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Neutron stars are nothing if not exotic. Most are the residues of massive stars that, after exhausting their thermonuclear fuel, die spectacularly as supernovae. Fifty percent more massive than our Sun, but only 20 kilometers in diameter, they are the densest objects in the universe. At the surface of a neutron star, the acceleration due to gravity is in excess of times that at the surface of Earth, so that the binding energy of a parcel of matter in a neutron star is about 10% of its rest mass. Hence mass accretion onto a neutron star, which releases over 100 MeV/nucleon, is a far more efficient energy conversion process than thermonuclear fusion, which releases approximately 8 MeV/nucleon. Theory suggests that neutron stars are about 95% neutrons in “chemical” equilibrium with a small admixture of protons and electrons: Some investigators have likened a neutron star to a giant nucleus with atomic mass about The pressure of strongly interacting degenerate neutrons at supranuclear densities is balanced by gravity to maintain a hydrostatic equilibrium in the canonical configuration described above. Neutron stars therefore are laboratories for both nuclear physics and general relativity.
The core of a massive star that lives for 10 million years collapses within one second, initiating a series of some of the most exotic and extreme events that occur in the universe.