ABSTRACT
The development of a theory of stellar evolution was one of the great successes of 20th century astrophysics. Most of stellar evolution can be understood using a quasistatic model, i.e., one in which a star evolves from one state of mechanical equilibrium to another. The constraints of thermodynamics mean that stars are always radiating energy away to their environment, and this can be understood as a primary driver of their evolution. The heat flow occurs in the form of blackbody radiation that leaves the stellar surface. The time scale to lose a significant amount of the internal store of thermal energy is ˜107 yr, much longer than the free-fall time, so the object can easily readjust to a new state of equilibrium. An interesting sidelight on stellar evolution is that, unlike nuclear reactions on Earth, stars that are supported primarily by thermal pressure have a natural built-in “safety valve” for their nuclear reactions.
