Neutron stars are highly dense stars that have a small size. This generates huge gravitational and magnetic field. After massive stars have finished burning their nuclear fuel, they undergo a supernova explosion. This explosion blows off the outer layers of a star into a supernova remnant. The central region of the star collapses under gravity. It collapses so much that protons and electrons combine to form neutrons. Therefore, the name "neutron star".
Neutron stars may also appear in binary systems. When a neutron star is in a binary system, astronomers are able to measure its mass using radio or X-ray telescopes. It has been calculated that a neutron star's mass is about 1.4 times the mass of the Sun. Measuring the mass of a binary systems containing an unknown object helps distinguish whether the object is a neutron star or a black hole, since black holes are more massive than neutron stars.
Pulsars are rotating neutron stars. Their rotation causes them to pulse. Pulsars were discovered 1967 by Jocelyn Bell Burnell and Antony Hewish of the Cambridge University as radio sources that blink at a constant frequency. Pulsars have jets of particles moving almost at the speed of light streaming out above their magnetic polls. Similar to Earth, the magnetic and rotational axes of a pulsar are also misaligned. Therefore, the beams of light from the jets sweep around as the pulsar rotates. Sometimes the term "spin-powered" is added to describe this kind of star (''spin-powered pulsar'').
A very different type of pulsar is seen by X-ray telescopes in some X-ray binaries. In this case, a neutron star and a normal star form the binary system. The strong gravitational force from the neutron star pulls material from the normal star. The material is funneled onto the neutron star at its magnetic poles. In this process, called accretion, the material becomes so hot that it produces X-rays. The pulses of X-rays are seen when the hot spots on the spinning neutron star rotate through our line of sight from Earth. These pulsars are sometimes called "accretion-powered pulsars" to distinguish them from the spin-powered pulsars. X-ray pulsars behave differently from rotation-powered pulsars, although it is accepted that both kinds are manifestations of a rotating magnetized neutron star, with the rotation cycle of the neutron star in both cases being identified with the pulse period. The major differences are that radio pulsars have periods in the order of milliseconds, and all radio pulsars(rotation-powered pulsars) are losing angular momentum and slowing down, while several of the X-ray pulsars have been shown to be continuously spinning faster, while others show either little change in period or display erratic spin-down and spin-up behavior.
A magnetar is a neutron star with an extremely strong magnetic field, the decay of which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma-rays. Magnetars have become accepted as explanation for observable objects known as soft gamma repeaters and anomalous X-ray pulsars.