|
Xi Ursae Majoris (Xi UMa, ξ Ursae Majoris, ξ UMa) is a star system in the constellation Ursa Major. On May 2, 1780, Sir William Herschel discovered that this was a binary star system, making it the first such system ever discovered. It was the first visual double star for which an orbit was calculated, when it was computed by Félix Savary in 1828. The two components are yellow G-type main-sequence stars. The brighter component, Xi Ursae Majoris A, has a mean apparent magnitude of +4.41. It is classified as an RS Canum Venaticorum type variable star and its brightness varies by 0.01 magnitudes. The companion star, Xi Ursae Majoris B has an apparent magnitude of +4.87. The orbital period of the two stars is 59.84 years, and they are currently separated by 1.2 arcseconds, or at least 10 AUs. Each component of this double star is itself a spectroscopic binary. B's binary companion, denoted Xi Ursae Majoris Bb, is unresolved, but the binary star is known to have an orbital period of 3.98 days. The masses of both A and B's companions (Ab and Bb) (deduced by the sum total mass of the system minus the likely masses of Aa and Ba determined by their class) indicate that they are probably MV stars (red dwarfs), Bb being on the cool end of the M spectrum, not much hotter than a brown dwarf.〔http://www.astro.illinois.edu/~jkaler/sow/alulaaus.html〕 It also has the proper name Alula Australis〔Piazzi, G., ''The Palermo Catalogue'', Palermo, 1814.〕 (and erroneously ''Alula Australe''〔Bečvář, A., ''Atlas Coeli (Atlas of the Heavens) II - Catalogue'', Plague, 1964.〕). ==Name and etymology== *The traditional name ''Alula'' (shared with ν UMa) comes from the Arabic phrase "the First Spring".〔(Richard Hinckley Allen :Star Names — Their Lore and Meaning - Ursa Major, the Greater Bear )〕 The term ''Australis'' meaning "the south side" in Latin. *In Chinese, (), meaning ''Three Steps'', refers to an asterism consisting of ξ Ursae Majoris, ι Ursae Majoris, κ Ursae Majoris, λ Ursae Majoris, μ Ursae Majoris and ν Ursae Majoris. Consequently, ξ Ursae Majoris itself is known as (, (英語:the Sixth Star of Three Steps)) and (, (英語:Star of Second Lower Step)).〔 ((Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 21 日 )〕 Orbit of Xi Ursae Majoris. 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「Xi Ursae Majoris」の詳細全文を読む スポンサード リンク
|
