Speedy Stars
If you look south after sunset the next few nights, you'll be facing the center of our Milky Way galaxy. It's above the "spout" of the teapot-shaped constellation Sagittarius, which is to the right of the Moon at nightfall.
Far beyond those stars, at a distance of about 27,000 light-years, lurks a giant black hole that's several million times as massive as the Sun. The black hole sometimes swallows gas, dust, and even entire stars. And it may be responsible for flinging stars out of the galaxy's heart on a one-way trip into intergalactic space.
Astronomers began discovering these "hypervelocity" stars only recently. The stars stood out because they traveled faster than any stars ever seen -- fast enough to completely escape the Milky Way.
Here's what some astronomers think may be happening. A binary star system -- two stars bound by their mutual gravitational pull -- skirts by the supermassive black hole. One star in the system enters orbit around the black hole, while the other star flies free of its companion. The first star eventually falls into the black hole, while the second star shoots away from the black hole at extreme speed.
Astronomers have discovered more than a dozen of these speedy stars. And in the year ahead, they'll surely discover more -- signs that the Milky Way's central black hole has a contrary nature: While it swallows up stars that wander too close, it shoots other stars completely out of the galaxy.
Script by Ken Croswell, Copyright 2008
For more information, visit StarDate's Black Holes Encyclopedia.
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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