A star system or stellar system is a system comprised of a star or group of stars, and, perhaps, planetary systems of smaller bodies (such as planets or asteroids), in gravitational association. The solar system is, properly, the star system comprised of our Sun and other bodies, such as the Earth, in orbit around it. (The word solar is taken from Sol, the Latin word for "Sun.")
Binary star systems
A star system of two stars is known as a physical double star, binary star, or binary star system. 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.
Systems with more than two stars
Systems with more than two stars are also possible; for example, a star cluster or galaxy is a kind of star system. Because of the large size of these systems, their dynamics are much more complicated than that of the binary star. However, it is also possible to have star systems with a small number (greater than two) of stars and simple orbital dynamics. These systems are called multiple star systems, or physical multiple stars.
Multiple star systems or physical multiple stars 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.
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. Nu Scorpii contains at least seven stars.
Some star systems are:
- Our own solar system, with the Sun at its centre (one star)
- Sirius (two stars)
- Alpha Centauri (three stars) [debated - see Proxima Centauri]
- 4 Centauri (four stars)
- Mizar (five stars)
- Castor (six stars)
- Nu Scorpii (seven stars)
- Cygnus X-1 (one star and one black hole)
- ↑ 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.