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	British scientists were surprised when they discovered a pulsar wobbling like a top By Lee Siegel Science Writer posted: 05:00 pm ET 02 August 2000

By Lee Siegel

Unlike old people, who can get a bit shaky on their feet, rapidly rotating stars called pulsars should not wobble as they age. So British scientists were surprised when they discovered a pulsar slowly wobbling like a top.

"This pulsar is 100,000 years old, and its still wobbling!" scientist Andrew Lyne declared in a statement issued by the University of Manchesters Jodrell Bank Observatory.

The 76-meter Lovell Telescope at the Jodrell Bank Observatory, University of Manchester, England.

Lyne, Ingrid Stairs and Setnam Shemar, all of Jodrell Bank, published the discovery in the August 3 issue of the journal Nature.

"There have been suggestions in the past 10 years that other pulsars have shown hints that they are wobbling," said astronomer Bryan Gaensler, a pulsar expert at the Massachusetts Institute of Technology. "But this is the first one in which there has been clear-cut evidence that this sort of behavior is occurring."

A pulsar is a rapidly rotating neutron star that emits lighthouse-like beams of radio waves. A neutron star is the incredibly dense, collapsed core of a star that exploded as a supernova.

"A neutron star us typically 20 kilometers (12 miles) in diameter -- about the size of a city -- weighs a million times the mass of the Earth and spins as fast as a top with predictable regularity," said the statement from Jodrell Bank.

It is believed that pulsars are made up largely of a superfluid of neutrons covered by a solid crust. Theories predict the interaction between the superfluid and the crust should cause any wobble or "precession" to damp out quickly. So researchers do not understand how pulsar PSR B1828-11 can wobble.

"Theorists are going to have to do some work to explain it," Stairs said.

Lyne, Stairs and Shemar detected the wobble by studying the 30 observations a year of PSR B1828-11 over 13 years with the observatorys 250-foot (76-meter) Lovell radio telescope. The pulsar is about 11,700 light-years or 69,000 trillion miles (111,000 trillion kilometers) from Earth.

The pulsar rotates 2.5 times each second, but also wobbles slowly but regularly with a period of about 1,000 days or 2.7 years.

"The motion is very much like the wobble of a top or gyroscope," Jodrell Bank said.

As a result, the pulsars radio pulse changes shape over time and the interval between pulses can become a bit shorter or longer.

The researchers believe the pulsar wobbles because it is "slightly squashed" instead of completely spherical. The resulting bulge causes the wobble by making the angle between the pulsars rotation axis and radio beam change with time, they theorize.

Lyne said the bulge must be tiny, with the pulsar departing from being a perfect sphere by only a 250th of an inch over 12 miles (0.1 millimeters in 20 kilometers).

"On Earth, this would mean no mountain could be higher than 3 centimeters (1.2 inches)," Lyne said.

Gaensler said authors of the new study "are very careful and, one by one, eliminated the other possibilities [that might explain the wobble]. The explanation they have come up with is the only alternative to explain the strange behavior seen in the data."

Radio astronomers study pulsars because their internal structure helps them learn about the fundamental physics of matter at pressures and densities much greater than can be created in a laboratory.