Of all aspects of stellar evolution the most important for close binary systems is that once a star has condensed out of a protostellar cloud it may repeatedly increase its dimensions by large fa ctors. During such an expansion matter can reach a point where the attraction of a binary companion is strong enough to pull it toward the other star. Since the stellar radii that are reached in this expansion can be of the order of an astronomical unit, this kind of interaction may take place even in rather wide binaries with periods of the order of years. At least during such a stage binary evolution must fo llow a path different fr om that of a single star. There are other types of interaction between the two stars that come into play much earlier, such as energy transfer by radiation or by matter, which leaves the. star as a stellar wind, and tidal fr iction or interaction via magnetic fields, but because of a built-in instability mass transfer is by far the most important fa ctor. In its simplest fo rm this instability is caused by the fa ct that each gram of transferred matter adds to the gravitational pull of the mass-gaining companion while weakening the attractive properties of the expanding star. Since the components of a binary are not fixed in space, their separation may change as well, depending on the amount of angular momenl:um transferred.From observations of close binaries it is well known that many rotate in synchronism with their revolution. This is expected because of the tidal interaction in these systems. Hence for ratios of masses and radii that are not too extreme the rotational angular momentum of both components can be neglected compared to the orbital angular momentum. Under these conditions the individual contributions to the total angular momentum are fixed by the masses of the two stars. Nevertheless, the one-dimensional picture of single-star evolution cannot be retained, since the initial conditions must include not only mass and chemical composition, but also the distance and mass of the companion star. Further complications are met during the stages of mass transfer. However, we should not fo rget that binary systems in 127 Annu. Rev. Astro. Astrophys. 1977.15:127-151. Downloaded from www.annualreviews.org by Moscow State University -Scientific Library of Lomonosov on 09/19/13. For personal use only.
Quick links to online contentFurther ANNUAL REVIEWS 128 THOMAS many cases provide rather accurate information about stellar masses and radii for comparison with evolutionary models.That several reviews on the theory of close binaries have appeared in the last five years (KopaI 1971, Paczynski 1971a, Martynov 1973, Plavec 1973, Rees 1974, Kraft 1975 is an indication of the great interest in this field. Many contributions were presented at the [AU Symposium No. 73 on "Structure and Evolution of Close Binary Systems" held in 1975 in Cambridge, England . The article by Paczynski (1971a) has served as a starting point for this review, which is certainly biased by the author's interest in the the...