# Homework Help: How many neutrons are in a neutron star with a diameter of 42.0 km?

1. Nov 17, 2013

### MichiganSt35

How many neutrons are in a neutron star with a diameter of 42.0 km?

I was under the impression it was just

(42.0e3/1e-15)^3 = 7.0048e58

but that's wrong??

2. Nov 17, 2013

### SteamKing

Staff Emeritus
It's not cool to post the same problem in multiple forums.

Neutron Stars are not composed 100% of neutrons. There is a small percentage of ordinary nuclei, particularly in the crust and atmosphere of the star. The deeper inside the star you go, the less likely you will encounter ordinary elementary particles.

http://en.wikipedia.org/wiki/Neutron_star

3. Nov 17, 2013

### cepheid

Staff Emeritus
That is certainly informative, but I don't know how relevant it is to the problem. How is the OP supposed to take into account the varying composition? How likely is it that this is more than just a density = mass/volume problem? MichiganSt35, you need some information about the average density of neutron stars, which is probably given in your notes or textbook. From this, you can estimate the total mass of the neutron star.

4. Nov 18, 2013

### SteamKing

Staff Emeritus
It's really hard to give advice when posters don't use the HW template. Is this a HW problem or idle curiosity? IDK. It could be a badly phrased or a confusing question. I rolled the dice and decided to inform the OP that regardless of the name, a neutron star is not a big ball of neutrons. The science of the interior of these objects is not settled in any event.