To work out what pulsars are, we must go back to massive stars exploding as supernovae, and to the “wild” idea of Fritz Zwicky.

Despite the obvious importance of Jocelyn Bell’s discovery, the mysterious burst of energy still hadn’t a name. So a journalist from the daily Telegraph proposed Pulsars as an abbreviation for “Pulsating Radio Star”, and that idea sounded really good to Jocelyn Bell and collaborators. It was clear that if already four pulsars were discovered in about 2 months and relatively close to the Sun, then that meant that millions of them were distributed throughout the Galaxy. But it wasn’t really clear what those objects actually were, except that they were very small, much less than the size of a planet, very energetic, and as massive as a star comparable to the Sun.

X-ray image of the pulsar in the Crab Nebula by the Chandra satellite. Credits: NASA

To work out what pulsars are, we must go back to massive stars exploding as supernovae. It used to be assumed that a supernova was the total catastrophic destruction of the star, with everything dispersed out into space. However, in the 1930s a Swiss astronomer called Fritz Zwicky reckoned that when a massive star dies, its core got shrunk right down to make these very dense balls, and it would be rich in neutrons, so it is called a “neutron star”. That was a very “wild” idea and was not taken very seriously at the time. But we now know following the discovery of pulsars that such objects actually exist, because it turns out that pulsars are indeed neutron stars and Zwicky was right.

Cosmic lighthouses

A neutron star is an incredibly compact thing that, due to angular momentum conservation during the core-collapse of the progenitor dying massive star, spins very quickly. They have a very intense magnetic field, about a thousand billion times stronger than the magnetic field of the Earth. Moreover, coming from the magnetic north and south pole, there is a beam of electromagnetic waves and particles, and as the star spins, this beam gets swept throughout the sky, like a lighthouse beam. Each time the beam sweeps across the Earth, we see a “pulse”. So if you see, for instance, a pulse going at ten times, or even one thousand times a second, it means that the neutron star is spinning at the same frequency.


Zwicky et al.; Astrophysical Journal, vol. 88, p.522-525

“Beautiful Minds, Series 1”. BBC Four. 25 April 2011

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