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\f0\fs32 \cf0 Hidden deep within a group of
ancient stars, there lurks a young and powerful
enigma. This is NGC 6624, a globular cluster
near our galaxy's center thought to be about
10 billion years old. NASA's Fermi Gamma-ray
Space Telescope detects high-energy radiation
from many globular clusters. Usually what
Fermi is seeing is the cumulative gamma rays
from all of the old pulsars in these clusters.
\
A pulsar is a rapidly spinning neutron star,
which is the small, incredibly dense remnant
of a much more massive star. A teaspoon of
matter from a neutron star weighs as much
Mount Everest, and a neutron star is so compact
that a ball about 15 miles across contains
more matter than our sun. Neutron stars spin
between 7 and 40,000 times a minute and form
with incredibly strong magnetic fields. \
Rapid spin and intense magnetic fields drive
powerful beams of electromagnetic radiation,
including gamma rays. As the pulsar rotates,
these beams sweep the sky like a lighthouse.
To a distant observer, the pulsar appears
to blink on and off. Pulsars slow down as
they age but some of the oldest pulsars spin
hundreds of times a second. Each of these
millisecond pulsars orbits a normal star.
\
Over time, the impact of gas pulled from the
normal star, has spun the pulsar up to incredible
speeds. This accretion may be the cause of
their weaker magnetic fields. Despite this,
these pulsars also emit gamma rays. But the
millisecond pulsar in NGC 6624 doesn't fit
neatly into this picture. It's so bright that
Fermi directly detects its gamma rays, and
so far it's the only one seen in a globular
cluster with such power. It's losing energy
so fast that it must be only around 25 million
years old--the youngest millisecond pulsar
ever found. It also possesses the strongest
magnetic field yet observed in a millisecond
pulsar. It's high energy output dooms it to
fade out quickly on astronomical time scales
and scientists wonder if this object represents
a new way to make millisecond pulsars. \
In three years, Fermi has detected more than
100 gamma-ray pulsars, shown here using animated
pulses fifty slower than actual speed. Recent
advances in data analysis helped Fermi reach
this milestone, and these techniques promise
to find many more gamma-ray pulsars. Some
of these are historical --the first gamma-ray
pulsars ever discovered. Others, like the
pulsar in NGC 6624, were first seen by radio
telescopes and then observed by Fermi. Some
were first spotted in radio after investigating
unknown sources detected by Fermi. And about
a third of gamma-ray pulsars were discovered
by Fermi on the basis of their gamma-ray pulsations
alone. Fermi's gamma-ray observations are
literally showing us these incredible stellar
lighthouses in a new light.}
