Stephen Hawking's research covered many areas, from the Big Bang to quantum gravity. But his biggest obsession, and his most important work, surrounded the mystery and power of black holes...

Did Stephen Hawking discover black holes? No. It was Albert Einstein's theories which first predicted that, if an object was small and dense enough, it would cause space-timeAnother of Einstein's most important theories: space and time are essentially the same thing, and they make up the fabric of the universe. This is why, when space curves to form a black hole, time also slows down (and eventually stops). to bend into infinity. This would create a hole in the fabric of the universe from which nothing - not even light - could escape.

Einstein himself was horrified by this idea, and was never convinced that they actually existed.

Do they? We are pretty sure they do, although it's difficult to spot them from Earth. (That is the trouble with things that cannot emit light.)

However, the first effects of a black hole were detected in 1971. There was a mysterious object emitting X-rays in the constellation Cygnus. It was at least 10 times as massive as the sun, but less than 100 miles across. The only possible candidate was a black hole, sucking the gas of a nearby star and producing X-rays as a result.

Scientists predict that we may finally see the first ever image of a black hole later this year, which would prove their existence once and for all.

How do they work? To understand black holes, first you have to understand Einstein's general theory of relativity: that the fabric of the universe (space-time) curves around heavy objects like a person sitting in the middle of a trampoline. Smaller objects are drawn towards them like rolling marbles. This is gravity.

A black hole is formed when a large star collapses under its own weight until it is an infinitely small size with an infinite density. This tiny point is called a singularityThe point at which something becomes infinite., and it causes a hole in space-time. Imagine, for example, if that person on the trampoline was heavier than three suns, pulling the fabric down around them. In black holes, this causes the laws of physics that we know about to break apart.

What did Stephen Hawking discover? Firstly, that the universe began as a singularity, before the Big BangThe moment the universe came into being. It is thought that this took place in a kind of explosion from a dimensionless singularity.. Secondly, that black holes are not quite as black as we thought. Before Hawking, it was believed that nothing could escape them. But then he began combining the very big physics of black holes with the very small physics of quantum mechanicsA fundamental theory in physics describing the properties of nature on an atomic scale. Unlike general relativity, which works when applied to large objects, quantum mechanics helps describe the world on a tiny level - at a scale where things can be in several places at once, and measurement can affect reality..

In quantum mechanics, supposedly empty space is actually full of "virtual particles". These are pairs of particles constantly fizzing in and out of existence. One is made of matter, and has a positive energy. The other is antimatter, and has a negative energy.

Hawking realised that if the negative particle was sucked into a black hole at its event horizon - the point of no return - then the positive particle could escape. The black hole would be emitting particles, and getting smaller as a result.

Then what? Eventually, it would become so small and hot that it would explode with the energy of a million one-megatonA million tons. hydrogen bombs, and disappear. Only radiation would be left.

This is known as Hawking radiation. The problem is, it's almost impossible to observe, and therefore difficult to prove. If it had been, Hawking would probably have won a Nobel PrizeOne of a set of prizes, laid out in the will of Swedish chemist Alfred Nobel, given each year to people who "have conferred the greatest benefit to humankind". .

What else did he discover? His other ideas and theories are still being tested and debated by scientists; we don't know if they are true. But he was always asking big scientific questions. Like: when something is sucked into a black hole, is the information lost forever? And can a single theory ever explain the whole universe?

Should we be afraid of black holes? Hawking certainly wasn't. "They're named black holes because they are related to human fears of being destroyed or gobbled up", he once said. "I don't have fears of being thrown into them. I understand them. I feel in a sense that I am their master."