When does a massive star become a supernova?
A massive star goes supernova when it runs out of nuclear fuel.
When a massive star depletes its supply of Hydrogen it starts fusing Helium. As the supply of Helium becomes depleted it starts fusing progressively heavier elements.
When the star's core is predominantly Iron then no further fusion reactions can take place as fusion reactions involving Iron and heavier elements consume energy rather than release energy.
Once the fusion reactions have stopped the core starts to collapse. If the core mass exceeds the Chandrasekhar limit or 1.44 solar masses, gravity is strong enough to overcome the electron degeneracy pressure. The atomic nuclei come together and protons are converted into neutrons by either beta plus decay or electron capture. This releases large numbers of electron neutrinos and the core temperature rises. If the star has enough Hydrogen in its outer layers, this will undergo rapid fusion which results in a supernova explosion.
The core will become a neutron star and if the mass is more than 15 solar masses, gravity will overcome neutron degeneracy pressure and collapse into a black hole.