10.
Counting Down
So
the pulsars were highly magnetised, rapidly spinning
neutron stars, left over from supernova explosions in
the distant past. Gold predicted that the rate of rotation
of these stars would gradually decrease as they radiated
energy, and accurate timing of the pulses proved this
to be the case. Pulsars slow down at rates of about
one millionth of a second per year. The ratio of a pulsar's
present speed to this slow-down rate is a good way of
telling how old it is.
By
the mid 1990s over 700 pulsars had been discovered and
catalogued. Gold's realisation that the first Cambridge
pulsars were spinning slower than many pulsars proved
correct, and in the early 1980s very fast pulsars were
discovered. These 'milli-second pulsars' spin nearly
a thousand times a second, and are thought to gain this
speed by pulling in material from a second star that
has drifted too close.
In
1974 Antony Hewish and Martin Ryle, head of the Cavendish
radioastronomy group, were awarded the Nobel Prize in
Physics "for their pioneering research in radio
astrophysics. Ryle for his observations and inventions,
and Hewish for his decisive role in the discovery of
pulsars." This was the first time a Nobel Prize
was awarded for work in astronomy. Jocelyn Bell received
her Ph.D. from Cambridge in 1969. In 1999 she was awarded
a CBE for her services to astronomy.
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