The Caldwell Objects

tounding, because NGC 6302 is a magnificent
object that can be seen in 7x35 binoculars.
Unfortunately, some sources muddy the waters
by listing the nebula's photographic magnitude
(12.8) in place of its far brighter visual magnitude
(9.6). Because of their peculiar spectra, which are
dominated by a few emission lines, this often
occurs with planetary nebulae, and undoubtedly
it has dissuaded many observers from trying to
find certain ones, NGC 6302 included. Robert
Burnham Jr. does not feature the Bug Nebula in
his Celestial Handbook; Walter Scott Houston
never discussed it in his "Deep-Sky Wonders"
column for Sky & Telescope (which ran monthly
for nearly half a century); and the Webb Society
does not include it in its Deep-Sky Observer's
Handbook.
In photographs taken with large telescopes
NGC 6302 indeed looks like a squashed bug. Its
long body trends east-west and comprises two
opposing cones or "wings" of gas. Separated by a
dark girdle of dust, these "wings" exemplify the
"butterfly" aspect of many bipolar planetary
nebulae. Each "wing" is made up of stringy veins
and clumps of material whose different elements
are being fluoresced by a hot central star. One
long tentacle-like extension trails off to the
northwest like a squashed bug's guts. Sun Kwok
(University of Calgary) believes that the various
forms of planetary nebulae — rings; butterflies;
and asymmetrical lobes — all can be explained
by one generic type of system seen at different
angles. In this model a dense torus of dust
surrounds the central star and channels its
outflowing gas through its open ends. Once free
of the ring the gas fans out, giving the planetary
a butterfly shape when seen edge-on. Kwok says
that bipolar nebulae were once deemed rare, but
"they are now believed to be more common than
we thought."
Try as they might, as of the year 2000 astronomers
had yet to image the Bug Nebula's
The Caldwell Objects
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central star, which ionizes its gases, making them
glow. If the star can indeed be seen at all at visual
wavelengths, it must be fainter than 20th
magnitude. The central star evades visual detection
probably because it is very blue and because
its light is dimmed by the dense torus of dust
separating the nebula's two lobes. The star's
presence has been detected, though, in the radio,
infrared, and ultraviolet portions of the
electromagnetic spectrum. The resulting data
imply that the star has an unusually high surface
temperature of about 380,000 degrees Kelvin.
Distance estimates for the nebula have ranged
from 3,900 to 7,800 light-years. In a 1993
Astrophysical Journal paper, Yolanda Gomez
(Autonomous National University of Mexico)
and her colleagues propose a value of 5,200 lightyears,
plus or minus a third. That would make
the object's physical dimensions 2.1 light-years
by 0.6 light-year or thereabouts. Gomez's team
also estimated the Bug Nebula's total mass to be
about 3 / 1 that 0 of the Sun, placing it among the
most massive planetary nebulae.
As I've said, the Bug is easy to spot under
dark skies. Again, it's nearly 4° due west of
Lambda (λ) Scorpii, so Telrad users have a direct
shot at it: place the eastern edge of your Telrad's
outermost circle on Lambda Scorpii and the Bug
will be on the same circle's western edge. Once I
knew where to look, the nebula was seen easily;
"no eye strain on this one," I wrote in my field
notes. Look for a solitary 9.6-magnitude "star"
surrounded by patches of dark nebulosity, which
populates this region. The object remains largely
stellar at 23x in the 4-inch, though with averted
vision it expands ever so slighdy. The Bug comes
alive at 72x, appearing as a definite high-surfacebrightness
glow elongated east to west. Even at
this modest power the nebula's true bipolar
nature can be detected without difficulty. Its
brightest section is intense and starlike, with a
forked "comet tail"
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