Pulsars: The Cosmic Clocks
TL;DR Summary
Rapidly spinning neutron stars emit beams of radiation that sweep across space like a lighthouse. These are pulsars.
LGM-1
In 1967, Jocelyn Bell Burnell discovered a strange radio signal pulsing with incredible regularity—every 1.337 seconds. The pulses were so precise that astronomers jokingly named the source "LGM-1" for "Little Green Men." However, they soon realized the source was completely natural.
The Cosmic Lighthouse
Pulsars are highly magnetized, rapidly rotating neutron stars. They emit beams of electromagnetic radiation from their magnetic poles. If the magnetic axis is not aligned with the rotation axis, these beams sweep through space as the star spins. If Earth lies in the path of the beam, we detect a pulse of radiation every time the beam sweeps past us.