The Theoretical Situation

The domain of this symposium is so wide – essentially anything at all having to do with stars – that it is impossible to do more than cursory justice to even a small portion of the matters to be discussed. My contribution will therefore be limited primarily to a discussion of the status of theoretical work bearing on the behavior of stars that evolve through the classical instability strip that extends from the region of Cepheids to the domain of RR Lyrae stars. Discussion of other extremely important variable stars such as cataclysmic variables, Mira and irregular variables, flare stars, β-Canis Majoris stars, and δ-Scuti and small amplitude variables will here be mentioned only in passing; presumably, most of these stars will be discussed at length by other speakers at this symposium. Further little attention will be paid to the thermal instability that is initiated in the helium-burning region of a double-shell-source star and to current thinking about the progenitors of type I and type II supernovae; presumably, these topics will be discussed in Warsaw in the symposium on advanced stages of evolution.