What Causes a Supernova?

The reason that stars aren't collapsing in the first place is this heat produced from fusion. Fusion is what's pushing back against gravity to keep a star in balance. And so once the star starts to cool and fusion starts to slow down, then that spells the end of it and it starts to collapse. You can just get a gravitational collapse, which leads to like a white dwarf or a neutron star or a black hole.