by Dave Finley, NRAO and
Elizabeth Thomson, MIT
July 12, 2000
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," leading astronomers,
and possibly particle physicists, to erroneous conclusions for the
past 30 years, according to researchers using the National Science
Foundation's Very Large Array (VLA) radio telescope.
Bryan Gaensler, at the Massachusetts Institute of Technology
(MIT) and Dale Frail, of the National Radio Astronomy Observatory (NRAO),
in Socorro, NM, 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 are being
published in the July 13 issue of the scientific journal Nature.
"This means that much of what we
thought we understood about the physics of pulsars and neutron
stars may be wrong," said 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
A Mosaic of VLA
images of Pulsar B1757-24 and the radio supernova remnant G5.4-1.2,
at different scales.
Gaensler and Frail studied a pulsar
15,000 light-years away in the constellation Sagittarius that has
travelled 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 the 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 travelled 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. Gaensler and
Frail compared a 1993 VLA 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 Frail.
Columbia University astronomer David
Helfand, who, with Robert Becker of the University of
California-Davis, first drew attention to the unusual nature of "The
Duck" in 1985, said he was "secretly delighted" with the new VLA
"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."
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
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 VLA 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 and how they work.
For example, 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," Helfand 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 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," Helfand 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 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.
A VLA image of Pulsar
B1757 and the radio supernova remnant G5.4-1.2, collectively known
as "The Duck."