2015-05-06 BBC:星光七纪元-31（在线收听）

 And with his colleagues, he did the calculations to prove it. The key was the conditions created in the final stages about massive stars fighting against its gravity. These stars are so massive and hot that they can go through a whole series of nuclear reactions. The ashes of one set of nuclear reactions becomes the fuel for the next set of nuclear reactions. The most massive stars are able to fuse heavier and heavier elements in the series of layers, creating the energy to resist the relentless inward pull of gravity. There is neon and magnesium and more oxygen, then the silicon and sulphur, and finally, in the middle, a core of Iron. And that’s where the fusion starts. With fusion at an end, there is no more energy to fight back. And gravity wins the battle. The star is doomed. When that ball of iron, which is a certain critical mass, about the size of the earth but much, much more massive, the electron pressure is no longer able to support it against the inward force of gravity, so it starts to collapse. It collapses to a ball, about the size of a city and then rebounds, and that rebounce hits the surrounding layers, launching a supernovae explosion. It’s the speed and violence of the collapse of the star’s iron core that triggers the supernovae.