A brief description of black holes for beginners.

1) They're black

2) They're holes

It's all down to escape velocity. Escape velocity is the speed at which an object without propulsion would have to be travelling to escape from a gravitational field. The Earth's escape velocity is around 7 miles per second. An object travelling upwards at a speed less than that would fall back to Earth.

Now, if the Earth were compressed into a smaller volume, the gravitational attraction at its surface would increase. That is because gravitational attraction of an object diminishes as the inverse square of the distance from the object. So, if you halve the diameter of the Earth, it's gravitational attraction at the surface would increase fourfold. Halve it again and gravity at the surface increases to 16 times the original. As the gravitational attraction increases, so does the escape velocity.

So, if an object is compressed enough there will come a point where the gravitational attraction will be so strong that the escape velocity will be equal to, or greater than, the speed of light. As nothing can travel faster than light, nothing would be able to escape the object's gravity. That includes light itself. If no light can escape then the object will appear black.

The distance from the centre of a black hole where the escape velocity equals the speed of light is known as the event horizon. Anything that crosses the event horizon will be on a 1-way trip to the centre of the black hole; there is no escape.

When a star with mass greater then 5 times that of our sun collapses, theory states that it should form a black hole.