When a star runs out of fuel to burn, it’s outer layers get ejected into space and it’s core collapses under gravity into a white dwarf. If the star was really big, more than about 8 times the mass of our Sun, then it continues to collapse, becoming denser and denser until the very atoms themselves are crushed. The enormous gravitational pressure overcomes the Weak Nuclear Force, so that electrons fall out of their orbit and touch the protons in the atomic nuclei. When this happens, the particles combine into neutrons, and the matter suddenly takes up a tiny fraction of the space it took before. The electron degenerate matter has now turned into neutron degenerate matter – a volume of stuff composed entirely of neutrons pressed up against each other – causing the star to collapse violently in less than a second until the core is completely converted and cannot shrink any further. This causes the outer layers, still falling inwards, to recoil violently outwards, taking with them the enormous amount of energy released by the collapse. This explosion, called a Supernova, is one of the single most violent events in the entire universe. What remains is a hot star composed almost entirely of neutrons, with perhaps a thin shell of electron degenerate matter on the surface. Neutron stars, as you might have guessed by this point, are much much denser than even white dwarfs: a liter (a cube with sides of 10cm, or about 4 inches) of neutron degenerate material would weight four hundred billion tonnes!
Written by Allen Versfeld
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