Merging Stars

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Many stars are double -- two stars that are bound together by gravity. The best-known example is Sirius, the brightest star in the night sky -- two stars that go around each other every 50 years.

Astronomers recently discovered a double star with a twist: It's destined to become single long before our Sun dies. The system is in Ursa Major, south of the bowl of the Big Dipper, which is high in the northeast this evening.

Both stars in the system are white dwarfs -- the hot, burned-out corpses of stars that were once like the Sun.

The white dwarfs in the newfound double star are as close together as Earth is to the Moon. Because they're so heavy, though, they orbit each other much faster than the Moon orbits Earth: once every 58 minutes.

What's more, the two stars are slowly drawing even closer together. That's because as they revolve around each other, they emit gravitational waves, which carry away some of their orbital energy. In less than 100 million years, the two small stars will slam together and merge to form a single star.

For a time, the star will be larger and will shine more brightly. Over time, though, it'll settle down. The star will have the combined mass of the two original white dwarfs. But it'll be smaller than either of its forebears, because its stronger gravity will squeeze the star tightly. And there will be one less double star in the universe.

Tomorrow: A famous but fickle meteor shower.

Script by Ken Croswell, Copyright 2010

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