Although a substantial fraction of stars belong to binary or multiple star systems, most of these are wide binaries; the average separation of the stars is tens or hundreds of times greater than the average distance of the earth from the sun. These are usually seen as visual binaries, if they are nearby. They are not usually variable in brightness for geometric reasons, because, with such large separations, they are unlikely to eclipse as seen from the earth, or to influence each other. Close binary stars, however, are usually variable in some way. That is because the probability of an eclipse is greater, for geometric reasons. And the degree of physical interaction is much greater.

Normally, close binaries will not be visual binaries, but techniques such as optical interferometry and adaptive optics will gradually enable astronomers to ‘see’ the two components of some close binaries. Visual binaries, spectroscopic binaries, and eclipsing binaries will increasingly overlap.

For an excellent introduction to close binary stars, see Hilditch (2001).

Overview

Eclipsing variables (figure 5.1) are binary stars in which the observer sees the orbit almost edge-on. One star periodically eclipses the other and, at these times, the total brightness of the pair decreases. The brightness change is a geometrical effect; there is not necessarily any physical or intrinsic change in the stars. (In close binary stars, however, there may be.)

Binary stars are of interest for several reasons. First of all, at least half of all stars are binary or multiple stars, so they are a normal occurrence in our universe.