In 1967, Jocelyn Bell was doing research in radio astronomy at Cambridge University under advisor Anthony Hewish.
She unexpectedly detected regular radio pulses from the sky with a period of about 1.3 seconds.
Bell and Hewish were astounded by the regularity of the pulses.
They jokingly wondered if the signals were the first transmissions received from an advanced alien civilization.
They named the first four of these strange sources of radio signals LGM 1, LGM 2, LGM 3, and LGM 4, with LGM standing for "Little Green Men".
These pulsars are actually rapidly spinning neutron stars.
These incredibly dense objects -- about 10 trillion times denser than a lead brick, with more mass than the sun -- produce gravity 100 billion times stronger than Earth.
A powerful magnetic field traps and accelerates charged particles.
Radio waves are beamed in a cone shape through space, like the rotating light from a lighthouse beacon.
If Earth lies in the sweep of the beam, we receive the signal once each rotation.