Neutron star

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What remains when a massive star's core collapses and stops. Roughly one and a half solar masses are compressed into a sphere on the order of twelve kilometres across, at densities comparable to an atomic nucleus, supported by neutron degeneracy together with the repulsive core of the strong interaction. The upper mass bound is not known precisely because it depends on the equation of state of matter at densities no laboratory can reach; the heaviest reliably measured neutron star, the pulsar PSR J0740+6620, has a gravitational mass of 2.08 plus or minus 0.07 solar masses, and independent analyses of the same NICER and XMM-Newton data gave equatorial radii near 12.4 km and near 13.7 km - a real disagreement at the level that matters for the physics. Neutron stars were hypothesised in 1934, two years after the neutron was discovered, and observed only in 1967. Not a place on Earth.