Neutron star Radiation from the pulsar PSR B1509-58 , a rapidly spinning neutron star, makes nearby gas glow in X-rays (gold, from Chandra ) and illuminates the rest of the nebula , here seen in infrared (blue and red, from WISE ). 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. [1] 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...