# Star system

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Star system

A star system or stellar system is a small number of stars which orbit each other,[1] 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 system may also be used to refer to a system of a single star together with a planetary system of orbiting smaller bodies.[2][3]

## 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, Procyon and Cygnus X-1, the last of which probably consists of a star and a black hole.

## Multiple star systems

Multiple star systems or physical multiple stars are systems of more than two stars.[4][5] Multiple various 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. These systems are smaller than open star clusters, which have more complex dynamics and typically have from 100 to 1,000 stars.[6]

### Dynamics

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.[7] This instability can be avoided if the system is what Evans[8] 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. In this case, the stars' motion will continue to approximate stable Keplerian orbits around the system's center of mass,[9] unlike the more complex dynamics of the large number of stars in star clusters and galaxies.

Artist's impression of the orbits of HD 188753, a triple star system

### Observation

Most multiple star systems known are triple; for higher multiplicities, the number of known systems with a given multiplicity decreases exponentially with multiplicity.[10] For example, in the 1999 revision of Tokovinin's catalog[5] of physical multiple stars, 551 out of the 728 systems described are triple. However, because of selection effects, our knowledge of these statistics is very incomplete.[11], §2.

Because of the dynamical instabilities mentioned earlier, triple systems are generally hierarchical: they contain a close binary pair which has a more distant companion. Systems with higher multiplicities are also generally hierarchical.[10] Systems with up to six stars are known; for example, Castor (Alpha Geminorum), which consists of a binary pair in a distant orbit of two closer binary pairs.[12] Another system known with six stars is ADS 9731, which consists of a pair of two triple systems, each of which is a spectroscopic binary in orbit together with a single star.[13]

## Examples

### Triple

• Polaris, the north star, is a triple star system in which the closer companion star is extremely close to the main star—so close that it was only known from its gravitational tug on Polaris A until it was photographed by the Hubble Space Telescope in 2006.
• Alpha Centauri is a triple star composed of a main binary yellow dwarf pair (Alpha Centauri A and Alpha Centauri B), and an outlying red dwarf, Proxima Centauri. A and B are a physical binary star, with an eccentric orbit in which A and B can be as close as 11 AU or as far away as 36 AU. Proxima is much further away (~15,000 AU) from A and B than they are to each other. Although this distance is still small compared to other interstellar distances, it is debatable whether Proxima is gravitationally bound to A and B.[14]
• HD 188753 is a triple star system located approximately 149 light-years away from Earth in the constellation Cygnus. The system is composed of HD 188753A, a yellow dwarf; HD 188753B, an orange dwarf; and HD 188753C, a red dwarf. B and C orbit each other every 156 days, and, as a group, orbit A every 25.7 years.

### Sextuple

• Castor[12]
• HD 139691[13]
• If Alcor is considered part of the Mizar system, the system can be considered a sextuple.

## References

1. ^ "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
2. ^ Astronomers discover a nearby star system just like our own Solar System, Joint Astronomy Centre, press release, July 8, 1998. Accessed on line September 23, 2007.
3. ^ Life unlikely in asteroid-ridden star system, Maggie McKee, NewScientist.com news service, July 7, 2004. Accessed on line August 27, 2009.
4. ^ p. 16, Understanding Variable Stars, John R. Percy, Cambridge: Cambridge University Press, 2007, ISBN 0521232538.
5. ^ a b 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.
6. ^ p. 24, Galactic Dynamics, James Binney and Scott Tremaine, Princeton University Press, 1987, ISBN 0691084459.
7. ^ 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.
8. ^ Stars of Higher Multiplicity, David S. Evans, Quarterly Journal of the Royal Astronomical Society 9 (1968), 388–400.
9. ^ Dynamics of multiple stars: observations, A. Tokovinin, in "Massive Stars in Interacting Binaries", August 16–20, 2004, Quebec (ASP Conf. Ser., in print).
10. ^ a b Statistics of multiple stars: some clues to formation mechanisms, A. Tokovinin, in the proceedings of IAU Symposium 200, The Formation of Binary Stars, Potsdam, Germany, April 10–15, 2000. Bibcode 2001IAUS..200...84T.
11. ^ Statistics of multiple stars, A. Tokovinin, in The Environment and Evolution of Double and Multiple Stars, Proceedings of IAU Colloquium 191, held 3–7 February 2002 in Merida, Yucatan, Mexico, edited by Christine Allen and Colin Scarfe, Revista Mexicana de Astronomía y Astrofísica (Serie de Conferencias) 21 (August 2004), pp. 7–14.
12. ^ a b Castor A and Castor B resolved in a simultaneous Chandra and XMM-Newton observation, B. Stelzer and V. Burwitz, Astronomy and Astrophysics 402 (May 2003), pp. 719–728.
13. ^ a b ADS 9731: A new sextuple system, A. A. Tokovinin, N. I. Shatskii, and A. K. Magnitskii, Astronomy Letters, 24, #6 (November 1998), pp. 795–801.
14. ^ Are Proxima and α Centauri Gravitationally Bound?, Jeremy G. Wertheimer, Gregory Laughlin, Astronomical Journal 132, #5 (November 2006), pp. 1995–1997.
15. ^ 4 Centauri, entry in the Multiple Star Catalog.
16. ^ The Binary Stars, R. G. Aitken, New York: Semi-Centennial Publications of the University of California, 1918.
17. ^ Vol. 1, part 1, p. 422, Almagestum Novum, Giovanni Battista Riccioli, Bononiae: Ex typographia haeredis Victorij Benatij, 1651.
18. ^ A New View of Mizar, Leos Ondra, accessed on line May 26, 2007.
19. ^ Nu Scorpii, entry in the Multiple Star Catalog.

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