The Caldwell Objects

Now let's tour the North America Nebula in
some detail. Start in Central America. The bright
strip of nebulosity running along the isthmus's
eastern (on the sky, that is!) coast is reminiscent
of the great volcanic spine that stretches the
length of that region on Earth; this "mountain
range" connects to the apex of a ghostly triangle
representing Mexico. The central region of
Mexico, in turn, is dimmer than its coasts.
Likewise, the central portion of the United States
is patchy, with a dim, grayish Y stretched across
it. The 10th-magnitude open cluster NGC 6997
roughly marks the location of Ohio. Through the
Genesis at 23x it's an elliptical gathering of about
two dozen suns sprinkled across 15' of sky; the
number of stars doubles at 72x, though you have
to strain to see the faintest ones. The cluster's
major axis runs east-west. Look for a tail of stars
that extends eastward from the cluster's eastern
end before curving north. Other faint arms of
stars branch out from the clus
ter's core to the north and south,
while its western end sports two
short and stubby legs. Seen in this
way, the cluster looks somewhat
like a human stick figure. The
cluster's distance is 2,000 lightyears,
while its age (roughly 100
million years) excludes the
possibility of an evolutionary
relationship with the nebula
complex.
Incidentally, confusion between
NGC 6997 and NGC 6996 —
supposedly also a cluster — is rife
in both the amateur and the
professional literature. The New
General Catalogue lists both objects
as open star clusters, and Brent
Archinal has verified that the
positions given
86
20
therein are correct. NGC 6996 lies nearly 1° to the
north of NGC 6997. So why the confusion? As
Archinal explains in his 1993 monograph, The
"Non-Existent" Star Clusters of the RNGC, the fault
may lie with the 1975 Revised New General
Catalogue of Nonstellar Astronomical Objects. This
work deemed NGC 6997 a "nonexistent" cluster
and NGC 6996 a bona fide one. Yet a casual
glance at the region through a telescope makes it
clear that there is an obvious cluster at NGC
6997's position and (at best) a subtle star
grouping at NGC 6996's. However, perhaps
because of the Revised New General Catalogue's
error, Uranometria 2000.0 put a cluster at NGC
6997's position but labeled it NGC 6996, while the
Millennium Star Atlas plotted NGC 6996 at its
correct location but omitted NGC 6997. The
second edition of Sky Atlas 2000.0 avoided the
issue entirely, omitting both clusters (though the
first edition correctly identifies and plots NGC
Deep-Sky Companions: The Caldwell Objects