The Caldwell Objects

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the Giraffe never sets when viewed from midnorthern
latitudes, it remains an inconspicuous
form under dark skies and a virtual washout
from many urban locations. Yet it is in this
wilderness of dim suns, tucked into the
constellation's southeastern quadrant (actually
within sniffing distance of the Great Bear's nose),
that Camelopardalis hides a precious
extragalactic treasure: the fertile spiral galaxy
NGC 2403.
NGC 2403 belongs to the Coma-Sculptor
Cloud of galaxies — a peppering of gravitationally
bound systems that includes our Local
Group and extends more than 10
million light-years beyond its fringe.
Having the mass of some 47 billion
Suns and spanning 98,000 lightyears
of space, NGC 2403 is one of
the Cloud's more physically
impressive members. From our
perspective, we see NGC 2403's
spiral form inclined 28° from edge
on. The galaxy displays two tiers of
arms. Those close to the nucleus are
tightly wound; farther out the arms
sweep wide and free.
Any photograph will show the
galaxy's face "exploding" with
clumps of starlight; some of these
are foreground stars, any one of which could
mislead an unsuspecting observer to think he or
she has stumbled upon an extragalactic
supernova. The galaxy's clumpy appearance,
however, is not due to foreground starlight
alone. After surveying the galaxy on plates taken
with the Palomar Mountain 200-inch reflector
during the 1950s, Edwin Hubble detected more
than 100 Η II regions lining the galaxy's spiral
arms. The largest of these nebulous star-forming
regions, NGC 2404, lies 6' north-northeast of the
nucleus; its 20" apparent diameter corresponds to
a true diameter
of 1,400 light-years at the galaxy's measured
distance of 14 million light-years. This Η II region
is larger than any known in our Milky Way. The
galaxy also contains many O B associations,
whose hot, blue stars hint at copious star
formation in the galaxy's recent past. The
distribution and number of NGC 2403's Η II
regions resemble those found in other galaxies,
such as M33, that are rife with gas and hot, young
stars. Hubble also identified 27 variable stars in
the system; as of 1960,10 of them had been
certified as Cepheid variable stars with periods
ranging from 18 to 54 days. In fact,
NGC 2403 is the first system beyond the Local
Group in which Cepheids — whose periodluminosity
relationship is an important tool for
measuring extragalactic distances — were
identified with the 200-inch.
Near-infrared images of NGC 2403 made in
the mid-1990s reveal a weak two-arm spiral
structure extending over as much as 180° in
azimuth to a radius of 6,500 light-years. Other
studies have determined that NGC 2403's most
active star-forming regions are not in its nucleus,
which appears quiescent, but along the outer
ridges of its spiral structure, where,
Deep-Sky Companions: The Caldwell Objects