The first known pulsar was discovered in August 1967 by student Jocelyn Bell (Belfast, Northern Ireland, UK, 15 July 1943) and her mentor Professor Antony Hewish (Fowey, Cornwall, England, UK, 11 May 1924). It had a pulse time of 1.3 seconds. They named the object responsible for this LGM, which stood for little green men, because it looked like a radio beacon that extraterrestrial intelligent life emitted. After some speculation a consensus was reached that the only object that could be responsible for these signals was a neutron star.
A pulsar (pulsating star) is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. This radiation can only be observed when the beam of emission is pointing towards the Earth, much the way a lighthouse can only be seen when the light is pointed in the direction of an observer, and is responsible for the pulsed appearance of emission. Neutron stars are very dense, and have short, regular rotational periods. This produces a very precise interval between pulses that range from roughly milliseconds to seconds for an individual pulsar.
Video: Jocelyn Bell Burnell (Beautiful Minds, 2010)