The Ultimate Heavy Metal Space Rock

by Nicholas Mee on July 24, 2017

Jocelyn Bell

It is fifty years since the Summer of Love. During that summer a young graduate student in Cambridge working on a newly developed radio telescope designed by Martin Ryle and Anthony Hewish noticed a bit of ‘scruff’ in her read-out. Jocelyn Bell looked further and realised that there was something strange going on.

The signal was repeating precisely every 1.337 seconds and it could not be from any local source as it was moving across the sky with the Earth’s rotation. There was no obvious astronomical phenomenon that could generate such regular and rapid pulses. Her first thought was that this might be a signal from an alien civilization. The radio source was provisionally given the half-serious acronym LGM1 (Little Green Men 1).
(It has since become known as CP1919 or PSR1919+21.) Bell recalled later: ‘we did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission. It is an interesting problem—if one thinks one may have detected life elsewhere in the universe, how does one announce the results responsibly?’

The periodic signal received by Jocelyn Bell in 1967.

The aliens were soon discounted as Bell discovered a second radio source pulsing away in the night sky. One alien civilization would be an incredible discovery, but two alien civilizations communicating in this way was simply too improbable. There had to be a natural solution. In the mean time these objects were dubbed pulsars.

When Hewish and Bell announced their discovery in 1968, Thomas Gold suggested that these objects might be rapidly spinning neutron stars. Fritz Zwicky and Walter Baade had proposed the existence of such objects in 1934, just two years after the discovery of the neutron. Their prophetic idea was that following a supernova explosion the remains of the star would be compressed into an object with the mass of a star but as dense as an atomic nucleus and composed entirely out of neutrons. (There is more about Zwicky and Baade and their supernova research here: The Crab and the Jellyfish.)

Gold argued that a neutron star would be spinning rapidly and its intense magnetic field would generate beams of radiation much like a rotating beacon. Initially, his theory was dismissed by the scientific community and he was even refused permission to present it to the first international conference on pulsars. Everything changed later in the year, however, when a pulsar with a period of 33 milliseconds was discovered in the Crab nebula, the remnant of a supernova seen in the year 1054.

In 1974 Ryle and Hewish were awarded the Nobel Prize in Physics for the discovery of pulsars. Jocelyn Bell did not receive the prize despite her leading role in the discovery.

The Crab nebula. Credit: ESO

Cosmic Lighthouses

Astronomers have now studied neutron stars for half a century. Much is known about them, but many mysteries remain. They are indeed created in supernova explosions and are believed to spin at close to one thousand times a second when newly created. As Gold first suggested, this generates a huge magnetic field, a trillion times that of the Earth. This in turn generates electric fields that accelerate electrons and other charged particles outwards in two intense beams of radiation that blast into space from the magnetic poles of the neutron star. Just as on Earth, the magnetic poles are not perfectly aligned with the rotation axis, so the beams sweep around the heavens like a cosmic lighthouse. Radio astronomers, such as Jocelyn Bell, detect a pulse of radio waves once every rotation when the pulsar beam points in our direction.

A representation of a neutron star showing the pulsar beams emanating from both magnetic poles.

The spinning magnetic field acts as a brake on the rotation of the neutron star, and the pulsar that it generates transmits the lost rotational energy to the surrounding nebula. Gradually the rate of rotation decreases. The neutron star within the Crab nebula formed almost one thousand years ago and now spins about thirty times a second. It has been calculated that the amount of rotational energy lost by the spinning neutron star matches the energy required to illuminate the nebula. Pulsars have a limited lifespan. In around one million years the period of rotation of the neutron star will have increased to about one second, at which point there will be insufficient energy to power the pulsar and it will disappear from view.

If a neutron star lives in a binary system, it may accrete material from its companion. This is quite likely to happen if the companion sheds its outer layers as it ages or inflates into a red giant. As material spirals on to its surface, the neutron star will be spun up and the long-dead pulsar may come back to life. This is thought to be the origin of millisecond pulsars, which are observed to have extremely short periods. Currently, the pulsar with the shortest known period of rotation is PSR J1748-2446ad, which spins an incredible 716 times a second.

Exotic Materials

The composition of neutron stars is more complicated than their name might suggest. Their radius is just 10-15 kilometres and their structure changes as we move inwards as indicated in the illustration below. They are thought to have a hot plasma atmosphere just a few centimetres thick surrounding an outer crust of white-dwarf-like matter consisting of heavy nuclei in a degenerate sea of electrons, with a density around a million times that of water.

The internal structure of a neutron star.

Moving inwards the density increases rapidly. When we reach the inner crust the proportion of neutrons in the nuclei dramatically increases along with the density of free neutrons until we reach a critical threshold at around 100 trillion times the density of water.

We now enter the outer core, which consists almost exclusively of neutrons plus a small quantity of protons, electrons and muons. No-one knows the physical structure of the material that forms the inner core at the heart of a neutron star. Various exotic possibilities have been proposed. These include mind-boggling suggestions such as: densely packed particles known as strange baryons, which are like heavy versions of protons and neutrons that contain strange quarks; a Bose-Einstein condensate of particles known as pions and kaons that are composed of quarks and antiquarks; or possibly some sort of quark-gluon plasma, which is an extreme state of matter currently being explored in the Large Hadron Collider.

There is an upper limit to the mass of a neutron star that is thought to be somewhere between two and three times the mass of the sun. Nothing can stop the total collapse of a neutron star whose mass exceeds this limit. Such a star must ultimately form a black hole.

A Cultural Icon

In 2016 the Institute of Physics decided to rename its Very Early Career Female Physicist Award the Jocelyn Bell Burnell Medal and Prize in recognition of the achievements of Professor Dame Jocelyn Bell Burnell and her discovery of pulsars during her PhD research.

The signal discovered by Jocelyn Bell has become a cultural icon thanks to the designer Peter Saville, who in 1979 used it as the cover art for the Joy Division album Unknown Pleasures. The beat of the first known cosmic lighthouse has been transformed into one of the most recognizable images in rock music. It reminds me of the track followed by the needle through the groove of a vinyl LP. Incidentally, Ian Curtis, lead singer and lyricist of Joy Division, grew up in the Cheshire town of Macclesfield close to the Jodrell Bank radio observatory that is now a world leading centre for research into pulsars. There is more about Jodrell Bank here: Quantum Wave: from Radar to Quasar. They have even turned the pulses of these spinning space rocks into audio files.

Further Information

A video related to this article is available here: Jocelyn Bell wins Breakthrough Prize on The Cosmic Mystery Tour YouTube Channel. Please don’t forget to subscribe to the YouTube channel.

If you would like to hear what a pulsar sounds like, recordings are available on the Jodrell Bank website here:

There is more about neutron stars in my book Higgs Force: Cosmic Symmetry Shattered.

An account of neutron stars with more mathematical details is given in The Physical World: An Inspirational Tour of Fundamental Physics by Nicholas Manton and Nicholas Mee.


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