Theory and application of stellar seismology

Observation of multiperiodic variability in stars offers a powerful tool to probe their internal structure, rotation and magnetism. This follows from the possibility of identification of observed frequencies with those calculated for realistic stellar models. In spherical stars oscillations are described in terms of two-dimensional discrete sets of acoustic- and gravity-modes. In realistic cases there is no universal relation among the frequencies. Thus, each frequency observed is an independent observable. In recent years comparisons between observed and theoretical frequencies were made for the sun and various variable stars. The most dramatic discrepancy occurs for double-mode Cepheids. Magnetism and/or rotation induce fine structure in the frequency spectrum. The width and structure of multiplets depend on the size of these effects weighted in a specific way for each mode. Recently, oscillation data were used to determine the behavior of the angular velocity inside the sun. For stars only some average rotation rates have been determined in this way.