The Caldwell Objects

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The Pincushion Cluster
NGC 3532
Type: Open Cluster
Con: Carina
RA: 11 h 05.5 m
Dec: -58° 44'
Mag: 3.0; 3.2 (O'Meara)
Diam: 50.0'
Dist: 1,600 light-years
Disc: Abbe Nicolas Louis de Lacaille (in his 1755 catalog), but
probably known since antiquity
J. H ERSCHEL: Chief star of a very large, round, loosely scattered
cluster of stars [of] 8[th to] 12th magnitude, which fills 2 or 3 fields.
A fine bright object, (h 3315)
M IDWAY BETWEEN THE F ALSE C ROSS AND the
Southern Cross lies one of the heavens' true
stellar spectacles, the 3rd-magnitude open star
cluster NGC 3532 (Caldwell 91). It is a burst of
visual splendor in the swirling tempest of the rich
Carina starcloud. Together with the Eta Carinae
Nebula (Caldwell 92, just 3° to the westsouthwest)
and IC 2602, the Southern Pleiades
(Caldwell 102), NGC 3532 is part of the most
magnificent tract of Milky Way in the entire
heavens. Doubtlessly it has been a visual
landmark ever since aboriginal people first began
looking to the heavens. Yet its discovery is
credited to an 18th-century European explorer,
Abbe Nicolas Louis de Lacaille; he included it in
his 1755 catalog as the 10th member of his Class
II (nebulous clusters). It seems almost
blasphemous to consider this object an 18thcentury
European revelation, and I'm certain
Lacaille himself would have agreed. That being
said, this cluster's brightest stars shine at
magnitude 8, so they are too faint to discern
individually without optical aid. To the naked
eye the cluster looks like an enhancement of
GC / NGC: Remarkable, cluster, extremely large, round,
little compressed, stars of magnitude 8 to 12.
the Milky Way, a fantastic knot in the fabric of
space — but not a star cluster. It took Lacaille's
meager ½-inch 8x telescope to reveal, for the first
time, that this diffuse glow was actually "a
prodigious number of faint stars forming a semicircle
of 20/25 [arc]minutes diameter."
Nearly three-quarters of a century later
James Dunlop trained his 9-inch telescope on the
southern skies and encountered NGC 3532,
which he described as "a very large cluster of
stars about the 9th magnitude, with a red star of
the 7-8th magnitude, north following [northeast
of] the centre of the cluster. Elliptical figure: the
stars are pretty regularly scattered." Dunlop's
description falls far short of declaring the object's
true glory, which John Herschel and, later,
William Pickering extolled. Herschel called NGC
3532 a "superb" cluster, "the most brilliant object
of the kind I have ever seen." And like Herschel
before him, Pickering saw it as the finest
irregular star cluster in the sky.
Today NGC 3532 is classified as a very rich,
intermediate-age open cluster. Recent estimates
date it at about 300 million years; that's
358 Deep-Sky Companions: The Caldwell Objects