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Achernar: Wikis


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Position Alpha Eri.png
The position of Achernar.
Observation data
Epoch J2000      Equinox J2000
Constellation Eridanus
Right ascension 01h 37m 42.85s [1]
Declination –57° 14′ 12.3″ [1]
Apparent magnitude (V) 0.50
Spectral type B3 Vpe
U-B color index −0.66
B-V color index −0.20
Variable type Lambda Eridani
Radial velocity (Rv) 16 km/s
Proper motion (μ) RA: 87.00 ± 0.58 [1] mas/yr
Dec.: −38.24 ± 0.50 [1] mas/yr
Parallax (π) 023.39 ± 0.57[1] mas
Distance 139 ± 3 ly
(43 ± 1 pc)
Absolute magnitude (MV) −2.77
Mass 6–8 M
Radius ~10 R
Luminosity 3,311(bolometric) L
Temperature 14,510 K
Rotation 225–300 km/s
Age 1–5 × 108 years
Other designations
α Eri, HR 472, CD -57°334, HD 10144, SAO 232481, FK5 54, HIP 7588.

Achernar (α Eri / α Eridani / Alpha Eridani), sometimes spelled Achenar, is the brightest star in the constellation Eridanus and the ninth-brightest star in the nighttime sky. Of the top ten apparent brightest stars (excluding our Sun): Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Rigel, Procyon, Achernar & Betelguese, Achernar is the hottest and bluest. It lies at the southern tip of the constellation.

The extreme rotation speed has flattened Achernar.

Achernar is a bright, blue, B3-type star of six to eight solar masses lying approximately 144 light-years (44 pc) away. Although classified as a main-sequence (dwarf) star, it is about 3,000 times more luminous than the Sun. Achernar is in the deep southern sky and never rises above 33°N. Achernar is best seen from the southern hemisphere in November; it is circumpolar below 33°S.

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.

It is the least spherical star in the Milky Way studied to date. (See “Achernar the Flattest star” in ‘Sky &Telescope’ P. 20 ‘Newsnotes’, September 2003.) Achernar spins so rapidly that its equatorial diameter is more than 50% greater than its polar diameter.

The name comes from the Arabic آخر النهر ākhir an-nahr "river's end".

It is known as 水委一 (Shuǐwěiyī, the First Star of the Crooked Running Water) in Chinese.



  1. ^ a b c d e van Leeuwen, F. (2007). "HIP 522". Hipparcos, the New Reduction. Retrieved 2009-12-04.  

External links

Coordinates: Sky map 01h 37m 42.8s, −57° 14′ 12″



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