Nothing escapes from a black hole

Last week we learned that black holes are formed when very big stars die and collapse under their own gravity. This week we look a little more closely at the spectacular galactic phenomenon.

Black holes earned their name from the fact that they are so dense and their gravity is so immense that nothing can escape from them, not even light. Any light that is emitted from them is dragged back into them. Because they are black, it is difficult for astronomers to spot them when they look out into space with their powerful telescopes. But there are a few tell-tale signs around the edges of black holes that make it possible for astronomers to locate them.

The most obvious sign of a black hole is when things are sucked into it. The closer the cosmic matter gets to the hole, the faster it is sucked in. The area around the black hole acts like a liquidiser, smashing everything up into small pieces and then sucking it into the black hole. This process takes places at tremendously high temperatures and X-rays are given off. If the black hole is very big and has lots of smashed up cosmic matter spinning around the mouth, then it is called a quasar and is one of the brightest things in the universe.

To understand how a black hole distorts time and space, you have to get to grips with Einstein's General Theory of Relativity. He predicted that a strong enough gravitational field would deflect light rays. This has actually been proved by images sent back by the Hubble telescope showing light from a star being bent as it travels past a black hole.

Einstein's famous theory linked speed and time. So if light rays are distorted as they travel past a black hole, then time will be as well.

For decades, black holes have been associated with death and destruction - huge, greedy mouths that sucked devoured everything. But in the last few years scientists have been changing their tack. They now think that black holes might actually trigger the birth of new galaxies.