# Relative population density of stars and galaxies

1. Feb 19, 2013

### thornluke

1. The problem statement, all variables and given/known data

The number of stars around the Sun, within a distance of 17 ly, is 75. The number of galaxies in the local group, within a distance of 4.0 x106 ly from the Sun, is 26.

Calculate the average popluation density, per ly3, of stars and galaxies.

2. Relevant equations

3. The attempt at a solution
Stars: 75/173
Galaxies: 26/(4.0 x106)3

But the answers I obtain from my attempt is incorrect :s

2. Feb 19, 2013

### voko

What is the shape of the volume element?

3. Feb 19, 2013

### thornluke

The question assumes that the Sun is a sphere. So I suppose the volume element is sphere.

4. Feb 19, 2013

### voko

What's the volume of the sphere?

5. Feb 19, 2013

### thornluke

I think my brain went dead, because I got it after saying that we assume the Sun as the volume of a sphere.

So basically ly is the radius and we divide 75 by the volume of the star and galaxy to get the two answers

6. Feb 19, 2013

### voko

Pedantry: it is not the volume of the star, and not the volume of the galaxy. It is the volumes of two balls of the specified radii.

:)

7. Feb 19, 2013

### cepheid

Staff Emeritus
Hah, I would say not pedantry at all! It's kind of a crucial detail that you're dividing the number of objects by the volume of the region. I hope the OP gets this now.