Neutron stars! And supernovae!

This is going to be a two-part discussion of the collapse of high(ish)-mass stars into neutron stars and the subsequent explosion we call a supernova.
It all starts with a heavy star, over around eight solar masses (random aside: it turns out that if these stars weren't constantly blowing away their outer layers, a star would only need to be around four solar masses to collapse into a neutron star). Over time, as the star ages, its core runs out of hydrogen to fuse. Here the massive star behaves kind of like the stars I mentioned yesterday, but instead of relying on degeneracy pressure to fuse helium, it's massive enough to be able to fuse helium stably, and keeps going. Over time, the star develops an onion structure, with layers of different elements and fusion processes. Close to the surface the star burns hydrogen. This is a very efficient reaction, because protons are much happier to be in helium nuclei than hydrogen, because of the higher "binding energy per nucleon" in helium. Unfortunately, that's a topic that will have to wait for another day. For now, take my word for the fact that hydrogen burning is very efficient, while the burning of higher-mass atoms is less so. Anyhow, layers closer and closer to the center are hotter and denser, and can thus fuse larger and larger nuclei. All the way down through silicon fusing into iron and nickel. But iron-56 is the most stable nucleus in the universe, so there is will take energy to fuse it into heavier atoms or break it up into smaller ones. This star therefore accumulates an iron core. Eventually, this iron core becomes so massive that the degeneracy pressure of the electrons inside it isn't enough to support it against gravity. At this point (at least in the example I saw recently), the core has a radius of around 10,000 km. Now that there's less supporting pressure, the star collapses inwards and gets hotter (this is a fascinating example of negative specific heat, in which a loss of energy produces an increase in temperature). This is hot enough to break down iron and helium in an endothermic reaction, which consumes much of the increase in temperature and means that the star's core is once again relying solely on degeneracy pressure for support. But the density is so high that degeneracy pressure just doesn't cut it anymore.
Instead, the core begins a phase called electron capture, in which the electrons and protons in the core are pushed together so hard that they merge into a neutron and a neutrino in the reaction $p+e\to n+\nu_e$. This electron capture happens in just a few seconds, and suddenly the core is just 20 km across. It has a density of $8\cdot10^{17}$ kilograms per cubic meter. That means that in just a cubic centimeter (the volume of a measly gram of water), the ex-iron-core has around the mass of a respectable mountain! This is called a neutron star, and apart from being generally awesome, they're also the most perfectly spherical objects in the universe (that we know of, anyway). What happens to the non-core part of the star is a topic I hope to explore in a post soon.

I probably won't be able to post tomorrow or over the weekend, but look forward to returning to awesome physics at the beginning of next week.