Death of High Mass Stars: Neutron Stars

Stars more massive than about 5 don't stop with helium burning. Hydrostatic equilibrium predicts that for a star supported by gas pressure (P=nkT), the larger the mass, the higher the temperature at the center of the star must be to support that mass. So massive stars have higher temperatures in their cores. We also know that the reason we need high temperatures to get fusion is that positively charged nuclei will repel away from each other before they can fuse unless they're moving very fast. It stands to reason, therefore, that if you're trying to fuse nuclei with more protons, you need higher temperatures to do it. That is, the more massive a star is, the heavier the nuclei which it can fuse.

Thus, as helium is used up in the core and a C-O core develops, that core does NOT collapse all the way to become a white dwarf. Before doing so, it becomes hot enough to fuse carbon into neon, and oxygen into sulfur and silicon. Finally, silicon gets fused into iron. Every time a heavier element is made, it sinks to the center of the star where it eventually becomes hot enough to undergo fusion. The ash of yesterday's burning becomes the fuel for today's. The result is an ``onion-skin'' structure for the star, with

1. hydrogen on the outside, and inner shells of
2. helium,
3. carbon/oxygen,
4. oxygen/neon/magnesium,
5. sulfur/silicon, and
6. an iron core at the very center.

Fusion occurs in all of the shells simultaneously, but can't occur in the core because you can't get energy from fusing iron into heavier elements. So the iron core cannot generate energy and begins to shrink.

In less than a day, silicon burning in the dying star produces so much iron that the iron core exceeds the Chandrasekhar limit. Because the core is not generating its own energy, it is supported only by electron degeneracy pressure, and once the mass of the core exceeds 1.4 it must collapse. In the process of the collapse, many of the electrons in the core are squeezed so tightly with the nuclei that they merge with protons to become neutrons. This reduces the electron degeneracy pressure further and accelerates the collapse. In less than a second, the core collapses into a ball of neutrons only about 10 km in radius--a neutron star. A neutron star is supported by gravity against a different kind of pressure, neutron degeneracy pressure. This is analogous to electron degeneracy pressure but requires much greater densities to become important. In fact, a neutron star is so dense that a cubic centimeter of it weighs as much as all the people on the planet Earth put together.

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