Extragalactic Astronomy -
Fall 2012
Homework #3 - due September 5
Classify these galaxies:
1. 
2. 
3. In Edwin Hubble's 1926 paper on "Extra-galactic nebulae" on which
pages does the table of "Classification of nebulae" appear? (You
may want to use ADS.)
4. The most luminous cD galaxies can have MB as large as
-25. What is this in terms of solar luminosities?
5. For a globular cluster with N ~ 105 stars, a
characteristic size of R ~ 10 pc, and a typical stellar velocity of V ~
10 km/s, what are the crossing time and relaxation time? Note
that if one keeps the numerical factors in the derivation of the
relaxation time (specifically the expression in the center of equation
3.8), there is a factor of 1/6 that should multiply the righthand sides
of equations 3.2 and 3.3. This was not important for elliptical
galaxies, but is important for globular clusters.
6. Which are brighter in the FIR, Sa or Sc galaxies?
7. An elliptical galaxy has a velocity dispersion of 297 km/s and a
diameter Dn of
27.1 arcseconds. How far away is it?
8. What is the most common type of galaxy with MB ~ -15 in the Virgo
cluster?