A Brief Introduction to Stellar Evolution

Abstract

With the aim of providing a reference frame for the study of stellar pulsations we describe the process known as stellar evolution. Evolution and pulsations are deeply related and the knowledge gained in one of them has an immediate impact on the other. First we describe the observational basis, presenting the Hertzsprung-Russell Diagram and other fundamental concepts. Then we describe the physical context of stellar evolution in which, quite fortunately, matter is very close to (but not in) thermodynamic equilibrium. This allows for a simplification of the problem of paramount importance. We describe the equation of state of stellar matter, paying attention on when we should expect the occurrence of partial and full ionization (fundamental for pulsations), and electron degeneracy. Then, we present the concept of hydrostatic equilibrium. As a natural consequence we consider barotropic structures, like polytropic spheres and cold white dwarfs, discussing the existence of the Chandrasekhar’s mass limit. As realistic stars are not cold but at finite temperature (they radiate energy in space!), in general they are nonbarotropic. So, we need to consider the conservation of energy and also its transport by radiation, convection and conduction. As it is well known, the engine of stars is nuclear reactions. We present the proton-proton and carbon-nitrogen-oxygen cycles of hydrogen burning and also the main helium burning reactions. Then, we make some brief comments on the methods for solving the full set of non-linear, partial differential equations of stellar evolution and also those needed for computing the changes of chemical composition. At this point we are in conditions to present stellar evolution as a direct consequence of these physical ingredients. We discuss the main stages of stellar evolution for a variety of objects: pre-main sequence, low and intermediate mass, white dwarfs, and finally massive stars. In this paper we restricted ourselves to the case of isolated and nonrotating objects evolving during their long lived stages. In our opinion, this provides a general basis for most of the usually considered pulsating stars.