The Caldwell Objects

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trending chain of stars surrounded by a magnificent
halo of unresolved suns. S Normae
immediately draws attention to itself with its
orange color. A 9th-magnitude star pops into
view at the extreme northern edge of the halo,
and a pair of 8th- and 9th-magnitude suns marks
the halo's southern boundary. Overall, the
binocular view reminds me of a granite Celtic
cross — the kind you'd find glowing in a
cemetery, as if moonlit on a foggy night. I cannot
see this background glow with the naked eye,
only the bright bar of stars; the background
swells into view only when binoculars and
averted vision are used.
NGC 6087 is a relatively young open cluster,
about 65 million years old. It has been of
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particular interest to astronomers because of its
association with its Cepheid lucida. Cepheids
have extremely stable periods of variability and
very reproducible light curves. In the early 20th
century Henrietta S. Leavitt discovered that
Cepheids have a very special relationship
between their intrinsic brightnesses, or luminosities,
and their periods of variability.
Basically, the brighter a Cepheid is, the longer it
takes to complete one cycle of brightening and
fading. By observationally determining the
period of a Cepheid variable, we learn its
intrinsic brightness. Then, by comparing how
bright it appears with how bright it really is, we
can deduce its distance. Applying the periodluminosity
rule to S Normae, we get a distance
of 1 kiloparsec, or 3,300
light-years.
At 23x in the Genesis
it's almost hard to believe
you're looking at a
star cluster with half the
angular size of the full
Moon, because NGC
6087's brightest stars do
not seem to blend
together very well.
Except for a 5'-wide
gathering of suns
close to S Normae, the cluster's remaining
constituents appear to be scattered like stellar
marbles, forming all manner of geometrical
patterns — right angles, lines, arcs, diamonds —
in a most haphazard way. Add the rich Milky
Way background and it's a visual mess. But if
you relax your gaze and let your eye roam
around the field you should see that the cluster's
edges fit within the confines of a boxlike
arrangement of field stars. It looks as if several
trucks loaded with stars had approached S
Normae from various directions, dropped their
wares on top of it, and then departed. In
Deep-Sky Companions: The Caldwell Objects