A supernova begins when the nuclear fusion stops at Iron in the core of a massive star. With no energy source to keep the core hot, gravity overwhelms the gas pressure and the core begins to collapse.

When the core reaches nuclear density the violent collapse of the core is halted, but the rest of the star has started to implode. The infalling material hits the collapsed core producing a shock wave.

This shock travels out to about 200 km before it is stalled by the continuing infall of stellar material raining down on the core. The region inside of the shock wave is unstable to convection, and current research has focussed on the possibility that convection in this region can help drive the shock the rest of the way out of the star.