Astronomy EssentialsBrightest Stars

Achernar is the End of the River

Star chart of winding linear constellation.
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.

The ninth brightest star in all the heavens, Achernar, is a well-known sight to observers in the Southern Hemisphere. But many northern stargazers know this star by name only. That’s because – despite its magnitude of 0.45, making it shine brightly as the brightest stars visible from Earth – it’s extremely far south on the dome of stars surrounding Earth. If you’re north of about 33 degrees north latitude, Achernar never rises above your horizon at all. And yet this star remains one of the sky’s most famous stars as the star at the end of the River.

The River is – of course – the constellation Eridanus, which is large and easy to see in a dark-enough sky, even if you’re fairly far north on Earth’s globe. That’s because the northern part of this constellation is located near the extremely prominent constellation Orion the Hunter. Eridanus appears to swell up in a great loop near Orion, then meander southward, finally – for most in the Northern Hemisphere – dropping out of sight below the southern horizon before it reaches its end.

But if you are far enough south – below 33 degrees north latitude – you’ll easily spot the River’s end as the bright star Achernar.

Come to know Eridanus, and someday – from some southerly latitude, maybe while on vacation – you’ll enjoy spotting Achernar!

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Star chart showing boundaries of constallations with Eridanus in the middle.
A more detailed chart showing Achernar’s location in southernmost Eridanus, via Wikimedia Commons.

How to see Achernar. For all practical purposes, you must be even further south – around 25 degrees north latitude – to see Achernar 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, seeing this star. 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.

And yet, if you are far-enough south, you can see Achernar easily. After all, this star is very bright!

Just as Eridanus marks the end of the River, so the River has a beginning, the star Beta Eridani or Cursa, which itself is easily visible from the Northern Hemisphere, very near Orion’s brightest star, Rigel.

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. If you have a dark sky, and are far enough south, you’ll easily see Achernar’s constellation Eridanus making its loop under the constellation Orion.

Oblique aerial view of a wide, shining river with large curves winding through a forest.
Earthly rivers are sometimes known for meandering. In the sky. the stars representing Eridanus the River – Achernar’s constellation – have a similar quality.

Achernar’s history and mythology. In fact, the name Achernar derives from an Arabic phrase meaning 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 considered the River’s end.

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, whose 1899 book on Star Names (Their Lore and Meaning) is still the best around.

A small yellow sun, plus 3 larger blue ovals representing flattened stars.
Artist’s concept contrasting the size, shape and color of our sun with several flattened stars, including Achernar.

Science of Achernar. Data from the Hipparcos mission placed Achernar at about 144 light-years away. It is a B3V star, meaning that it belongs to the main sequence of stars.

Still, Achernar is much hotter and brighter than our sun.

In fact, it’s nearly 1,100 times as bright, visually, as our sun. 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 to 8 times that of our sun, and its average diameter is nearly 8 to 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 kelvins (about 14,200 to 19,000 C or around 26,000 to 32,000 F).

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

An oval blue star, with gas extending from its equatorial region.
A simple artist’s concept of the star Achernar. It’s flattened because – despite being about 6 to 8 times more massive than our sun, it rotates or spins on its axis about 15 times more rapidly, completing one rotation in 2 Earth-days, in contrast to the sun’s 25-day rotation. Observations have suggested a disk of ionized gas at the star’s equator. Image via Wikimedia Commons.

Bottom line: Achernar – the ninth brightest star – is the southernmost bright star in the constellation Eridanus the River.

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November 25, 2019
Astronomy Essentials

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