Epoch J2000 Equinox J2000
|Right ascension||01h 37m 42.84548s|
|Declination||–57° 14′ 12.3101″|
|Apparent magnitude (V)||0.46 (0.40 - 0.46)|
|Spectral type||B6 Vep|
|U−B color index||−0.66|
|B−V color index||−0.16|
|Radial velocity (Rv)||+16 km/s|
|Proper motion (μ)||RA: 87.00 ± 0.58 mas/yr
Dec.: −38.24 ± 0.50 mas/yr
|Parallax (π)||23.39 ± 0.57 mas|
|Distance||139 ± 3 ly
(43 ± 1 pc)
|Absolute magnitude (MV)||–2.77|
|Radius||7.3 × 11.4 R☉|
|Surface gravity (log g)||3.5 cgs|
|Rotational velocity (v sin i)||250 km/s|
|Age||1–5 × 108 years|
Achernar //, also designated Alpha Eridani (α Eridani, abbreviated Alpha Eri, α Eri), is the brightest star in the constellation of Eridanus and the tenth-brightest star in the night sky. Of the ten apparent brightest stars in the nighttime sky,[nb 1] Achernar is the hottest and bluest in color, being of spectral type B. Lying at the southern tip of Eridanus, the star has an unusually rapid rotational velocity, causing it to become oblate in shape. It is actually part of a binary star system with the second star, known informally as Achernar B, being smaller, of spectral type A and orbiting Achernar at a distance of roughly 12 astronomical units (AU).
The traditional name, sometimes spelled Achenar, originally comes from the Arabic آخر النهر ākhir an-nahr, meaning, "The End of the River". However, it seems that this name originally referred to Theta Eridani instead, which now goes by the similar name Acamar, with the same etymology. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Achernar for this star.
Alpha Eridani is the star's Bayer designation.
Achernar is a bright, blue star with about seven times the mass of the Sun. As determined by the Hipparcos astrometry satellite, it is approximately 139 light-years (43 pc) away. It is a main sequence star with a stellar classification of B6 Vep, but is about 3,150 times more luminous than the Sun. Achernar is in the deep southern sky and never rises above the horizon beyond 33°N, roughly the latitude of Dallas, Texas. Achernar is best seen from the southern hemisphere in November; it is circumpolar above (i.e. south of) 33°S, roughly the latitude of Santiago. On this latitude, e.g. the south coast of South Africa (Cape Town to Port Elizabeth) when in lower culmination it is barely visible to the naked eye as it is only 1 degree above the horizon, but still circumpolar. Further south, it is well visible at all times during night.
Until about March 2000, Achernar and Fomalhaut were the two first-magnitude stars furthest in angular distance from any other first-magnitude star in the celestial sphere. Antares, in the constellation of Scorpius, is now the most isolated first-magnitude star, although Antares is located in a constellation with many bright second-magnitude stars, whereas the stars surrounding Achernar and Fomalhaut are considerably fainter.
Infrared observations of the star using an adaptive optics system on the Very Large Telescope show that Achernar has a companion star in a close orbit. This appears to be an A-type star in the stellar classification range A0V–A3V, which suggests a stellar mass of about double that of the Sun. The separation of the two stars is roughly 12.3 AU and their orbital period is at least 14–15 years.
As of 2003, Achernar is the least spherical star in the Milky Way studied to date. It spins so rapidly that it has assumed the shape of an oblate spheroid with an equatorial diameter 56% greater than its polar diameter. The polar axis is inclined about 65° to the line of sight from the Earth. Since it is actually a binary star, its highly distorted shape may cause non-negligible departures of the companion's orbital trajectory with respect to a Keplerian ellipse. A similar situation occurs for the star Regulus.
Because of the distorted shape of this star, there is a significant temperature variation by latitude. At the pole, the temperature may be above 20,000 K, while the equator is at or below 10,000 K. The average temperature of the star is about 15,000 K. The high polar temperatures are generating a fast polar wind that is ejecting matter from the star, creating a polar envelope of hot gas and plasma. The entire star is surrounded by an extended envelope that can be detected by its excess infrared emission, or by its polarization. The presence of a circumstellar disk of ionized gas is a common feature of Be stars such as this. The disk is not stable and periodically decretes back into the star. The maximum polarization for Achernar's disk was observed in September 2014, and it is now decreasing.
Due to precession, Achernar lay much further south in ancient times than at present, being 7.5 degrees of the south pole around 3400 BCE (decl 82º40')  and still lying at declination -76 by around 1500 BCE. Hence the Ancient Egyptians could not have known it. Even in 100 CE its declination was around -67, meaning Ptolemy could not possibly have seen it from Alexandria - whereas Acamar was visible as far north as Crete. So Ptolemy's "end of the river" was certainly Acamar. Achernar was not visible from Alexandria until about 1600 CE.
The first star catalogue to contain Achernar accompanied the chart of Eridanus in Johann Bayer's Uranometria. The provenance of the chart is rather interesting, as Bayer did not observe it himself, and is attributed to Keyser and the voyages of the Dutch. Thus, it was the last first magnitude star observed and the only one unknown to the ancient Greeks.
Achernar will continue to move north in the next few millennia, rising from Crete about 500 years hence before reaching its maximum northern declination between the 8th and 11th millennia, when it will be visible as far north as Germany and southern England.
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- Evans, D. S. (June 20–24, 1966). "The Revision of the General Catalogue of Radial Velocities". In Batten, Alan Henry; Heard, John Frederick. Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30. University of Toronto: International Astronomical Union. Bibcode:1967IAUS...30...57E. Retrieved 2009-09-10.
- Kervella, P.; Domiciano de Souza, A.; Bendjoya, Ph. (June 2008), "The close-in companion of the fast rotating Be star Achernar", Astronomy and Astrophysics, 484 (1): L13–L16, arXiv:, Bibcode:2008A&A...484L..13K, doi:10.1051/0004-6361:200809765
- Carciofi, A. C.; et al. (March 2008), "On the Determination of the Rotational Oblateness of Achernar", The Astrophysical Journal, 676 (1): L41–L44, arXiv:, Bibcode:2008ApJ...676L..41C, doi:10.1086/586895
- Kervella, P.; et al. (January 2009), "The environment of the fast rotating star Achernar. II. Thermal infrared interferometry with VLTI/MIDI", Astronomy and Astrophysics, 493 (3): L53–L56, arXiv:, Bibcode:2009A&A...493L..53K, doi:10.1051/0004-6361:200810980
- "Achernar -- Be Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2010-02-16
- "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
- "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
- (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 27 日
- Hamacher, Duane W.; Frew, David J. (2010). "An Aboriginal Australian Record of the Great Eruption of Eta Carinae" (PDF). Journal of Astronomical History & Heritage. 13 (3): 220–34. arXiv:. Bibcode:2010JAHH...13..220H.
- Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; et al. (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics, 323: L49–L52, Bibcode:1997A&A...323L..49P
- Perryman, Michael (2010), "The Making of History's Greatest Star Map", The Making of History's Greatest Star Map:, Astronomers’ Universe, Heidelberg: Springer-Verlag, Bibcode:2010mhgs.book.....P, doi:10.1007/978-3-642-11602-5, ISBN 978-3-642-11601-8
- See "Achernar the Flattest star" in ‘Sky & Telescope’ P. 20 ‘Newsnotes’, September 2003.
- Carciofi, A. C.; et al. (December 2007), "Achernar: Rapid Polarization Variability as Evidence of Photospheric and Circumstellar Activity", The Astrophysical Journal, 671 (1): L49–L52, arXiv:, Bibcode:2007ApJ...671L..49C, doi:10.1086/524772
- Cotton, D. V.; et al. (January 2016). "The linear polarization of Southern bright stars measured at the parts-per-million level". Monthly Notices of the Royal Astronomical Society. 455: 1607–1628. arXiv:. Bibcode:2016MNRAS.455.1607C. doi:10.1093/mnras/stv2185 (inactive 2015-12-25).
- calculated by Stellarium 0.13, an open source sky mapping app. http://www.stellarium.org
- | Star Tales – Eridanus
- Achernar at solstation.com
- Surface temperature and synthetic spectral energy distributions for rotationally deformed stars