The Caldwell Objects

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have revealed a small near-infrared star cluster
coincident with a bright knot of hydrogen-alpha
emission near the southern edge of the OB
association (itself several degrees across). This
cluster's location is tantalizing, for it lies near a
sharp spike of ionized gas that delineates the
interface between the ΗII region and the
molecular cloud. The cluster may be the result of
star formation that was triggered by a front of
ionized gas, which pushed out from the OB
association into the molecular cloud.
Employing your telescope, you will find this
"invisible" action occurring halfway between the
5th-magnitude star 30 Cephei and the Bubble
Nebula (Caldwell 11). Sharpless 2-155 also can be
found by voyaging a little more than 3° due west
and a tad north of the famous open star cluster
M52 in
Cassiopeia. By placing M52 just out
of my Genesis's nearly 3° field, I can
make the Sharpless 2-155 field come
into the eyepiece from the west.
Another tiny nudge northwestward
brings the 6.5-magnitude star near
the brightest part of the nebula into
the center of my field of view. Notice
I was careful to state that the star —
not the nebula — is what appears in
the eyepiece. Sharpless objects are for
the most part notoriously faint.
Indeed, no visual description of
Sharpless 2-155 is readily available to
the casual observer. The 50' x 30'
object does not appear in the first
edition of either Sky Atlas 2000.0 or
Uranometria 2000.0. Stewart Sharpless
noticed this nebula on National
Geographic Society - Palomar
Observatory Sky Survey (POSS) plates,
and he recorded it in his 1959
Catalogue of
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Η II Regions. Observing guides like The Deep Sky
Field Guide to Uranometria 2000.0 and Sky
Catalogue2000.0 do offer photographic descriptions,
informing us that the object is obvious in
red-light photographs, with the northern half of
the crescent-shaped nebula (which has both
amorphous and filamentary structure) being
most noticeable. Indeed, the Cave Nebula is
occasionally photographed by amateur astronomers
and showcased in magazines like Sky Si
Telescope. But what visual impression does it
make through a small telescope, if any?
In a cursory literary search I did learn that
amateurs in Helsinki, Finland, found the Cave
visible in a 14-inch telescope with the help of an
ultra-high-contrast (UHC) filter. It can be seen
with smaller, unfiltered apertures — but just
barely. Through the 4-inch Genesis the
Deep-Sky Companions: The Caldwell Objects