A double star is a pair of stars that are held together by the force of gravitation and revolve around their common center.
Double (or binary) stars are very frequent.
Orbital periods, ranging from minutes in the case of very close couples to thousands of years in the case of distant couples, depend on the separation between the stars and their respective masses.
There are also multiple stars, systems in which three or four stars rotate in complex paths. Lira looks like a double star, but through a telescope it looks like each of the two components is a binary system.
The observation of double star orbits is the only direct method that astronomers have to weigh the stars.
In the case of very close couples, their gravitational attraction can distort the shape of the stars, and it is possible for gas to flow from one star to another in a process called "mass transfer."
Through the telescope many double stars that seemed simple were detected. However, when they are very close, they are only detected if their light is studied by spectroscopy. Then the spectra of two stars are seen, and their movement can be deduced by the Doppler effect in both spectra. These couples are called spectroscopic binaries.
Most of the stars we see in the sky are double or even multiple. Occasionally, one of the stars of a double system may hide the other when viewed from Earth, which results in an eclipsing binary.
In most cases, it is believed that the components of a double system have originated simultaneously, although other times, a star can be captured by the gravitational field of another in areas of high stellar density, such as star clusters, giving place to double system.
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