The Caldwell Objects

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members do extend beyond the cluster's putative
edge. Perhaps this exercise simply demonstrates
just how difficult it is to determine the extent of
an open cluster in a rich Milky Way region. It also
highlights the imprecision of supposedly "hard"
astronomical data. Archinal elaborates: "First of
all, there appears to be a basic definition problem
here regarding what the diameter of a cluster
should be. This is another one of those cases
where the NGC designation really applies to
something that's not quite the same thing as the
modern astro-physically accepted cluster. Most of
the stars are concentrated in a 20-30' diameter
area, but possibly with outlier stars going out to
1°. So, depending on how one wants to define
diameter, all the values cited are correct! It all just
boils down to deciding whether you prefer the
'original' diameter, the approximate obvious
visual diameter (my 22' seems reasonable), or the
maximum diameter, to include all possible
cluster members (1° or more)."
Our astronomical predecessors had no way
of determining the distances to the stars, so their
perception of a star cluster was simply that —
stars clustered together on the plane of the sky.
When Lacaille discovered NGC 2516, he used a
½-inch telescope at 8x, so his view would have
been similar to mine through 7x35 binoculars —
a wishbone of stars with a fuzzy "bottom."
Through my 7x35s about a dozen stars can be
seen to 9th magnitude. So Lacaille probably did
include the northwestern branch of the wishbone
in his own "very close group of 10-12 stars."
NGC 2516 has 103 known members, several
of which are red giants: an orange, magni-tude-
6.7 K1III star (R.A. 7 h 58 m 09.8 s ; Dec. -60° 49' 00")
near the core of the cluster's densest part; the 5.2magnitude
Μ0ΙΙ irregular variable V460 Carinae
(R.A. 7 h 59 m 37.6 s ; Dec. -60° 35' 13"); and a
magnitude-5.7 K4II star (R.A. 7 h 56 m
386
18.6 s ; Dec. -60° 31' 35"). Given the historical discrepancies
regarding the cluster's size, one can
only wonder which of these reddish stars was
Herschel's "orange-coloured star . . . in [the]
middle of [this] large and magnificent cluster."
At 23x in the 4-inch NGC 2516's densest part
appears boxy, like the core of the Beehive Cluster
(M44 in Cancer), and many streams of irregularly
bright stars radiate from it. The cluster also
contains several dark voids, and with a little
imagination I can envision a cat getting ready to
sleep on a cushion of starlight whose creases are
the shadowy recesses of darkness. This cushion's
brightest part is a wide crescent of mixed and
colorful suns. Try defocusing the image ever so
slightly, and look for a dark rosette centered on
this cushion of stars. At 72x the "cat" is
surrounded by three prominent pools of
blackness, each of which looks like crushed black
velvet. These pools are outlined by sparkling
jewels. Yet they are not ink-black, for they
shimmer softly, as if they were reflecting the
starlight surrounding them. Use high
magnification to look for a "comet tail" of dim
suns flowing to the southwest.
Deep-Sky Companions: The Caldwell Objects