Homework Help: Colliding stars

1. Jan 12, 2012

dani234

1. The problem statement, all variables and given/known data
The sun moves through the collection of stars in it's neighborhood with velocity v=10 km/s.
(A)Assuming n*=1 pc^(-3) and that all starts have the same radius R*, what is the typical time we must wait before the sun collides with another star? Express your answer first in terms ov variables v, n*, and R*, then plug in typical values to obtain a number.
(B) what is the typical distance traveled before a collision aka mean free path. (l=1/σn)
(C) How long would we have to wait before the sun comes close enough to be substantially affected by it gravitationally? (distance that causes gravitation effects is decided by student but must be justified)

2. Relevant equations

3. The attempt at a solution

I had thought that I could solve part A by calculating the distance to the next star by using the density n* but since part B asks to find that I didn't know if that was unacceptable and if there was another way of doing the problem. Also for the mean free path I am unsure if I should multiply the cross sectional area (σ) by two for each star, or if I should only use it once. I'm sure part C follows quite easily from part A as long as I can justify my distance till gravitational effects.