Astronomers have just discovered their 1000th pulsar but what they would really like is to find one orbiting a black hole.

Pulsars were discovered in 1967 as unexpected pulsing radio sources in the sky. At first the possibility was considered that they were beacons from an alien civilisation.

The first one was even for a time designated LGM-1 or Little Green Man 1. But soon astronomers discovered they were natural objects.

The sound of three pulsars

Pulsars are a stage at the end of the lives of some stars. It is what is left over after a titanic stellar explosion, when one star can outshine a galaxy of its companions.

This stellar remnant is a star the size of a city, just 20 km across. It is so dense that a piece of it the size of a sugar cube would weigh 100 million tonnes.

Remarkably they also spin rapidly and give off a pulse or pulses during each rotation, hence the name pulsar.

Australian research

A new hunt for pulsars being carried out in Australia is finding them faster than ever before, about one every hour.

Finding a pulsar an hour

"This is thanks to the power of a new instrument on our radio telescope, the multibeam system, which has slashed the time it takes to scan the sky," said Dr Richard Manchester co-leader of the project.

"We are particularly interested in young pulsars," said Professor Vicky Kaspi of the Massachusetts Institute of Technology.

"Their signals tend to show sudden changes, which is a sign of a 'starquake' taking place.

The timing of the pulses varies considerably. The fastest one pulses 600 times a second, the slowest about one pulse every few seconds.

It is believed that they are formed spinning rapidly and slow down as they get older.

Signals from distant pulsars also reveal conditions in the depths of the Galaxy.

"The space in between the stars is threaded with invisible clouds of electrons that blur the pulsar signals," said Dr Fernando Camilo of the Jodrell Bank radio observatory in England.

Observations of fast pulsars scattered around the sky may even help astronomers detect so-called gravity waves.

These are so far unseen ripples in the fabric of space caused by violent events in the universe.