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Unicorn Stars

February 24, 2010

Seen from afar, the brightest stars of Monoceros, the unicorn, aren't all that impressive. The top two shine less than one percent as bright as the brightest star in the night sky. In fact, from the light-polluted skies of most cities, they're pretty much invisible.

From close range, though, both of them are quite impressive. One of them is a stellar giant -- an aging star that's far bigger and brighter than the Sun. And the other is a triple star -- three stars bound in a mutual orbit. All three are also more impressive than the Sun.

The giant is known as Alpha Monocerotis. Although it's much younger than the Sun, it's also more massive, so it's already nearing the end of its life. It's consumed its original hydrogen fuel and is now burning through the helium "ash" that it created. This change has caused the star to puff up like a big balloon, getting so bright that it's visible across about 145 light-years of space.

The triple system -- Beta Monocerotis -- is almost five times farther. But it looks just as bright because its three stars are all brilliant blue-white beacons. They're several times more massive than the Sun, so they'll live even shorter lives than Alpha. At the end they, too, will puff up, so Beta Monocerotis will look much more impressive.

If you have clear, dark skies, look for these stars to the left of brilliant Orion. The hunter is high in the south by mid-evening, marked by his prominent belt.

Script by Damond Benningfield, Copyright 2009

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