# Avg. density of neutron star

1. Mar 27, 2004

### ACLerok

Okay, I am told to find the average density of a neutron star with the same mass of the sun but with a radius of only 20km. I figured this would be pretty simple so I just would divide the Sun's mass (found using Google) by the volume of the neutron star which would be (4/3)*pi*r^3 with r being 20000. But it's telling me this is incorrect. Can someone please tell me what's wrong?

2. Mar 28, 2004

The problem is either in mismatching units or not using the proper value for the mass of the sun.

Make sure that all of your units are kg and m (or whatever they specify). Also check to see if your program--I assume that's what you're using, since something is telling you it's wrong--has a specific mass for the sun that it wants to use.

3. Mar 28, 2004

### marcus

cookiemonster has said all that needs saying but I didnt hear the beginning of the conversation and just saw the headline and I am curious

maybe the book has a typo and the book is wrong, so lets check the book.

the sun's mass is 2E30 kilogram
the radius you are told to use is 2E4 meters
so if you cube that you get 8E12 cubic meters

if you divide 2E30 by 8E12 and then by 4pi/3, then how can you be wrong?

the first division gives 25E16 kilograms per cubic meter
and then you have to divide by 4pi/3 which is a little over 4
so it should come out somewheres around 6E16
what can go wrong here?
if the book says something different the book must be wrong

neutron stars are fun because the more massive ones are smaller
(with ordinary things like rocks and SUVs the more massive ones are larger)

in standard metric units the unit of density is a kilogram per cubic meter (roughly the density of air around us, not the density of water) and it is easy to forget this and get confused----I am not saying you did but it happens a lot.