Brian J. Reed
Binary and Variable Stars
Most stars do not exist in isolation. Most have stellar companions and orbit one another. Occasionally it is possible to see the two components, either with the naked eye or through a telescope. When this is the case, the star is called a visual binary. Not all stars that look close together are true binary stars, however. Some stars are not associated with one another and exist at vastly different distances but because they lie in the same direction from the Earth, they look close together on the sky. True binaries are ones in which two stars are gravitationally bound to one another. The time it takes for binary stars to orbit one another is highly variable. It depends upon many factors such as the mass of the two components, the ratio of their individual masses, how far apart they are and what stage in their evolution they have reached. Some stars whiz around one another in mere days whilst other takes centuries. Many double stars cannot be seen as visual binaries. This could be for a number of different reasons. Perhaps the star is too far away for the separate components to be resolved or perhaps the star is relatively near by but the two components are too close together. Another possibility is that one component is much fainter than the other and is outshone as a result. A number of different techniques exist to detect the non-visual binary stars such as watching for the star to wobble! The components of a binary star system orbit one another around their common center of mass. This means that neither star is stationary. If this oscillating motion can be detected relative to the background of stars then a small or dim companion star is obviously in orbit around the larger, brighter one. These are binaries known as astrometric binaries. Another method of finding binaries is to study their spectra. The spectral absorption lines may indicate the presence of two stars, each...