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.

  1. What is the only thing that can hold off gravity for eternity?
  2. Why are molecular clouds the most likely site for star formation?
  3. Why do we not see H-burning stars with mass less than 0.08 solar masses?
  4. No pulsar has yet to be detected at the site of the supernova 1987A. How might you reconcile this fact with astrophysical theory?
  5. What makes a star move off the Main Sequence?
  6. If mass is added to a white dwarf, what happens to its radius?
  7. What is the meaning of the term ``Iron Catostrophe"?
  8. Why is it important to you that massive stars explode? What would be different if they did not explode?
  9. What would happen if you slowly added 1 solar mass of material to a 1 solar mass white dwarf?
  10. Why are stars like the sun rotating very slowly, while massive stars typically rotate very fast?
  11. If white dwarfs are so much smaller than main sequence stars, why can we still see them?
  12. 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:

  1. 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.}
    1. Calculate the change in stellar radius required by the observed change in period assuming the angular momentum of the neutron star is conserved.
    2. 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.
  2. 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.
  3. How hot is the gas that is shocked by the passage of a supernova blastwave?
    1. 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.
    2. 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?
    3. 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.