A neutron star is a compact object composed mainly of neutrons, having a typical mass of 1.4 M⊙and a typical radius of 10 km. Neutrons are abundantly produced through electron capture by protons in the final gravitational collapse of the dense Fe-core of a massive star. At densities 1014 g cm−3 the high pressure of the degenerate gas of neutrons resists further collapse, producing a neutron star, provided the mass of the compact object is less than the Oppenheimer-Volkoff limit of 2–3 M⊙; otherwise, the object collapses to a black hole. Rapidly rotating neutron stars emit beams of electromagnetic radiation and are known as pulsars .