

Question 28

The remnant left behind by a white-dwarf supernova is a neutron star.

Question 29

A 10 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

Question 30

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

Question 31

Which of the following statements 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. However, he or she will fade from view as the light he or she emits becomes more and more redshifted.
B) If we watch a clock fall toward a black hole, we will see it tick slower and slower as it falls towards to the black hole.
C) The event horizon of a black hole represents a boundary from which nothing can escape.
D) 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.
E) 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.

Question 32

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 pressure in the core, the core will collapse to 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 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.

Question 33

Neutron stars are the densest objects that we can observe in the universe.

Question 34

There is no upper limit to the mass of a neutron star.

Question 35

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

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

Question 36

Consider an X-ray binary system in which a compact object, surrounded by an accretion disk, is in a binary orbit with another star. All of the following statements about such accretion disks are true except:

A) X-rays are emitted by the hot gas in the accretion disk.
B) The accretion disk consists of material that spills off the companion star.
C) The compact object may be either a neutron star or a black hole.
D) Several examples of flattened accretion disks being "fed" by a large companion star can be seen clearly in photos from the Hubble Space Telescope.
E) The radiation from an accretion disk may vary rapidly in time.

Question 37

What is the origin of short gamma ray bursts?

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

Question 38

Our Sun will likely undergo a nova event in about 5 billion years.

Question 39

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

A) Light doesn't have mass; therefore, it is not affected by gravity.
B) Light escaping from a compact massive object, such as a neutron star, will be redshifted.
C) Light escaping from a compact massive object, such as a neutron star, will be blueshifted.
D) 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.
E) 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.

Question 40

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.

Question 41

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

Question 42

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

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

Question 43

Briefly describe how a nova event occurs.

Question 44

What would happen if a small piece of neutron star material (say the size of a paper clip) struck the Earth?

Question 45

All massive-star supernovae leave behind black holes as remnants.

Question 46

Why do white-dwarf supernovae all have the same maximum luminosity?

Question 47

Light escaping from white dwarfs will show a gravitational redshift.

Question 48

Briefly describe what you would see if your friend plunged into a black hole.

Question 49

What is an X-ray burster? What causes the X-ray bursts?

Question 50

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, black hole singularity, neutron star, white dwarf
C) Main sequence star, neutron star, white dwarf, black hole singularity
D) Black hole singularity, main sequence star, white dwarf, neutron star

Question 51

Suppose you find an X-ray binary system that shows X-ray bursts. Is it possible that this system consists of a red giant and a black hole? Why or why not?

Question 52

Why would the earth's orbit be unaffected were the Sun to suddenly become a black hole?

Question 53

Could our Sun ever undergo a nova or a white-dwarf supernova event? Why or why not?

Question 54

What is the evidence that gamma-ray bursts originate from beyond the Milky Way Galaxy?

Question 55

No visible light can escape a black hole, but things such as gamma rays, X-rays, and neutrinos can.

Question 56

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

Question 57

"Negatively Defined" Objects: Summarize the best observational evidence that astronomers have for the existence of stellar mass black holes. Contrast this with the observational evidence that pulsars exist. Do you agree or disagree with the statement that the best evidence we have for stellar mass black holes is that we have detected objects for which an alternate explanation is lacking? How does your answer influence your confidence in the existence of black holes? Does the strong observational confirmation of Einstein's general theory of relativity (see Special Topic General Relativity and Spacetime in Section 13.3) increase your confidence in the existence of black holes? Should it?

Question 58

Why does the size of a white dwarf decrease with increasing mass?

Question 59

Evidence that Pulsars are Neutron Stars: Suppose a friend of yours insists that pulsars are artificial time-signals constructed by aliens. List and explain all of the observational evidence that pulsars are actually natural phenomena, namely rapidly spinning neutron stars.

Question 60

Studying the Singularity: The singularity at the center of a black hole is predicted to be a region of zero volume and infinite density that contains all of the black hole's mass. It is a point at which all currently known physical laws break down. Yet in a black hole, this "terrible point" is hidden from view behind an event horizon that prevents any knowledge about the singularity reaching the outside universe. Astronomers continue to spend considerable effort trying to understand the nature of these singularities, objects for which observational input, it would seem, will be forever lacking? Are these astronomers practicing science? Argue both yes and no. Which do you find convincing?

Question 61

A white dwarf supernova leaves behind:

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

Question 62

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.

Question 63

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

Question 64

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

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

Question 65

According to our modern understanding, what is a nova?

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

Question 66

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) radio light.
D) visible light.
E) emission lines.

Question 67

A typical neutron star is more massive than our Sun and about the size (radius) of ________.

A) a small asteroid (10 km in diameter)
B) Earth
C) the Moon
D) Jupiter

Question 68

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

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

Question 69

Order the following objects in increasing size (radius):

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

Question 70

Order these objects by size (radius) from smallest to largest:

A) 1 solar-mass white dwarf, 0.5 solar-mass white dwarf, a 3 solar-mass neutron star, a 3 solar-mass black hole.
B) 3 solar mass neutron star, 3 solar mass black hole, 1 solar mass white dwarf, 0.5 solar mass white dwarf.
C) 0.5 solar mass white dwarf, 1 solar mass white dwarf, a 3 solar mass neutron star, a 3 solar mass black hole.
D) 3 solar mass black hole, 3 solar mass neutron star, 1 solar mass white dwarf, 0.5 solar mass white dwarf.
E) 3 solar mass black hole, 3 solar mass neutron star, 0.5 solar mass white dwarf, 1 solar mass white dwarf. F) 0.5 solar mass white dwarf, 1 solar mass white dwarf, a 3 solar mass black hole, a 3 solar mass neutron star.

Question 71

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

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

Question 72

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

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

Question 73

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

Question 74

Pulsars are thought to be ________.

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

Question 75

What is an accretion disk?

A) A disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole
B) Any flattened disk in space, such as the disk of the Milky Way Galaxy
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

Question 76

A white dwarf is ________.

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

Question 77

A typical white dwarf is ________.

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

Question 78

A neutron star is ________.

A) the remains of a star that died in a massive star supernova (if no black hole was created)
B) the remains of a star that died by expelling its outer layers in a planetary nebula
C) a star made mostly of elements with high atomic mass numbers, so that they have lots of neutrons
D) an object that will ultimately become a black hole

Question 79

Which stars are more common?

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

Question 80

The maximum mass of a white dwarf is ________.

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