The Caldwell Objects

Nicolas Louis de Lacaille, however, is credited
with first describing it as a star cluster. It is the
fifth object listed in his 1755 catalog's Class II
(Nebulous Clusters). In his ½-inch 8x telescope
he saw it as a "small star group."
Whenever I take my first naked-eye glance at
this cluster, I immediately see a double star,
oriented northwest-to-southeast, whose components
are separated by about 20'; these "stars"
are Omicron Velorum and what is actually the
combined light of a magnitude-5.5 star
andamagnitude-4.9 star, themselves separated by
less than 2'. With a little more time, a magnitude-
5.5 star pops into view about ½° southsouthwest
of Omicron Velorum. These stars form
a tight naked-eye triangle. Now return to
Omicron Velorum with averted vision. Does it
look fuzzy? It should, because it is the brightest
of three naked-eye stars that form a tiny (10'long)
arc running from northeast to southwest.
Can anyone resolve these stars with the unaided
eye? Try to do so in the twilight.
In 7x35 binoculars the cluster's main body
comprises about a dozen suns. These stars form
an asterism that, to me, looks like the
85
stick figure of a woman extending an arm to the
southeast. In her hand is a pillow (the close pair
of magnitude-5.5 suns and others). And atop that
pillow is a diamond tiara formed by a tight
sprinkling of 8th- to 11th-magnitude suns. It
looks like a tiny cluster-within-a-clus-ter. The
view is better yet in the Genesis at 23x. The star
immediately south of aqua-tinted Omicron
Velorum burns with a lemony hue. Among the
roughly two dozen suns readily visible in the 1°wide
cluster, several pairs can be seen, and many
of these make up the tiara. If we accept the
distance tabulated on page 339, this loose cluster
is spread across 9 light-years of space. The stars
are scattered unevenly, and no point forms a
distinct visual center. In fact, Archinal's latest
data shift the cluster's center northward to
Omicron Velorum, creating an even more
disturbingly asymmetric picture. Imagine how
different the view of the night sky would be from
hypothetical planets orbiting various stars in this
lopsided cluster.
Contrary to some reports, IC 2391 does have
appeal under higher magnification. At 72x, for
instance, about a dozen suns burn forth amid a
bright trapezoidal asterism, inside of which lies a
smattering of various scintillating suns. It looks
as if Orion's Trapezium has been magnified 1,000
times over without any loss of image quality.
Omicron Velorum and its two bright
counterparts each sport one companion or more.
The pillow-and-tiara region also stands apart,
surrounded by blackness.
Before moving on, hop about 1° due east of
Omicron Velorum to the 12'-wide open cluster
NGC 2669. Also known as Harvard 3 (H3) on
some charts, it's a neat little 6th-mag-nitude
cluster of about a dozen or so suns, with four
prominent pairs forming a mirror-image Greek
letter lambda (λ). Can you see it with the
unaided eye?
The Caldwell Objects 341