How do supernovae begin?
A supernova explosion begins when the core of a large star collapses under gravity.
Large stars start out by fusing hydrogen into helium. As the hydrogen runs out they start fusing progressively heavier elements. Each of the fusion processes produce energy until iron is produced iron fusion requires more energy than is produced.
Stars need fusion reaction to maintain hydrostatic equilibrium and prevent collapse under gravity. When the core is largely iron, fusion stops and the core collapses under gravity. Depending on the mass of the core it will either collapse into a neutron star or a black hole.
As the core collapses it generates heat. This can start fusion reactions in the layers around the core. It is also thought that massive numbers of neutrinos which get created when protons get transformed into neutrons during collapse can also have an effect.
The fusion reactions in the layers around the core can involve a significant amount of the available material is a short period of time. This results in a massively violent supernova explosion.
Another form of supernova can occur if a white dwarf star is a binary star with a red giant partner. The white dwarf can consume material from its partner. Once a sufficient amount of material has been acquired by the white dwarf it starts to collapse. This triggers a fusion reaction in its mainly carbon and oxygen core. The reaction typically involves the fusion of a significant pat of the core in a short time leading to a supernova explosion.