How to see it

The 9th brightest star in all the heavens, Achernar is a well-known sight to observers in the Southern Hemisphere, but known only by name to the great majority of observers in the Northern Hemisphere. It shines brightly with a magnitude of 0.45 – in other words, as brightly as the brightest stars – but it is so far south on the dome of stars surrounding Earth that it never rises above the horizon from any location north of about 33 degrees N. latitude. And in fact, for all practical purposes, you must be further south – around 25 degrees N. latitude – to see it well. That is a line drawn around the entire globe passing through, say, Miami in the U.S. state of Florida and Taipei in China.

Nowhere in North America has it easy. For example, from Key West, Achernar rises only about 8 degrees above the southern horizon. Even farther south, from the southern tip of Hawaii’s Big Island, Achernar never quite makes it to 14 degrees.

In a dark sky, you can see that Achernar marks the end of a great stream of stars known to the ancients as a celestial River. This is the constellation Eridanus the River, and Achernar is its brightest star. In fact, the name Achernar means End of the River.

On most nights of the year, Achernar cannot be seen from anywhere in North America, but around October 20 it skirts the southern horizon around midnight, never getting very high. Then as the months pass, it is seen earlier at night – around 10 p.m. in November, 8 p.m. in December and just after sunset in January. Being far to the south with no bright stars around it, Achernar stands out in its isolation. Fomalhaut, about 39 degrees to the upper right and another isolated first magnitude star, is the next best thing to a guide star for finding Achernar.

History and Myth

Officially Alpha Eridani, Achernar is the brightest star in the southern constellation Eridanus, the River. As the southernmost star in that long contorted constellation, its name derives from an Arabic phrase meaning “the end of the river.”

Interestingly, in early classical times the name Achernar was given to the star we now know as Theta Eridani, or Acamar. At that time Acamar was the brightest star of the constellation visible from Greece, and thus was thought be the “end” of the river. When voyagers discovered the brighter star farther to the south, it became Achernar, and the former Achernar became Acamar. Apparently both names derive from the same phrase, “Al Ahir al Nahr” according to Richard Hinckley Allen, and mean the same thing.

So Eridanus has two ends, as well as a beginning, the star Beta Eridani or Cursa, which itself is easily visible from the northern hemisphere, very near Orion’s brightest star, Rigel.

Achernar, the flattest star known

Science

Data from the Hipparcos mission puts Achernar at about 144 light-years away. It is a B3V star, meaning that it belongs to the ranks of “normal” stars known as the “main sequence,” but it is much hotter and brighter than the sun.

In fact, it is nearly 1100 times as bright, visually, as our neighborhood star. Brighter, hotter (and bluer) than the sun, Achernar produces more energy in the non-visible ultraviolet (UV) wavelengths. When you take this into consideration, it pumps out some 3,000 to as much as 5,000 times the solar energy level. The discrepancy is due to an uncertainty in the amount of UV radiation it produces.

Achernar’s mass is 6-8 times that of our sun, and its average diameter is nearly 8-10 times that of the sun. But, while our sun spins on its axis once about every 25 days, Achernar completes one rotation in slightly more than two days, or nearly 15 times faster than our sun. This fast rotation produces an odd, flattened shape, first discovered by the European Southern Observatory’s Very Large Telescope (VLT) in 2003. Up close, Achernar would look more like a blue M&M, while our sun would look more like an orange. Read more about Achernar’s flattened shape from ESO.

This flattening of Achernar makes an exact surface temperature for this star hard to determine, because the flattening actually causes the star’s poles to be hotter than the equator. Estimates range from about 14,500 to 19,300 k (around 26,000 to about 32,000 F).

Achernar’s position is RA: 01h 37m 42.8s, dec: -57° 14′ 12″.