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The Caldwell Objects

The Caldwell Objects

The Caldwell Objects

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in that triangle (which, together with the brighter<br />

star, almost points directly to the cluster). All<br />

these stars are plotted on our finder chart below.<br />

NGC 1851 should be obvious even in 7x35<br />

binoculars as a slightly swollen "star" of pearly<br />

light. From Hawaii the cluster is just visible to the<br />

unaided eye with averted vision.<br />

At 23x in the 4-inch the cluster looks like the<br />

bright head of a comet. With direct vision a faint<br />

coma of light surrounds a dense, round core.<br />

Switch to averted vision and traces of arms show<br />

in the outer coma. Although the cluster spreads<br />

out to 12' in photographs, visually it's half that<br />

size. Now focus your attention on the bright 2'wide<br />

central condensation, which should appear<br />

starlike with a glance. <strong>The</strong> longer you look, the<br />

more the core will break down into individual<br />

elements; it's like opening packages within<br />

packages of stars. <strong>The</strong> halo immediately<br />

surrounding the core reveals definite arms and<br />

some resolved stars at the periphery of the halo.<br />

Do you see a yellowish<br />

cast to the core?<br />

Much can be seen<br />

with a 4-inch at 72x.<br />

Although the cluster's<br />

horizontal-branch stars<br />

shine at 16th magnitude,<br />

its brightest members are<br />

three magnitudes brighter.<br />

This means that NGC 1851<br />

can be partially resolved<br />

in small telescopes. Try<br />

looking at the cluster with<br />

your left eye, then your<br />

right eye, and see if there's<br />

a difference. That's what<br />

John Herschel did during<br />

his second observation of<br />

this object. He also found<br />

290<br />

73<br />

the stars "barely visible in strong twilight." I find<br />

the cluster looks asymmetric to the north, with<br />

two fuzzy wings of faint stars, many of which are<br />

resolved. <strong>The</strong> fuzzy wings remind me of those<br />

visible in M13, the Great Hercules Cluster. Two<br />

spidery arms extend eastward, jutting out of a<br />

parabolic envelope of scintillating haze. <strong>The</strong><br />

western side of the outer halo appears detached<br />

from the core, separated by a lane of darkness.<br />

Deep-Sky Companions: <strong>The</strong> <strong>Caldwell</strong> <strong>Objects</strong>

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