Pulsars -- those spinning, superdense neutron stars that send powerful lighthouse beams of radio waves and light flashing through the universe -- have been lying about their ages. This has led astronomers, and possibly particle physicists, to erroneous conclusions for the past 30 years, according to researchers at MIT and the National Radio Astronomy Observatory (NRAO).
Bryan Gaensler, a Hubble postdoctoral fellow at MIT's Center for Space Research, and Dale Frail of the NRAO studied a pulsar that was thought to be 16,000 years old, but found instead that it is at least 40,000 years old and may be as old as 170,000 years. The results of their research were published in the July 13 issue of Nature.
"This means that much of what we thought we understood about the physics of pulsars and neutron stars may be wrong," said Dr. Gaensler. "Neutron stars are the densest objects in the universe and provide important physical tests of our most basic understanding of matter. Much of this theory is based on a belief that we could accurately estimate their ages. Our research indicates that these objects may be 10 times older than we thought, and this could force much reevaluation."
Drs. Gaensler and Frail studied a pulsar 15,000 light-years away in the constellation Sagittarius that has traveled outside the shell of debris from the supernova explosion that created it. The pulsar and the shell, known as a supernova remnant, together are dubbed "the Duck" because of their unusual appearance. Stars much more massive than our sun end their normal lives in violent supernova explosions, leaving behind an extremely dense neutron star. Some of these neutron stars produce the beams of electromagnetic radiation that characterize pulsars.
For the pulsar, designated B1757-24, to have traveled from the center of the supernova remnant to its present position in 16,000 years, it would have to have moved at about 1,000 miles per second, a particularly high speed compared to other pulsars. Drs. Gaensler and Frail compared a 1993 image of the region to one they made last year to measure the pulsar's change in position over a known time, and thus to calculate its speed. They were surprised to find the pulsar moved at a maximum of about 350 miles per second.
"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 Dr. Frail.
Columbia University Professor David Helfand, who with Professor Robert Becker of the University of California at Davis first drew attention to the unusual nature of "The Duck" in 1985, said he was "secretly delighted" with the new measurements. "I was skeptical of the high velocity" attributed to the object earlier, he said. The new work, he said, "clearly cautions us that a present snapshot of a system does not always give a full picture of its history."
AGE GAUGE
For years, astronomers have estimated the age of a pulsar by measuring the rotation period of its neutron star and the tiny amount by which that rotation slows down over time. The neutron star's powerful magnetic field acts as a giant dynamo, emitting electromagnetic radiation as the star rotates. That 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 its "characteristic age," which has been presumed to be the true age.
That presumption now is called into question. 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 Dr. Frail. The discrepancy could require astronomers to reexamine many of their previous conclusions about neutron stars and how they work.
For example, Professor Helfand pointed out that there are cases in which astronomers concluded that a pulsar and supernova remnant, while nearby, are not related because the pulsar's characteristic age was much younger than the age calculated for the supernova remnant. "We now ought to re-examine those cases," he said.
While the older age for B1757-24 poses problems for some astrophysical theories, the pulsar's slower speed actually helps current theory in one area, according to Professor Helfand. Neutron stars are thought to get a "kick" because the supernova explosion that creates them is not symmetrical. "The high velocity for this object was difficult to explain through that theory," he said. "The lower speed is easier to explain."
If the characteristic ages are not, in fact, the true ages of the neutron stars, the implications extend beyond astronomy to particle physics. "Neutron stars, as the densest objects in the universe, provide a unique laboratory for physics. Physicists look at neutron stars as a way of showing how matter acts under these extreme conditions. Part of what they need to know in order to draw proper conclusions is the age of the neutron star. If that changes, so do many of their theories," said Dr. Gaensler, whose work is supported by a Hubble Fellowship awarded by the Space Telescope Science Institute.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
In 1999, Dr. Gaensler was named the Young Australian of the Year.
A version of this article appeared in MIT Tech Talk on August 9, 2000.
