The Caldwell Objects

88
members. Archinal lists 103 cluster members
within a 12'-wide area. NGC 5823 extends 12
light-years in space. (Very conveniently, each
arcminute equals one light-year at the cluster's
distance of 3,400 light-years.) NGC 5822, by the
way, is 1,600 light-years closer to us than is NGC
5823, and a bit older; with an age of 600 to 800
million years, it is more or less coeval with the
Hyades.
Now that the cluster's identity has been
established, we can sit back and enjoy the view.
At 23x in the 4-inch NGC 5823 looks boxy, with
its brightest stars — actually foreground stars —
lining its ragged edges. Inside this box the
cluster's main stars trace a distinct tuliplike
figure. At 72x the stars outlining the tulip appear
most prominent. The base of the tulip is a fine Cshaped
curve of equally bright suns whose open
end faces northeast. A tiny wave of starlight just
northeast of the C completes the flower. Using
my imagination I can transform the tulip into a
skull half-buried in the stellar sands, while a
prominent row of stars to the north (and oriented
east-west)
350
becomes a bone lying nearby. Magnifying the
cluster will bring out its fainter members, but at
the cost of diminishing the cluster's overall
appeal. As John Herschel noted, the cluster is
"not much compressed in the middle," so
magnifying the view just spreads out this already
loose aggregation of faint suns. Be sure to look
for a tiny clustering of five or six roughly 13thmagnitude
suns a few arcminutes due west of the
cluster core. It is cleanly separated from the main
body of stars and looks rather like a stick figure
of a fish.
Of course, the entire region around NGC
5823 and its neighboring clusters deserves
attention. A little more than 2° to the northwest of
NGC 5823 lies yet another fine open cluster, NGC
5749. This roughly 8th-magni-tude glow contains
a wishbone-shaped aster-ism of 10th- and 11thmagnitude
stars at its core. Unfortunately, this 7'wide
cluster was omitted from the second edition
of Sky Atlas 2000.0, but it appears on the chart on
page 349. And if you'd like to test your color
sensitivity, Zeta Lupi is a nice double star for
small-telescope users. Its magnitude-3.5 CK0III)
primary shines with a brilliant yellow cast, while
its magnitude-6.7 (F8V) secondary seems to shine
with a rust-colored hue. (Of course, star color is
highly subjective; the color difference might be
the result of the brightness thresholds for
stimulating different color perceptions in the eye,
rather than intrinsic to the stars. What do you
see?) The secondary is 72" west of the primary (or
so things stood in 1991, the most recent date for
which the stars' relative positions are tabulated in
the Washington Double Star Catalog).
Deep-Sky Companions: The Caldwell Objects