"Guest Star" Makes a Mess of Pulsar Theories
SAN DIEGO (Jan. 11, 2001) -- A "guest star" that visited the night sky more than 1,600 years ago
may help astronomers better understand the processes that cause massive stars
to blast themselves to bits, according to astronomers who are studying the ancient
visitor.
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| Observations by the Chandra X-Ray Observatory of a pulsar at the center of the supernova remnant known as G11.2-0.3 provide strong evidence that it was formed in the supernova of 386 AD, which was witnessed by Chinese astronomers. If confirmed, this will be only the second pulsar to be clearly associated with a historic event. [NASA/McGill/V. Kaspi et al.] |
Chinese astronomers recorded a bright new star, which they called a guest star,
in the spring of 386 AD. It remained visible in the night sky for about three
months. Modern astronomers believe the star was a supernova -- the titanic explosion
of a star several times as massive as our own Sun.
In the 1970s, astronomers discovered an expanding cloud of gas and dust at the
same position as the guest star. Known by its catalog number, G11.2-0.3, this
spherical shell is called a supernova remnant, and it's the material expelled
into space when the star exploded.
In 1997, using an orbiting Japanese X-ray telescope, astronomers discovered
a pulsar inside the cloud -- the crushed, rapidly spinning core of a supernova.
But techniques for estimating a pulsar's age suggested it was about 24,000 years
old -- far too old to be associated with the supernova of 386.
Last year, though, using the more sensitive Chandra X-Ray Observatory, one
of NASA's orbiting telescopes, a team of astronomers headed by Victoria Kaspi
of McGill University in Montreal found compelling evidence that the pulsar is
directly associated with the surrounding shell of gas and dust, and must therefore
have been born in the same supernova blast that created G11.2-0.3. If so, it
means the technique astronomers have used to estimate pulsar ages for several
decades doesn't always work, so many pulsars could be much younger than thought.
Cosmic Beacons
When a supernova explodes, it blasts its outer layers into space, creating clouds
of gas and dust like G11.2-0.3. But its core is crushed inward by the blast,
creating a neutron star -- a ball of material only a few miles across, but containing
several times as much mass as our own Sun. The explosion sets the neutron star
to spinning, and as it spins it beams energy into space like a lighthouse, so
it appears to "pulse" on and off several times a second -- hence the
name "pulsar."
The most famous pulsar, inside the Crab Nebula, is the corpse of a supernova
that Chinese astronomers recorded in 1054, and until this discovery was the
only pulsar whose age could be pinpointed. Astronomers have measured the Crab's
rotation rate and the rate at which it's slowing down with great accuracy. From
those numbers they developed models that allow them to estimate the ages of
other pulsars by measuring how fast they spin. The technique assumes that all
pulsars spin very rapidly when they're born.
But this technique yielded an age of 24,000 years for the pulsar created in
the supernova of 386. Since the pulsar is actually much younger, it must have
been spinning more slowly than expected when it was born -- only a little faster
than its current rate of 14 revolutions per second.
Kaspi's team is studying the pulsars and gas clouds from several other supernovae,
including some found in other historic records. These new observations may help
refine the theories for measuring pulsar ages. -- Damond Benningfield
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