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A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two, three, four, or even more stars are called ''multiple star systems''. These systems, especially when more distant, often appear to the unaided eye as a single point of light, and are then revealed as double (or more) by other means. Research over the last two centuries suggests that half or more of visible stars are part of multiple star systems.〔Filippenko, Alex, ''Understanding the Universe'' (of ''The Great Courses'' on DVD), Lecture 46, time 1:17, The Teaching Company, Chantilly, VA, USA, 2007〕 The term ''double star'' is often used synonymously with ''binary star''; however, double star can also mean ''optical double star''. Optical doubles are so called because the two stars appear close together in the sky as seen from the Earth; they are almost on the same line of sight. Nevertheless, their "doubleness" depends only on this optical effect; the stars themselves are distant from one another and share no physical connection. A double star can be revealed as optical by means of differences in their parallax measurements, proper motions, or radial velocities. Most known double stars have not been studied sufficiently closely to determine whether they are optical doubles or they are doubles physically bound through gravitation into a multiple star system. Binary star systems are very important in astrophysics because calculations of their orbits allow the masses of their component stars to be directly determined, which in turn allows other stellar parameters, such as radius and density, to be indirectly estimated. This also determines an empirical mass-luminosity relationship (MLR) from which the masses of single stars can be estimated. Binary stars are often detected optically, in which case they are called ''visual binaries''. Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known. They may also be detected by indirect techniques, such as spectroscopy (''spectroscopic binaries'') or astrometry (''astrometric binaries''). If a binary star happens to orbit in a plane along our line of sight, its components will eclipse and transit each other; these pairs are called ''eclipsing binaries'', or, as they are detected by their changes in brightness during eclipses and transits, ''photometric binaries''. If components in binary star systems are close enough they can gravitationally distort their mutual outer stellar atmospheres. In some cases, these ''close binary systems'' can exchange mass, which may bring their evolution to stages that single stars cannot attain. Examples of binaries are Sirius and Cygnus X-1 (Cygnus X-1 being a well known black hole). Binary stars are also common as the nuclei of many planetary nebulae, and are the progenitors of both novae and type Ia supernovae. ==Discovery== The term ''binary'' was first used in this context by Sir William Herschel in 1802,〔 when he wrote: By the modern definition, the term ''binary star'' is generally restricted to pairs of stars which revolve around a common center of mass. Binary stars which can be resolved with a telescope or interferometric methods are known as ''visual binaries''.〔(【引用サイトリンク】 Visual Binaries )〕 For most of the known visual binary stars one whole revolution has not been observed yet, they are observed to have travelled along a curved path or a partial arc. The more general term ''double star'' is used for pairs of stars which are seen to be close together in the sky.〔''The Binary Stars'', Robert Grant Aitken, New York: Dover, 1964, p. ix.〕 This distinction is rarely made in languages other than English.〔 Double stars may be binary systems or may be merely two stars that appear to be close together in the sky but have vastly different true distances from the Sun. The latter are termed ''optical doubles'' or ''optical pairs''. Since the invention of the telescope, many pairs of double stars have been found. Early examples include Mizar and Acrux. Mizar, in the Big Dipper (Ursa Major), was observed to be double by Giovanni Battista Riccioli in 1650〔''The Binary Stars'', Robert Grant Aitken, New York: Dover, 1964, p. 1.〕〔(Vol. 1, part 1, p. 422, ''Almagestum Novum'' ), Giovanni Battista Riccioli, Bononiae: Ex typographia haeredis Victorij Benatij, 1651.〕 (and probably earlier by Benedetto Castelli and Galileo).〔(A New View of Mizar ), Leos Ondra, accessed on line May 26, 2007.〕 The bright southern star Acrux, in the Southern Cross, was discovered to be double by Father Fontenay in 1685.〔 John Michell was the first to suggest that double stars might be physically attached to each other when he argued in 1767 that the probability that a double star was due to a chance alignment was small.〔pp. 10–11, ''Observing and Measuring Double Stars'', Bob Argyle, ed., London: Springer, 2004, ISBN 1-85233-558-0.〕〔pp. 249–250, (An Inquiry into the Probable Parallax, and Magnitude of the Fixed Stars, from the Quantity of Light Which They Afford us, and the Particular Circumstances of Their Situation ), John Michell,''Philosophical Transactions (1683–1775)'' 57 (1767), pp. 234–264.〕 William Herschel began observing double stars in 1779 and soon thereafter published catalogs of about 700 double stars. By 1803, he had observed changes in the relative positions in a number of double stars over the course of 25 years, and concluded that they must be binary systems;〔(Account of the Changes That Have Happened, during the Last Twenty-Five Years, in the Relative Situation of Double-Stars; With an Investigation of the Cause to Which They Are Owing ), William Herschel, ''Philosophical Transactions of the Royal Society of London'' 93 (1803), pp. 339–382.〕 the first orbit of a binary star, however, was not computed until 1827, when Félix Savary computed the orbit of Xi Ursae Majoris.〔p. 291, French astronomers, visual double stars and the double stars working group of the Société Astronomique de France, E. Soulié, ''The Third Pacific Rim Conference on Recent Development of Binary Star Research'', proceedings of a conference sponsored by Chiang Mai University, Thai Astronomical Society and the University of Nebraska-Lincoln held in Chiang Mai, Thailand, 26 October-1 November 1995, ''ASP Conference Series'' 130 (1997), ed. Kam-Ching Leung, pp. 291–294, .〕 Since this time, many more double stars have been catalogued and measured. The Washington Double Star Catalog, a database of visual double stars compiled by the United States Naval Observatory, contains over 100,000 pairs of double stars,〔"Introduction and Growth of the WDS", (The Washington Double Star Catalog ), Brian D. Mason, Gary L. Wycoff, and William I. Hartkopf, Astrometry Department, United States Naval Observatory, accessed on line August 20, 2008.〕 including optical doubles as well as binary stars. Orbits are known for only a few thousand of these double stars,〔(Sixth Catalog of Orbits of Visual Binary Stars ), William I. Hartkopf and Brian D. Mason, United States Naval Observatory, accessed on line August 20, 2008.〕 and most have not been ascertained to be either true binaries or optical double stars.〔(The Washington Double Star Catalog ), Brian D. Mason, Gary L. Wycoff, and William I. Hartkopf, United States Naval Observatory. Accessed on line December 20, 2008.〕 This can be determined by observing the relative motion of the pairs. If the motion is part of an orbit, or if the stars have similar radial velocities and the difference in their proper motions is small compared to their common proper motion, the pair is probably physical. One of the tasks that remains for visual observers of double stars is to obtain sufficient observations to prove or disprove gravitational connection. 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「Binary star」の詳細全文を読む スポンサード リンク
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