Nowhere to Hide 
Black holes should be the ultimate contestants at hide-and-seek. After all, their surface gravity is so strong that not even light can escape them, so they really are completely dark. Yet that gravitational pull also frequently gives them away. Just as fields of flowers attract bees, black holes attract gas. As the gas spirals toward a black hole, it gets extremely hot, so it shines brightly — especially in X-rays. And that gives the black hole away.
In fact, astronomers have used X-ray telescopes in space to discover four likely black holes in M31, the Andromeda galaxy. M31 is high in the west at nightfall, above the Great Square of Pegasus. Under dark skies, it’s visible to the unaided eye as a small, hazy patch of light.
The new black holes are all in globular clusters — densely packed balls of old stars. Some theories have suggested there shouldn’t be any black holes in such clusters, because they’d get kicked out by gravitational interactions with other stars.
Yet the X-ray objects found in M31’s clusters can’t be explained by anything other than black holes that are ingesting large amounts of gas — gas stolen from companion stars. All the black holes are likely a few times as massive as the Sun, although one could be 30 times the Sun’s mass. That means they’re all the collapsed cores of once-mighty stars.
So while the black holes are completely dark, they’ve given themselves away — by taking gas from their companions.
Script by Damond Benningfield, Copyright 2012