A team using CSIRO's Parkes radio telescope has just found the thousandth pulsar known to science.

The telescope holds the international record for having discovered the largest number of these small spinning stars since the first was found in 1967. The new survey is clocking them up more than ten times faster than any previous search, anywhere - about one for each hour the telescope is used - and has already found more than 200.

"This is thanks to the power of a new instrument on the telescope, the multibeam system, which has slashed the time it takes to scan the sky," said co-leader of the pulsar team, CSIRO's Dr Dick Manchester.

Even surveys like this can find only a fraction of the 300 000 pulsars thought to live in our Galaxy. "Many have signals that are too weak to pick up, or their beams are not pointing towards us," explained Dr Manchester.

The survey is an international collaboration between astronomers from the University of Manchester, UK; the CSIRO Australia Telescope National Facility; the Massachusetts Institute of Technology, USA; and the Osservatorio Astronomico di Bologna, Italy.

A pulsar is the collapsed core of a massive star, only 20 kilometres across, born when the original star explodes at the end of its life.

Like an egg, a pulsar has a hard external crust covering a fluid interior. This fluid 'neutron matter' is so dense that a piece the size of a sugar cube has a mass of 100 million tonnes. Deep in the pulsar's innards the density is so great that matter may exist only as exotic subatomic particles.

A pulsar is ringed by a strong magnetic field. Electrons flung around by the field put out a beam of radio waves. As a pulsar spins, its beam sweeps repeatedly over the Earth and is seen as a pulsating radio signal.

Just as biologists hunt for new species to build up a picture of the Earth's biodiversity, astronomers hunt for new pulsars to understand 'astrodiversity'.

"There are many different types of pulsar, and we have only a few examples of some types," said Dr Manchester. "One of the main aims of the survey is to find more examples of these rare types and perhaps other types not even known or anticipated at present."

"In this survey's first hundred pulsars we found one orbiting another neutron star - this is only the sixth such object known."

"Most of all we'd like to find a pulsar orbiting a black hole, to test ideas about black-hole physics. Theories predict that one pulsar in a thousand should be in such a system," he said

"We are particularly interested in young pulsars," said team member Professor Vicky Kaspi of Massachusetts Institute of Technology. "Their signals tend to glitch - show sudden changes - which is a sign of a 'starquake' taking place, and we can use this to study their interiors."

"As well, some young pulsars could be counterparts of high-energy X-ray and gamma-ray sources. We've detected many such sources but can't identify them with any particular objects."

The more pulsars we find, the better we can understand how they are born and evolve. "We think most of the pulsars in the Galaxy are weak. Not many of these have been found, and so our current estimates of how many pulsars exist and how often they are born are rather uncertain," said Dr Manchester.

Studying a large population of pulsars also means we can better understand what makes zthem 'tick'. "Like people, pulsars are all individuals - they have different signal characteristics," said Dr Manchester. "We want to get beyond those idiosyncrasies to understand how pulsars actually emit their signals."

And beyond this is the very question of what pulsars are. "The centre of a pulsar is denser than an atomic nucleus," said Dr Manchester. The equations that describe pulsar matter put a limit on how fast a pulsar can spin without it breaking apart. The fastest pulsar we know of spins around 600 times a second. If we found one spinning faster - say, at 1200 times a second - that would better pin down what pulsars are made of."

Signals from distant pulsars also reveal the conditions in the depths of the Galaxy, said Dr Fernando Camilo of the University of Manchester. "The space between the stars is threaded through with magnetic fields and invisible giant clouds of electrons," he explained. "These blur pulsar signals that travel through them. From the nature of the blur we can reconstruct the conditions in space. Already our survey has doubled the known number of really distant pulsars - those more than 20 000 light-years from the Sun - which are going to allow us to probe out to those distances."

A network of particularly 'fast-ticking' pulsars could even help us to 'see' gravity waves, says Professor Matthew Bailes of Swinburne University of Technology, who is doing another pulsar search with the Parkes telescope.

The pulsars would be like ocean buoys that rise and fall as a wave passes by. "A passing gravity wave would slightly alter the time the pulsar's signal takes to reach us," said Professor Bailes.

The team members of the Parkes multibeam pulsar survey are: Professor Andrew Lyne, Dr Fernando Camilo, Ms Nuria McKay and Mr Dan Sheppard, Jodrell Bank Observatory, University of Manchester; Professor Vicky Kaspi and Mr Froney Crawford, Massachusetts Institute of Technology; Dr Nichi D'Amico, Osservatorio Astronomico di Bologna; and Dr Dick Manchester and Dr Jon Bell, CSIRO Australia Telescope National Facility.

The Parkes radio telescope is operated by the CSIRO Australia Telescope National Facility.