1.1: Introduction

The development of a relatively complete picture of the structure and evolution of the stars has been one of the great conceptual accomplishments of the twentieth century. While questions still exist concerning the details of the birth and death of stars, scientists now understand over 90% of a star's life. Furthermore, our understanding of stellar structure has progressed to the point where it can be studied within an axiomatic framework comparable to those of other branches of Physics. It is within this axiomatic framework that we will study stellar structure stellar spectra - the traditional source of virtually all information about stars.

This book is divided into two parts: stellar interiors and stellar atmospheres. While the division between the two is fairly arbitrary, it is a traditional division separating regimes where different axioms apply. A similar distinction exists between the continuum and lines of a stellar spectrum. These distinctions represent a transition zone where one physical process dominates over another. The transition in nature is never abrupt and represents a difference in degree rather than in kind.

We assume that the readers know what stars are, that is, have a working knowledge of the Hertzsprung-Russell diagram and of how the vast wealth of knowledge contained in it has been acquired. Readers should understand that most stars are basically spherical and should know something about the ranges of masses, radii, and luminosities appropriate for the majority of stars. The relative size and accuracy of the stellar sample upon which this information is based must be understood before a theoretical description of stars can be believed. However, the more we learn about stars, the more the fundamentals of our theoretical descriptions are confirmed. The history of stellar astrophysics in the twentieth century can be likened to that of a photographer steadily sharpening the focus of the camera to capture the basic nature of stars.

In this book, the basic problem of stellar structure under consideration is the determination of the run of physical variables that describe the local properties of stellar material with position in the star. In general, the position in the star is the independent variable(s) in the problem, and other parameters such as the pressure P, temperature T, and density ρ are the dependent variables. Since these parameters describe the state of the material, they are often referred to as state variables. Part I of this book discusses these parameters alone. In Part II, when we arrive near the surface of the star, we shall also be interested in the detailed distribution of the photons, particularly as they leave the star.

Although there are some excursions into the study of nonspherical stars, the main thrust of this book is to provide a basis for understanding the structure of spherical stars. Although the proof is not a simple one, it would be interesting to show that the equilibrium configuration of a gas cloud confined solely by gravity is that of a sphere. However, instead of beginning this book with a lengthy proof, we simply take the result as an axiom that all stars dominated by gravity alone are spherical.

We describe these remarkably stable structures in terms of microphysics, involving particles and photons which are largely in equilibrium. Statistical mechanics is the general area of physics that deals with this subject and contains the axioms that form the basis for stellar astrophysics. Our discussion of stellar structure centers on the interaction of light with matter. We must first describe the properties of the space in which the interaction will take place. It is not the normal Euclidean three-dimensional space of intuition, but a higher-dimension space. This higher-dimension space, called phase space, includes the momentum distribution of the particles which make up the star as well as their physical location.