The Caldwell Objects

in that triangle (which, together with the brighter
star, almost points directly to the cluster). All
these stars are plotted on our finder chart below.
NGC 1851 should be obvious even in 7x35
binoculars as a slightly swollen "star" of pearly
light. From Hawaii the cluster is just visible to the
unaided eye with averted vision.
At 23x in the 4-inch the cluster looks like the
bright head of a comet. With direct vision a faint
coma of light surrounds a dense, round core.
Switch to averted vision and traces of arms show
in the outer coma. Although the cluster spreads
out to 12' in photographs, visually it's half that
size. Now focus your attention on the bright 2'wide
central condensation, which should appear
starlike with a glance. The longer you look, the
more the core will break down into individual
elements; it's like opening packages within
packages of stars. The halo immediately
surrounding the core reveals definite arms and
some resolved stars at the periphery of the halo.
Do you see a yellowish
cast to the core?
Much can be seen
with a 4-inch at 72x.
Although the cluster's
horizontal-branch stars
shine at 16th magnitude,
its brightest members are
three magnitudes brighter.
This means that NGC 1851
can be partially resolved
in small telescopes. Try
looking at the cluster with
your left eye, then your
right eye, and see if there's
a difference. That's what
John Herschel did during
his second observation of
this object. He also found
290
73
the stars "barely visible in strong twilight." I find
the cluster looks asymmetric to the north, with
two fuzzy wings of faint stars, many of which are
resolved. The fuzzy wings remind me of those
visible in M13, the Great Hercules Cluster. Two
spidery arms extend eastward, jutting out of a
parabolic envelope of scintillating haze. The
western side of the outer halo appears detached
from the core, separated by a lane of darkness.
Deep-Sky Companions: The Caldwell Objects