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AAO/ROE, photo by
(Used with permission)
Larger labelled image of the Pleiades,
or a black and white photo of Achernar
from Per Gröningsson.
Achernar is a blue-white star that is
larger, brighter, and hotter than
Sirius A, like six of the brightest
stars of the Pleiades Cluster at left.
Achernar, or Alpha Eridani, is located around 144 light-years (ly) from Sol (HIPPARCOS Plx of 22.68, +/- e_Plx of 0.57 mas). The star constitutes the southern end (1:37:42.8-57:14:12.3, ICRS 2000.0) of Constellation Eridanus, the River, of which it is the brightest. In the venerable Star Names: Their Lore and Meaning, Richard Hinckley Allen noted that the star's proper name, Achernar, is derived from "Al Ahir al Nahr" from the Arabic for "The End of the River," as it now marks the southern end of this great celestial river that once ended at Theta Eridani, which was renamed from the original Achernar to Acamar after Alpha Eridani was seen by European explorers in the 16th century. Achernar is the ninth brightest star in Earth's night sky, beating out Betelgeuse because of its variability. However, the star lies only 32 degrees from the South Pole and so is not well known to those living in the northern hemisphere.
Achernar is hotter,
brighter, and bluer
than Vega or Sirius A,
but is slightly cooler
and smaller than Spica.
See a discussion of
the "main sequence"
as part of stellar
evolution and death.
Achernar is a blue-white main sequence star of spectral and luminosity type B3 Vpe (Hiltner et al, 1969), that previously had been classed as bright as a subgiant. The star has a mass that is six to eight times greater than Sol's (see Petr Harmanec, 1988; and James Kaler), 14.4 (+/- 0.4, polar) to 24.0 (+/- 0.8, equatorial) times its diameter (ESO; and Domicano de Souza et al, 2003), and 1,070 times its visual luminosity and at least 2,900 to 5,400 times its bolometric luminosity (depending on the estimate of ultraviolet radiation). A fast-spinning hot star, it is also very young with no more than a few hundred million years, like Regulus A. Ejecting mass at a rate thousands of times greater than Sol, Achernar's high spin velocity of 225 to 300 kilometers per second has helped to turn it into a "Be" (B-emission) star, which has an expanding circumstellar envelope (CSE) of gas circulating around its equator with "episodic Balmer lined in emissions" when its CSE is enlarged by mass ejections (Domiciano de Souza et al, 2003; and Oegerle and Polidan, 1984). As a very young high-mass star, Achernar is a fast rotator with a rotational period of hours and a substantial magnetic field around one kG. Useful catalogue numbers and designations for this star include: Alp or Alf Eri, Eri, HR 472*, Hip 7588, HD 10144, CP(D)-57 334, SAO 232481, and FK5 54.
According to Professor Jim Kaler's Stars page on Achernar, the star's high spin velocity of at least 250 kilometers per second contributes to its status as a "Be" (B-emission) star that has a belt of emitting gas circulating in its equator, causing the star to lose mass at a rate thousands of times greater than Sol's. Achernar is also a member of a peculiar class of Lambda Eridani-type stars that show small but very regular periodic light variations (with a period of 1.26 days) that may be caused by actual complex pulsations or by rotation and dark "starspots" (Balona et al, 1987). Although Achernar is a massive star, it is still young enough to be fusing hydrogen into helium in its core and may be small enough to evolve off the sequence as a massive white dwarf like Sirius B.
In July 2003, a team of astronomers (including Armando Domiciano de Souza, Lyu Abe, Farrokh Vakili, Pierre Kervella, Slobodan Jankov, Emmanuel DiFolco, and Francesco Paresce) announced that Achernar is much more oblate than originally expected, with an equatorial radius that is more than 50 percent larger than its polar one -- a surprisingly high axial ratio of 1.56 ± 0.05 (ESO press release; and Domiciano de Souza et al, 2003). According to the ESO team, the angular size of Achernar's elliptical profile is 0.00253 ± 0.00006 arcsec (major axis) and 0.00162 ± 0.00001 arcsec (minor axis), respectively. At its measured distance, the star's equatorial radius is estimated to be 12.0 ± 0.4 and Solar radii, while the upper value of its polar radius was calculated to be 7.7 ± 0.2 times the Sol's -- or 8.4 and 5.4 million kilometers (5.2 and 3.4 million miles), respectively. The ESO's estimates were upper values that are dependent on the actual inclination of the star's polar axis to their line-of-sight from Earth, and so they may well be slightly smaller. On the other hand, the high degree of flattening measured for Achernar cannot be reproduced by common models of stellar interiors unless certain phenomena are assumed, including meridional circulation on the surface ("north-south streams") and non-uniform rotation at different depths inside the star. A side effect of the extreme oblateness would be a high rate of mass loss from the surface which is enhanced by the rapid rotation through the centrifugal effect. (See Kenneth Croswell's discussion of the relative oblateness of Regulus A with Altair and Acherar.)
Accounting for its intense ultraviolet radiation, the distance from Achernar where an Earth-type planet would be "comfortable" with liquid water is center around 54 to 73 AUs -- beyond orbital distance of Pluto in the Solar System. At such distances from the star, such a planet would have an orbital period of around 160 to 260 Earth years. If there is life on any Earth-type planet that has had time to cool to hold water orbiting very youthful Achernar, it is likely to be primitive single-cell, anaerobic (non-oxygen producing) bacteria under constant bombardment by meteorites and comets as Earth was for the first billion years. Since there is unlikely to be free oxygen in the atmosphere of such a planet, it probably would not have an ozone layer (O3) although Achernar puts out a lot more hard radiation (especially ultraviolet) than Sol. Astronomers would find it very difficult to detect an Earth-sized planet around this star using present methods.
The following table includes some star systems known to be located within 10 light-years (ly), plus more bright stars within 10 to 20 ly, of Achernar.
|Star System||Spectra &|
|CP-59 127||G6 V||3.4|
|CP-56 318||K5 V||4.9|
|CP-58 138||K2 V||10|
|* plus bright stars *||. . .|
|CP-60 182||F8 V||12|
|HR 460||F2 V||12|
|CP-61 58||G1-2 V||15|
|CP-61 124||G1 V||17|
|CD-51 311||F5 V||19|
Try Professor Jim Kaler's Stars site for other information about Achernar at the University of Illinois' Department of Astronomy. Older catalogue information on Achernar is also available from www.alcyone.de.
Up-to-date technical summaries on this star can be found at: the HIPPARCOS Catalogue using the VizieR Search Service mirrored from the Centre de Données astronomiques de Strasbourg (CDS); NASA's ADS Abstract Service for the Astrophysics Data System; the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access; and the NSF-funded, arXiv.org Physics e-Print archive's search interface.
Eridanus, the River, wends its way from the Hunter's foot of Orion then southwest to the southern circumpolar zone to enclose a larger area of sky than any other constellation. For more information on stars and other objects in Constellation Eridanus and an illustration, go to Christine Kronberg's Eridanus. Another illustration is available at David Haworth's Eridanus.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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