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September 5, 2015

One of the oddest mythological creatures in the sky crawls low across the south on September evenings: Capricornus, the sea-goat. It’s depicted as the head and body of a goat connected to the tail of a fish. To modern eyes, though, it’s just a wide, narrow triangle, with the longest line at the top.

The bright star at the right-hand point of the triangle is known as Algedi, from an Arabic name that means “the kid.”

Alpha Capricorni 1 and 2 [SDSS]Alpha Capricorni 1 and 2 [SDSS]If you look carefully, you’ll see that it’s actually two stars, not one. They’re known as Alpha-1 and Alpha-2 Capricorni.

The only relationship between the two, though, is their position in the sky — they line up in the same direction from Earth. Otherwise, there’s no connection between them at all.

As seen from Earth, the brighter star is Alpha-2. And in fact, it is impressive. It’s nearing the end of its life, so it’s swelling up to giant proportions. It’s many times bigger than the Sun, and dozens of times brighter.

Alpha-2 looks brighter than Alpha-1, but that’s only because it’s much closer — about a hundred light-years away, versus almost 700 light-years for Alpha-1.

Alpha-1 is nearing the end of its life, too, so it’s also puffing up. It’s about 40 times wider than the Sun, and almost a thousand times brighter.

Before long, both stars will expel their outer layers into space, leaving only their dead cores — and the sea-goat will lose a bit of its luster.

More about Capricornus tomorrow.


Script by Damond Benningfield, Copyright 2015

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