Categories: Stars

Star Facts: Achernar

Achernar (Alpha Eridani) is the most luminous star in the constellation Eridanus, marking the southern termination of the River, and is the 9th most luminous star in the entire night sky. Being a hot, blue class B star, Archenar is also among the ten stars in the sky with the highest apparent visual magnitude; in fact, of the top ten most luminous stars, Archenar is the hottest and bluest.

Quick Facts

• Constellation: Eridanus
• Coordinates: RA 01h 37m 43s|Dec -57° 14′ 12
• Distance to Earth: 139 light years
• Star Type: Blue (B3Vpe)
• Mass: 6.7 solar masses
• Radius: 7.934 million km (solar radii)
• Apparent Magnitude: 0.46 (0.40 – 0.46)
• Luminosity: 3,150 sol
• Surface Temperature: 15,000K
• Rotational Velocity: 250 km/s
• Age: 100 to 500 million years
• Other Designations: a Eri, CD -57°334, FK5 54, HD 10144, HIP 7588, HR 472, SAO 232481


Although the top of the constellation Eridanus start near Rigel in Orion, its brightest star Achernar, derived from the Arabic for “The End of the River”, lies all the way at its southern end, and is therefore rarely visible in the Northern Hemisphere. In fact, while Achernar is visible between latitudes of +32° and -90°, it is best viewed in the southern hemisphere from positions south of latitude 33 degrees S where it is circumpolar. From locations such as Cape Town, however, the star barely reaches an elevation of 1 degree above the horizon, although it remains circumpolar.

Physical Properties

Located about 139 light years away as determined by the Hipparcos satellite, Archenar is blue-white supergiant star (B3 Vpe) about seven times as massive as the Sun, and at least 1,350 times as luminous. Archenar is also the least round star yet discovered in the Milky Way, with its 250 km/sec rotational velocity having deformed the star into an exaggerated oblate shape, and its equatorial diameter a full 56% bigger than its polar diameter.


This high degree of rotational velocity and distortion causes considerable temperature variations. In fact, although the star’s average temperature is around 15,000K, its polar regions may experience temperatures higher than 20,000K, while the equatorial regions may be cooler than 10,000 or so Kelvins. The result of the high temperatures in the polar regions is a highly energetic solar wind that is blowing off material that eventually forms a relatively dense envelope of hot gas and plasma around the star. This type of gaseous envelope is a common feature around Be-class stars, but is nonetheless not stable, with some of the material contained in it sometimes falling back onto the star.

Binary System

Archenar has been shown to be the primary component of a close binary system. Infrared observation with the VLT (Very Large Telescope) has revealed an A0V–A3V-class star with about twice the Sun’s mass that is orbiting the primary star once every 14 to 15 years, at a separation of about 12.3 AU. However, that figure is by no means certain, since the highly distorted shape of the primary star could conceivably cause major perturbations of the companion star’s orbit.

Peter Christoforou :