# Entropy of a star

## Main Question or Discussion Point

How is the entropy of a star like the sun or a neutron star calculated?

## Answers and Replies

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to calculate the thermodynamical entropy, I suppose that you could use the famous Boltzmann entropy equation:
S=k*ln W
W is the number of distinguishable arrangements of atoms/molecules.
If you are able to figure out W, work done!

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The entropy of a black hole is $$S=\frac{Akc^3}{4hG}$$ where A is the area of the black hole, k the Boltzman's constant, h the Planck's constant, c the speed of light and G the gravitational constant
(http://nrumiano.free.fr/Estars/bh_thermo.html)

I'm not sure you can use the same formula for a (neutron) star though.

See also: http://www.madsci.org/posts/archives/sep99/937295445.As.r.html

(Google is the answer to a lot of questions!!)

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russ_watters
Mentor
Since we know the power output of the sun and we know the enthalpy and entropy of a fusion reaction, its just an easy ratio.