Astr 421 Fall 2004
Galaxies Sylvain Veilleux
September 24, 2004 DUE: October 6, 2004
Problems 1 - 4.
Sparke & Gallagher, Problems 2.4, 2.5, 2.8, and 2.15
The following question is about interstellar reddening in external galaxies. Let's consider two different situations.
Case 1: Stars are distributed uniformly throughout a thin layer of thickness D with a layer of absorbing material of optical depth between the stars and the observer ( is the optical depth in the visual band).
Case 2: Same as Case 1, but now the absorbing medium is distributed uniformly with the stars.
Assume D is negligible compared to the distance to the observer, and the absorbing material obeys the so-called Whitford law with R = 3 where is called the ``color excess''.
Hint: Take a look at Section 3-11 of Mihalas & Binney (1981), attached with this problem sheet.
Note: The calculation is only academic since real dust grains have an albedo of about 50%, so they scatter much of the light rather than absorb it.