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Alpha Ophiuchi (α Oph, α Ophiuchi) is the brightest star in the constellation Ophiuchus. It has the traditional name Ras Alhague, often condensed to Rasalhague. Alpha Ophiuchi is a binary star system with an orbital period of about 8.62 years. The orbital parameters were only poorly known until 2011 when observations using adaptive optics produced a better orbital fit, allowing the individual masses of the two components to be determined. The primary component, Alpha Ophiuchi A, has a mass of about 2.4 times the mass of the Sun, while the secondary, Alpha Ophiuchi B, has 0.85 solar masses.〔 Estimates of the mass of the primary by other means range from a low of 1.92 to 2.10 solar masses, up to 2.84 or even 4.8 solar masses.〔 The mass of the secondary suggests that it has a stellar classification in the range K5V to K7V, which indicates it is a main sequence star that is still generating energy by the thermonuclear fusion of hydrogen at its core. The pair reached periastron passage, or closest approach, around April 19, 2012, when they had an angular separation of 50 milliarcseconds.〔 This star system has a combined apparent magnitude of +2.08 and is located at a distance of about from the Earth. The stellar classification of A5 III indicates that the primary is a giant star that has evolved away from the main sequence after consuming the hydrogen at its core. It is radiating about 25 times the luminosity of the Sun and has an effective temperature of about 8,000 K, giving it the characteristic white hue of an A-type star.〔〔 Alpha Ophiuchi A is a rapidly rotating star with a projected rotational velocity of 240 km s−1.〔 It is spinning at a rate of about 88.5% of the velocity that would cause the star to break up.〔 The resulting equatorial bulge is about 20% larger than the polar radius, giving the star the shape of an oblate spheroid. Because of this distorted shape, the poles have an effective temperature about 1,840 K greater than along the equator.〔 The axis of rotation of the star is inclined about to the line of sight from the Earth, so that it is being observed from nearly equator-on.〔 The spectrum of Alpha Ophiuchi shows an anomalously high level of absorption of the lines for singly-ionized calcium (Ca II). However, this is likely the result of interstellar matter between the Earth and the star, rather than a property of the star or circumstellar dust.〔 ==Name and etymology== *The name Ras Alhague is from the Arabic رأس الحواء (''raʾs al-ḥawwāʾ''), meaning "the Head of the Serpent collector".〔 *The Chinese name 候 (Mandarin: Hòu) meaning ''Astrologer'', because this star is marking itself and stand alone in ''Astrologer'' asterism, Heavenly Market enclosure (see : Chinese constellation). 〔 (AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 25 日 )〕 候 (Hòu) westernized into How in R.H. Allen's work, meaning "the Duke". 〔 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「Alpha Ophiuchi」の詳細全文を読む スポンサード リンク
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