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In observational astronomy, a double star is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope. This can happen either because the pair forms a binary star, i.e. a binary system of stars in mutual orbit, gravitationally bound to each other, or because it is an ''optical double'', a chance alignment of two stars in the sky that lie at different distances.〔''The Binary Stars'', Robert Grant Aitken, New York: Dover, 1964, p. 1.〕 Binary stars are important to stellar astronomers as knowledge of their motions allows direct calculation of stellar mass and other stellar parameters. Since the beginning of the 1780s, both professional and amateur double star observers have telescopically measured the distances and angles between double stars to determine the relative motions of the pairs. If the relative motion of a pair determines a curved arc of an orbit, or if the relative motion is small compared to the common proper motion of both stars, it may be concluded that the pair is in mutual orbit as a binary star. Otherwise, the pair is optical.〔 Multiple stars are also studied in this way, although the dynamics of multiple stellar systems are more complex than those of binary stars. There are three types of paired stars: # optical doubles—unrelated stars that appear close together through chance alignment with Earth # visual binaries—gravitationally-bound stars that are separately visible with a telescope # non-visual binaries—stars whose binary status was deduced through more esoteric means such as occultation (eclipsing binaries), spectroscopy (spectroscopic binaries), or anomalies in proper motion (astrometric binaries). Conceptually, there is no difference between the latter two categories, and improvements in telescopes can shift previously non-visual binaries into the visual class, as happened with Polaris in 2006. Thus it is only the inability to observe the third group telescopically that makes the difference. ==History== Mizar, in Ursa Major, was observed to be double by Giovanni Battista Riccioli in 1650〔〔(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 identification of other doubles soon followed: Robert Hooke discovered one of the first double-star systems, Gamma Arietis, in 1664, while the bright southern star Acrux, in the Southern Cross, was discovered to be double by Fontenay in 1685.〔 Since that time, the search has been carried out thoroughly and the entire sky has been examined for double stars down to a limiting apparent magnitude of about 9.0.〔See ''The Binary Stars'', Robert Grant Aitken, New York: Dover, 1964, pp. 24–25, 38, and p. 61, The present status of double star astronomy, K. Aa. Strand, ''Astronomical Journal'' 59 (March 1954), pp. 61–66, .〕 At least 1 in 18 stars brighter than 9.0 magnitude in the northern half of the sky are known to be double stars visible with a telescope.〔''The Binary Stars'', Robert Grant Aitken, New York: Dover, 1964, p. 260.〕 The unrelated categories of optical doubles and true binaries are lumped together for historical and practical reasons. When Mizar was found to be a binary, it was quite difficult to determine whether a double star was a binary system or only an optical double. Improved telescopes, spectroscopy,〔Fraunhofer, 1814〕 and photography are the basic tools used to make the distinction. After it was determined to be a visual binary, Mizar's components were found to be spectroscopic binaries themselves.〔Pickering, 1889〕 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「Double star」の詳細全文を読む スポンサード リンク
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