Pulsars and neutron stars

Pulsars are among the most regular but shocking objects in the universe sending out massive bolts of electricity across space.

These compact stars are in fact super condensed to an extent that’s hard to comprehend for us Earth dwellers. What we're looking at is the final death throes of dying stars. The mass of a battleship in neutron star material occupies the space of a pinhead.

The white dwarf

A white dwarf, which is a star about the size of the Earth but with a mass similar to that of the Sun, is prevented from shrinking further by 'electron degeneracy pressure' – under the laws of quantum mechanics, free electrons can be packed only so closely together and not more.

Neutron stars: smaller than London, heavier than the Sun

In some stars, usually more massive than white dwarfs, this barrier is overcome when the electrons combine with protons to form neutrons, which can pack together 2000 times more closely. A neutron star has one and a half times the mass of the Sun, but is only about 20 kilometres across.

Neutron stars are created when the core of a dying, massive star collapses, triggering the explosive ejection of the outer parts of the star in a supernova. Although neutron stars are small and cannot generate light by fusion, some can be observed at great distances by an entirely different kind of radiation, a regularly pulsating radio signal. These are the pulsars.


What are pulsars?

Pulsars were discovered in 1967 by Jocelyn Bell (now Dame Jocelyn Bell Burnell) and her thesis supervisor Anthony Hewish. Pulsar radio emission is very distinctive, a uniform series of pulses, spaced with great precision at periods of between a few milliseconds and several seconds. Over 700 radio pulsars are known. Some pulsars have also been detected by optical, X-ray and gamma-ray telescopes.

The regularity of the pulses is phenomenal: observers can now predict the arrival times of pulses a year ahead with an accuracy better than a millisecond. How can a star behave as such an accurate clock? The only possibility for so rapid and so precise a repetition is for the star to be very small, rotating rapidly and emitting a beam of radiation which sweeps round the sky like a lighthouse, pointing towards the observer once per rotation. The only kind of star which can rotate fast enough without bursting from its own centrifugal force is a neutron star.

Electricity generating stations

Pulsars are rapidly rotating, very strongly magnetised neutron stars, with fields of strength reaching 1000 million Tesla (10 million million Gauss. For comparison, the Earth's magnetic field measures less than 1 Gauss). These extreme properties result from the compression of the original star's core, which would have had a weaker magnetic field and slower rotation. They make neutron stars into powerful electric generators, capable of creating and accelerating charged particles to energies of a thousand million million Volts. These particles are the source of the beams of radiation in radio, light, X-rays and gamma-rays. They also carry away much of the pulsar's energy in the form of a fast wind. Their energy comes from the rotation of the star, which must therefore be slowing down. This slowing down can be detected as a lengthening of the pulse period. Typically a pulsar rotation rate slows down by one part in 10 million each year.

How many pulsars are there in our galaxy?

Pulsars are found mainly in the disc of the Milky Way, within about 500 light-years of the plane of the Galaxy. A complete survey of the pulsars in the Galaxy is impossible as faint pulsars can only be detected if they are nearby. Radio surveys have now covered almost the whole sky, and over 1,000 pulsars have been located. Extrapolating from the small sample of detectable pulsars, it is estimated that there are around 200,000 pulsars in the whole of our Galaxy.

How long until the next supernova arrives?

If we know the total population (200,000) and the lifetime (20,000,000 years), we can deduce that a new pulsar must be born in our galaxy roughly every 100 years (assuming that the population remains steady).

If pulsars are born in supernova explosions, then the rate of supernovae must be at least as high as the pulsar birth rate. In fact the supernova rate is probably also around one every 100 years or higher. 

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