Achernar

Achernar /ˈeɪkərnɑːr/[12] is the primary (or 'A') component[13] of the binary system[7] designated Alpha Eridani (α Eridani, abbreviated Alpha Eri, α Eri), which is the brightest star in the constellation of Eridanus, and the ninth-brightest in the night sky. The two components are designated Alpha Eridani A (the primary) and B (the secondary, also known informally as Achernar B). As determined by the Hipparcos astrometry satellite,[14][15] it is approximately 139 light-years (43 pc) from the Sun.[1]

Not to be confused with Achenar.
Achernar
Location of Achernar (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Eridanus
Right ascension  01h 37m 42.84548s[1]
Declination –57° 14′ 12.3101″[1]
Apparent magnitude (V) 0.40 - 0.46[2]
Characteristics
Spectral type B6 Vep[3]
U−B color index −0.66[4]
B−V color index −0.16[4]
Variable type Be[2]
Astrometry
Radial velocity (Rv)+16[5] km/s
Proper motion (μ) RA: 87.00 ± 0.58[1] mas/yr
Dec.: −38.24 ± 0.50[1] mas/yr
Parallax (π)23.39 ± 0.57[1] mas
Distance139 ± 3 ly
(43 ± 1 pc)
Absolute magnitude (MV)–1.46[6]
Details
Mass6.7[7] M☉
Radius7.3 × 11.4[8] R☉
Luminosity3,150[8] L☉
Surface gravity (log g)3.5[9] cgs
Temperature~15,000[9] K
Rotational velocity (v sin i)250[9] km/s
Age37.3[10] Myr
Other designations
α Eri, CD -57°334, FK5 54, HD 10144, HIP 7588, HR 472, SAO 232481,[11] 70 Eri, 2 G. Eri, 水委一
Database references
SIMBADdata

Of the ten apparent brightest stars in the night-time sky,[nb 1] Alpha Eridani is the hottest and bluest in color, due to Achernar being of spectral type B. Achernar has an unusually rapid rotational velocity, causing it to become oblate in shape. The secondary is smaller, of spectral type A, and orbits Achernar at a distance of roughly 12 astronomical units (AU).

Nomenclature

α Eridani (Latinised to Alpha Eridani) is the system's Bayer designation. The designations of the two components - Alpha Eridani A and B - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[16]

The system bears the traditional name of Achernar (sometimes spelled Achenar), derived 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 latterly was known by the similar traditional name Acamar, with the same etymology.[17] The IAU Working Group on Star Names (WGSN) approved the name with the spelling Achernar for the component Alpha Eridani A on 30 June 2016 and it is now so included in the List of IAU-approved Star Names.[13][18][19]

In Chinese caused by adaptation of the European southern hemisphere constellations into the Chinese system, 水委 (Shuǐ Wěi), meaning Crooked Running Water, refers to an asterism consisting of Achernar, ζ Phoenicis and η Phoenicis. Consequently, Achernar itself is known as 水委一 (Shuǐ Wěi yī, English: the First Star of Crooked Running Water.)[20]

The indigenous Boorong people of northwestern Victoria named it Yerrerdetkurrk.[21]

Namesake

USS Achernar (AKA-53) was a United States Navy attack cargo ship.

Properties
Extreme rotation speed has flattened Achernar.

Achernar is in the deep southern sky and never rises above the horizon beyond 33°N, roughly the latitude of Dallas, Texas. It 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.

Achernar is a bright, blue star with about seven times the mass of the Sun.[7] It is a main sequence star with a stellar classification of B6 Vep, but is about 3,150 times more luminous than the Sun. Infrared observations of the star using an adaptive optics system on the Very Large Telescope show that it 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.[7]

As of 2003, Achernar is the least spherical star in the Milky Way studied to date.[22] 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.[8] 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,[9] or by its polarization.[23] The presence of a circumstellar disk of ionized gas is a common feature of Be stars such as this.[23] 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.[24]

Historical visibility

Due to precession, Achernar lay much farther south in ancient times than at present, being 7.5 degrees of the south pole around 3400 BCE (decl 82º40')[25] 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 Theta Eridani was visible as far north as Crete. So Ptolemy's "end of the river" was certainly Theta Eridani. Alpha Eridani was not visible from Alexandria until about 1600 CE.

Until about March 2000, Achernar and Fomalhaut were the two first-magnitude stars farthest from any other, their nearest neighbors being each other. Antares is now the most isolated first-magnitude star, although Antares is located in a constellation (Scorpius) with many bright second-magnitude stars, whereas the stars surrounding Alpha Eridani and Fomalhaut are considerably fainter.

The first star catalogue to contain Achernar in the chart of Eridanus is Johann Bayer's Uranometria.[26] Bayer did not observe it himself, and it is attributed to Keyser and the voyages of the Dutch. Thus it was the only first-magnitude star not listed in Ptolemy's Almagest.[27]

Alpha Eridani 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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See also

Notes
  1. The ten brightest stars in the nighttime sky in terms of apparent magnitude are, from brightest to least brightest, Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Rigel, Procyon, Achernar and Betelgeuse

References
  1. van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  2. Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1: 02025. Bibcode:2009yCat....102025S.
  3. Nazé, Y. (November 2009). "Hot stars observed by XMM-Newton. I. The catalog and the properties of OB stars". Astronomy and Astrophysics. 506 (2): 1055–1064. arXiv:0908.1461. Bibcode:2009A&A...506.1055N. doi:10.1051/0004-6361/200912659.
  4. Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.
  5. Evans, D. S. (June 20–24, 1966). "The Revision of the General Catalogue of Radial Velocities". In Batten, Alan Henry; Heard, John Frederick (eds.). Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30. University of Toronto: International Astronomical Union. Bibcode:1967IAUS...30...57E.
  6. Moujtahid, A.; Zorec, J. (2000). "The Visual Absolute Magnitude of the Central Objects in Be Stars". The be Phenomenon in Early-Type Stars. 214: 55. Bibcode:2000ASPC..214...55M.
  7. 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:0804.3465. Bibcode:2008A&A...484L..13K. doi:10.1051/0004-6361:200809765.
  8. Carciofi, A. C.; et al. (March 2008). "On the Determination of the Rotational Oblateness of Achernar". The Astrophysical Journal. 676 (1): L41–L44. arXiv:0801.4901. Bibcode:2008ApJ...676L..41C. doi:10.1086/586895.
  9. 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:0812.2531. Bibcode:2009A&A...493L..53K. doi:10.1051/0004-6361:200810980.
  10. Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (2011). "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun". Monthly Notices of the Royal Astronomical Society. 410 (1): 190. arXiv:1007.4883. Bibcode:2011MNRAS.410..190T. doi:10.1111/j.1365-2966.2010.17434.x.
  11. "Achernar -- Be Star". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2010-02-16.
  12. "Achernar". Merriam-Webster Dictionary.
  13. "Naming Stars". IAU.org. Retrieved 16 December 2017.
  14. 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.
  15. 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.
  16. Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  17. Richard Hinckley Allen (1 January 1963). Star Names: Their Lore and Meaning. Courier Corporation. ISBN 978-0-486-21079-7.
  18. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  19. "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  20. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 27 日
  21. Hamacher, Duane W.; Frew, David J. (2010). "An Aboriginal Australian Record of the Great Eruption of Eta Carinae". Journal of Astronomical History & Heritage. 13 (3): 220–34. arXiv:1010.4610. Bibcode:2010JAHH...13..220H.
  22. "Flattest Star Ever Seen". ESO. Retrieved 2018-11-29.
  23. 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:0710.4163. Bibcode:2007ApJ...671L..49C. doi:10.1086/524772.
  24. 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 (2): 1607–1628. arXiv:1509.07221. Bibcode:2016MNRAS.455.1607C. doi:10.1093/mnras/stv2185.
  25. calculated by Stellarium 0.13, an open source sky mapping app. http://www.stellarium.org
  26. "Historical image of Eridanus". Retrieved 2017-01-13.
  27. "Ian Ridpath - Star Tales – Eridanus". Retrieved 2017-01-13.

Further reading
  • Lovekin, C. C.; Deupree, R. G.; Short, C. I. (2006). "Surface Temperature and Synthetic Spectral Energy Distributions for Rotationally Deformed Stars". The Astrophysical Journal. 643 (1): 460–470. arXiv:astro-ph/0602084. Bibcode:2006ApJ...643..460L. doi:10.1086/501492.

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