Like all stars, large ones are stable as long as there is a sufficient amount of hydrogen (or helium) to fuse. This fusion process is what prevents them from collapsing in on themselves. However, once the main elements have been fused up to iron, the star becomes unstable. Eventually, it may supernova and leave a black hole; a singularity that sucks in light and matter that enters the event horizon.
The star prevents collapsing in on itself with fusion. When it goes supernova, it expels a large amount of its mass. If the remaining bit is enough to create a black hole that is so dense that fusion cannot balance the gravitational force, then how did the star exist in the first place and why wasn't it dense enough to form a black hole?
Best Answer
It wasn't a black hole because the density wasn't sufficiently high. The density was lower than what is needed for a black hole because the volume was larger. The volume was larger because the atoms (mostly hydrogen) were kept away from each other by the pressure produced by the fusion processes. Once the fusion processes stop, this source of repulsion between the atoms disappears, the volume shrinks, the density goes up, and the black hole threshold may be surpassed.