Achernar

NASA Observatorium


Achernar is hotter,
brighter, and bluer
than Vega or Sirius A,
but is slightly cooler
and smaller than Spica,
which includes B-type
giant and dwarf stars.


See a discussion of
the "main sequence"
as part of stellar
evolution and death.

The Star

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.

VLTI + VINCI,
Paranal Observatory, ESO



Larger illustration.




Due to its fast rotation,
Achernar is around half
as much wider than it is
tall (more from ESO).

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.

VLTI + VINCI,
Paranal Observatory,
ESO


Larger illustration.


Models of the star's
3-D shape vary the
assumed inclination
values (see ESO).

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.