Jocelyn Bell Burnell and the Discovery of Pulsars

Jocelyn Bell Burnell discovered pulsars (a specific type of neutron star) and got zero credit for it until recently. Here's her story.

Sabrina Stierwalt, PhD,
September 11, 2018
Episode #301
image of a pulsar star

In 1974, the Nobel Prize in Physics was awarded to two men, Anthony Hewish and Sir Martin Ryle, for the discovery of pulsars, the dead remnants of massive stars left behind after the massive supernova explosions that ended their lives, a kind of star which had previously only been theorized to exist. However, the bulk of the work that led to the discovery from seven years earlier had actually been done by Jocelyn Bell Burnell, Hewish’s graduate student and one of the few female astronomers at the time.

Despite her incredibly important contributions being overlooked, Bell Burnell remained an active researcher. Last week, 50 years after her work as a graduate student, she earned the $3 million Special Breakthrough Prize in Fundamental Physics for her discovery of pulsars which revolutionized our understanding of the universe. Even more revolutionary, her personal experience has inspired her to donate the whole thing.

What Are Pulsars?

Pulsars are rapidly rotating dead stars that get their name because we detect them by their flashes of radio emission. But they only appear to pulse because their light is beamed into a search light that sweeps across our telescopes as they rotate. The most commonly used analogy is that of a lighthouse whose beacon of light scans the vast ocean for anyone who might be looking to detect it. The emission doesn’t actually blink on and off, it just appears to from our vantage point.

So how did pulsars get this way? Stars like our Sun, a relatively low mass star, spend much of their lives supporting themselves by burning through their supply of hydrogen. This act of fusion produces radiation which can exert an outward pressure which fights back against gravitational collapse. Stars that are more massive than our Sun move on to burn heavier and heavier elements, working their way down the periodic table of elements until they get to iron.

For elements heavier than iron, energy is no longer produced during their fusion but instead energy is required for them to form. So without this steady input of energy, stars halt the fusion process and thus no longer have a way to fight back against gravitational collapse. The outer layers of the star come crashing down on its core before being blown away in a supernova explosion.

Meanwhile, the core itself has become incredibly compact, so much so that electrons and protons have been squeezed together to form neutrons, the non-charged particles that like to hang out in the nuclei of atoms. Neutrinos are also formed in this process but they quickly escape. Eventually the neutrons are so tightly packed together that they cannot be forced any closer to their neighbors and so exert a pressure that again fights back against gravitational collapse, leaving a dead corpse star known as a neutron star.

Neutron stars are typically 10 to 20 kilometers across, which means they pack roughly one and a half times the mass of our Sun into a space comparable to the size of Manhattan. From another perspective, one teaspoon of neutron star weighs around 10 million tons.

The light emitted by a neutron star is beamed in two directions so that it shines out of opposite sides of the star instead of in all directions (like the light from our Sun) due in part to the star’s strong magnetic fields. Now, I’ll let you in on a little secret. When astronomers can’t explain a certain phenomenon, we often say, “it’s probably related to the magnetic fields” because magnetic fields can be quite challenging to study. But in this case, it really is true! The magnetic fields around neutron stars have been compressed and, as particles spiral around these very strong magnetic field lines, they produce emission leading to this relativistic beaming effect.

If the beam of a neutron star happens to be pointed in the right direction so that it sweeps over us here on Earth—like that lighthouse would over a ship in the ocean—then we call that neutron star a pulsar.

Who Discovered Pulsars?

In the 1960s, Jocelyn Bell Burnell and her advisor Anthony Hewish, along with a small team of other astronomers at the University of Cambridge, built a radio telescope in hope of finding quasars, powerful radio sources from the distant universe. And they did find about 100 quasars by studying the paper read outs produced by their spectrometer—computers were not readily available for this kind of research at the time so there was no way to write a handy search algorithm.


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