The Caldwell Objects

ightness by summing the contributions of the
stars in each cluster as recorded on photographic
plates. The differences among published values
may reflect (1) differences in the numbers of stars
used, and (2) whether the total magnitude
includes any bright stars superimposed on the
clusters. (On photographic plates the
overexposed images of bright stars will lead to
spuriously bright total magnitude estimates.)
Motivated by these discrepancies, in February
1998 I estimated the brightness of the individual
clusters with the naked eye, both in deep twilight
and under a dark sky, and got an average visual
magnitude of 4.5 for NGC 869 and 5.7 for NGC
884. Brent Archinal suspects that these visual
estimates are close to what one would get by
including the stars that are too bright to be
measured on photographs.
The Double Cluster resides in a Milky Way
spiral arm (the Perseus Arm) that lies farther
from the galactic center than the one containing
our solar system (the Orion Arm). A recent study
places the clusters 7,300 light-years
The Caldwell Objects
14
from Earth. These clusters
probably formed from a
single cloud of dust and
gas, perhaps 13 million
years ago. We see the
Double Cluster through
thick clouds of interstellar
dust that line the plane of
the Milky Way. Were it not
for this dust, we would see
the Double Cluster shining
1.6 magnitudes (4.4 times)
brighter. The obscuration is
unfortunate because, of the
300 or so suns that populate
each of the two
clusters, many are hot, bluewhite
supergiants; yet interstellar dimming lets
even the brightest of these appear no brighter
than magnitude 6.5. Imagine the grandeur of the
Double Cluster if it were at the distance of the
Pleiades (407 light-years). It would recall the
starscape described in Isaac Asimov's story,
"Nightfall." The twin mounds of blue supergiants
would each span 5 Moon diameters and shine
300 times brighter than they actually do in our
skies. One-quarter of the northern sky would be
filled with the concentrated splendor of 600 suns,
the brightest of which would shine with the
brilliance of Vega. The earliest skywatchers
would have succumbed to the Double Cluster's
presence, looked to it for meaning, and planned
their affairs in step with its eternal rhythms.
Poets and artists would have looked upon these
twin celestial cities to fuel their passions, or
perhaps simply to dream.
But we have to settle for the more subtle
view. To the naked eye, each cluster has a central
condensation and a mottled outer halo. Try
resolving some of the stars in the Double
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