A star system or stellar system is a small number of stars that orbit each other, bound by gravity. A large number of stars bound by gravitation is generally called a star cluster. Star systems are not to be confused with planetary systems, which include planets and similar items.
A stellar system of two stars is known as a binary star, binary star system or physical double star. If there are no tidal effects 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 orbit around the center of mass of the system.
A multiple star consists of three or more stars that appear from Earth to be close to one another in the sky. This may result from the stars being physically close and gravitationally close to each other, in which case it is a physical multiple star, or this closeness may be apparent, in which case it is an optical multiple star. Physical multiple stars are also commonly called multiple stars or multiple star systems. Most multiple star systems are triple stars. Systems with four or more components are less likely to occur. Multiple-star systems are called triple, trinary if they contain three stars; quadruple or quaternary if they contain four stars; quintuple or quintenary with five stars; sextuple or sextenary with six stars; septuple or septenary with seven stars. These systems are smaller than open star clusters, which have more complex dynamics and typically have from 100 to 1,000 stars. Most multiple star systems known are triple. Multiple-star systems can be divided into two main dynamical classes: hierarchical systems which are stable and consist of nested orbits that don't interact much and so each level of the hierarchy can be treated as a Two-body problem, or the trapezia which have unstable strongly interacting orbits and are modelled as an n-body problem, exhibiting chaotic behaviour.