(notes on Pulsars, Quark stars and other strange beasts)
Author - Julia McKillop - August 1996
In 1934, Walter Baade and Fritz Zwicky proposed that when a star goes supernova and it's core collapses, the result would be a neutron star - a superdense ball of matter with a mass similar to that of the Sun but with a diameter of only 10-15 km, compared to the Sun's diameter of 14x10e5 km. Since volume depends on the cube of the diameter and density on volume, this means the ball of matter is some 10e5 times as dense as the Sun.
This theoretical prediction was confirmed by Jocelyn Bell and Anthony Hewish when they discovered pulsars in 1967. They were observing sources of highly regular radio pulses , with periods of 220 msec to about a second. A number of theories were proposed to account for these, but the one that best fitted was that they were produced by a rotating, highly magnetised neutron star.
This was suggested by Thomas Gold in 1968 who proposed that charged particles escaping from the star's surface emit radiation as they are accelerated along it's magnetic axis. When this beam of radiation crosses our line of sight, we detect pulses of radio waves, the regularity being the result of the enormous rotational inertia of the star.
The model predicts that the pulsar will gradually slow down with it's period becoming longer as charged particles carry away energy. This has indeed been observed for the pulsar in the heart of the crab Nebular.
More than 700 pulsars have now been found and their connection with rotating neutron stars is generally accepted. There seem to be two classes : 'normal' pulsars which are solitary stars with periods of around 1 second and 'millisecond' pulsars which have binary companions, with much shorter periods of about 1.5 to 60 msec..
The most rapidly rotating pulsars found so far, B1936+21 and B1957+20, have periods of about 1.5 msec.. Theories suggest that a neutron star may have an outer crust of electrons, protons and neutrons at a density of about 10e9/cubic meter. Towards the centre, the density increases so much that the electrons and protons are forced together to become neutrons (neutronium) at about 10e11kg/cubic meter.
A slight diversion....above a temperature of about 10e11 K, quarks are able to 'boil off' from baryons and mesons in a "quark soup". Something similar may also happen at lower temperature in high density nuclear matter such as that found in the middle of a neutron star. If this does happen, the cores of pulsars would consist of quark matter, and they should perhaps be called quark stars rather than neutron stars (Physics World, Oct.1995 p 37).
It seems likely, due to their mode of formation, that all neutron stars rotate rapidly. They collapse from rotating stars and will keep most of the star's original angular momentum, thus as they become smaller, they will rotate faster. Similarly they should also retain the star's magnetic field.