For much of their long lives, the giant black holes in the centers of most galaxies are quiet. But every once in a while -- perhaps every 10,000 years or so -- they flare brightly as they grab a snack: a passing star.
Despite their fearsome reputations, black holes don't devour entire galaxies of stars. Instead, a black hole waits until a star passes within its gravitational influence. When that happens to a black hole of the right mass, the black hole squashes the star, stretches it like taffy, then rips it apart. Much of the star's hot gas then forms a wide disk around the black hole. The gas gets hotter as it spirals closer to the black hole, so it produces X-rays and ultraviolet light.
Orbiting observatories have caught several supermassive black holes in the act of devouring stars. The most recent came a year and a half ago, when the GALEX satellite detected a flare-up from a galaxy that's four billion light-years away.
The galaxy is bigger and heavier than the Milky Way. And the black hole in its center is about 10 times as massive as the Milky Way's. The flare-up lasted about 18 months, as the star was pulled apart and its gas was slowly ingested by the black hole.
The galaxy is in Bootes, which is high overhead this evening. Look for its brightest star, yellow-orange Arcturus. The galaxy is too far away to see without a big telescope, but its black hole is waiting for another star to pass close enough to become a stellar snack.
More about black holes tomorrow.
Script by Damond Benningfield, Copyright 2008
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