Bell and Hewish discover pulsars
While pursuing her PhD at Cambridge University, Jocelyn Bell's advisor was Antony Hewish, a radio astronomer. Hewish and his graduate students in 1967 completed a radio telescope specially designed to observe the scintillation (twinkling) of stars, particularly quasars. That summer, they observed an unusual signal at a wavelength of 3.7m -- unusual in that it corresponded to a sharp burst of radio energy at a regular interval of about one second. These were not like signals from other known sources such as stars, galaxies, or solar wind.
Bell realized that the unusual reading, "scruff" as she called it, was regular (every 1.3373011 seconds) and synchronized with sidereal (star) time and not Earth time. That insight, plus the ruling out of various earthbound sources such as pirate radio and police transmissions, suggested that the signal was extraterrestrial. Might it be a message sent by intelligent beings from another world? They couldn't rule out "little green men" at this point, and they half-kiddingly named the signal LGM1. But that left them with a dilemma: Who could they tell? And what would they say?
Bell and Hewish kept tracking their discovery and before they felt they had to announce the find to anyone, Bell found another pulsing signal. Only this one was slightly different -- it pulsed at 1.2 second intervals. And it came from the other side of the sky. These two differences made it extremely unlikely that the source was intelligent beings.
They announced the discovery, though they still hadn't determined the nature of the source. The astronomical puzzle was quickly taken up by physicists, who offered a few explanations. By 1968 most opinion settled on neutron stars as the best solution. This came from a theory put forth by Robert Oppenheimer and Fritz Zwicky in the 1930s, predicting that when a massive star died, it would collapse into an incredibly dense, spinning body, a neutron star.
Bell and Hewish's discovery was the first known evidence for this, and the pulsing signal source became known as a pulsar. They changed the name of the signal from LGM to CP, for Cambridge pulsar. Many more pulsars have been found since the first. They are believed to be rapidly rotating neutron stars with intense electromagnetic fields, which emit radio waves from north and south poles. From far off, the spinning pulsar's radio emission is perceived in pulses, much as one perceives a light signal from the rotating lantern of a lighthouse. The discovery of pulsars was exciting because it was a first step in veryifying the existence of a very strange theoretical phenomenon: black holes.
In 1968 Bell earned her PhD -- pulsars appeared in the appendix of her dissertation. In 1974 Hewish received the Nobel Prize for the discovery of pulsars.