Hydra

StarDate: February 7, 2009

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In the night sky, big isn't always beautiful. In fact, the largest constellation of all slithers across the sky tonight. It stretches a quarter of the way around the sky. Yet it's also one of the faintest constellations, with only one moderately bright star.

Hydra, the sea serpent, begins to climb into the sky around dark. Its head is far to the lower right of the Moon. But it takes about six hours for the rest of its body to wriggle into view, so its tail doesn't rise until after midnight.

Hydra's brightest star is Alphard -- a name that means "the solitary one." And it's certainly a solitary light in a fairly dark region of the sky.

But the star might more accurately be known as "the solitary two" because it's a binary -- two stars locked in a mutual orbit. The star that's visible to our eyes is classified as a giant. It's nearing the end of its life, so it's puffed up to enormous proportions. If it took the Sun's place in our own solar system, it would extend halfway out to the orbit of Mercury.

Its companion has long since passed that phase of life. It swelled up, then cast its outer layers into space, leaving only its hot, dense core -- a white dwarf.

The same fate awaits the Sun -- a fate in which big is beautiful. First it'll puff up to become a giant, so it'll shine hundreds of times brighter than it does now. Then it'll lose its outer layers and become a white dwarf -- a faint ember lost in the vastness of space.

Script by Damond Benningfield, Copyright 2008

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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