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A 'multiple star' consists of three or more stars which appear from the Earth to be close to one another.
This closeness may be merely apparent, in which case the multiple star is 'optical', or result
from the stars being physically close and gravitationally bound to each other, in which case
it is 'physical'.^[1][2][3] Physical multiple stars are ''multiple star systems''. Multiple stars may be called 'triple' if they have three stars, 'quadruple' with four, and so on. A physical triple star is also called 'trinary', 'ternary' or a 'triple star system'.
In a triple star system, each star orbits the center of mass of the system. Usually, two of the stars form a close binary star and the third is further away; this configuration is called 'hierarchical'. Multiple star systems containing more than three stars are also usually hierarchical.^2, §2.4.

Contents

Examples

References

See also

External links

Examples

★ HR 3617 is a multiple star with three component stars, HR 3617A, HR 3617B, and HR 3617C. A and B form a physical binary star, while C is optical.

★ 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.^[4]

★ HD 188753 is a physical 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.

★ 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.

References

1. Hipparcos: Double and Multiple Stars, web page, accessed May 26, 2007.
2. MSC - a catalogue of physical multiple stars, A. A. Tokovinin, ''Astronomy and Astrophysics Supplement Series'' '124' (July 1997), pp. 75–84.
3. Binary and Multiple Stars, web page, accessed May 26, 2007.
4. Are Proxima and α Centauri Gravitationally Bound?, Jeremy G. Wertheimer, Gregory Laughlin, ''Astronomical Journal'' '132', #5 (November 2006), pp. 1995–1997.

See also

★ Binary star

★ Double star

★ Star system

External links

★ Triple star system, APOD

★ Alpha Centauri system, APOD

★ Alpa Centauri, APOD, 2002 April 25