Star System

A star system or stellar system is a system comprised of a star or group of stars, and, perhaps, planetary systems of smaller bodies (such as planets or asteroids), in gravitational association. The solar system is, properly, the star system comprised of our Sun and other bodies, such as the Earth, in orbit around it. (The word solar is taken from Sol, the Latin word for "Sun.")
== Binary star systems ==
A star system of two stars is known as a physical double star, binary star, or binary star system. If there are no tidal effects, no perturbation from other forces, and no transfer of mass from one star to the other, such a system is stable, and both stars will trace out an elliptical orbit around the center of mass of the system indefinitely.
== Systems with two or more stars ==
Systems with more than two stars are also possible; for example, a star cluster or galaxy is a kind of star system. Because of the large size of these systems, their dynamics are much more complicated than that of the binary star. However, it is also possible to have star systems with a small number (greater than two) of stars and simple orbital dynamics. These systems are called multiple star systems, or physical multiple stars.
=== Nomenclature ===
Multiple star systems or physical multiple stars are called triple, trinary or ternary if they contain three stars; quadruple or quaternary if they contain four stars; quintuple with five stars; sextuple with six stars; septuple with seven stars; and so on.
=== Triple star systems ===
Triple systems are by far the most common type of multiple system. For example, in the 1999 revision of Tokovinin's catalog of physical multiple stars, 551 out of the 728 systems described are triple. In accordance with the hierarchical principle, triple star systems generally contain a close binary pair which has a more distant companion.
== Examples ==
Some star systems are:
* Our own solar system, with the Sun at its centre (one star)
* Sirius (two stars)
* Alpha Centauri (three stars) [debated - see Proxima Centauri]
* 4 Centauri (four stars)
* Nu Scorpii (seven stars)
* Cygnus X-1 (one star and one black hole)
== See also ==
* Solar system
* Binary star
* Star cluster
* Triple star
* Planetary system
* Galaxy