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Alpha Arae (α Arae, α Ara) is the second brightest star in the southern constellation of Ara. With an average apparent visual magnitude 2.93,〔 it is readily visible to the naked eye from the southern hemisphere. This star is close enough to the Earth that its distance can be estimated using parallax data collected during the Hipparcos mission. It is around away, with a 7% margin of error. The visual magnitude of the star is diminished by 0.10 magnitudes as a result of extinction from intervening gas and dust.〔 ==Properties== Alpha Arae has a stellar classification of B2 Vne,〔 indicating that it is a massive B-type main sequence star. The 'n' notation in the suffix indicates that the absorption lines in the star's spectrum appear spread out and nebulous because of the Doppler effect from rapid rotation. The measured projected rotational velocity has been measured as high as .〔 Meilland et al. (2007) estimate the pole of the star is inclined by 55° to the line of sight, yielding an equatorial azimuthal velocity of . This is close to the critical velocity where the star would start to breakup.〔 The rapid rotation is causing a pronounced equatorial bulge of about 2.4–2.7 times the polar radius.〔 It is a Be star, as indicated by the 'e' notation in the star's classification. This indicates that emission lines are observed in the spectrum, which is coming from a disk of material ejected from the star because of its rapid rotation.〔(To Be or Not to Be: Is It All About Spinning?: VLTI Discerns How Matter Behaves in Disc Around a Be Star ), ESO press release 35/06, September 20, 2006. Accessed on line December 12, 2008.〕 In 2003 and 2005, Alpha Arae was observed by infrared interferometry, using the MIDI and AMBER instruments at the VLT Interferometer. The results, published in 2005 and 2007, appear to show that Alpha Arae is surrounded by a dense equatorial disk of material in Keplerian (rather than uniform) rotation, and that it is losing mass by a polar stellar wind with a terminal velocity of approximately 1,000 km/s. There is also some evidence that Alpha Arae is orbited by a companion at 0.7 AU.〔〔First VLTI/MIDI observations of a Be star: Alpha Arae, O. Chesneau et al., ''Astronomy and Astrophysics'' 435, #1 (May 2005), pp. 275–287, , .〕 This star has around 9.6〔 times as much mass as the Sun and an average of 4.5〔 times the Sun's radius. It is emitting 5,800〔 as much luminosity as the Sun from its outer envelope at an effective temperature of 18,044 K.〔 This heat gives Alpha Arae the blue-white hue that is characteristic of B-type stars. It is a variable star with a magnitude that varies between 2.76m and 2.90m.〔〔 Alpha Arae has a visual companion star, ''CCDM J17318-4953B'', located approximately 50 arcseconds away along a position angle of 168°, with an apparent visual magnitude of about 11.〔Entry 17318-4953, (The Washington Double Star Catalog ), United States Naval Observatory. Accessed on line November 26, 2008.〕 The two stars appear close to each other by coincidence and are not physically close in space.〔 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 Arae」の詳細全文を読む スポンサード リンク
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