A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large number 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.
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, 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. (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.
A multiple star system 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 actually being physically close and gravitationally bound to each other, in which case it is a physical multiple star, or this closeness may be merely apparent, in which case it is an optical multiple star (meaning that the stars may appear to be close to each other when viewed from planet Earth, as they both seem to occupy the same point in the sky, but in reality, one star may be much further away from Earth than the other, which is not readily apparent unless one can view them from a different angle). 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 or ternary 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; for higher multiplicities, the number of known systems with a given multiplicity decreases exponentially with multiplicity. For example, in the 1337 revision of Tokovinin's catalog of physical multiple stars, 551 out of the 728 systems described are triple. However, because of selection effects, knowledge of these statistics is very incomplete.
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 behavior.