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}} Achernar (α Eri, α Eridani, Alpha Eridani), sometimes spelled Achenar, is the brightest star in the constellation Eridanus and the tenth-brightest star in the night sky. Of the ten apparent brightest stars in the nighttime sky, Achernar is the hottest and bluest in color, being of spectral type B. Lying at the southern tip of Eridanus, the star has an unusually rapid rotational velocity, causing it to become oblate in shape. Achernar is actually a binary star system,〔 with the second star known as ''Achernar B''. The second star is smaller and orbits Achernar A at a distance of roughly 12 astronomical units (AU). Achernar B is of spectral type A. ==Physical properties== Achernar is a bright, blue star with about seven times the mass of the Sun.〔 As determined by the Hipparcos astrometry satellite,〔〔 it is approximately away.〔 It is a main sequence star with a stellar classification of B6 Vep, but is about 3,150 times more luminous than the Sun. Achernar is in the deep southern sky and never rises above the horizon beyond 33°N, roughly the latitude of Dallas, Texas. Achernar is best seen from the southern hemisphere in November; it is circumpolar above (i.e. south of) 33°S, roughly the latitude of Santiago. On this latitude, e.g. the south coast of South Africa (Cape Town to Port Elizabeth) when in lower culmination it is barely visible to the naked eye as it is only 1 degree above the horizon, but still circumpolar. Further south, it is well visible at all times during night. Until about March 2000, Achernar and Fomalhaut were the two first-magnitude stars furthest in angular distance from any other first-magnitude star in the celestial sphere. Antares, in the constellation of Scorpius, is now the most isolated first-magnitude star, although Antares is located in a constellation with many bright second-magnitude stars, whereas the stars surrounding Achernar and Fomalhaut are considerably fainter. Infrared observations of the star using an adaptive optics system on the Very Large Telescope show that Achernar has a companion star in a close orbit. This appears to be an A-type star in the stellar classification range A0V–A3V, which suggests a stellar mass of about double the Sun's mass. The separation of the two stars is roughly 12.3 AU and their orbital period is at least 14–15 years.〔 As of 2003, Achernar is the least spherical star in the Milky Way studied to date.〔See ''"Achernar the Flattest star"'' in ''‘Sky & Telescope’'' P. 20 ''‘Newsnotes’'', September 2003.〕 It spins so rapidly that it has assumed the shape of an oblate spheroid with an equatorial diameter 56% greater than its polar diameter. The polar axis is inclined about 65° to the line of sight from the Earth.〔 Since it is actually a binary star, its highly distorted shape may cause non-negligible departures of the companion's orbital trajectory with respect to a Keplerian ellipse. A similar situation occurs for the star Regulus. Because of the distorted shape of this star, there is a significant temperature variation by latitude. At the pole, the temperature may be above 20,000 K, while the equator is at or below 10,000 K. The average temperature of the star is about 15,000 K. The high polar temperatures are generating a fast polar wind that is ejecting matter from the star, creating a polar envelope of hot gas and plasma. The entire star is surrounded by an extended envelope that can be detected by its excess infrared emission.〔 The presence of a circumstellar disk of ionized gas is a common feature of Be stars such as this.〔 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「Achernar」の詳細全文を読む スポンサード リンク
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