By Lee Siegel
Astronomers just discovered what Hollywood has known all along: some
stars lie about their age.Scientists traditionally estimated the ages of pulsars using the rate at which the superdense stars spin more slowly as they age. But in the July 13 issue of the journal Nature, researchers conclude
pulsars may be much older than believed. 
Radio-emission map of the tip of the nebula powered by pulsar B1757-24, marked by the ellipse near the head of the nebula.
Bryan Gaensler of the Massachusetts Institute of Technology and Dale Frail of the National Radio Astronomy Observatory in New Mexico studied pulsar B1757-24, thought to be 16,000 years old. They determined the pulsar really is between 39,000 and 170,000 years old.
"This pulsar has been lying to us about its age," Frail said.
After a giant old star explodes as a
supernova, it leaves behind a supernova remnant -- an expanding shell of matter and energy. It also can produce a superdense neutron star that is called a pulsar and emits regular pulses of radio waves, like beams from a lighthouse. Astronomers detect pulsar B1757-25s radio waves eight times a second.A pulsar and our sun have similar mass. Yet a pulsar is only about a dozen miles wide (20 kilometers). The sun is 865,000 miles wide (1.4 million kilometers).
The supernova explosion that produced B1757-24 kicked the pulsar out of the supernova remnants shell. If the pulsar were only 16,000 years old the "characteristic age" suggested by the rate it spins the pulsar would have hurtled through space at 1,000 miles (1,610 kilometers) per second to reach its observed position.
But the study found the pulsar moves only 350 miles (560 kilometers) per second, meaning it took much longer to reach its present location and thus is much older.
The pulsar is about 15,000 light-years (88,200 trillion miles or 141,900 trillion kilometers) from Earth, Gaensler said. It is surrounded by a glowing, bubble-like
nebula of particles flying away from the spinning star.~
Gaensler and Frail used 27 Very Large Array radio telescope dishes in New Mexico to measure the speed of the nebula and pulsar as they moved away from the
supernova remnant between 1993 and 1999. The astronomers also measured the distance the nebula traveled since the supernovas initial explosion.
A radio-telescope image of "The Duck," which includes a supernova remnant with pulsar B1757-24 emerging from the right side enshrouded in an ionized gas nebula.
With the pulsars speed and distance, Gaensler and Frail calculated it was born at least 39,000 years ago.
Other calculations indicated the position of the pulsar and the shapes of the nebula and supernova remnant are best explained if the pulsar is up to 170,000 years old.
"Men who dont know the age of their girlfriend can make a reasonable guess by finding out the age of her parents," astronomer John Seiradakis, of the University of Thessaloniki, Greece, said in a commentary accompanying the study. "This is what Gaensler and Frail have done in order to getthe age of a pulsar observed to be fleeing its parent supernova."
"Roughly 100,000 years would be a reasonable guess" for the pulsars true age, Gaensler said. "Pulsars live billions of years. So were suggesting its a twenty-something instead of a teenager. We originally wanted to use a line about spinning spinsters, but that was judged politically incorrect."
Seiradakis said the study might explain why many apparently young pulsars are not seen with supernova remnants. The pulsars may be so old the supernova shells would have dispersed.
Frail and Gaensler said if other pulsars are older than they seem, astronomers may have to reconsider theories about how pulsars work, the fraction of supernovas that produce pulsars instead of black holes, and how supernovas explode asymmetrically to eject pulsars.
"We want to understand how stars explode because those explosions made us; the atoms in you and me were stardust," Gaensler said. "An important component of our understanding of matter is how it behaves under extreme densities [in pulsars]. We also use pulsars to test out understanding of space and time."
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