pulsar

A stellar source of radio waves, characterized by the rapid frequency and regularity of the bursts of radio waves emitted. The time between successive pulses is milliseconds for pulsars in binary systems and up to 4 seconds for the slowest. Some pulsars emit pulsed radiation in other bands of the electromagnetic spectrum, including visible light, as well as radio waves. A pulsar is a rotating neutron star, with a mass similar to the Sun's but a diameter of only about 10 kilometres. The pulses occur because the neutron star is rotating very rapidly: a beam of radio emission sweeps past an observer once per rotation. The pulses are very regular, apart from the occasional glitch, and all single pulsars are slowing down as they lose rotational energy. Some X-ray pulsars are in binary systems where complex dynamical effects cause the spin rate to speed up, and these millisecond pulsars are the fastest known. Millisecond pulsars not currently in binary systems are thought to have once belonged to pairs that have been split apart. Most have been discovered in globular clusters, where stars are densely packed and gravitational interactions can easily occur. At least one pulsar appears to have another neutron star as a companion, and another has two or three planet-sized companions. Their presence is deduced from variations in the arrival time of pulses. Pulsars are formed in supernova explosions, though only two - the Crab Pulsar and the Vela Pulsar - are within currently observable supernova remnants.