Theory of Cepheid pulsation - Excitation mechanisms

The various excitation mechanisms (eight in all) that have been proposed to account for the vibrational instability of variable stars, are surveyed. The most widely applied one is perhaps the “envelope ionization mechanism.” This can account for most of the essential characteristics of the “instability strip.” A simple explanation of the period-luminosity relation of classical Cepheids is given. A few outstanding problems in pulsation theory are also listed.