Specifically white dwarfs in a type 1a supernova. While not the biggest supernovae around, they're unambiguously nuclear in nature (whereas black holes play into the others). What happens is that the force of gravity in a white dwarf eating mass from some other star overcomes the electron degeneracy pressure (fancy way of saying the "force that keeps electrons from sitting on top of eachother"), forcing the charged particles to bunch up together as neutrons in order to obey the Pauli Exclusion Principle.