The Caldwell Objects

RA: 22 h 56.8 m
Dec: +62° 37'
Mag: 10.0 (O'Meara)
Dim: 50' χ 30'
Dist: ~2,800 light-years
W HEN CLOCKS STRIKE THE MIDNIGHT HOUR
on August 11th, a look to the northern sky will
reveal King Cepheus executing his highest reign
over that celestial realm. That's also a propitious
time to start watching the King get pelted by
stones — meteors whipped across the Milky Way
from that seemingly angry hero Perseus, rising in
the northeast. If one of these meteors were to fly
from Perseus to Cepheus along the northern edge
of the dense Milky Way, it would skirt the
famous variable star Delta (δ) Cephei. There, the
Milky Way suddenly turns back on itself and
forms a hollow, a cave of sorts. It's as if the
imaginary meteor had suddenly bored a hole into
our galaxy's star-studded disk. And it's at the
mouth of this "cave" where telescopic
adventurers can seek out a highly elusive visual
"monster" — the emission nebula Sharpless 2-155
(Caldwell 9), ironically known as the Cave
Nebula for its own hollowed-out shape.
A word of clarification. Although Stewart
Sharpless included the Cave Nebula in his 1953
Catalogue of Emission Nebulae, he did not call it the
Cave Nebula; he cataloged it simply as a large
emission nebula. In fact, calling Sharpless 2-155
the Cave Nebula is a historical no-no. I refer you
to the December 1908 Monthly Notices of the Royal
The Caldwell Objects
Disc: noted by Stewart Sharpless on photographic
plates and included in his Catalogue of Emission
Nebulae (1953)
Astronomical Society, where, in a paper titled " A
New 'Cave-Nebula' in Cepheus," Max Wolf
reports:
I have the pleasure to announce the discovery of a
new interesting nebula in the midst of the
constellation Cepheus.
H ERSCHEL: None.
GC/NGC: None.
The nebula was found by Dr. [August] Kopff on a
plate taken by him with my Bruce-telescope on the
night of October 21st, 1908.
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The "Cave-Nebula in Cepheus" that Wolf
goes on to describe is not Sharpless 2-155 but a
nebula that now bears the designation vdB 152; it
is located 9° northwest of Sharpless 2-155 and 31/2°
east of Beta (β) Cephei. What's key is that Wolf
wrote that this new nebula "forms an important
addition to this class of nebulae." And what class
of nebulae might that be? "This nebula," Wolf
continues, "is a good example of the singular
phenomenon of cave-formation amongst Milky
Way stars. In some respects it shows the general
characteristics of other cave nebulae ..."
(emphasis mine).
Sharpless 2-155 is but one of several Caves
in the heavens.
But let's follow modern custom and apply
the Cave Nebula moniker to Sharpless 2-155. The
Cave Nebula, it turns out, is part of an extensive
region of ionized hydrogen gas — an Η II
region — associated with the Cepheus Β giant
molecular cloud. Part of the cloud is illuminated
by a pack of hot, young stars known as the
Cepheus OB3 association. Energetic photons from
these stars strip the surrounding hydrogen atoms
of their electrons, causing the gas in the visible
nebula to glow. The now-defunct Rosat satellite
has revealed some 40 point sources of X-rays in
the association, the majority of which are Τ Tauri
stars — hot young suns still partially swaddled in
dusty blankets of gas. Radio and near-infrared
observations also
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