If A Black Hole Is Black, How Do We Know It Is There?

Black holes are very difficult to observe because they are so small and neither emit nor reflect light. The presence of a black hole is not by visual means, but by the incredible forces it creates in outer space.

A black hole is betrayed by the effect of its peripheral gravitational fields on nearby matter. As the matter is sucked inexorably towards the black hole, it becomes super-heated and gives off powerful X-ray radiation before disappearing into the abyss. It is these bursts of X-ray radiation that are the tell-tale sign that a black hole is in the vicinity.

In 1994, the Hubble Space Telescope collected evidence of a massive black hole in the M87 galaxy. Despite having the mass of two to three billion Suns, it is packed into an area no larger than our own Solar System. The giveaway was the presence and behaviour of a gas cloud that swirls around the black hole at incredibly high speed.