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November 27, 2016

If you’re an Earthling, you probably think that all stars are nice round balls. After all, our star, the Sun, certainly is. But if you lived around another star, you might have a different opinion, because some stars spin so fast that they bulge out at the middle like a squashed beachball.

A prime example is the luminary of the constellation Eridanus, which flows low across the south tonight. Achernar spins so fast that its equator bulges outward. It’s about 10 million miles in diameter through the equator, but only about seven million through its poles. For comparison, the Sun\'s diameter is a little less than a million miles no matter how you measure it.

Achernar is 140 light-years from Earth, and it’s one of the 10 brightest stars in the sky. It’s hotter than the Sun, which makes its surface look blue — if you’re lucky enough to be able to see it. The trouble is, Achernar is so far south that, for most Americans, it stays below the horizon. Only those in Hawaii and southern Texas and Florida can see it.

Although Achernar is brilliant, its constellation is not. The star is at the southern end of Eridanus, which represents a river. In fact, the star’s name derives from Arabic words that mean “end of the river.” If you have a dark sky, you can see most of the rest of Eridanus. But you’ll have to travel south to see its brightest star, Achernar — a star that spins so fast that it has a bulging waistline.

Tomorrow: cosmic bombardment.


Script by Ken Croswell, Copyright 2016

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