In the dark night sky, we see many stars and it looks like some stars are very close together. Proximity of two stars is apparent in some cases due to the effect of projection of stars according to an observer. Nevertheless, we also see stars that are gravitationally bound and their proximity on the night sky is not just the effect of projection. When both stars are visible, sequence of observations would indicate that the stars move around a common center of gravity.

            Observations indicate that majority of stars have a companion, thus that stars are in more than 50% of cases united in a system of two or more stars. Multiple systems consist of stars, which are gravitationally bound and move around a common center of gravity. Systems of multiplicity three an higher are frequent and are representing approximate 20% of the total stellar population, but higher is the number of stars in system, lower is the number of known systems. Double stars, or in a shorter-term binaries, are important because they are numerous and we can compare them among themselves. They are the main source of our knowledge on basic characteristics of stars, because we get their values of masses, temperatures luminosities, radii… with modeling.

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