# Homework Help: Gravitation problem (estimate numbers of stars in our galaxy)

1. May 13, 2012

### U.Renko

1. The problem statement, all variables and given/known data
The sun mass MS = 2.0 E30 kg revolves around the center of the milky way which has a total extension of 2.2 E20 m. The sun takes 2.5 E8 years to complete one revolution. Estimate the number of star in our galaxy based on this data.

Suppose that the distribution is spherically simetric and the sun is in the very outskirts of the galaxy.

2. Relevant equations

$F = \frac{-GmM}{r^2}$

3. The attempt at a solution

I'm not sure where to start.
What I did think is use the mass of the galaxy somehow (which I found as result in another exercise, but could be done by Kepler's third law, since we have the period and the distance to center.) MG = 1.5 E12 times the mass of sun = 3 E42 kg.

I feel that I have to integrate something, but how could that give me a discrete number of stars...

2. May 13, 2012

### tiny-tim

Hi U.Renko!
I'll guess that you're supposed to assume that all stars have the same mass as the sun.