As viewed from Earth, there are a number of pairs of stars that appear very close together and are often described as double stars. In some cases, this results from a coincidental alignment of two stars that are in reality very distant from one another — this is known as an optical double. Alternatively, the stars may be close enough to interact gravitationally so that they orbit one another — this is known as a binary star system. Multiple star systems featuring more than two stars orbiting one another are also possible. It is thought that as many as 85% of stars in our galaxy are in multiple star systems, including binaries.
There are a number of examples of double stars of both kinds. Zeta Lyra, near the bright star Vega, is an example of an optical double star — the two stars are distinguishable with binoculars. Sirius A and B, and Alpha Centauri A and B are examples of binary star systems. Both of these systems appear as a single star to the naked eye.
Where it is possible to resolve a binary system into two separate stars, either with the naked eye or through a telescope, the system is known as a visual binary. These can generally be distinguished from optical doubles by observation of the stars’ movements relative to one another. In rare cases, the stars, as viewed from the Earth, can eclipse one another, which results in a periodic reduction in the stars’ light. Algol, in the constellation of Perseus, is an example where the dimming, which takes place every few days, is actually visible to the naked eye. Where eclipsing takes place, it is possible to calculate the diameters of both stars from the duration of the dimming.
In many cases, a double star appears as one star, even when viewed through a powerful telescope, because the component stars are very close together and/or very distant. There are a number of methods that can be used to determine if a star is in fact a binary system comprising two stars. Different types of star produce recognizable spectra, due to their differing compositions. If analysis of the emission spectrum of a distant, luminous object indicates two distinct types of stellar spectrum, this would indicate that it is a double star system containing two different star types.
Another method of detecting binary stars, again involving stellar spectra, uses the Doppler effect. In a double star system, at any one time it is likely that one of the stars will be moving toward or away from the Earth, as the stars orbit one another. The spectrum of the star will be blue-shifted when it is approaching and red-shifted when it is receding. This situation will periodically reverse, indicating an orbital motion, even if only one of the stars can actually be observed.
Double stars may be sufficiently close to one another for the gravity of one star to pull in material from the other. These are known as contact binaries. This situation can occur when one of the stars expands to the point where some of its outer material encroaches into the zone where the other star’s gravitational pull is stronger. One such example is the binary star system Beta Lyrae, also known as Sheliak.