There are a number of pairs of stars that appear very close together when viewed from Earth and are commonly referred to as double stars. In some cases, this is caused by a coincidental alignment of two stars that are actually very far apart — this is referred to as an optical double. Alternatively, the stars could be close enough to interact gravitationally and orbit each other, forming a binary star system. It’s also possible to have multiple star systems with more than two stars orbiting each other. It’s estimated that up to 85% of the stars in our galaxy are in multiple star systems, including binary systems.
There are numerous examples of both types of double stars. An optical double star, Zeta Lyra, is located near the bright star Vega and can be distinguished with binoculars. Binary star systems include Sirius A and B, as well as Alpha Centauri A and B. To the naked eye, both of these systems appear as a single star.
The term “visual binary” refers to a binary system that can be resolved into two separate stars with the naked eye or through a telescope. Observation of the stars’ movements relative to one another can usually distinguish these from optical doubles. As seen from Earth, the stars can occasionally eclipse one another, resulting in a periodic dimming of the stars’ light. Algol, in the Perseus constellation, is an example of where the dimming, which occurs every few days, can be seen with the naked eye. The diameters of both stars can be calculated from the duration of the dimming where eclipsing occurs.
Because the component stars are very close together and/or very distant, a double star often appears as one star, even when viewed through a powerful telescope. There are several methods for determining whether or not a star is part of a binary system with two stars. Because of their different compositions, different types of stars produce recognizable spectra. If a distant, bright object’s emission spectrum reveals two distinct types of stellar spectrum, it’s a double star system with two different star types.
The Doppler effect is used in another method of detecting binary stars, which also involves stellar spectra. As the stars orbit one another in a double star system, one of the stars will most likely be moving toward or away from the Earth at any given time. When the star is approaching, its spectrum will be blue-shifted, and when it is receding, it will be red-shifted. Even if only one of the stars can be seen, this situation will periodically reverse, indicating orbital motion.
Double stars may be close enough together for one star’s gravity to pull material in from the other. Contact binaries are what they’re called. When one of the stars expands to the point where some of its outer material encroaches into the zone where the gravitational pull of the other star is stronger, this situation can arise. The binary star system Beta Lyrae, also known as Sheliak, is one such example.