Despite their confusingly similar names, these are very different celestial objects. A pulsar (originally short for ‘pulsating star’) is a rapidly spinning neutron star – the remnant of a supernova explosion. It has a powerful magnetic field, shooting out jets of radiation that sweep across space like lighthouse beams – when they line up with Earth they appear as a rapidly repeating burst of light, radio waves and other radiations. A quasar (from ‘quasi-stellar radio source’) is in fact a distant galaxy with a fluctuating blaze of light and other radiations coming from its central regions. The activity in these galaxies is caused by a giant black hole at their very heart, pulling in material from its surroundings, tearing it to shreds and heating it up to tremendous temperatures before swallowing it up.
Answered by Giles Sparrow