Alpha Eridani - Achernar

Achernar is the brightest star in the constellation Eridanus, and at magnitude 0.45 is the 9th brightest star in the night sky. It lies 57 degrees below the celestial equator at the southern tip of the River Eridanus, visible only to those who live below 32 degrees north latitude. It is easily seen only from the tropics and southern hemisphere; observers in the southernmost parts of the United States may get a glimpse of Achernar low on the horizon during the month of October.

History and Mythology

Achernar's name comes from the Arabic phrase "akhir an-nahr", meaning "the river's end." The constellation Eridanus is a meandering flow of mostly faint stars that originates with Cursa (β Eri), and ends at Achernar (α Eri). Achernar is so far south that it was not originally part of this long, thin constellation. Eridanus originally ended at Acamar (θ Eri), which was originally named Achernar. After α Eri was seen by European explorers in the 16th century, θ Eri was renamed Acamar; Achernar took θ Eri's name, and the great celestial river was allowed to flow farther south.

Physical Properties

At a distance of 144 light years, Achernar is a hot class B3 Vpe main sequence star, fusing hydrogen into helium in its deep core. It is the hottest of the nine brightest stars in the sky; various temperature measures range from 14,500 to 19,300 K. Its overall luminosity is 2,900 to 5,400 times the Sun's (depending on the estimate of ultraviolet radiation, which in turn depends on the temperature). Achernar is massive, containing 6 - 8 solar masses of material. It is also very young, no more than a few hundred million years old, and will eventually become a massive white dwarf. Achernar is also a "λ Eri" variable that shows tiny, but very regular, light variations over a period of 1.26 days. These may be caused by actual complex pulsations, or by rotation bringing dark starspots in and out of view.

Interferometer measures show Achernar to be distinctly oblate, the result of a 225 km/sec rotation speed. Its major and minor axes are measured, respectively, to be 12.0 and 7.7 times the Sun's. In fact, it is the least spherical star in the Milky Way studied to date. Its average radius of 9.9 times the Sun's gives it a rotation period of 2.2 days. The temperature measurement problem has to do with Achernar's high spin velocity. In an oblate star, the poles are hotter than the equator, so there is a range of temperatures across its surface. Fast rotation also makes Achernar a "Be" or "B-emission" star, generating a hot belt of emitting gas around in its equator. In fact, Achernar is losing mass at a rate thousands of times the Sun's.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]