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.
- Is the photometric color index B-V of an O star larger or
smaller than that of the sun?
- O and B stars have relatively few absorption lines in their spectra. Why?
- Why did the USA spend 2 billion dollars to build and launch the Hubble
Space Telescope?
- Why are we able to get a few days advance warning about the effects of a solar
flare on communications here on earth?
- If there were no source of energy in the core of the sun, what would happen
to the size of the sun?
- 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)
- Give two examples of evidence telling us that the corona is hot.
- 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:
- The surface temperature of the sun is about 5,800 K. The temperature of the corona is 2,000,000 K.
- Sunspots are roughly 5 times dimmer than the normal stellar surface.
What is the temperature of sunspots?
- What would be the luminosity of the solar corona if it emitted like a black body?
- Why don't we see the sun as a black body at the temperature of the corona?
- 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.
- What is the energy of an OIII photon?
- 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?)
- 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.
- The stellar object BL Lac was originally classified as a variable
star. It has an apparent magnitude, m=15.
- If it is at a distance of 100 pc, what is its absolute magnitude?
- It is observed to flare to m=12.5. By what factor has the luminosity
changed? Does this depend on distance?
- 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?
- 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.
- What is the optical depth at the wavelength of Ly-alpha of the
gas producing the anomolous absorption line?
- What can you say about the velocity and direction of the gas producing this line?
- 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?