PY 228: Stellar Astrophysics
The second midterm exam will be on March 30.
The following is a sample second midterm, taken from last spring semester.
PLEASE NOTE: The midterm this semester will be easier than this midterm.
Section I. 5 points each. Please give a brief explanation for your answer.
- What is the only thing that can hold off gravity for eternity?
- Why are molecular clouds the most likely site for star formation?
- Why do we not see H-burning stars with mass less than 0.08 solar masses?
- No pulsar has yet to be detected at the site of the supernova 1987A.
How might you reconcile this fact with astrophysical theory?
- What makes a star move off the Main Sequence?
- If mass is added to a white dwarf, what happens to its radius?
- What is the meaning of the term ``Iron Catostrophe"?
- Why is it important to you that massive stars explode? What would
be different if they did not explode?
- What would happen if you slowly added 1 solar mass of material
to a 1 solar mass white dwarf?
- Why are stars like the sun rotating very slowly, while massive
stars typically rotate very fast?
- If white dwarfs are so much smaller than main sequence
stars, why can we still see them?
- The luminosity of stars is generated by burning H into
heavier atoms, yet at the end of the life of a massive star, the material
in the core is broken back down to H atoms. If the end product (protons)
is the same as the original product (protons), where did all the energy radiated away
during the life of the star come from?
Section II. 20 points each. Do only 2 of the following 3 questions:
- The history of gamma-ray astronomy is dominated by a single
event: a gamma-ray burst observed on March 5, 1979. Since the burst
was observed from the direction of the Large Magellanic Cloud, we can
assume its distance is that of the LMC and derive a luminosity of
5x10**44 erg/s. The burst lasted only one-tenth of a second.
Unconfirmed reports claim that a pulsed emission was detected from
this source prior to the burst, and that the period of the pulsation was
longer than the pulse period observed during the burst (8 seconds) by
an amount of 10**-6 seconds. Show that the change in gravitational
energy associated with this change in rotational period (if we are
talking about a neutron star) is sufficient to power the March 5 gamma-ray
burst.}
- Calculate the change in stellar radius required by
the observed change in period assuming the angular momentum of
the neutron star is conserved.
- Calculate the amount of gravitational energy released
by this change in radius, and compare with the total energy observed
from the March 5 gamma-ray burst.
- Brown dwarfs are stars that are too small to ignite H burning
in their core, yet they still produce a substantial luminosity through
gravitational contraction. For a ``star'' of 0.02 solar masses,
0.05 solar radii, and luminosity L = 10**27 ergs/s, calculate
the rate at which this brown dwarf shrinks. Use the Virial theorem
result that 1/2 of the change in gravitational energy is radiated away.
- How hot is the gas that is shocked by the passage
of a supernova blastwave?
- The energy of a supernova explosion is derived from
the release of gravitational potential energy in forming a neutron
star. Calculate the total energy available for the supernova
explosion.
- If 1% of this energy goes into the kinetic energy of
the ejected envelope of mass 6 solar masses, how fast is the envelope
moving?
- Assume all of this kinetic energy goes into thermal
energy of the (hydrogen) gas to calculate the temperature of
the interstellar gas that is shocked by the supernova blastwave.