# Black Holes - whirlpools in space

Astronomers think that they have found some strange objects out in space. They have called them BLACK HOLES.

You can think of a Black Hole as a sort of invisible whirlpool that sucks in everything around it - I mean everything, even light. The gravitational field of a black hole is rather like that shown in the diagram – once you have slipped over what is called the 'event horizon' you fall into the centre.

Once you have been sucked into a Black Hole you can never get out again. It's all to do with ESCAPE VELOCITY. This is the speed that you have to reach in order to escape from something like a planet, moon or star.

If you jump in the air on the Earth you fall back to the ground again. This is because of the gravity of the Earth. However if you jump up very fast - 40000 km/hour (25000 miles/hour) then you will never come down. You have reached the ESCAPE VELOCITY of the Earth

Escape velocity of a black hole
The pull of gravity of a Black Hole is so huge that its escape velocity is as big as the speed of light. That's why it's black, light that goes in can't get out.

Since nothing can go faster than light – or even reach the speed of light if a spaceship got pulled into a Black Hole it would be trapped there forever.

A black hole formed from the death of a heavy star

A black hole is usually the result of the collapse of a star that had a mass of between 5 and 100 times that of our Sun. At the end of its life the star contracts so that all its mass is enclosed in a very small volume so that the gravitational pull near its surface would be enormous.
Black Holes may be as big as a star or even larger so one day somebody may run into one. If they do we will never see them again.

How we can 'see' a black hole
We can't actually observe a black hole directly but we can 'see' black holes because of its effect on its surroundings. The material close to the event horizon of a black hole forms a disc. This disc is compressed and the motion of the gas within the disc means that the gas gets heated up and glows giving out X rays and ultraviolet radiation.

## Gravitational lensing

The effects of Black Holes on light from distant galaxies have been observed. The gravitational pull of a black hole is so great that it actually distorts the space around it – in this distorted space light actually curves and we get what astronomers call gravitational lensing.

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