Sir James Chadwick discovered the neutron as an elementary particle in 1932, earning the 1935 Nobel Prize in Physics for this work. Astronomers Walter Baade and Fritz Zwicky proposed the existence of a neutron star as early as 1933. In their prediction, the release of gravitational binding energy in the formation of a neutron star is what powers the visible supernovae.
The first neutron star was observed in 1965 in the Crab Nebula. In 1967, the discovery of a regular radio pulse from space resulted in the first pulsar discovery, which was later determined to be the magnetic field of a rotating, isolated neutron star. Much of the major work into neutron stars has focused on pulsars.
A typical neutron star is between 30,000 to 70,000 times smaller than the sun, but much more massive, reaching up to over twice the mass of the sun. A simple density calculation shows that the neutron star is, therefore, much more dense than the sun ... so much so, in fact, that their density is comparable to the density of an atomic nucleus (on the scale of 1017 kg/m3).
If a star is too massive - around 5 solar masses - then the gravitational force of the collapse is so powerful that a black hole is created instead of a neutron star.
