Why are neutron stars so heavy?

Neutron stars are created when a star around eight to ten times the mass of our Sun runs out of fuel. The outward pressure generated by fusion reduces rapidly, allowing gravity to pull the star in on itself and trigger a supernova, where the outer layers of a star’s atmosphere get blown into space.

The remaining matter continues to collapse under gravity, forcing electrons and protons to be squashed together and become neutrons. The neutron star will have less mass than its parent star (typically about 1.4-times the mass of the Sun), but this mass will be confined by gravity to a region of approximately 20 kilometres (12 miles) across, leading to an incredibly dense object. It is this density (a teaspoon full of neutron star would have a mass of about a billion tons) that truly defines a neutron star.

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