Question: What is a black hole ?

James Smallcombe answered on 20 Oct 2020: last edited 20 Oct 2020 8:14 am

A black hole is a object that is formed when matter gets so dense, that the gravity (or the curvature of space-time) outside its surface is strong enough that not even light can escape.
The object itself disappears from view and leaves a lightless black sphere in space, the sphere isn’t a physical thing its more like a shadow, its the line beyond which nothing can escape, the “event horizon”.

The matter density needed is about 20,000,000,000,000,000,000 kg per cubic metre, which is a lot. There are a few different way we think they can form, which generally involve stars collapsing under their own weight, when the gravity pulling them together overcomes the forces pushing outward.