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Friday, 14 July, 2000, 09:02 GMT 10:02 UK
Pulsars 'lie about their ages'
A pulsar bursts out of its shell betraying its true age
By BBC News Online science editor Dr David Whitehouse

Pulsars, those spinning, superdense neutron stars that send powerful flashes of radio waves across space, may be much older than was thought.

According to new research, one pulsar with the mass of our Sun but only about the size of a city, that was believed to be 16,000 years old, is at least 40,000 years old and may be as old as 170,000 years.

"This means that much of what we thought we understood about the physics of pulsars and neutron stars may be wrong," said Dr Bryan Gaensler of the Massachusetts Institute of Technology, US.

"Neutron stars are the densest objects in the Universe and provide important physical tests of our most basic understanding of matter.

"Our research indicates that these objects may be ten times older than we thought, and this could force much re-evaluation."

Bryan Gaensler and Dale Frail, of the US National Radio Astronomy Observatories in New Mexico, studied a pulsar 15,000 light-years away in the constellation Sagittarius that had travelled outside the shell of debris from the explosion it had created.

Older object

For the pulsar, designated B1757-24, to have travelled from the centre of the supernova remnant to its present position in 16,000 years, it would have to have moved at about 1,600 km per sec (1,000 miles per second), a particularly high speed compared to other pulsars.

Gaensler and Dale were surprised to find the pulsar moved at a maximum of about 560 km per second (350 miles per second).

Close-up on the escaping pulsar
"This means the pulsar took much longer to reach its current position, and so it is a much older object than we had believed," said Frail.

For years, astronomers have estimated the age of a pulsar by measuring its rotation period and the tiny amount by which that rotation slows down over time.

Loss of energy slows the star's rotation, according to the standard theory used for nearly three decades. A calculation based on the neutron star's rotation period and its rate of slowing produces what astronomers call the "characteristic age," which has been presumed to be the true age.

It is that assumption that is now being questioned with the large difference between B1757-24's "characteristic age" and the age required by the new measurements. "This pulsar has been lying to us about its age," said Frail.

The discrepancy could require astronomers to re-examine many of their previous conclusions about neutron stars.

The research is published in the journal Nature.

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