# Hydrostatic equilibrium for stars question!

1. Dec 31, 2008

### coffeem

Hi, I have attatched the question. I have done part a using: m(r) = integral(4pir^2rhodr).

Does any one have any suggestions for parts a and b?

Thanks

p.s. if anyone knows of a freeware program for paints for macs i would appretiate it rather than having to create pdf's lol

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2. Jan 3, 2009

### Thaakisfox

for the second part use the equation for hydrostatic equilibrium:

$$\vec{f}-\text{grad}p =0$$

Where f force exerted on the volume (in this case gravity).

So write out the gradient operator in spherical coordinates and determine the gravitational field in an arbitrary point inside the star, and you are done.

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