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Description: In a “real” (physical) double star (also known as a binary star system), the individual stars are gravitationally bound to one another and periodically orbit the common center of gravity with orbital times ranging from fractions of a day to many millennia. For the so-called telescopic double stars (which appear as a pair of stars in the telescope), the periods are predominantly in the range of 50 to 500 years. As they orbit each other, each star has its own elliptical orbit, the semi-major axis of which is inversely proportional to its mass in relation to the second star. The orbital speeds and the distance between the two stars change in the rhythm of the orbital period, as the second animation shows. The first animation shows the uniform movement in the rare case of two circular paths. The individual stars that belong to optical double stars, on the other hand, have no physical connection to one another and are specifically not at a similar distance from Earth. As a random phenomenon, optical double stars are only of interest to amateur observers; In specialist publications, the term “double star” is therefore consistently used to mean “physical double star”.[2] Here the term “foreground star” or “Background star” is used without naming such a star together with the description object as an (optical) double star. Similarly, a multiple star (also multiple system or multiple star system) consists of three or more stars. In triple systems, a binary star is orbited by a distant companion; in quadruple stars, two close binary stars usually orbit each other (e.g. Epsilon Lyrae). For even larger systems, there are several options for how the system can be structured. The most component-rich system known is the six-fold star Castor in Gemini. (Wikipedia)