Neutron stars
A neutron star is the collapsed core of a large (10–29 solar masses) star. Neutron stars are the smallest and densest stars known to exist. Though neutron stars typically have a radius on the order of 10 km, they can have masses of about twice that of the Sun. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past the white dwarf star density to that of atomic nuclei. Most of the basic models for these objects imply that neutron stars are composed almost entirely of neutrons, which are subatomic particles with no net electrical charge and with slightly larger mass than protons. They are supported against further collapse by neutron degeneracy pressure, a phenomenon described by the Pauli exclusion principle. If the remnant has too great a density, something which occurs in excess of an upper limit of the size of neutron stars at 2–3 solar masses, it will continue collapsing to form a black hole.
Neutron stars that can be observed are very hot and typically have a surface temperature around 6×105 K. They are so dense that a normal-sized matchbox containing neutron-star material would have a mass of approximately 13 million tonnes, or a 2.5 million cubic metre chunk of the Earth (a cube with edges of about 135 metres). Their magnetic fields are between 108 and 1015 times as strong as that of the Earth. The gravitational field at the neutron star's surface is about 2×1011 times that of the Earth.
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