A fast-spinning, highly magnetized neutron star, formed (in most cases) following a supernova explosion, that sends out regular directional pulses of radiation as it rotates, in the manner of a lighthouse beam; the pulsar effect is seen if the beam happens to sweep in our direction. Pulsars were found originally at radio wavelengths but have since been observed at optical, X-ray, and gamma-ray energies; the first was discovered in 1967 by Jocelyn Bell Burnell. The emission comes from the acceleration of electrons to near-light speed above the pulsar's magnetic poles, which are displaced from the object's geographic poles. Pulses last on the order of microseconds, while the interval between pulses, known as the pulse period, is typically 0.25 to 2 seconds, but can be short as 1 to 10 milliseconds in the case of millisecond pulsars. The pulse period gradually lengthens as the neutron star loses rotational energy, though some young pulsars are prone to glitches, probably due to starquakes, when period abruptly changes. Precise timing pulses has revealed the existence of binary pulsars and of two pulsars that have sets of planet-sized companions (see pulsar planets).
The discovery of millisecond pulsars, with pulse periods of less than 10 ms (the record holder is PSR 1937+21, also the first to be found, with a period of 1.56 ms) was initially puzzling. The reason for this is that the youngest known pulsar is the one in the Crab pulsar with a period of 33 ms. Since pulsars slow down with age, how could older pulsars have shorter periods? The answer seems to be that millisecond pulsars have been "spun up," thereby becoming what are called recycled pulsars, by the transfer of matter from a companion. Almost all the 90 or so known millisecond pulsars have been found to be part of binary systems in which the partner is a white dwarf, the presumption being that the pulsars was rejuvenated by matter transfer while their companions were still in the red dwarf phase.
Although most pulsars are thought to form during supernovae, mounting evidence suggests that some originate as white dwarfs. Matter transfer is again the underlying phenomenon at work. If a white dwarf acquires enough mass from a close companion and doesn't somehow manage to get rid of it, as in a nova outburst, for example, it will eventually collapse to become a neutron star and may be seen as a pulsar.
