|
A star system or stellar system is a small number of stars that orbit each other,〔"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〕 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. 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. A multiple star system consists of three or more stars that appear from Earth to be close to one another in the sky. This may result from the stars actually being physically close and gravitationally bound to each other, in which case it is a ''physical'' multiple star, or this closeness may be merely apparent, in which case it is an ''optical'' multiple star (meaning that the stars may appear to be close to each other when viewed from planet Earth, as they both seem to occupy the same point in the sky, but in reality, one star may be much further away from Earth than the other, which is not readily apparent unless one can view them from a different angle). Physical multiple stars are also commonly called ''multiple stars'' or ''multiple star systems''.〔p. 16, ''Understanding Variable Stars'', John R. Percy, Cambridge: Cambridge University Press, 2007, ISBN 0-521-23253-8.〕〔〔(Hipparcos: Double and Multiple Stars ), web page, accessed October 31, 2007.〕〔(MSC – a catalogue of physical multiple stars ), A. A. Tokovinin, ''Astronomy and Astrophysics Supplement Series'' 124 (July 1997), pp. 75–84.〕〔(Binary and Multiple Stars ), web page, accessed May 26, 2007.〕 Most multiple star systems are ''triple stars''. Systems with four or more components are less likely to occur.〔 Multiple-star systems are called ''triple'', ''trinary'' or ''ternary'' if they contain three stars; ''quadruple'' or ''quaternary'' if they contain four stars; ''quintuple'' or ''quintenary'' with five stars; ''sextuple'' or ''sextenary'' with six stars; ''septuple'' or ''septenary'' with seven stars. These systems are smaller than open star clusters, which have more complex dynamics and typically have from 100 to 1,000 stars.〔p. 24, ''Galactic Dynamics'', James Binney and Scott Tremaine, Princeton University Press, 1987, ISBN 0-691-08445-9.〕 Most multiple star systems known are triple; for higher multiplicities, the number of known systems with a given multiplicity decreases exponentially with multiplicity.〔(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, 10–15 April 2000. Bibcode (2001IAUS..200...84T ).〕 For example, in the 1999 revision of Tokovinin's catalog〔(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 ).〕 of physical multiple stars, 551 out of the 728 systems described are triple. However, because of selection effects, knowledge of these statistics is very incomplete.〔(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, section 2〕 Multiple-star systems can be divided into two main dynamical classes: hierarchical systems which are stable and consist of nested orbits that don't interact much and so each level of the hierarchy can be treated as a ''Two-body problem'', or the trapezia which have unstable strongly interacting orbits and are modelled as an ''n-body problem'', exhibiting chaotic behavior.〔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.〕 ==Hierarchical systems== Most multiple-star systems are organized in what is called 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.〔(Stars of Higher Multiplicity ), David S. Evans, ''Quarterly Journal of the Royal Astronomical Society'' 9 (1968), 388–400.〕 Each level of the hierarchy can be treated as a ''two-body problem'' by considering close pairs as if they were a single star. In these systems there is little interaction between the orbits and the stars' motion will continue to approximate stable〔 Keplerian orbits around the system's center of mass,〔(Dynamics of multiple stars: observations ), A. Tokovinin, in "Massive Stars in Interacting Binaries", 16–20 August 2004, Quebec (ASP Conf. Ser., in print).〕 unlike the unstable trapezia systems or the even more complex dynamics of the large number of stars in star clusters and galaxies. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Star system」の詳細全文を読む スポンサード リンク
|