Thanks to James B. Kaler. These contents are the property of the author and are reproduced from original without the author's express consent because of fair use and valid educational purposes.
ACHERNAR
(Alpha Eridani). There are 21 classical "first magnitude" stars in the sky. Of these, 10 are so bright that in modern times they had to be placed into even brighter categories, seven into "zeroth" magnitude (the brightest of which is Alpha Centauri) and two, Canopus and Sirius, into the exclusive "minus-first magnitude" group. Of these 10, Achernar ranks number nine, right behind Procyon in Canis Minor and just beating out Betelgeuse in Orion. Achernar, however, is nowhere nearly as well known to northerners, as it is a deep southern star, visible only to those who live below 32 degrees north latitude, and easily noted only from the tropics. The name, from an Arabic phrase, means "the end of the river," as appropriate for the star that ends the southerly flow of Eridanus, the River, the celestial depiction of "Ocean Stream," a meandering flow of mostly faint stars that originates with Cursa, on which Orion rests his foot. Appropriate to its brilliance, Achernar is also the Alpha star, while Cursa, number two, is the Beta. Achnernar is so far south that it was not originally part of this long, thin constellation, which originally ended at Acamar (Theta Eridani), from which Achernar took its name when the river was allowed in more modern times to flow farther to the south. Achernar, a hot, blue class B star, is the hottest of the top ten, rather handily beating out Rigel in Orion. Yet surprisingly, for such a bright star, its temperature is not well known, various measures running from 14,500 to 19,300 Kelvin. From its distance of 144 light years, the lower temperature gives a luminosity 2900 times that of the Sun, while the upper gives 5400 (the difference caused by different estimates of the amount of ultraviolet radiation). The radius then ranges from 8.5 to 6.6 times solar, the former agreeing well with direct measures of angular diameter. The temperature problem probably has to do with the Achernar's high spin velocity of at least 250 kilometers per second, which helps turn it into a "Be," or "B- emission" star that has a belt of emitting gas circulating in its equator, Achernar losing mass at a rate thousands of times that of the Sun. Achernar is also a member of a peculiar class of "Lambda Eridani" stars that show tiny but very regular periodic light variations that may be caused by actual complex pulsations or by rotation and dark "starspots." No one really knows. We do know, however, that Achernar is massive, containing six to eight times the solar mass. It is now normally fusing hydrogen into helium in its deep core and will eventually die as a massive white dwarf like Sirius-B.