An Introduction to Modern Astrophysics

EVOLUTION ONTHE MAIN SEQUENCE

In Section 10.6 we learned that the existence of the main sequence is due to the nuclear reactions that convert hydrogen into helium in the cores of stars. The evolutionary process of protostellar collapse to the zero-age main sequence was discussed in Chapter 12. In this chapter we will follow the lives of stars as they age, beginning on the main sequence. This evolutionary process is an inevitable consequence of the relentless force of gravity and the change in chemical composition due to nuclear reactions.

Stellar Evolution Timescales

To maintain their luminosities, stars must tap sources of energy contained within, either nuclear or gravitational. Pre-main-sequence evolution is characterized by two basic timescales: the free-fall timescale (Eq. 12.26) and the thermal Kelvin–Helmholtz timescale (Eq. 10.24). Main-sequence and post-main-sequence evolution are also governed by a third timescale, the timescale of nuclear reactions (Eq. 10.25). As we saw in Example 10.3.2, the nuclear timescale is on the order of 1010 years for the Sun, much longer than the Kelvin–Helmholtz timescale of roughly 107 years, estimated in Example 10.3.1. It is the difference in timescales for the various phases of evolution of individual stars that explains why approximately 80% to 90% of all stars in the solar neighborhood are observed to be main-sequence stars (see Section 8.2); we are more likely to find stars on the main sequence simply because that stage of evolution requires the most time; later stages of evolution proceed more rapidly. However, as a star switches from one nuclear source to the next, gravitational energy can play a major role and the Kelvin–Helmholtz timescale will again become important.

Width of the Main Sequence

Careful study of the main sequence of an observational H–R diagram such as Fig. 8.13 or the observational mass–luminosity relation (Fig. 7.7) reveals that these curves are not simply thin lines but have finite widths. The widths of the main sequence and the mass–luminosity relation are due to a number of factors, including observational errors, differing chemical compositions of the individual stars in the study, and varying stages of evolution on the main sequence.