# I Sun and hydrostatic equilibrium

1. Jul 6, 2016

### Andreas C

I've been trying to make an estimate for the pressure of the sun's photosphere using some sort of hydrostatic equilibrium equation, but I can't seem to get one that even comes close to agreeing with observations. Does anyone know what the right formula is and what the right way to approach the problem is?

2. Jul 7, 2016

### Andreas C

Yeah, I don't think the formula I used is the right one, and I CAN'T seem to find the right one, so does anyone know which one it is?

3. Jul 9, 2016

### Ken G

The photosphere is where you see through a single mean-free path of an optical photon. So you need to know the opacity there, to know how much stuff is in a single mean-free path. So look up the cross section per gram in the solar photosphere, then know that the weight per area of the gas you are seeing through, to see the photosphere, is the surface gravity times the mass per area, where the mass per area is 1 over the cross section per gram. So that's the pressure-- g over kappa, where g is the acceleration of gravity at the solar surface, and kappa is the cross section per gram there. Think of that as g times the mass per area that you are seeing through.