Star system

The Algol three-star system imaged in the near-infrared by the CHARA interferometer with 0.5 mas resolution in 2009. The shape of Algol C is an artifact.^{[citation needed]}

Algol A is being regularly eclipsed by the dimmer Algol B every 2.87 days. (Imaged in the H-band by the CHARA interferometer. Sudden jumps in the animation are artifacts.)

Artist's impression of the orbits of HD 188753, a triple star system.

A star system or stellar system is a small number of stars that orbit each other,^[1] bound by gravitational attraction. A large group of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems. Star systems are not to be confused with planetary systems, which include planets and similar bodies (such as comets).

A star system of two stars is known as a binary star, binary star system or physical double star. 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 barycenter of the system indefinitely.^{[citation needed]} (See Two-body problem). Examples of binary systems are Sirius, Procyon and Cygnus X-1, the last of which probably consists of a star and a black hole.