Stellar Atmosphere

the outer layer of stars which forms the spectrum of their radiation. A distinction is made between the atmosphere proper—the layer in which the line spectrum originates—and the deeper photosphere which provides a continuous spectrum. However, there is no clear boundary between them. Under the photosphere, whose luminosity determines the star’s brightness, there are unob-servable deep layers containing the source of the energy. The energy is transferred through the photosphere primarily by radiation. For stars with constant brightness, radiation of each elementary volume of the photosphere occurs at the expense of the radiant energy absorbed by this volume (radiation equilibrium). Constructing models of the stellar atmosphere (calculating the distribution of the density, pressure, temperature, and other physical properties of an atmosphere as a function of depth) makes it possible to theoretically determine the distribution of energy in the star’s continuous and line spectra. A comparison of the theoretical and the observed spectra of stars of different classes is the criterion for the validity of the theoretical assumptions. The fundamental information about the stars—chemical composition, motions in the atmosphere, rotation, and magnetic fields—is obtained by studying their spectra.

One of the most important parameters in the theory of stellar atmospheres is the absorption coefficient of the stellar matter because it determines the photosphere’s geometrical depth. For hot stars, a basic role is played by the absorption of radiant energy by hydrogen atoms; for very hot stars there is the added absorption by helium and scattering by free electrons. In the atmospheres of cold stars an important role in the absorption of radiant energy is played by negative hydrogen ions. The chemical composition of the outer layers of stellar atmospheres is determined by comparing the observed and theoretical (obtained by the growth curve method or from stellar atmosphere models) equivalent width of absorption lines—that is, the width of the portion of the continuous spectrum adjacent to the line whose energy is equal to the energy absorbed in the line. The most common elements are hydrogen and helium, followed by carbon, nitrogen, and oxygen. The number of atoms of all the metals amounts to approximately one ten-thousandth of the number of atoms of hydrogen. By the 1%0’s stellar models for all spectral classes had been computed in detail and their observed spectra generally explained quite well. In general, the chemical compositions of stellar atmospheres are all alike; however, significant deviations are observed which are associated both with the peculiar state of the atmospheres (magnetic stars and close binary stars) and with actual differences in chemical composition (red giant stars, metallic “helium,” “barium,” and “lithium” stars, and others) probably brought about by evolutionary processes. Such stars and star groups are being studied especially intensively.