Neutron stars: the smallest and densest stars in the universe

Simulated view of a neutron star. Due to its strong gravity, the background is gravitationally lensed, making it appear distorted.
A neutron star is the collapsed core of a giant star which before collapse had a total mass of between 10 and 29 solar masses. Neutron stars are the smallest and densest stars, not counting black holes, hypothetical white holes, quark stars and strange stars. Neutron stars have a radius on the order of 10 kilometres (6.2 mi) and a mass of about 1.4 solar masses.They result from the supernova explosion of a massive star, combined with gravitational collapse , that compresses the core, packing the neutrons until they are occupying the smallest space possible
This is similar to the process that produces a black hole, which occurs with more massive objects; however, no black hole has been detected below 5 solar masses.
What occurs between that neutron star upper limit and black hole lower limit is still a mystery - but one that massive neutron stars could shed some light on.

Population and distances

Central neutron star at the heart of the Crab Nebula.
At present, there are about 2,000 known neutron stars in the Milky Way and the Magellanic Clouds, the majority of which have been detected as radio pulsars. Neutron stars are mostly concentrated along the disk of the Milky Way although the spread perpendicular to the disk is large because the supernova explosion process can impart high translational speeds (400 km/s) to the newly formed neutron star. Some of the closest known neutron stars are RX J1856.5−3754, which is about 400 light-years from Earth, and PSR J0108−1431 at about 424 light years.

Neutron star (RX J0806.4-4123)

Neutron star (RX J0806.4-4123)Add caption
Neutron star (RX J0806.4-4123) with a disc of warm dust that produces an infrared signature as detected by the NASA/ESA Hubble Space Telescope. The disc was not directly images, but one way to explain the data is by hypothesising a disc structure that could be 29 billion kilometres across. The disc would be made up of material falling back onto the neutron star after the supernova explosion that created the stellar remnant.