Most large galaxies are likely to host millions of black holes — the dark remnants of dead stars. Not surprisingly, though, the black holes are hard to find because they emit no energy of their own. But some black holes steal gas from a companion star. The gas forms a hot disk around the black hole, which is visible to X-ray telescopes in space.
In the last couple of years, astronomers have used this technique to discover about three dozen black holes in M31, the Andromeda galaxy. It’s the closest big galaxy to our own, at a distance of two-and-a-half million light-years. It’s high in the east this evening, and looks like a small, faint smudge of light.
Most of the black holes were discovered through surveys conducted with the space-based Chandra X-Ray Observatory. They were confirmed with other telescopes in space and on the ground.
The black holes are all probably about 5 to 10 times as massive as the Sun. Their powerful gravity pulls hot gas off the surface of a close, puffy companion star. As the gas spirals into the black hole it gets hotter and hotter, so it shines brightly in X-rays.
One black hole put on an especially powerful display. Gas was flowing toward the black hole too fast for the disk around it to take in. So the disk produced a brilliant eruption. Much of the energy formed a “beam” that aimed our way — turning the black hole into a brilliant cosmic beacon.
More about M31 tomorrow.
Script by Damond Benningfield, Copyright 2013
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