# About the reference for equation of state

Well, I am a post graduate studying mathematics. Current I am working on the mathematical theory of star(not an expert thought). When I am making some assumption on the equation of state (for a star or sun of course), I simply sum up the radiation pressure, idea gas pressure and degenerate pressure together. In some sense I believe this choice is an appropriate approximate to some model. But when someone ask me why, I am kind of stuck, even thought some of the lecture note posted online supports my guess(for example, http://www.jb.man.ac.uk/~smao/starHtml/equationState.pdf). But I am still wondering, is there any standard text book introducing something like this? I know Chandrasekhar's book has already told something, but it looks like it did not really state that I can simply sum all the pressure together linearly. Thank you in advance.