Epsilon Aurigae

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A bright star is about to begin a slow fade -- one that should last for almost two years.

Epsilon Aurigae is a binary -- two massive objects bound to each other by their mutual gravity. One of the objects is a big, heavy star. The other appears to be a dark cloud of dust that's shaped like a pancake. This dark companion is about to pass in front of the star, making it look fainter.

The star is a supergiant. It's about 15 times as massive as the Sun, and so big that if it took the Sun's place in our solar system, it would swallow up Earth. The star is also hotter than the Sun. The combination of its size and temperature makes it extremely bright.

The dark companion is about as massive as the star, but much wider -- a billion miles across. Astronomers suspect that there's a star in the middle of it, but they've never seen it. And the exact size and composition of the disk are unclear.

Now, though, there's a chance to clear up some of that mystery. The two objects orbit each other once every couple of decades. And once per orbit, the disk partially eclipses the star -- it basically cuts it in two. The next eclipse is expected to get underway this week; more about that tomorrow.

Epsilon Aurigae is bright enough to see with the unaided eye. It's to the right of the bright star Capella, which is high in the east-northeast at first light. Over the next few months, Epsilon will fade to about half its current brightness -- obscured by a dark companion.

Script by Damond Benningfield, Copyright 2009

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