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The Crab Nebula

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Darrell Heath | Astronomy Columnist

The night of July 4th,
in the year 1054, seems
to have been one of considerable
excitement for
Chinese astronomers; it
was on this evening that
they meticulously recorded
the appearance of a “guest
star”, a star that suddenly
appears where there wasn’t
one before. Aside from the
Chinese sky gazers we also
have records from Arab
and Japanese astronomers
of this extraordinary event.
Sadly, and somewhat uncharacteristically,
there are
no records from European
observers. This may have
been due to the fact that
the Church, where most of
the literate people of the
Western world were employed
at the time, was in
the middle of a great schism
and, consequently, the goings
on of the night sky took
a back seat to more pressing
matters. Surely they must
have seen it though because
our other sources record
it as being so bright that it
could be observed during
the day and they state that
it was six times as bright
as the planet Venus. Even
more astounding, it could
be seen in the daytime for
a whopping 23 days and by
night for approximately two
years! What the people of
that time were seeing was
not the appearance of a new
star however but the death
of an old one by way of a
supernova, the violent and
explosive death throes of a
massive star.
A supernova occurs
when a massive star runs
out of fusible material within
its core. No longer able to
conduct thermonuclear fusion
the star’s outer layers
crash down upon an iron
rich core which results in the
explosion we call a supernova.
The star’s innards are
blasted out in all directions
as a chemically enriched
cloud of gas and dust, a
nebula, which may one day
go into the making of yet
another star, possibly even
planets, and maybe even life
itself. While the explosion
happened over 5,000 years
ago we can still see that shell
of gas and dust expanding
ever outwards and doing
so at speeds estimated at
around 3 million mph!
During the 1960’s the
United States Air Force detected
an unusual fluctuating
radio source coming
from an area on the sky
that corresponded with the
location of the Crab Nebula.
We now know that radio
source to be the remnant
core of the star that gave
rise to the nebula: a type
of neutron star known as a
pulsar. Neutron stars are the
highly compressed cores of
massive, dead stars. These
exotic chunks of degenerate
matter (a thimbleful of this
stuff would weigh around
100 million tons on Earth!)
are only a few miles across
and in the case of the Crab
Pulsar spinning 30 times a
second. As it spins around it
blasts electromagnetic radiation,
including radio waves,
our way. For reasons we
don’t quite understand, but
probably has to do with the
pulsar’s wind of subatomic
particles interacting with
magnetic fields, there have
been recent outbursts of
powerful gamma rays from
the Crab Nebula.
Look for the Crab Nebula
high overhead by midnight
this month in the constellation
of Taurus the Bull. Print
out a map of the sky at www.
skymaps.com to locate this
supernova remnant. You
will often see it designated
on such maps and in books
by its catalog name M1, or
Messier 1. You will need a
dark sky and optical aid to
see it but don’t expect it
to look anything like those
glorious Hubble images or
those photos taken by backyard
astroimagers. To the
eye it will only appear as
a gray smudge of light, it
takes a camera to bring out
the Crab’s awesome beauty
of tangled filaments of gas
and dust.
We humans have only
been seriously thinking
about recycling materials
within recent times but the
universe has been doing it
for billions of years and the
Crab Nebula is a glorious
example of that.