Question: What is a black hole? What causes a black hole to form?

Dominic Doyle answered on 14 Mar 2017:

A black hole is essentially a star that has collapsed under the force of gravity into itself. This happens at the end of it’s life when the nuclear fusion reactions that make the star shine (like our Sun) have stopped. As the star collapses it becomes so dense that it’s gravitational field is then strong enough to even prevent light escaping – hence it becomes effectively “black”!