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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 and south. 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 River
Ocean, a meandering flow of mostly faint stars that originates with
Kursa, 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 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 in part by different estimates of the amount
of ultraviolet radiation). The radius then ranges between 8.5 to
6.6 times solar. Interferometer measures show the star to be
distinctively flattened, the result of a minimum 225 kilometer-per-
second rotation speed. The minor and major axes are respectively
measured to be 7.6 by 11.8 Suns across, for an average of 9.9 Suns,
which agrees better with that derived from the lower temperature.
The higher temperature, however, is more in tune with that
indicated by the spectral class. The temperature problem probably
has to do with the Achernar's high spin velocity, 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. As a result, the diameter and
temperature are both hard to determine. 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.