Introduction

This chapter deals with several observed phenomena in binary star systems and depends on the material developed in dynamics (Vol. I, Chap. 2) and Chaps. 3, 5, and 6 of this volume. The material related to accretion disks developed here will be needed in the modelling of active galactic nuclei in Vol. III.

Overview

The discussion of stellar evolution in Chaps. 2–6 concentrated on the star as a single dynamical entity, uninfluenced by its surroundings. The evolutionary phenomena change significantly and a variety of new effects come into play if the star is a member of a binary system that consists of two stars gravitationally bound to each other. We saw in Chap. 3 that star formation takes place in giant molecular clouds in the ISM. The chances that a given star is gravitationally bound to another star is fairly high under such circumstances and – in fact – well over half of all the stars in the sky are members of binary or multiple star systems. It is therefore necessary to study the effect of a close companion on the evolution of a star.

Such an effect clearly depends on how close the two stars are. When the stars are reasonably far away (compared with the sum of their radii at any stage in their evolution) they are said to form a detached binary system, and the influence of one star on another is minimum.