The double (or binary stars are very frequent. A double star is a pair o stars that are held together by the force of gravitation and rotate around their common Center.
The orbital periods ranging from minutes in the case of very close couples up to thousands of years in the case of distant pairs, depend on the separation between the stars and their respective masses.There are also multiple-star systems in which three or four star revolve in complex paths. Lira seems a double star, but through a telescope you can see how each of the two components is a binary system.
The observation of the orbits of double stars is the only direct method that astronomers have to weigh the stars.
For close couples, its gravitational attraction can distort the shape of the stars, and is possible to flow gas from one star to another in a process called "transfer of masses".Through the telescope is detectean many double stars that seemed simple. However, when they are very close, are detected only if we study its light through spectroscopy. Then the spectra of two-star look, and its movement can be deduced by the Doppler effect in both spectra. These couples are called binary spectroscopic.
Most of the stars that we see in the sky are double or even multiple. Occasionally, one of the stars of a dual system can hide the other when observed from the Earth, which leads to a binary eclipsante.
In the majority of cases, it is believed that the components of a double system originated at the same time, although sometimes, a star can be captured by the gravitational field of another in areas of high stellar density, such as star clusters, giving rise to the double system.
Translation for educational purposes authorized by: Astronomía: Tierra, Sistema Solar y Universo