Stars in binary systems evolve in the way predicted by model calculations for single stars, as long as the two components of the binary are sufficiently far apart that they do not interfere with each other. An evolving star expands, however, and the components of many binaries are close enough to each other so that they may begin to interfere in the early stages of evolution. When this happens, mass may be transferred from one component to the other, or be lost (together with its associated angular momentum) from the system. Astronomers believe that the wide variety of observed binary systems can be explained as the result of the way in which these processes affect binaries of various initial properties at different stages of the evolution of the component stars. The theory is not yet, however, fully quantitative. Some discussion of the origin and very early evolution of binaries is included, as is also a discussion of some binaries that do not yet seem to fit into the scheme.
