How do supernovae work?

A type 1 supernova occurs when a white dwarf star accretes enough mass to trigger carbon fusion. White dwarf stars are compact stellar remnants that are supported by electron degeneracy pressure instead of thermal pressure, making them incredibly dense.

If enough mass is dropped on top of a white dwarf, the temperature will rise enough to trigger carbon fusion. Because the dense white dwarf cannot expand to regulate the heat from fusion, all of the carbon rapidly begins to fuse. The energy from so much fusion at once blows the star apart.

Type Ia supernovae are a special case that occurs in binary systems. One star will cease fusion and become a white dwarf before its companion. When the other star enters the giant phase, it will begin dumping mass onto the white dwarf until it triggers a supernova. Since they always occur in the same way, type Ia supernovae are used as standard candles to measure large distances.

A type II supernova, on the other hand, occurs when a high mass star begins fusing iron. The energy from iron fusion is not enough to support the star against gravity, so it begins to collapse, becoming extremely dense. The density is enough to trap neutrinos, which are a byproduct of fusion. The expanding neutrinos and the shockwave from the collapsing star "bouncing" off of the core, are enough to explode the star into space.