A star system or stellar system is a small number of stars which 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.
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.
Multiple star systems 
Multiple star systems or physical multiple stars are systems of more than two stars. 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. These systems are smaller than open star clusters, which have more complex dynamics and typically have from 100 to 1,000 stars.
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. In this case, the stars' motion will continue to approximate stable Keplerian orbits around the system's center of mass, unlike the more complex dynamics of the large number of stars in star clusters and galaxies.
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 1999 revision of Tokovinin's catalog 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., §2.
Because of the dynamical instabilities mentioned earlier, triple systems are generally hierarchical: they contain a close binary pair with a more distant companion. Systems with higher multiplicities are also generally hierarchical. 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. 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.
- Sirius, a binary consisting of a main-sequence type A star and a white dwarf.
- Procyon, which is similar to Sirius.
- Mira, a variable
- Delta Cephei, a Cepheid variable
- Epsilon Aurigae, an eclipsing binary.
- 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.
- Gliese 667, which houses the "super-Earth" Gliese 667Cc is a triple star system.
- 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 farther away (~15,000 AU) from A and B than they are to each other. Although this distance is small compared to other interstellar distances, it is still uncertain whether Proxima is gravitationally bound to A and B.
- 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.
- Xi Tauri (ξ Tau, ξ Tauri) is a triple star system in the constellation Taurus. Xi Tauri a spectroscopic and eclipsing triple star. It consists of three blue-white B-type main sequence dwarfs. Two of the stars are in a close orbit and revolve around each other once every 7.15 days. These in turn orbit the third star once every 145 days. The mean combined apparent magnitude of the system is +3.73 but, because the stars eclipse one another during their orbits, it is classified as a variable star, and its brightness varies from magnitude +3.70 to +3.79. Xi Tauri is approximately 222 light years from Earth.
- 4 Centauri
- Mizar is often said to have been the first binary star discovered when it was observed in 1650 by Giovanni Battista Riccioli, p. 1; , but it was probably observed earlier, by Benedetto Castelli and Galileo. Later, spectroscopy of its components Mizar A and B revealed that they are both binary stars themselves.
- HD 98800
- The Kepler 64 system has the planet PH1 (discovered in 2012 by the Planet Hunters group, a part of the Zooniverse) orbiting two of the four stars, making it to be the first known planet to be in a quadruple star system. 
- Beta Tucanae 
- HD 139691
- If Alcor is considered part of the Mizar system, the system can be considered a sextuple.
See also 
- "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
- p. 16, Understanding Variable Stars, John R. Percy, Cambridge: Cambridge University Press, 2007, ISBN 0-521-23253-8.
- 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.
- p. 24, Galactic Dynamics, James Binney and Scott Tremaine, Princeton University Press, 1987, ISBN 0-691-08445-9.
- 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.
- Dynamics of multiple stars: observations, A. Tokovinin, in "Massive Stars in Interacting Binaries", 16 August–20, 2004, Quebec (ASP Conf. Ser., in print).
- 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, 10 April–15, 2000. Bibcode 2001IAUS..200...84T.
- 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.
- 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.
- ADS 9731: A new sextuple system, A. A. Tokovinin, N. I. Shatskii, and A. K. Magnitskii, Astronomy Letters, 24, No. 6 (November 1998), pp. 795–801.
- Are Proxima and α Centauri Gravitationally Bound?, Jeremy G. Wertheimer, Gregory Laughlin, Astronomical Journal 132, No. 5 (November 2006), pp. 1995–1997.
- Does triple star orbit directly affect orbit time, Jeremy Hien, Jon Shewarts, Astronomical News 132, No. 6 (November 2011)
- 4 Centauri, entry in the Multiple Star Catalog.
- The Binary Stars, R. G. Aitken, New York: Semi-Centennial Publications of the University of California, 1918.
- Vol. 1, part 1, p. 422, Almagestum Novum, Giovanni Battista Riccioli, Bononiae: Ex typographia haeredis Victorij Benatij, 1651.
- A New View of Mizar, Leos Ondra, accessed on line 26 May 2007.
- Planet Hunters
- Nu Scorpii, entry in the Multiple Star Catalog.
- Triple star system, APOD
- Alpha Centauri system, APOD
- Alpha Centauri, APOD, 2002 April 25
- General news on triple star systems, TSN, 2008 April 22