Dead stars may masquerade as ingenues

When it comes to lying about their ages, certain stars are masters of spin. The stellar deception could throw into disarray theories describing some of the densest objects in the universe.

Radio image of the pulsar B1757-24 (arrow) fleeing its parent supernova. Gaensler and Frail

That’s the lesson two astronomers say they’ve learned from studying a seemingly young pulsar, the rapidly rotating corpse of a massive star. Left over from a supernova explosion, a pulsar crams as much mass as the sun into a sphere only 20 kilometers wide. The resulting pressure squeezes its electrically charged building blocks—electrons and protons—into a superdense ball of neutrons.

For years, astronomers have estimated the ages of pulsars by measuring how fast they rotate. As they spin, pulsars emit jets of radiation and particles that sweep past Earth like a lighthouse beam. The cast-off energy slows the pulsars down.

Astronomers calculate a pulsar’s age from estimates of its initial spin and measurements of the rate at which it’s slowing.

For a pulsar called B1757-24, that age computes to a youthful 16,000 years, but that figure may reflect some spin doctoring. The pulsar, which got a kick from the supernova that created it, now lies far from the site of the explosion. For the pulsar to be so young, it must have been traveling 1,600 km per second, four times the average speed for pulsars, notes Bryan M. Gaensler of the Massachusetts Institute of Technology.

He and Dale A. Frail of the National Radio Astronomy Observatory in Socorro, N.M., set out to measure the velocity of the pulsar by a more direct method. Last year, using the Very Large Array radio telescope near Socorro, they took an image of a bright nebula surrounding the pulsar and compared it with an image taken in 1993. The movement of the nebula reveals that the pulsar’s actual velocity is only 560 km/s.

The lower velocity means that to have traveled as far as it has from its supernova source, B1757-24 must be at least 39,000 years old and perhaps an even more mature 170,000 years old, the astronomers assert in the July 13 Nature. Other studies also indicate that a pulsar’s apparent age is not always its true age, Gaensler says.

Determining the ages of pulsars is critical for estimating not only how often supernovas explode, but also for calculating the number of pulsars in our galaxy and their average life span, notes Gaensler. If many pulsars are older than their so-called characteristic age—the age deduced from their spin rate—theorists may have to revise models describing the stability and survival of such extremely dense matter.

“There’s been 30 years of analysis and modeling . . . of pulsars, and all of that has been underpinned by the assumption that a pulsar’s characteristic age is at least roughly right,” says Gaensler. It will take time, he says, “to work out just what aspects of our understanding . . .may have to be altered if we have to change or abandon that assumption.”

Aleksander Wolszczan of Pennsylvania State University in State College takes a more conservative view. “I would be cautious to attempt revolutionizing the whole pulsar field based on just one contradictory example, no matter how spectacular,” he says.

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