<P> A neutron star is the collapsed core of a large star which before collapse had a total of between 10 and 29 solar masses . Neutron stars are the smallest and densest stars known to exist . Though neutron stars typically have a radius on the order of 10 kilometres (6.2 mi), 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 . Once formed, they no longer actively generate heat and cool over time; however, they may still evolve further through collision or accretion . 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 . Neutron stars are supported against further collapse by neutron degeneracy pressure, a phenomenon described by the Pauli exclusion principle, just as white dwarfs are supported against collapse by electron degeneracy pressure . If the remnant star has a mass greater than about 3 solar masses, it continues collapsing to form a black hole . </P> <P> Neutron stars that can be observed are very hot and typically have a surface temperature around 7005600000000000000 ♠ 600 000 K. They are so dense that a normal - sized matchbox containing neutron - star material would have a mass of approximately 3 billion tonnes, or a 0.5 cubic kilometre chunk of the Earth (a cube with edges of about 800 metres). Their magnetic fields are between 10 and 10 (100 million to 1 quadrillion) times as strong as that of the Earth . The gravitational field at the neutron star's surface is about 7011200000000000000 ♠ 2 × 10 (200 billion) times that of the Earth . </P>

What are the densest stars known to exist