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Achernar (The End Of The River).

Updated: Jul 1, 2023

No clear nights for a fortnight, so no new image this week. However, I thought for this weeks blog, we would have a look at something that you cannot see. No, I have not lost my mind (although after the last 5 days, it was close). No, it's quite simple, this object cannot be seen at all in the Northern hemisphere, in fact, a lot of people have never heard of this object. As you have probably guessed by the headline, the object is Achernar, a blue/white main sequence star in the constellation, Eridanus (The River). Indeed, this star marks the end of the river. Unfortunately, as I have said, we cannot see this star in the North, at all, never have and will probably never do so. As you can see by the graphic below, the river runs from Orion, above the bright star Rigel, goes past Canis Major to the left and finally ends far South with Achernar.

Achernar is a binary star system and the brightest in the constellation of Eridanus. It is the ninth-brightest star in Earth's night sky. The two stars, Achernar A (seven times the mass of the sun) and Achernar B (which is smaller), rotate about 6.7 Earth-sun distances or astronomical units apart. Achernar's apparent magnitude is 0.46. Even though the star is in the top 10 stars in the sky, Achernar is often not mentioned by Northern Hemisphere observers, because as I have said it is invisible above 32 degrees North latitude. This is the approximate location of El Paso in Texas, Nagasaki in Japan, and Tel Aviv in Israel.

Achernar A has a very rapid spin rate which astronomers estimate is about 155 miles per second (250 km/s). This rapid rotation was first spotted in 2003 by observations with the VLT (Very Large Telescope). The force of the spin flattens the star considerably, with its equator bulging about 50 percent larger than its poles. This high degree of "flattening" is a first in observational astrophysics, and is a real puzzle for theoretical astrophysicists. Indeed, because of the spin, the diameter and temperature of the star are both difficult to determine. Incredibly, the poles are hotter than the equator which means that there is no single defining temperature for the star!

Artists rendition of the "flattenning" of the

star, Achernar A.

This star system bears the traditional name of Achernar, sometimes spelled Achenar, derived from the Arabicآخر النهر ākhir an-nahr, meaning "The End of the River". Chinese astronomers called the star Achernar, 水委一 (Shuǐ Wěi yī, which in English means "the First Star of Crooked Running Water". The indigenous Boorong people of northwestern Victoria in Australia, named it Yerrerdetkurrk. Due to precession, Achernar lay much farther south in ancient times than at present, being 7.5 degrees off the south pole around 3400 BC declination −82° and still lying at declination −76° by around 1500 BC. Hence, the ancient Egyptians could not have known it. The first star catalogue to contain Achernar in the chart of Eridanus is Johann Bayer's Uranometria. Bayer did not observe it himself, and it is attributed to Pieter Dirkszoon Keyser and the first voyages of the Dutch to the East Indies. Thus it was the only first-magnitude star not listed in Ptolemy's Almagest. Achernar will continue to move north in the next few millennia, reaching its maximum northern declination between the 8th and 11th millennia, when it will be visible as far North as Germany and Southern England, which as I said previously, means we will never see it, unless you are lucky enough to travel that far South.

Artists rendition of the comparative sizes of Achernar and other stars compared to our Sun.

Now, as well as all the above amazing facts about Achernar, I can think of another claim to fame, especially if you are a seasoned video gamer. In the second Elite game by David Braben, called Frontier Elite II ( a brilliant space combat and trading game), Achernar is the primary residence of the emperor, and the seat of Imperial power. The emperor also runs a faction within the game called the Imperial Navy, with which you can do some great missions such as nuking a base (how lifelike). Ha, I remember it so well on my Commodore Amiga 500. Superb. Now, how many professional astronomers know about that? Not many I will wager..

Screenshot of the video game Frontier Elite II. Note, the spelling of Achernar in the game.

Well, that's it for this blog. Hope you have enjoyed it as much as I have enjoyed writing it. As always, please feel free to comment good or bad on this blog. Talk next time. Until then stay free and hope for some good skies.. We live in hope!!

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Kez Bloor
Kez Bloor
Apr 29, 2023

Achenar and Achernar, that was supposed to say..


Kez Bloor
Kez Bloor
Apr 29, 2023

Haha, Elite 2 was the first thing I thought of as I was reading the blog, yes the home of the Empire in David Braben’s universe. Their given temperature for the star is most conservative! Also as suggested, the spelling difference between Archenear and Archenar was noticeable. Amazing how ’flattened’ Archenar is to other stars, must be an incredible sight closer up. Great article and images.

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