NEUTRONIUM. “Ordinary neutron stars should have a fluid made almost entirely of neutrons beneath the crust. This is neutronium, a mysterious and extremely dense substance. Just a spoonful of it would weigh more than Earth’s tallest mountain. But this neutronium doesn’t exist just anywhere — it exists at the center of a neutron star where immense pressures can cause the neutrons to melt into quarks. Neutrons are not, after all, elementary particles. Quarks are.”
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