# Luminosity physics question

1. Jan 31, 2007

### Logarythmic

1. The problem statement, all variables and given/known data
Show that the Hubble profile of surface brightness

$$I(r) = I_0 \left(1+\frac{r}{R}\right)^{-2}$$

leads to an infinite total luminosity. r is the distance from the center and R is a constant.

2. The attempt at a solution
For large r this is related as

$$I(r) \propto r^{-2}$$

but how is the surface brightness related to the total luminosity?

2. Jan 31, 2007

### dextercioby

Can't you find it in the textbook ? Even though the problem seems to be purely mathematical (basically a computation which gives infinity as its result), you have problems in the conceptual domain. Well, the/my advice is to look again in the book.

3. Jan 31, 2007