What two things must happen for a black hole two form?

Answer 1

In order to appreciate why a black hole forms, you need to be familiar with some basic concepts of mass and density. Mass is basically all the matter that makes up an individual object (protons, neutrons, other sub atomic particles etc). Density is the amount of space or volume that that matter takes up. for example, if you were to compare a pound of feathers to a pound of rocks, the pound of rocks will take up considerably less space as it is more dense. Now looking at a larger scale, we have a star. Stars are very dense, very large, and very heavy. As stars age they go through all of their hydrogen, and start to consume heavier elements such as helium, producing denser elements still in the process. Eventually all these heavy particles cause the stars to collapse in on itself, and if it large enough, produce a supernova, and if it is large enough still, to create a black hole. The creation of the black hole is a result of all that suns matter, being compressed into a tiny fraction of its previous size. Imagine instantly compressing an entire sky scraper into the size of a pin head, and that is close to the kind of density required to make a black hole. If you've ever noticed that a heavy, tiny object will rip through a garbage bag, the same concept holds true on the galactic level. In this case however, the garbage bag is the fabric of the universe itself. The star's collapse condensed so much matter into so small a space that it literally ripped a hole in the universe. (although the matter never leaves the universe, it is condensed into an impossibly small area called a singularity)

Answer 2

For a black hole to form, two things must happen: a massive star must undergo a supernova explosion, and the core that remains must collapse under its own gravity. This collapse creates a singularity surrounded by an event horizon, defining the black hole.