When a very massive star, reaching the final stage of its life, exhausts its nuclear fuel it explodes as a supernova. The outer parts of the star are expelled violently into space, while the core completely collapses under its own weight. The outer gaseous layers, mainly hydrogen, still surrounding the core would rush outward, generating a shock wave.
If the core remaining after the supernova is very massive (more than 2.5 times the mass of the Sun), no known repulsive force inside a star can push back hard enough to prevent gravity from completely collapsing the core into a black hole.
If this animation is not awesome, I don’t know what is.
Watch the episode of the Cosmic Journeys: The Largest Black Holes in the Universe here.