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

**Problem 5.**

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''.

- (a)
- The observer has measured integrated colors in U, B, and V
for the slab of stars (seen face on), but these colors are naturally
affected by reddening. Calculate the
*apparent*value of R and S for Cases 1 and 2 in the limits 0 and . - (b)
- Mention a couple of realistic situations in which this variation
in R and S might actually occur.

*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.