The Caldwell Objects

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Theta Carinae Cluster;
Southern Pleiades
IC 2602
Type: Open Cluster
Con: Carina
RA: 10 h 42.9 m
Dec: -64° 24'
Mag: 1.9
Diam: 100'
Dist: 492 light-years
Disc: Abbe Nicolas Louis
de Lacaille, included in his
1755 catalog
H ERSCHEL: None.
GC/NGC: None.
IC: Cluster, coarse, includes θ Carinae.
D ROP A VISUAL PLUMB LINE 4¾° DUE
SOUTH of the magnificent Eta (η) Carinae
Nebula and it will collide with another brilliant
deep-sky wonder, IC 2602, the dazzling Theta (θ)
Carinae Cluster (Caldwell 102). Louis de Lacaille
discovered this mine of celestial diamonds
during his great mid-18th-century exploration of
the southern skies. The object is included in his
1755 catalog, where it is the 9th listed under his
Class II category ("nebulous clusters"). Lacaille
called the object "Theta Argus" (Carina is the keel
of the now-defunct constellation Argo Navis) and
estimated its brightness as 3rd magnitude.
Through his diminutive ½-inch 8x telescope
Lacaille saw Theta Carinae "surrounded by many
6/7/8 mag [stars] like the Pleiades." And that is
why on modern charts IC 2602 is labeled "The
Southern Pleiades."
With a declination of-64° 23.7', IC 2602 is
only above 5½° my Volcano, Hawaii, horizon
when at upper culmination. Yet even at that
402
meager altitude it is clearly visible to the unaided
eye. The cluster is one of three major glows in the
rich Carina Milky Way, the other two being NGC
3532 (Caldwell 91) and the Eta Carinae Nebula
(Caldwell 92). Together with the dense starclouds
of the attendant Milky Way, the sight of these
three objects is without question the most
stunning in the entire heavens. Several of IC
2602's stars are visible to the naked eye, but just
how many depends upon the cluster's altitude,
your site's darkness, and your ability to resolve
stars packed tightly together. Photometry of two
15'-wide fields, conducted in 1991 with the 1meter
Swope telescope at Las Campanas
Observatory in Chile, revealed that half the stars
visible to the telescope in IC 2602 were
background stars — not surprising for an open
cluster in a rich Milky Way field. Brent Archinal
says IC 2602 harbors at least 60 definite members
spread across 100'; some 30 of these are brighter
than
Deep-Sky Companions: The Caldwell Objects