Homework                                             Star Death

 

 

MULTIPLE CHOICE.  Choose the one alternative that best completes the statement or answers the question.

1)  Degeneracy pressure is the source of the pressure that stops the crush of gravity in all the following except

A) a brown dwarf.

B) a very massive main-sequence star.

C) a white dwarf.

D) the central core of the Sun after hydrogen fusion ceases but before helium fusion begins.

E) a neutron star.

 

 

2)  White dwarfs are so called because

A) they are supported by electron degeneracy pressure.

B) they are both very hot and very small.

C) they are the end-products of small, low-mass stars.

D) they are the opposite of black holes.

E) it amplifies the contrast with red giants.

 

 

3)  A teaspoonful of white dwarf material on Earth would weigh

A) a few tons.

B) about the same as the earth.

C) about the same as Mt. Everest.

D) a few million tons.

E) the same as a teaspoonful of Earth-like material.

 

 

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

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

 

 

5)  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, 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, so more mass cannot be added without breaking the degeneracy pressure.

C) The more massive the white dwarf, the higher its temperature and hence the greater its degeneracy pressure. At about 1.4 solar masses, the temperature becomes so high that all matter effectively melts, even individual subatomic particles.

D) White dwarfs come only from stars smaller than 1.4 solar masses.

 

 

6)  What is the ultimate fate of an isolated white dwarf?

A) As gravity overwhelms the electron degeneracy pressure, it will explode as a supernova.

B) As gravity overwhelms the electron degeneracy pressure, it will explode as a nova.

C) The electron degeneracy pressure will eventually overwhelm gravity and the white dwarf will slowly evaporate.

D) As gravity overwhelms the electron degeneracy pressure, it will become a neutron star.

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

 

 

7)  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?

A) 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.

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) 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.

 

 

8)  Which of the following statements about novae is not true?

A) 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.

B) A star system that undergoes a nova may have another nova sometime in the future.

C) When a star system undergoes a nova, it brightens considerably, but not as much as a star system undergoing a supernova.

D) A nova involves fusion taking place on the surface of a white dwarf.

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

 

 

9)  What kind of pressure supports a white dwarf?

A) electron degeneracy pressure

B) thermal pressure

C) radiation pressure

D) neutron degeneracy pressure

E) all of the above

 

 

10)  What is the upper limit to the mass of a white dwarf?

A) 2 solar masses

B) 1.4 solar masses

C) There is no upper limit.

D) 1 solar mass

E) There is an upper limit, but we do not yet know what it is.

 

 

11)  How does a 1.2-solar-mass white dwarf compare to a 1.0-solar-mass white dwarf?

A) It is supported by neutron, rather than electron, degeneracy pressure.

B) It has a higher surface temperature.

C) It has a smaller radius.

D) It has a lower surface temperature.

E) It has a larger radius.

 

 

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

A) the earth

B) the Sun

C) a football stadium

D) a basketball

E) a small city

 

 

13)  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 pulsar

E) a white dwarf star with a red giant binary companion

 

 

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

A) A massive-star supernova happens only once, while a white-dwarf supernova can repeat periodically.

B) The spectrum of a massive-star supernova shows prominent hydrogen lines, while the spectrum of a white-dwarf supernova does not.

C) The light of a white-dwarf supernova fades steadily, while the light of a massive-star supernova brightens for many weeks.

D) We cannot yet tell the difference between a massive-star supernova and a white-dwarf supernova.

E) A massive-star supernova is brighter than a white-dwarf supernova.

 

 

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

A) either a white dwarf or a neutron star

B) either a neutron star or a black hole

C) always a neutron star

D) always a black hole

E) always a white dwarf

 

 

16)  What is the upper limit to the mass of a neutron star?

A) 1 solar mass

B) There is an upper limit less than 3 solar masses, but we do not yet know precisely what it is.

C) 1.4 solar masses

D) precisely 2 solar masses

E) There is no upper limit.

 

 

17)  A teaspoonful of neutron star material on Earth would weigh 

A) a few tons.

B) more than the earth.

C) about the same as a teaspoonful of Earth-like material.

D) more than the Moon.

E) more than Mt. Everest.

 

 

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

A) the Sun

B) the earth

C) a city

D) a football stadium

E) a basketball

 

 

19)  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 reasons) to appear in your hometown?

A) It would rapidly sink to the center of the earth.

B) The combined mass of the earth and the neutron star would cause the neutron star to collapse into a black hole.

C) The entire mass of the earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star.

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.

 

 

20)  From an observational standpoint, what is a pulsar?

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

B) 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

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

 

 

21)  From a theoretical standpoint, what is a pulsar?

A) a star that is burning iron in its core

B) a neutron star or black hole that happens to be in a binary system

C) a rapidly rotating neutron star

D) a star that alternately expands and contracts in size

E) a binary system that happens to be aligned so that one star periodically eclipses the other

 

 

22)  What causes the radio pulses of a pulsar?

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

B) The star's orbiting companion periodically eclipses the radio waves emitted by the main pulsar.

C) The star vibrates.

D) A black hole near the star absorbs energy and re-emits it as radio waves.

E) The star undergoes periodic explosions of nuclear fusion that generate radio emission.

 

 

23)  How do we know that pulsars are neutron stars?

A) Pulsars and neutron stars look exactly the same.

B) We have observed massive-star supernovae produce pulsars.

C) No massive object, other than a neutron star, could spin as fast as we observe pulsars spin.

D) Pulsars have the same upper mass limit as neutron stars do.

E) none of the above

 

 

24)  What is the ultimate fate of an isolated pulsar?

A) It will slow down, the magnetic field will weaken,  and it will become invisible.

B) As gravity overwhelms the neutron degeneracy pressure, it will explode as a supernova.

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.

 

 

25)  What is the basic definition of a black hole?

A) any object made from dark matter

B) a dead star that has faded from view

C) any compact mass that emits no light

D) a dead galactic nucleus

E) any object from which the escape velocity equals the speed of light

 

 

26)  How does the gravity of an object affect light?

A) Light doesn't have mass; therefore, it is not affected by gravity.

B) Light coming from a compact massive object, such as a neutron star, will be redshifted.

C) Visible light coming 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.

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 coming from a compact massive object, such as a neutron star, will be blueshifted.

 

 

27)  How does a black hole form from a massive star?

A) 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.

B) A black hole forms when two massive main-sequence stars collide.

C) 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.

D) Any star that is more massive than 8 solar masses will undergo a supernova explosion and leave behind a black-hole remnant.

E) If enough mass is accreted by a neutron star, it will undergo a supernova explosion and leave behind a black-hole remnant.

 

 

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

A) 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.

B) If we watch a clock fall toward a black hole, we will see it tick slower and slower as it falls nearer to the black hole.

C) 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 (or reflects) becomes more and more redshifted.

D) A black hole is truly a hole in spacetime, through which we could leave the observable universe.

E) If you fell into a black hole, you would experience time to be running normally as you plunged rapidly across the event horizon.

 

 

29)  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:

A) the accretion disk consists of material that spills off the companion star.

B) several examples of flattened accretion disks being "fed" by a large companion star can be seen clearly in photos from the Hubble Space Telescope.

C) the central object about which the accretion disk swirls may be either a neutron star or a black hole.

D) the radiation from an accretion disk may vary rapidly in time.

E) X rays are emitted by the hot gas in the accretion disk.

 

 

30)  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?

A) spectral lines from the companion star that alternately shift to shorter and longer wavelengths

B) bright X-ray emission that varies on a time scale of a few hours

C) visible and ultraviolet light from the companion star

D) sudden, intense X-ray bursts

 

 

31)  What is the Schwarzschild radius of a 100 million-solar-mass black hole? The mass of the Sun is about 2 multiply 10 to power of (exponent) kg, and the formula for the Schwarzschild radius of a black hole of mass M is:

            _elementsubscript_element = 2GM/c to power of (exponent)   (G = 6.67 multiply 10 to power of (exponent) m to power of (exponent)/kg multiply s to power of (exponent);  c =  3 multiply 10 to power of (exponent) m/s)

A) 3 million km

B) 3 km

C) 300 million km

D) 3,000 km

E) 30 km

 

 

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

A) black hole B)  white dwarf

C) neutron star D)  none of the above

 

 

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

A) It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.

B) There are no binary black holeshorizontaleach one is isolated.

C) 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.

D) An object can become a black hole only once, and a black hole cannot evolve into anything else.

E) It is the edge of the black hole, where one could leave the observable universe.

 

 

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

A) Physicists have created minature black holes in the lab.

B) Astronomers have detected X rays from accretion disks around black holes.

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

D) Astronomers have sent spacecraft through the event horizon of a nearby black hole.

E) Astronomers have analyzed the light from matter within the event horizon of many black holes.

 

 

35)  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?

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 show that gamma-ray bursts come randomly from all directions in the sky.

D) Observations from the Compton Gamma-Ray Observatory have allowed us to trace gamma-ray bursts to pulsating variable stars in distant galaxies.

E) Observations from the Compton Gamma-Ray Observatory show that gamma-ray bursts occur too frequently to be attributed to neutron stars.

 

 

36)  Why do astronomers consider gamma-ray bursts to be one of the greatest mysteries in astronomy?

A) 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

B) 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 we know they come from pulsating variable stars but don't know how they are created

D) because they are so rare

E) because current evidence suggests that they come from our own Milky Way, but we have no idea where in the Milky Way they occur

 

 

An advanced civilization lives on a planet orbiting a close binary star system that consists of a _elementsubscript_element red giant and a _elementsubscript_element black hole. Assume that the two stars are quite close together, so that an accretion disk surrounds the black hole. The planet on which the civilization lives orbits the binary star at a distance of 10 AU.

37)  Sometime within the next million years or so, their planet is likely to be doomed because

A) their planet receives most of its energy from the red giant. However, this star will soon be completely devoured in the accretion disk and thus will no longer exist.

B) tidal forces from the black hole will rip the planet apart.

C) the planet's orbit gradually will decay as it is sucked in by the black hole.

D) the red giant will probably undergo a supernova explosion within the next million years.

E) jets of material shot out of the accretion disk will shoot down their planet.

 

 

38)  One foolhardy day, a daring major (let's call him Tom) in their space force decides to become the first of his race 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 cause significant damage)? (Hint: The key word here is first. Be sure to consider the distances from the black hole at which each of the noted effects is likely to become damaging.)

A) the crush of gravity at the singularity embedded within the black hole

B) the tidal forces due to the black hole

C) the sucking force from the black hole, which will cause his head to explode

D) the strong acceleration as he descends towards the black hole

E) the X rays from the accretion disk

 

 

39)  Through a bizarre (and scientifically unexplainable) fluctuation in the spacetime continuum, a copy of a book titled Iguoonos: How We Evolved appears on your desk. As you begin to read, you learn that the book describes the evolution of the people living in the star system described above. In the first chapter, you learn that these people 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) As a result of traumatic experiences of their evolutionary ancestors, they dislike television.

B) They believe that the presence of two stars in their system was critical to their evolution.

C) They evolved on a different planet in a different star system and moved to their current location.

D) Their immediate ancestors were chimpanzees.

E) 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.

 

 

40)  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 black hole

C) a rapidly spinning pulsar

D) a white dwarf

E) everything will be pretty much the same as it is now

 

 

 

 

 

MULTIPLE CHOICE.  Choose the one alternative that best completes the statement or answers the question.

1)  B

 

 

2)  B

 

 

3)  A

 

 

4)  C

 

 

5)  B

 

 

6)  E

 

 

7)  D

 

 

8)  E

 

 

9)  A

 

 

10)  B

 

 

11)  C

 

 

12)  A

 

 

13)  E

 

 

14)  B

 

 

15)  B

 

 

16)  B

 

 

17)  E

 

 

18)  C

 

 

19)  C

 

 

20)  D

 

 

21)  C

 

 

22)  A

 

 

23)  C

 

 

24)  A

 

 

25)  E

 

 

26)  B

 

 

27)  C

 

 

28)  A

 

 

29)  B

 

 

30)  D

 

 

31)  C

 

 

32)  C

 

 

33)  A

 

 

34)  C

 

 

35)  C

 

 

36)  A

 

 

37)  D

 

 

38)  E

 

 

39)  C

 

 

40)  D