In 1933, the Indian Subrahmanyan Chandrasekhar realized that the Pauli exclusion principle had only a limited ability to fight against the squeeze of gravity. As pressure in the star increases, the Pauli exclusion principle states that ekectrons inside must move faster and faster to avoid one another. But there's a speed limit: electrons cannot move faster than the speed of light, so if you put enough pressure on a lump of matter, electrons cannot move fast enough to stop the matter from collapsing. Chandrasekhar showed that a collapsing star that has about 1.4 times the mass of our sun will have enough gravity to overwhelm the Pauli exclusion principle. Above this Chandrasekhar limit a star's gravity will pull on itself so strongly that electrons can't stop its collapse. The force of gravity is so great that the star's electrons give up their struggle once and for all; the electrons smash into the star's protons, creating neutrons. The massive star winds up being a gigantic ball of neutrons: a neutron star.
( Charles Seife )
[ Zero: The Biography of a ]
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