- F.H. Shu, The Physical Universe, Chapter 4
- G.B. Rybicki and A.P. Lightman, Radiative Processes in Astrophysics,
sections 1.3 and 1.5
- L.D. Landau and E.M. Lifshitz, Statistical Physics (3rd ed Part I),
sections 37,38,56,57
- T. Padmanabhan, Theoretical Astrophysics, vol I, Chapter 1