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Wednesday, 2 August, 2000, 18:11 GMT 19:11 UK
Astronomers investigate pulsar puzzle
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By BBC News Online science editor Dr David Whitehouse

Astronomers have found a superdense egg-shaped star spinning in space.

The pulsar is the size of a city, yet weighs more than a million times that of the Earth. It spins and pulses nearly three times a second.

A detailed observation of its radio pulses suggests that it is not absolutely spherical but slightly flattened.

Because the star is made of material normally found at the centre of atoms, the observations should help scientists understand how matter behaves under extreme conditions.

Slightly squashed

The discovery of the pulsar was made using the world famous Lovell Telescope at the Jodrell Bank Observatory, UK.

Pulsars are rapidly spinning neutron stars. They are typically not much more 10 km (6 miles) in diameter, but still weigh more than our own Sun.

A pulsar produces beams of radio waves that sweep like a lighthouse across the Universe. When a beam crosses the Earth, a pulse of radiation is detected.

Jodrell Bank astronomers Ingrid Stairs, Andrew Lyne and Setnam Shemar studied 13 years of data from the pulsar PSR B1828-11. They discovered that the star wobbles like a gyroscope with a period of about 1,000 days, as well as flashing 2.5 times a second.

The wobble causes the pulse to change its shape, and makes the time between pulses vary.

In an article in the journal Nature, the astronomers say that the neutron star is not perfectly spherical but slightly squashed.

Neutron superfluid

The deformation is not great. Dr Stairs said: "This star departs from being a perfect sphere by only 0.1 mm in 20 km. To the same scale on the Earth this would mean that no mountain could be higher than 3 cm."

The surprising aspect to the discovery is not the small size of the wobble but the fact that the wobble is seen at all.

The interior of a pulsar is made up largely of a neutron superfluid, with a solid crust.

Current theories predict that the interaction between the superfluid and the crust should cause any precession to die out extremely quickly.

"But this pulsar is one hundred thousand years old, and it's still wobbling," said Professor Andrew Lyne.

"We really don't understand how this precession can be happening, and theorists are going to have to do some work to explain it."

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See also:

14 Jul 00 | Sci/Tech
Pulsars 'lie about their ages'
08 Jun 00 | Sci/Tech
Bad places in space
17 Jul 00 | Sci/Tech
Jodrell Bank faces uncertain future
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