Star system
From Wikipedia, the free encyclopedia
- This article is about stars in outer space. For the Hollywood star system, see star system (film).
A star system or stellar system is a small number of stars that orbit each other,[1] 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.
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[edit] 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.
[edit] 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.
[edit] Nomenclature
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.
[edit] Dynamical theory
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.[2] This instability can be avoided if the system is what Evans[3] 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.
[edit] 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[4] 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.
[edit] Higher multiplicities
Many systems with more than three stars are known to exist. Castor (Alpha Geminorum) contains at least six stars. This consists of a binary pair in a distant orbit of two close binary pairs.
[edit] Examples
Some multiple star systems are:
- Alpha Centauri (three stars) [debated - see Proxima Centauri]
- 4 Centauri (four stars)
- Mizar (four stars)
- Nu Scorpii (five stars)
- Castor (six stars)
[edit] See also
- Solar System
- Binary star
- Triple star system
- Quadruple star system
- Planetary system
- Extrasolar planet
[edit] References
- ^ "Star system" in Modern Dictionary of Astronomy and Space Technology. A.S. Bhatia, ed. New Delhi: Deep & Deep Publications, 2005. ISBN 81-7629-741-0
- ^ Multiple Stellar Systems: Types and Stability, Peter J. T. Leonard, in Encyclopedia of Astronomy and Astrophysics, P. Murdin, ed., online edition at the Institute of Physics, orig. ed. published by Nature Publishing Group, 2001.
- ^ Stars of Higher Multiplicity, David S. Evans, Quarterly Journal of the Royal Astronomical Society 9 (1968), 388–400.
- ^ MSC—a catalogue of physical multiple stars, A. A. Tokovinin, Astronomy and Astrophysics Supplement Series 124 (1997), 75–84; online versions at VizieR and the Multiple Star Catalog.
