The ACHERNAR constellation - myths, facts and location

By Jake Riley, SR Senior Editor

The 9th luminous, binary star system in all the heavens. Achernar is a well-known sight to observers in the Southern Hemisphere. It shines as brightly as the brightest stars with a magnitude of 0.45.

The two stars, Achernar A (seven times the mass of the sun) and Achernar B (the smaller one), rotate about 6.7 the earth-sun’s distances. Achernar is sometimes not sighted by Northern Hemisphere observers, despite its position in the top 10 stars in the sky, perhaps because it is invisible above 32 degrees north latitude.

History and Mythology of Achernar

The name Achernar is derived from an Arabic phrase meaning ‘the end of the river’, however the star’s official name is Alpha Ariadne. The star we now know as Theta Eridani, or Acamar got the name Achernar in the early classical times. At that time, Acamar was the brightest star of the constellation visible from Greece.

The former Achernar became Acamar and the brightest star father became Achernar after the voyagers discovered it.

Originally Achernar was not considered part of the constellation. Its location is:

  • Right ascension: 1 hour 37 minutes 42.9 seconds

  • Declination: -57 degrees 14 minutes 12 seconds

Science of Achernar

Much hotter and brighter than the sun, it is a B3V star, denoting that it belongs to the list of “normal” stars known as the “main sequence”. It is nearly 1100 times as bright, visually, as our neighborhood star. It produces more energy in the non-visible ultraviolet (UV) wavelengths. It pumps out some 3,000 to as much as 5,000 times the solar energy level.

It’s brighter, hotter (and bluer) than the sun. Its mass is 6-8 times and average diameter nearly 8-10 times respectively that of our sun.

The Achernar’s Flattened Shape

While our sun spins on its axis once about every 25 days, Achernar’s rotation lasts up to two more days, or approximately 15 times quicker than our sun. This fast rotation of Achernar results in an odd, flattened shape.

The force of the spin flattens the star considerably, with its equator bulging about 50 percent larger than its poles. The shape was first spotted in 2003 with a very large telescope by the European Southern Observatory. A closer look at Achernar, it would look more like a blue M&M, while our sun would look more like an orange.

The flattening actually causes the star’s poles to be hotter than the equator, making an exact surface temperature for this star hard to determine.

How to See Achernar?

To see Achernar well, you must be even further south – around 25 degrees N. latitude. On most nights of the year, this star cannot be seen easily anywhere near North America. But around October 20 it circumnavigates the southern horizon around midnight. Then as the month’s days pass, Achernar can be seen around in November at 10 p.m., 8 p.m. in December and just after the sunset in January.

But with a dark sky and far enough to South, you can see Achernar easily. After all, this star is very bright!