How does a Supernova start?

After the hydrogen in the core has been exhausted through nuclear fusion, the core begins to contract under it's own gravity (radiation pressure had been helping to stop this while fusion was going on). It will do this several times, stopping to fuse helium, oxygen, and carbon on the way, until the core fuses something to iron and can go no further. Then the core contracts again and the remaining photons destroy some of the iron through photodestruction leading to an implosion which officially begins the supernova process.