The
Pencil Nebula
By David Malin (David Malin Images)
 NGC
2736, the Pencil Nebula was first noted by John
Herschel on March 1, 1835, during his four-year
stay in South Africa. He observed NGC 2736 (=
h3145) only once. He described it "...as
an extraordinary long narrow ray of excessively
feeble light". Though there are much more
conspicuous and extensive regions of the Vela
SNR visible on modern wide angle photographs
they were not seen by Herschel, probably because
of their more diffuse nature and the rich starfield
in which they lie. However, he did find several
more objects in this part of the sky that eventually
appeared as extended objects in the New General
Catalogue (NGC) of 1888, so he was looking hard.
The
Pencil Nebula is almost exactly on the Galactic
equator. This places it several degrees from
where one might expect to find an outlying wisp
of the Vela supernova remnant if you assume
that the arc of nebulosity seen in this image
of the Vela supernova remnant is the north-west
quadrant of a roughly spherical shell outlining
the expansion of the supernova explosion.
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It is also much further east
of where one might expect to find nebulosity
if you assume that the Vela pulsar is close
to the center of the nebulous shell. However,
it is known that the Vela pulsar has a highproper
motion. Even so, from the purely optical images
alone, the association of the Pencil nebula
with the Vela supernova remnant begins to look
a little doubtful.
 As
is so often in astronomy, there is more here
than meets the eye, and at X-ray wavelengths,
the full extent of the Vela SNR becomes evident
in pictures from the ROSAT satellite (at left).
The Pencil nebula coincides with a 'blister'
on eastern edge the X-ray image. This blister
is one of several symmetrical features on the
X-ray photograph that probably originate in
the supernova explosion itself. However, its
corresponding western counterpart, far away
from the Galactic equator, is very diffuse and
extended. The eastern part, which is close to
the optical position of the Pencil nebula, is
compact and bright at X-ray wavelengths. This
is probably because it is expanding into much
denser interstellar material close to the Galactic
plane.
All this suggests that much of
the Vela SNR is hidden by dust at optical wavelengths,
and that we are fortunate to have a clear patch
through which we see it. The nebula is probably
'reddened' by intervening interstellar matter,
which will have the effect of diminishing the
blue light from the excited gas rather more
than the red light.
For more information, visit
the
Vela
Supernova Remnant File
by Bill Blair (JHU)
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