Introduction

This chapter discusses the structure of stars that are in steady state. Concepts described in Vol. I, Chaps. 5–7, will be used extensively here. The models described here will be needed in several subsequent chapters dealing with stellar evolution, compact remnants, and binary stars.

Equations of Stellar Structure

A self-gravitating body of mass M and radius R will have gravitational potential energy of U ≈ −(GM2/R). If such a body is in equilibrium with the gas pressure balancing the gravity, the virial theorem implies that it will have temperature T such that NkBT ≈ (GM2/R that is, T ≈ (GMmp/kBR). For a sufficiently large value of M/R, this temperature can be high enough to ignite nuclear reactions at the centre of the body. The nuclear energy generated near the centre will be transported by radiation and convection towards the outer regions and will eventually escape from the body. This will establish a temperature gradient inside the body such that, in steady state, the energy produced by nuclear reactions is equal to the energy radiated away from the outer surface. Such a steady-state situation can last as long as the conditions in the body allow the generation of nuclear energy inside it. Observations suggest that the stars belong to such a category of self-gravitating bodies that are essentially powered by the nuclear reactions.