Introduction to Pulsars

Pulsars were famously discovered by Jocelyn Bell Burnell, a PhD student at the University of Cambridge. Jocelyn Bell used a radio telescope, in Cambridge, to look across the sky. There was one very puzzling object, which was sending out radio waves in pulses, every 11/3 seconds! Normally, when we receive pulses of radiation, it is because the object is rotating.
Problem is, a star (like our Sun), definitely can’t rotate as fast as once every 11/3 seconds! Jocelyn, and the team of astronomers that she was working with, began to think that there were aliens- or ‘little green men’- sending out signals to try and communicate with Earth. This object was called LGM1- standing for little green men 1. After more observations, more of these objects sending out pulses of radio waves were discovered, in different parts of the Universe- which made it less likely that they pulses were from extra-terrestrial life.

The pulses of radio waves were very evenly spaced, and always lasted the same amount of time.

Advanced: In fact, the pulsars gradually slow down over time. As this happens, the length of the pulse increases. The pulse length increases at a rate equal to the derivative of the period (P) with time (t) (called the ‘period derivative’):


Astronomers had three options for what pulsars could be: binary systems, pulsating stars or rotating stars. By carrying out detailed observations and calculations, astronomers concluded that pulsars were rotating neutron stars.

Advanced: A. Neutron stars rotate with precise clock-like behaviour because of conservation of angular momentum.

Next page: Neutron Stars