Black hole

A black hole is a region of space in which the gravitational field is so powerful that nothing can escape after having fallen past the event horizon.

Most accretion disks and gas jets are not clear proof that a stellar-mass black hole is present, because other massive, ultra-dense objects such as neutron stars and white dwarfs cause accretion disks and gas jets to form and to behave in the same ways as those around black holes. But they can often help by telling astronomers where it might be worth looking for a black hole.

Photo from http://flicker.com/photoes/hoovery

Information from http://en.wikipedia.org/wiki/Blackhole