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August 1, 2013

Brilliant Vega, one of the brightest stars in all the night sky, stands high in the east as night falls, and passes directly overhead a couple of hours later. It’s the leading light of Lyra, the harp.

Four other stars form a small parallelogram to the south of Vega. One point of that figure is the star Sheliak, which is also known as Beta Lyrae. Although it looks much fainter than Vega, that’s only because it’s roughly 35 times farther away.

Sheliak fades and brightens twice every 13 days. That’s because it consists of two stars that are in a tight orbit around each other. Their orbit lines up in such a way that we see the two stars eclipse each other twice during each orbit. When the brighter star covers the fainter star, the brightness drops by about a third. And when the fainter star covers the brighter star, the brightness drops by half.

The two stars are so close together that one of them is pulling hot gas off the surface of the other — enough gas every 50,000 years to make a star as heavy as the Sun. The gas piles up in a thick disk around the stellar thief before dropping onto the star’s surface.

In time, the flow of gas will shut down. That will leave the two stars to age in peace — for a while. Eventually, the star that’s taking gas will begin to puff up. It’ll loosen its grip on its outer layers, so the other star will take some of its gas — reclaiming what it had lost over the millennia.

Tomorrow: the Moon runs a gauntlet.


Script by Damond Benningfield, Copyright 2013

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