The Caldwell Objects

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the notice of the unaided eye? The problem may
be one of contrast. With a casual glance the
faintly luminous background of the Milky Way
tends to fill the gap between the cores, making
the split less distinct. Atmospheric clarity also
may be a critical factor. When the Double Cluster
is seen from nearly 14,000 feet atop Hawaii's
Mauna Kea, a distinct lane of darkness separates
the two components, even with direct vision. At
an altitude of 4,200 feet, however, resolving the
components requires a slight effort. The odds
seem to worsen at sea level. You can avoid the
issue by looking for the Double Cluster at the
start of astronomical twilight, about 45 minutes
after sunset; it emerges at about the same time as
the bright-
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est stars in Cassiopeia. Before the sky darkens
completely the two cores pop clearly into view
for several minutes, without any interference
from the Milky Way.
It is also at this time that one can make an
accurate estimate of each component's brightness.
Published magnitudes for the individual clusters
are wide-ranging. The Observing Handbook and
Catalogue of Deep-Sky Objects lists the components'
visual magnitudes as 3.5 (NGC 869) and 3.6
(NGC 884). Visual Astronomy of the Deep Sky pegs
them at 4.4 and 4.7, respectively. And both the
Deep Sky Field Guide to Uranometria 2000.0 and
Star Clusters list values of 5.3 for NGC 869 and 6.1
for NGC 884. Most sources determine a cluster's
total apparent
Deep-Sky Companions: The Caldwell Objects