Although there are many classifications of supernovae, there are two basic mechanisms that result in stars blowing themselves apart. The first involves the accumulation of material into a white dwarf star, the extremely dense final evolutionary state of most normal stars. This extra material could come from a merger with or by simple accumulation from a close companion star. As this material accumulates on the white dwarf, its core temperature increases until a runaway nuclear reaction occurs. In a fraction of a second most of the white dwarf undergoes nuclear fusion, blowing the star apart in the process.
The other way a supernova can ‘ignite’ is by the collapse of a high-mass stellar core. As the star reaches the end of its life the nuclear fuel becomes exhausted, the energy source switches off and the pressure holding the star up against gravity disappears. The core collapses almost instantaneously, causing a catastrophic release of energy that destroys the star. If the core mass is high enough, however, the star may become a neutron star or black hole with little radiated energy.