Question: give me the definition of a black hole

Sam Connolly answered on 13 Jun 2014:

Technically, a black hole is any object whose ‘event horizon’ is outside the object. The event horizon is the distance from the centre of an object where the gravity is so strong that light can’t escape. If this distance is outside the surface of the object, it’s a black hole.

You might have heard of the Earth’s ‘escape velocity’, which is the speed you have to go to get off the earth, by escaping it’s gravity – on Earth it is about 25000 miles per hour, for the Sun it’s over a million miles an hour! The escape velocity is higher for heavier things (which is why the Sun’s is higher), but also if you have the same amount of stuff in a smaller space (a denser object) because the closer you can get to the centre of an object the higher the gravity. A black hole is anything with an escape velocity which is bigger than the speed of light. The speed of light is about 50 million miles per hour, so you need something very heavy in a very small space to get a black hole!