In a thread about the newly visible supernova in the M82 galaxy, MetaFilter user Ivan Fyodorovich offered up this plain-English explanation of what happens when a star dies and goes supernova. It’s a great read.
It will take it just 6 months to burn up its oxygen. Again, when there’s not enough oxygen being fused to generate energy to balance the pressure of gravitational contraction, the star begins to shrink, almost doubling the temperature, tripling the density, and causing the silicon (which was produced by the oxygen fusion) to begin fusing, in its own complicated sequence involving the alpha process, with the end result of nickel-56 (which radioactively decays into cobalt-56 and iron-56). This, as before, balances against the gravitational pressure and returns the star to equilibrium.
And now it will take merely 1 day to burn up its silicon. Finally, when there’s not enough silicon being fused to generate energy to balance the pressure of gravitational contraction, the star begins to shrink.
This time, however, the core of the star is mostly nickel and iron, and they cannot ordinarily be fused into heavier elements, so as the star shrinks and the temperature and density increase, there is no nuclear fusion ignition of the nickel and iron to counteract the contraction. Here the limit of pressure and density is the electron degeneracy pressure, which is the resistance of electrons being forced to occupy the same energy states, which they can’t.