The Caldwell Objects

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anomalously slow decline that it was
rediscovered years after its initial
apparition. Its slow decay is now
attributed to interactions between the
supernova ejecta and particles that had
been cast off earlier by the progenitor
star. X-ray observations with the nowdefunct
Rosat satellite support this
theory.
Like IC 342 (Caldwell 5) and NGC
2403 (Caldwell 7), NGC 6946 is a
member of the Coma-Sculptor Cloud of
galaxies. Shining at magnitude 8.8, NGC
6946 lies only 11° from the central plane
of the Milky Way, whose dust and gas
dim its light by 1.6 magnitudes. Images
taken through large telescopes reveal
the system to be quite large. Its
estimated linear diameter is 58,000 lightyears,
and it shines with a total
luminosity of 30 billion Suns.
We see NGC 6946 inclined 42° from face-on;
its clumpy spiral arms are flung wildly open.
Recent observations of these arms have caused
some confusion. In theory astronomers expect to
find a spiral's strongest magnetic fields at the
inner edges of the visible spiral arms. A 1990s
map of NGC 6946's polarized radio emissions did
indeed trace out two well-defined magnetic arms.
But these magnetic arms lay between the visible
ones. This finding defied then-prevailing
theories. However, University of Chicago
theorists Zuhui Fan and Yu-Qing Lou have since
proposed that these out-of-phase magnetic arms
are caused by a second, rather sluggish set of
density waves that peak between the visible
arms.
It's a shame that this astrophysical dynamo
is not equally electrifying visually. Robert
Burnham Jr. has little to say about NGC 6946's
visual impression, except that "owing to the low
56
surface brightness, the small central nucleus is
the only detail which appears clearly to the visual
observer." Surprisingly, Christian Luginbuhl and
Brian Skiff do not include NGC 6946 in their
Observing Handbook and Catalogue of Deep-Sky
Objects, and, though it is listed in Roger Clark's
Visual Astronomy of the Deep Sky, the galaxy's
magnitude is misrepresented there as 11.1, a full
2.3 magnitudes fainter than the currently
accepted value.
Certainly NGC 6946 is an object whose faint
features can be fully appreciated only under a
dark sky. The galaxy is nicely situated only 2°
southwest of Eta (η) Cephei, just 2 /3° from the
brighter, more visually appealing open cluster
NGC 6939 (magnitude 7.8). Seen through the 4inch
at 23x these objects make a magnificent pair:
the cluster starts to be resolvable at low power
and is cleanly sliced through the middle by a
dramatic dark lane bordered
Deep-Sky Companions: The Caldwell Objects