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What is a pulsar?

A pulsar is a highly magnetized, rapidly rotating neutron star. Weighing more than our Sun, yet only 20 km in diameter, these incredibly dense objects produce radio beams that sweep the sky like a lighthouse. Since their discovery in 1967, over 1700 pulsars have been found, and they provide a wealth of information about neutron star physics, general relativity, the Galactic gravitational potential and magnetic field, the interstellar medium, celestial mechanics, planetary physics and even cosmology. Here at WVU, we use pulsars to carry out research into many of these areas using a wide variety of telescopes in different areas of the electromagnetic spectrum.

Most of the stars we see in the night sky are in equilibrium, resisting the relentless pressure to collapse under their own gravity with radiation pressure generated from nuclear reactions within their cores. About once every second in the Universe, and once a century in our Galaxy, a star several times the mass of the Sun runs has used up all its nuclear fuel and explodes in a dramatic event known as a supernova. The core of the star collapses violently sending shock waves back through its outer layers which get blasted out into space. In 1054 AD, Chinese astronomers were fortunate enough to witness such an explosion whose remnant, shown on the right, is today known as the Crab Nebula.

In a landmark paper on the nature of supernovae written in 1934, Walter Baade and Fritz Zwicky proposed that the core collapse would leave a neutron star - a superdense, exotic star with the same mass as our sun but a diameter of only 10-15 km. Confirmation of this idea came unexpectedly with the discovery of pulsars by Cambridge astronomers Jocelyn Bell and Anthony Hewish in 1967. The first few of these highly periodic sources of pulsed emission had periods ranging between 200 ms and 1 second and presented theorists with a challenge as to their origin. Indeed, at one stage, the theories put forward greatly outnumbered the pulsars! The discovery of a 33 ms pulsar in the Crab nebula in 1968, however, ruled out all but one model - a rapidly rotating, highly magnetized neutron star.

Further reading:
Living Review article on Binary and Millisecond Pulsars