Star system

A star system or stellar system is a group of stars (and possibly smaller bodies such as planets or asteroids) that orbit one another (a system with planetary bodies orbiting stars, are referred to as solar systems or planetary systems). Although this is similar to the definition of a star cluster, the term is generally used to describe a group of a small number of stars, often two or three, emphasizing the gravitational influence they have upon one another. A star system has a definitive barycenter which the stars in the system orbit about, and well defined orbital behaviour that have some semblance of positional prediction over multiple orbits, unlike the quasi-orbits of stars in a cluster.

Binary star systems

A star system of two stars is known as a binary star. Such star systems are stable in the absence of influence from external forces; see Two-body problem.

Multiple star systems

A star system with three stars is a known as a triple star. Star systems with three or more stars may be unstable, possibly resulting in the ejection of one or more stars. Scientists have found modeling such star systems more difficult than binary star systems due to their chaotic nature.

Triple star systems

Triple star systems generally have a close binary pair and a far orbiting companion.

Quadruple star systems

Quadruple star systems generally have two close binary pairs whose barycenters orbit a common system barycenter in distant orbits.

Examples

Well known star systems include:

Alcyone (star) (four stars)
Alpha Centauri (three stars)
Castor (six stars)
Mizar (six stars)
Sirius (two stars)