# Eddington luminosity in various plasmas

So I know that the Eddington luminosity is given by:

$$L_{Eddington} = 4\pi GMc/\kappa$$.

I want to calculate this luminosity for a plasma of purely ionized helium, as well as for an electron-positron plasma.

How do I find $$\kappa$$ in each of these cases? I know sometimes this is approximated as $$m/\sigma_T$$, but I'm not sure when to invoke this approximation, and what the respective mass would be.

• electron-positron plasma Do you really mean an electron-proton plasma? – PM 2Ring May 22 '19 at 14:57
• No, I meant electron-proton, so that's been fixed. – Ken G May 22 '19 at 23:26