A guide to the mysterious light of quasars

An image of a galactic core (at the centre), with four images of the same quasar surrounding it in an Einstein Cross. By ESA/Hubble & NASA, CC BY 3.0

It was the 1960s and Maarten Schmidt faced an observation, unlike anything he’d seen before.

Quasars had been discovered at the end of the last decade as mysterious radio sources in all-sky radio surveys. Whilst Penzias and Wilson were busy evicting pigeons from their radio antenna, trying to find the origin of a mysterious omnipresent hum, these radio signals came from very small…