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Star system is a collection of stars.
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 called a Star cluster or Galaxy, although they are star systems of a broad definition.
Binary star systems.
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 and Cygnus X-1, the latter probably consisting of a star and a black hole.
Multiple star systems.
Multiple star systems or physical multiple stars are systems of several stars. These systems have relatively simple orbital Dynamics compared with Star clusters and galaxies with their large numbers of stars. Triple star systems are the most common multiple star systems.
Nomenclature star systems.
Multiple star systems 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.
Dynamical theory of star systems.
Theoretically, modelling a multiple star system is more difficult than modelling a binary star, as the dynamical system involved, the N-body problem, may exhibit chaotic behavior. Many configurations of small groups of stars are found to be unstable, as eventually one star will approach another closely and be accelerated so much that it will escape from the system. This instability can be avoided if the system is what Evans has called hierarchical. In a hierarchical system, the stars in the system can be divided into two smaller groups, each of which traverses a larger orbit around the system's center of mass. Each of these smaller groups must also be hierarchical, which means that they must be divided into smaller subgroups which themselves are hierarchical, 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.
Many systems with more than three stars are known to exist. Castor (Alpha Gemininorum) contains at least six stars. This consists of a binary pair in a distant orbit of two close binary pairs.
Examples of star formation.
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