Exam III Material

Question 1

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

a) White dwarfs form only from stars smaller 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, 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) The upper limit to the masses of white dwarfs was determined through observations of white dwarfs, but no one knows why the limit exists.
e) Above this mass, the electrons would be pushed together so closely they would turn into neutrons and the star would become a neutron star.

Answer 1

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.

Question 2

The Schwarzschild radius of a body is:

a) the distance from its center at which nuclear fusion ceases.
b) the distance from its surface at which an orbiting companion will be broken apart.
c) the maximum radius a white dwarf can have before it collapses.
d) the maximum radius a neutron star can have before it collapses.
e) the radius of a body at which its escape velocity equals the speed of light.

Answer 2

e) the radius of a body at which its escape velocity equals the speed of light.

Question 3

Which kinds of stars are most common in a newly formed star cluster?

a) O stars
b) G stars
c) M stars

Answer 3

c) M stars

Question 4

The typical size of interstellar dust particles is ______; and they consist mainly of _______.

a) 1 cm; silicates and carbon compounds
b) 1 mm; hydrogen and helium
c) about a micrometer or less; silicates and carbon compounds
d) about a nanometer or less; hydrogen and helium

Answer 4

c) about a micrometer or less; silicates and carbon compounds

Question 5

By mass, the interstellar medium in our region of the Milky Way consists of:

a) 70% Hydrogen, 30% Helium.
b) 70% Hydrogen, 28% Helium, 2% heavier elements.
c) 70% Hydrogen, 20% Helium, 10% heavier elements.
d) 50% Hydrogen, 50% Helium.
e) 50% Hydrogen, 30% Helium, 20% heavier elements.

Answer 5

b) 70% Hydrogen, 28% Helium, 2% heavier elements.

Question 6

The most abundant molecule in molecular clouds is:
a) H2
b) He2
c) CO
d) H2O
e) HHe

Answer 6

a) H2

Question 7

What is the range of timescales for star formation?

a) from 1 million years for the most massive stars up to 10 million years for the least massive stars
b) from 1 million years for the most massive stars up to 100 million years for the least massive stars
c) from 1 million years for the least massive stars up to 10 million years for the most massive stars
d) from 1 million years for the least massive stars up to 100 million years for the most massive stars
e) about 30 million years for all stars, whatever mass

Answer 7

b) from 1 million years for the most massive stars up to 100 million years for the least massive stars

Question 8

What is the smallest mass a newborn star can have?

a) 8 times the mass of Jupiter
b) 80 times the mass of Jupiter
c) 800 times the mass of Jupiter
d) about 1/80 the mass of our Sun
e) about 1/800 the mass of our Sun

Answer 8

b) 80 times the mass of Jupiter

Question 9

What are the letters that follow the spectral sequence OBAFGKM?

a) NP
b) YZ
c) LT
d) CD
e) UV

Answer 9

c) LT

Question 10

What is the greatest mass a newborn star can have:

a) 10 solar masses.
b) 20 solar masses.
c) 50 solar masses.
d) 150 solar masses.
e) 300 solar masses.

Answer 10

d) 150 solar masses.

Question 11

Which element has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission?

a) hydrogen
b) oxygen
c) silicon
d) iron
e) uranium

Answer 11

d) iron

Question 12

What happens when the gravity of a massive star is able to overcome neutron degeneracy pressure?

a) The core contracts and becomes a white dwarf.
b) The core contracts and becomes a ball of neutrons.
c) The core contracts and becomes a black hole.
d) The star explodes violently, leaving nothing behind.
e) Gravity is not able to overcome neutron degeneracy pressure.

Answer 12

c) The core contracts and becomes a black hole.

Question 13

Which event marks the beginning of a supernova?

a) the onset of helium burning after a helium flash in a star with mass comparable to that of the Sun
b) the sudden outpouring of X rays from a newly formed accretion disk
c) the sudden collapse of an iron core into a compact ball of neutrons
d) the beginning of neon burning in an extremely massive star
e) the expansion of a low-mass star into a red giant

Answer 13

c) the sudden collapse of an iron core into a compact ball of neutrons

Question 14

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 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.

Answer 14

d) a very massive main-sequence star.

Question 15

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.

Answer 15

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

Question 16

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.

Answer 16

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.

Question 17

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 brightens for many weeks.
e) We cannot yet tell the difference between a massive-star supernova and a white-dwarf supernova.

Answer 17

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

Question 18

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.

Answer 18

e) Either a neutron star or a black hole.

Question 19

What is the basic definition of a black hole?

a) Any compact mass that emits no 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.

Answer 19

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

Question 20

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.

Answer 20

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

Question 21

What is interstellar reddening?

a) Interstellar dust absorbs more red light than blue light, making stars appear redder than their true color.
b) Interstellar dust absorbs more red light than blue light, making stars appear bluer than their true color.
c) Interstellar dust absorbs more blue light than red light, making stars appear redder than their true color.
d) Interstellar dust absorbs more blue light than red light, making stars appear bluer than their true color.
e) The spectral line shift due to a star’s motion through the interstellar medium.

Answer 21

c) Interstellar dust absorbs more blue light than red light, making stars appear redder than their true color.

Question 22

If you wanted to observe stars behind a molecular cloud, in what wavelength of light would you most likely observe?

a) Ultraviolet.
b) Visible.
c) Infrared.
d) X-ray.
e) Gamma-ray.

Answer 22

c) Infrared.

Question 23

When does a protostar become a true star?

a) When the star is 1 million years old.
b) When the central temperature reaches 1 million Kelvin.
c) When nuclear fusion begins in the core.
d) When the thermal energy becomes trapped in the center.
e) When the stellar winds and jets blow away the surrounding material.

Answer 23

c) When nuclear fusion begins in the core.

Question 24

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 slow down, 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.

Answer 24

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

Question 25

Levels of the Sun’s composition:

Answer 25

Core
Radiation Zone
Convection Zone
Photosphere
Chromosphere
Corona
Solar Wind