Achernar (α Eridani) Facts

Achernar is a blue main-sequence star with a stellar classification of B6 Vep. It is a spectral type B star.

Achernar is the brightest star in the constellation of Eridanus and the ninth brightest star in the night sky. Out of the ten brightest stars in the sky, Achernar is the hottest and bluest in color.

Key Facts & Summary

  • Achernar is a blue main-sequence star with a stellar classification of B6 Vep. It is a spectral type B star.
  • It has an unusually rapid rotational velocity of around 250 km / 155 mi per second. Because of this, Achernar seems to have an oblate shape.
  • Achernar is around 37.3 million years old and it is 3.150 times brighter than our Sun.
  • Achernar has around 6.3 solar masses, a radius of about 7.3 x 11.4 solar radii, and an average diameter of more than 10 times that of our Sun.
  • Achernar spins around 15 times faster than our sun, completing one rotation in around 2 days.
  • The average temperature on Achernar’s surface is at around 15.000 K, or around 3 times hotter than our sun.
  • Because of its rapid spinning that’s causing its distortion, temperatures vary. At its poles, Achernar is estimated to have around 20.00 K, while at the equator around 10.000 K.
  • Achernar is the least spherical star in the Milky Way that has been currently studied.
  • Due to its high temperatures at its poles, Achernar’s generating a fast polar wind that is ejecting matter. This creates a polar envelope of hot gas and plasma.
  • This disk of ionized gas is a common feature of Be variable stars.
  • Achernar has a companion star in close orbit. It appears it is an A-type star in the stellar classification range A0V-A3V – this suggests it has double than the mass of our Sun.
  • The stars are separated by around 12.3 AU, and their orbital period is at least 14 – 15 years.

Since Achernar is visible to the naked eye it was known to the ancients. The star’s name derives from an Arabic phrase and it roughly translates to “End of the River.”

In the early classical times, the name Achernar was given to the star we know now as Theta Eridani, today Acamar. When it was studied, Acamar was at its brightest in its constellation that was visible from Greece. It was considered the River’s end but this changed.

As voyagers traveled and discovered a brighter star farther to the south, it became Achernar and it marked the southernmost end of the constellation, and the former Achernar became Acamar. Both names derive from the same Arabic phrase “Al Ahir Al Nahr.” The river refers to the constellation of Eridanus.


Achernar is believed to have formed around 37.3 million years ago from a molecular cloud or nebula of gas and dust. Gravity pulled the swirling gas and dust and resulted in the big, bright, bluest star that we today now call Achernar. Its fast-spinning feature may be the result of a collision in the past.

Distance, Size, and Mass

Achernar is around 139 light-years / 43 parsecs away from the Sun. The blue star is around 6.3 times more massive than our sun and has around 7.3 or more solar radii.

Achernar has a diameter 10 times greater than our sun’s diameter, and because it spins so incredibly fast, its equatorial diameter is around 56% greater than its polar diameter.

Other Characteristics

Achernar has an apparent visual magnitude of around 0.40 – 0.46. Its absolute magnitude is at – 1.46.  As such, it is a variable type Be star which is at least 3.150 times brighter than our Sun.

Achernar has a surface gravity of around 3.5 cgs and it is almost three times hotter than our sun with average temperatures at around 15.000 Kelvins. It emits around 3.000 to 5.000 times more UV rays than the Sun.

Because it spins at an incredible velocity of around 250 km / 155 mi per second, the star’s shape is oblate and temperatures aren’t distributed evenly.

At the poles, Achernar is almost four times hotter than our sun, at average temperatures of around 20.00 K, while at the equator, around 10.000 K, or almost two times hotter than our sun. It spins around 15 times faster than our sun, completing one rotation on its axis in 2 days.

Because the Polar Regions have such high temperatures, a highly energetic solar wind is blowing off material from the star. This matter eventually forms a relatively dense envelope of hot gas and plasma around the star.

Some of the ejected material falls back onto the star. Every 14 or 15 years, an A0V-A3V star with about twice the mass of the Sun, is orbiting Achernar. Estimates suggest that these stars are usually separated by around 12.3 AU. But, due to Achernar’s highly distorted shape, it may cause major perturbations in the companion star’s orbit.


Achernar is located in the constellation of Eridanus “The River”, marking the river’s south end. The star cannot be seen from locations north of latitude – 33° N.

Observers in the south of latitude 33° N, see Achernar lay in the same area of the sky as the Large and Small Magellanic Clouds – satellite galaxies of the Milky Way. The constellation of Eridanus is the sixth largest constellation in the night sky, and the best time to observe it and its components, is in the month of December.

The Future

Achernar is ejecting mass at a rate thousands of times greater than our Sun. It will, in time, lose much of its mass and it’s possible that its fast spinning will slow down. 

If something occurs that would make this star spin even faster, it will end up destroying itself. Since Achernar young, it’s still fusing hydrogen in its core, and it may be small enough to evolve off the main sequence into a massive white dwarf.

Did you know?

  • Achernar was much farther south in the ancient times than it is now, this happened due to axial precession. It has been estimated that the blue star was 7.5 degrees south around 3.400 B.C.
  • Due to Achernar’s position in the night sky, the ancient Egyptians couldn’t see the star and as such, it remained unknown to them.
  • Achernar is the only first-magnitude star that was not listed in Ptolemy’s famous Almagest.
  • Achernar continues to move north. Rising from Crete 500 years hence before reaching its maximum northern declination between the 8th and 11th millennia When this will happen, the star will be visible from as far north as Germany and southern England.
  • To the ancient Chinese, Achernar was known as First Star of Crooked Running Water – it referred to the asterism known as Crooked Running Water, which Achernar forms along with Zeta Phoenicis and Eta Phoenicis.
  • Achernar is one of the 58 navigational stars that have a special status in the field of celestial navigation due to their brightness and because they are easily recognizable.
  • The Boorong people of Australia called the star Yerrerdetkurrk. This was the mother-in-law of Totyarguil, represented by the constellation Aquila, who never let him see her, heading the local cultural taboo.


  1. Wikipedia
  3. Space
  4. Earthsky
  5. Astronomytrek
  6. Star-facts

Image source: