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River’s End

January 13, 2018

Because the Sun is a spinning ball of gas, it’s wider through its equator than through the poles. The difference is so small, though, that you really can’t see it.

For the brightest star of Eridanus, the river, though, it’s a different story. Achernar is more than half again as wide through its equator as through the poles. So it looks not like a ball, but like an M&M.

Achernar is much bigger and heavier than the Sun, and thousands of times brighter. And it has a smaller companion star. They orbit each other once every 15 years or so.

Achernar is so flattened because it’s spinning like crazy: A spot at its equator rotates at about 1.3 million miles per hour, hundreds of times faster than the Sun.

That fast rotation has a couple of interesting consequences. For one thing, because Achernar’s poles are much closer to its core — the source of the star’s energy — they’re thousands of degrees hotter than the equator. That heat creates “winds” of gas from Achernar’s poles. Gas also is flung away from the equator by the fast rotation. That surrounds the star with a cloud of gas and dust — a veil around a stellar whirling dervish.

Achernar is at the southern tip of Eridanus. In fact, its name means “river’s end.” It’s so far south that it’s not visible north of about Dallas. The rest of Eridanus is in better view. It begins next to the foot of Orion, which is in the southeast at nightfall, then curls to the right and down to the horizon.


Script by Damond Benningfield

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