A neutron star is a highly compact supernova remnant formed from the gravitational collapse of a massive star, in which the electromagnetic repulsion between electrons and protons is no longer sufficient to withstand the strength of the star’s own gravitational field.
As the atoms in the collapsing star’s core are forced together, the protons combine with the electrons to become neutrons. Normal atomic matter is mostly empty space; however in a neutron star, the atomic nuclei are forced together by gravity, forming an extremely dense form of matter know as neutronium. One marble-sized amount of this material, of one cubic centimetre in volume (1.2 centimetres across), would weigh around 12 billion tonnes!
Neutron stars must be larger than 1.1 times the mass of the Sun in order to overcome the electromagnetic force during their formation, and under around 3 solar masses in order to prevent further gravitational collapse to a black hole. This means that neutron stars are typically around 24 to 26 kilometres (about 15 to 16 miles) in diameter but have a mass of more than 2×1027 tonnes.
The existence of neutron stars was first proposed by Walter Baade and Fritz Zwicky in 1934.
Pulsars
Pulsar are spinning neutron stars with high magnetic fields that produce jets of electromagnetic radiation from their poles. If these polar jets sweep across the Earth as the neutron star spins, they are detectable as pulses – like cosmic lighthouses – hence the name pulsar, from a contraction of the words pulsating star. The timing of these pulses is very precise, and varies between pulsars from a few seconds to a few microseconds.
The first pulsar was discovered on 28 November, 1967, by Jocelyn Bell Burnell and Antony Hewish, who originally considered the possibility that the pulses might be a signal produced by extra-terrestrial intelligence. Shortly before the discovery, also in 1967, Franco Pacini had predicted that rotating neutron stars might produce jets of radiation in this manner.
This first pulsars discovered produced pulses at radio wavelengths; however, pulsars have since been observed that emit radiation at higher wavelengths, from microwaves to visible light to gamma rays.