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Pulsars

Particle Physics and Astronomy Research Council

Royal Greenwich Observatory
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Information Leaflet No. 8: 'Pulsars'.

Compact Stars:

Red dwarfs, white dwarfs, neutron stars, black holes: this is a list of objects in which each is smaller, denser and more extreme in its physical conditions than the one before. The compaction is a result of the familiar force of gravity, but the condensed stars which result are outside our common experience. A matchbox size piece of white dwarf material would contain the same mass as a battleship, while the same mass of neutron star material occupies the space of a pinhead. A black hole is so collapsed that size and density no longer have any meaning.

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 'degenerate electron pressure' --- free electrons cannot be packed more closely together. In some stars, usually more massive than white dwarfs, this barrier is overcome by the combination of the electrons with protons to form neutrons, which pack together even more closely, giving a neutron star. A neutron star has about the same mass as the Sun, but is only about 30 kilometres across. So small a star has a tiny surface area, and cannot emit much of the thermal radiation which makes normal stars shine; nevertheless some neutron stars 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 Anthony Hewish and Jocelyn Bell at the radio astronomy observatory (now the Nuffield Radio Astronomy Observatory) at Cambridge. Their characteristic radio emission is a uniform series of pulses, spaced with great precision at periods between a few milliseconds and several seconds. Over 300 are known, but only two, the Crab Pulsar and the Vela Pulsar, emit detectable visible pulses. These two are also known to emit gamma ray pulses, and one, the Crab, also emits X-ray pulses.

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 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 by its own centrifugal force is a neutron star.

Pulsars are very strongly magnetised neutron stars, with fields of strength reaching 100 million Tesla (1 million million Gauss, compared with less than 1 Gauss for the Earth's magnetic field). The rapid rotation therefore makes them powerful electric generators, capable of accelerating charged particles to energies of a thousand million million Volts. These charged particles are, in some way as yet unknown, responsible for the beam of radiation in radio, light, X-rays and gamma rays. 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 a million each year: the Crab Pulsar, which is the youngest and most energetic known, slows by one part in two thousand each year.

How Many Pulsars In Our Galaxy?

Pulsars are found mainly in 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 weak pulsars can only be detected if they are nearby. Radio surveys have now covered almost the whole sky, and over 300 pulsars have been located. Their distance can be measured from a delay in pulse arrival times observed at low radio frequencies; the delay depends on the electron density in interstellar gas and on the distance travelled. Extrapolating from this small sample of detectable pulsars it is estimated that there are at least 200,000 pulsars in the whole of our Galaxy. Allowing for those pulsars whose lighthouse beams do not sweep across in our direction, the total population must reach one million.

Each pulsar radiates for about four million years; after this time it has lost so much rotational energy that it cannot produce detectable radio pulses. If we know the total population (1,000,000) and the lifetime (4,000,000 years), we can deduce that a new pulsar must be born every four years (assuming that the population remains steady).

Very recently pulsars have been found in globular clusters. They are believed to have been formed there by accretion of matter onto white dwarf stars in binary systems. Other pulsars are born in supernova explosions. If all pulsars were born from supernova explosions we could predict that there should be a supernova in our Galaxy every four years. These are spectacular events, and we would expect to see more of them if one occurs every four years. The last directly observed supernova in our Galaxy was Kepler's supernova of AD 1604, but we do know that others occur which are less spectacular or which are hidden from us by interstellar dust clouds. It is not yet clear whether the birth rate of pulsars and the rate of supernovae can be fully reconciled or how many outside globular clusters may be formed in binary systems.

The Crab Pulsar:

The Crab Nebula is the visible remnant of a supernova explosion which was witnessed in AD 1054 by Chinese and Japanese astronomers. Near the centre of the Nebula is the Crab Pulsar, which is the most energetic pulsar known. It rotates 30 times per second, and it is very strongly magnetised. It therefore acts as a celestial power station, generating enough energy to keep the entire Nebula radiating over practically the whole of the electromagnetic spectrum.

The Crab Pulsar radiates two pulses per revolution: this double pulse profile is similar at all radio frequencies from 30 MHz upwards, and in the optical, X-ray and gamma ray parts of the spectrum, covering at least 49 octaves in wavelength.

It's visible light is powerful enough for the pulsar to appear on photographs of the Nebula, where it is seen as a star of about magnitude 16. Normal photographs smooth out the pulses, but stroboscopic techniques can show the star separately in it's 'off' and 'on' conditions.

The Binary Pulsar and General Relativity:

Many stars are members of binary systems, in which two stars orbit around each other with periods of some days or years. If one of the stars is a neutron star, the orbiting pair can be so close that the gravitational attraction between them is very high, and some unusual effects can be observed. Several binary systems are known in which the other star is a giant; in these cases the neutron star can attract gas from the outer parts of its companion, and a stream of gas falls with great energy on to the surface of the neutron star. These systems are observed as X-ray sources. Some of the X-ray sources show periodic variations as the neutron star rotates: these are the so-called 'X-ray pulsars'.

One binary system, known as PSR 1913+16, consists of two neutron stars, so close together that their orbital period is only 775 hours. No gas streams between these stars, which interact only by their mutual gravitational attraction. The orbit of one of them can be described in great detail, because it is a pulsar.
The period of this pulsar is 59 milliseconds, and it produces a very stable series of pulses with an unusually low slow-down rate. It is, in fact, an accurate clock moving very rapidly in a strong gravitational field, which is the classical situation required for a test of Einstein's General Theory of Relativity.

According to non-relativistic, or Newtonian, dynamical theory, the orbits of both stars should be ellipses with a fixed orientation, and the orbital period should be constant. Measurements of the arrival time of the pulses have shown significant differences from the simple Newtonian orbits. The most obvious is that the orbit precesses by 42 degrees per year.
There is also a small, but very important, effect on the orbital period, which is now known to be reducing by 89 nanoseconds (less than one ten-millionth of a second) each orbit.

The reducing orbital period represents a loss of energy, which can only be accounted for by gravitational radiation. Although gravitational radiation itself has never been observed directly, the observations of PSR 1913+16 have provided good proof of its existence. It is appropriate that this discovery, which is a further confirmation of the predictions of the General Theory of Relativity, was announced in 1979, which was the centenary of Einstein's birth.