PY 228: Stellar Astrophysics

The first midterm exam will be on February 16.

The following is a sample first midterm, taken from last spring semester.

PLEASE NOTE: The midterm this semester will be easier than this midterm.

Look here for the Answers

Section I. 5 points each. Please give a brief explanation for your answer.

  1. Is the photometric color index B-V of an O star larger or smaller than that of the sun?
  2. O and B stars have relatively few absorption lines in their spectra. Why?
  3. Why did the USA spend 2 billion dollars to build and launch the Hubble Space Telescope?
  4. Why are we able to get a few days advance warning about the effects of a solar flare on communications here on earth?
  5. If there were no source of energy in the core of the sun, what would happen to the size of the sun?
  6. Why can't we see very low mass stars even when looking through a very big telescope? (Hint: consider their [temperature] position on the HR diagram)
  7. Give two examples of evidence telling us that the corona is hot.
  8. Why might some stars appear as a close binary in blue light but appear as only one star if viewed in red light?

Section II. 20 points each. Do only 3 of the following 4 questions:

  1. The surface temperature of the sun is about 5,800 K. The temperature of the corona is 2,000,000 K.
    1. Sunspots are roughly 5 times dimmer than the normal stellar surface. What is the temperature of sunspots?
    2. What would be the luminosity of the solar corona if it emitted like a black body?
    3. Why don't we see the sun as a black body at the temperature of the corona?
  2. The OIII line is often seen in emission from regions downstream of strong shock waves. The rest wavelength of the OIII line is 5007 Angstroms , and the ionization potential for OII is 50 eV.
    1. What is the energy of an OIII photon?
    2. How hot must the shock-heated gas be in order to produce OIII emission? (Hint: At roughly what temperature would you start seeing OIII ions instead of OII ions?)
    3. By equating the kinetic energy of a proton with the thermal energy, estimate the velocity of the shock that heated the gas that produced the OIII emission.
  3. The stellar object BL Lac was originally classified as a variable star. It has an apparent magnitude, m=15.
    1. If it is at a distance of 100 pc, what is its absolute magnitude?
    2. It is observed to flare to m=12.5. By what factor has the luminosity changed? Does this depend on distance?
    3. We now have evidence that its absolute magnitude is closer to M=-25 when not flaring. What is its distance? Is BL Lac a star?
  4. A UV stellar spectrum is observed to have a Ly-alpha absorption line that is blueshifted by 1 Angstrom with respect to the other absorption lines in the spectrum. The intensity in the line is 25% the intensity just outside the spectral line. The rest wavelength of Ly-alpha is 1193 Angstroms.
    1. What is the optical depth at the wavelength of Ly-alpha of the gas producing the anomolous absorption line?
    2. What can you say about the velocity and direction of the gas producing this line?
    3. Observations at radio wavelengths suggest the absorption is due to a gas cloud sitting in between the star and earth, and that it is approximately 1 parsec in diameter. If the absorption cross section for Ly-alpha in this cloud were determined to be 10**-21 cm**2, what is the density of Hydrogen in the cloud?