Introduction

This chapter deals with several time-dependent stellar phenomena and – in particular – with the time evolution of stellar structures. It uses the results of the last chapter extensively and also draws on the material covered in Chaps. 5, 8, 10, and 12 of Vol. I.

In the last chapter we discussed the time-independent equilibrium configuration for stars, which were treated as self-gravitating bodies with ongoing nuclear reactions in the core. These stars have characteristic masses in the range (0.1–60) M⊙ and central temperatures that are higher than ~107 K. Because nuclear reactions can fuel an object for only a finite period of time, of the order of tnuc ≈ 1010(M/M⊙)−2.5 yr [see Eq. (2.31) of Chap. 2], it is clear that any particular star must have formed at some finite time in the past. Similarly, the nuclear reactions will be able to provide a steady state for the star for only a finite period into the future. The structure of the star must evolve over time scales comparable with the nuclear-reaction time scale.

In studying such evolution, there are three phases that are best addressed individually. To begin with, we have to understand how the stars of different masses form out of gas in the interstellar medium (ISM). Second, we should follow the structural changes in the star as the nuclear reactions that power the star evolve in time.