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ACHERNAR (Alpha Eridani). There are 22 classical "first magnitude" stars in the sky. Ofthese, nine are so bright that in modern times they had to beplaced into even brighter categories, seven into "zeroth" magnitude(the brightest of which isAlphaCentauri) and two (Canopus andSirius) into the exclusive "minus-firstmagnitude" group. Of these nine,Achernar ranks last, right behindProcyon inCanisMinor and just beating outHadar insouthern Centaurus. (Because of itsvariability,Betelgeuse inOrion can sometimes make zeroth magnitudeas well.) Achernar, however, is nowhere nearly as well known tonortherners, as it is a deep southern star, visible only to thosewho live below 32 degrees north latitude, and easily noted onlyfrom the tropics and south. The name, from an Arabic phrase, means"the end of the river," as appropriate for the star that ends thesoutherly flow ofEridanus, the River,the celestial depiction of River Ocean, a meandering flow of mostlyfaint stars that originates withCursa, onwhich Orion rests his foot. Appropriate to its brilliance,Achernar is also the Alpha star, while Cursa, number two, is theBeta. Achnernar is so far south that it was not originally part ofthis long, thin constellation, which originally ended atAcamar (Theta Eridani), from which Achernartook its name when the river was allowed in more modern times toflow farther to the south. Achernar, a hot class B (B3) dwarf, isthe hottest of the top nine, rather handily beating outRigel in Orion. Yet surprisingly, for sucha bright star, its temperature is not well known, various measuresrunning from 14,500 to 19,300 Kelvin. From its distance of 140light years (second Hipparcos reduction, the lower temperature gives a luminosity 2700 timesthat of theSun, while the upper gives 5100(the difference caused in part by different estimates of the amountof ultraviolet radiation). The radius then ranges between 8.2 and 6.4 times solar. Interferometer measures show the star to bedistinctively flattened, the result of a minimum 225kilometer-per-second rotation speed. The projected minor and major axes arerespectively measured to be 7.5 by 11.6 Suns across(which gives a rotation period of under 2.1 days), whichagrees better with that derived from the lower temperature. Thehigher temperature, however, is more in tune with that indicated bythe spectral class. The temperature problem has to do withAchernar's high spin velocity, which helps turn it into a "Be," or"B-emission," star that has a belt of emitting gas circulating inits equator, Achernar losing mass at a rate thousands of times thatof the Sun. As a result, the diameter and temperature are bothhard to determine. Equally, if not more, important, in an oblatestar, the poles are hotter than the equator such that there reallyis no single defining temperature. Achernar is also a member of apeculiar class of "Lambda Eridani" stars that show tiny but veryregular periodic light variations that may be caused by actualcomplex pulsations or by rotation and dark "starspots." No onereally knows. We do know, however, that Achernar is massive,containing six to eight times the solar mass. It is now normallyfusing hydrogen into helium in its deep core and will eventuallydie as a massivewhite dwarf likeSirius B.All this action is watched by a closecompanion that averagesroughly a dozen Astronomical Units away from Achernar proper withan orbital period estimated to be around 12 years. Infraredobservations suggest that it is a warm class A (A1-A3) dwarfsimilar toVega or Sirius. Written byJim Kaler 12/24/99; last updated1/28/11). Return toSTARS.
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