Most stars spend most of their lives on the main sequence. A star like the Sun, for example, after spending a few tens of millions of years in formation (stages 1—6 in Chapter 19), resides on or near the main sequence (stage 7) for 10 billion years before evolving into something else. (Sec. 19.2) That "something else" is the topic of this chapter.

Virtually all the low-mass stars that have ever formed still exist as stars. The coolest M stars—red dwarfs—burn so slowly that not one of them has yet left the main sequence. Some of them will burn steadily for a trillion years or more. (Sec. 17.9) Conversely, the most massive O and B stars evolve away from the main sequence after only a few tens of millions of years. Most of the high-mass stars that have ever existed perished long ago. Between these two extremes, many stars are observed in advanced stages of evolution, their properties quite different from when they formed. By combining these observations with theoretical models, astronomers have built up a comprehensive picture of how stars evolve.

On the main sequence, a star slowly fuses hydrogen into helium in its core. This process is called core hydrogen burning. In Chapter 16 we saw how the proton—proton fusion chain powers the Sun. (Sec. 16.5.3) More Precisely 20-1 describes another sequence of nuclear reactions, of great importance in stars more massive than the Sun, that accomplishes the same basic result as the proton—proton chain, but in a very different way.

A star's equilibrium during the main-sequence phase is the result of a balance between gravity and pressure, in which pressure's outward push exactly counteracts gravity's inward pull (Figure 20.1). Eventually, however, as the hydrogen in the core is consumed, the balance starts to shift, and both the star's internal structure and its outward appearance begin to change: the star leaves the main sequence. You should keep Figure 20.1 in mind as you study the various stages of stellar evolution described next. Much of a star's complex behavior can be understood in these simple terms.

Figure 20.1 In a steadily burning star on the main sequence, the outward pressure of hot gas balances the inward pull of gravity. This is true at every point within the star, guaranteeing its stability.

Once a star begins to evolve away from the main sequence, its days are numbered. The post—main-sequence stages of stellar evolution—the end of a star's life—depend critically on the star's mass. As a rule of thumb, we can say that low-mass stars die gently, whereas high-mass stars die catastrophically. The dividing line between these two very different outcomes lies around eight times the mass of the Sun, and in this chapter we will refer to stars of more than 8 solar masses as "high-mass" stars. Realize, though, that within both the "high-mass" and the "low-mass" (that is, less than 8 solar masses) categories there are substantial variations.

Rather than dwelling on the many details, we will concentrate on a few representative evolutionary sequences. We begin by considering the evolution of a fairly low mass star like the Sun. The stages described in the next few sections pertain to the Sun as it nears the end of its fusion cycle 5 billion years from now. In fact, most of the qualitative features of the discussion apply to any low-mass star, although the exact numbers vary considerably. Later, we will broaden our discussion to include all stars, large and small.