The
Origin of Pulsars
Pulsars
are rapidly-rotating neutron stars; the collapsed cores of supergiant
stars that have exploded as supernovae. They are exceedingly
dense, weighing more than our Sun, but the size of a city. They
are highly magnetised. As the star spins, radio waves emerge
as a beam above the magnetic poles. Sometimes these beams will
sweep across the Earth's position in space and we see regular
pulses of energy, much like the flashes from a lighthouse.
Discovering
Pulsars
Thanks to the great sensitivity of the Lovell Telescope, Jodrell
Bank has been at the forefront of pulsar research since their
discovery at Cambridge in 1967. Over three quarters of the more
than 1000 pulsars now known have been discovered by Jodrell
Bank astronomers, often in collaboration with astronomers at
the 64-metre Parkes Telescope in Australia. This telescope has
recently been equipped with a 13-beam receiver system; the very
low-noise amplifiers and much of the data acquisition hardware
has been built by Jodrell Bank engineers. The survey using this
fast, highly sensitive, system at Parkes is detecting one new
pulsar for each hour of observation.
Millisecond
Pulsars
One of the most surprising discoveries ever made by the Lovell
Telescope was the finding of a very rapidly rotating pulsar
in the globular cluster M28. As pulsars age they slow down and
finally cease to radiate. Globular clusters are very old, and
thus it was not expected that any would be found there. However,
it seems that a close encounter between a neutron star and one
of the myriads of stars in the cluster had "spun-up" and re-activated
this pulsar whose period is now measured in milliseconds - hence
the name. Many other globular clusters, like 47 Tuc (left),
are now known to harbour millisecond pulsars. The fastest known
pulsar rotates 642 times per second! Such pulsars are exceedingly
good timekeepers and may even be more stable than the world's
best atomic clocks.
The
Crab Pulsar
A small 13-metre telescope continuously monitors the pulsar
in the Crab Nebula, M1. (see left) Formed when its progenitor
star was seen to explode in 1054, the Crab Pulsar is now spinning
just less than 30 times a second and is gradually slowing down.
However, every 18 months or so it is seen to increase its rotational
speed suddenly, an event known as a "glitch". This is due to
a change in the deep interior of the neutron star where some
of the neutrons exist as a superfluid. By studying these events
with a timing precision of one part in a trillion it is possible
to probe the interiors of these astounding objects.
Speeding
through space
With simultaneous observations by the Lovell Telescope and MERLIN,
pulsar positions can be measured very precisely. Observations
over a number of years have shown that some are moving through
space at speeds of more than 500 km/sec! It appears that many
new-born pulsars will be travelling so fast that they will escape
the gravitational pull of our Galaxy and be lost into the depths
of space.
For more on pulsar research see the Pulsar Group Research Pages
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