Absorption lines in stellar spectra show differences in shape and strength according to the physical conditions in the star's atmosphere. Some of the toughest and most fascinating problems arise in the study of the interplay of the line absorption with the temperature, pressure, radiation, and magnetic and velocity fields of the gas. We are not yet able to calculate the full interlinking of these variables. On the other hand, the panoramic view of spectral-line behavior can be understood in relatively simple terms which we shall presently describe.

The most fundamental point to bear in mind is that the strength of line absorption depends on the number of absorbers producing that absorption. Thus the atomic level populations are of primary concern. But since the number of absorbers also means along the line of sight through the visible depths of the atmosphere, the path length is equally important. Specifically, if the continuous absorption is strong, the path length will be short and vice versa. In effect, the depth of the atmosphere changes with the amount of continuous absorption. In this way, the ratio of the line absorption to the continuous absorption is seen to be the main factor to consider.

One of the goals of stellar atmosphere studies is to understand the various line profiles and line strengths shown by stars. Another is to use our knowledge of line behavior to interpret the fundamental properties of stars, for example, the measurement of effective temperature, surface gravity, radii, and chemical composition. Our attention is directed to the first of these goals in this chapter, while the others are taken up in subsequent chapters.