In 1934, two astronomers, Walter Baade and Fritz Zwicky, proposed the existence of a new form of star, the neutron star, which would be the end point of stellar evolution. They wrote:
… with all reserve we advance the view that a supernova represents the transition of an ordinary star into a neutron star, consisting mainly of neutrons. Such a star may possess a very small radius and an extremely high density.
These prophetic remarks seemed at the time to be beyond any possibility of actual observation, since a neutron star would be small, cold and inert, and would emit very little light. More than 30 years later the discovery of the pulsars, and the realisation a few months later that they were neutron stars, provided a totally unexpected verification of the proposal.
The physical conditions inside a neutron star are very different from laboratory experience. Densities up to 1014 g cm−3, and magnetic fields up to 1015 gauss (1011 tesla), are found in a star of solar mass but only about 20 kilometres in diameter. Again, predictions of these astonishing conditions were made before the discovery of pulsars. Oppenheimer & Volkoff in 1939 used a simple equation of state to predict the total mass, the density and the diameter; Hoyle, Narlikar & Wheeler in 1964 argued that a magnetic field of 1010 gauss might exist on a neutron star at the centre of the Crab Nebula; Pacini in 1967, just before the pulsar discovery, proposed that the rapid rotation of a highly magnetised neutron star might be the source of energy in the Crab Nebula.