The Caldwell Objects

with the fiery reddish light of hydrogen-alpha
emission.
Few places in the night sky reveal this wavedriven
drama in details large and bright enough
to be seen in amateur telescopes. But one region
places all the performers on stage, ready for you
to critique: it's the Ara OB1 Association, a 1°wide
swath of cosmic splendor centered on the
naked-eye open cluster NGC 6193 (Caldwell 82).
You can find this sparkling cluster nearly 7°
southwest of Zeta 1 , 2 (ζ 1,2 ) Scorpii, itself a
member of yet another stellar grouping, the
Scorpius OB1 Association, whose nucleus is the
open star cluster NGC 6231 (Caldwell 76). NGC
6193 is still shedding its swaddling clothes,
which can be seen best in photographs as the part
emission, part reflection nebula NGC 6188. Cold,
opaque clouds of dusty gas make NGC 6193's
neighborhood one of the most visually intriguing
in the Milky Way. In color photographs the area
becomes an elaborate mosaic of cosmic clouds
and starlight, replete with baby blue
82
suns, turbulent black eddies, red walls of glowing
gas, and diaphanous veils of reflected
starlight. Recent studies have shown that the 18light-year-wide
cluster is also on the border of a
vast (310-light-year-wide) expanding cloud of
atomic hydrogen gas. The dark clouds
surrounding NGC 6193 appear to be part of two
molecular complexes, one of which might be
associated with the cluster.
If you live under dark southern skies, finding
NGC 6193 is no chore. It's simply 2½° eastsoutheast
of magnitude-4.5 Epsilon (ε) Normae.
Most catalogs list the cluster's magnitude as 5.2,
about the same as that of M35 in Gemini. But
NGC 6193 is 44 percent smaller than M35, so it's
more condensed and thus easier to see. From
Hawaii I found NGC 6193 obvious to my naked
eye as a magnitude-4.8 "star"; thus my brightness
estimate is 0.4 magnitude (44 percent) greater
than the commonly published value. The
cluster's brightest star, HD 150136, shines at
magnitude 5.7 and is also visible to the unaided
eye. Through 7x35 binoculars the cluster looks
like a bright wedge of Milky Way with a pair of
stars marking its southeastern apex. At 23x an
irregular scattering of suns begins to reveal itself,
though it's hard to determine which belong to
NGC 6193 because the region is so rich with
stars. (Brent Archinal lists only 14 stars as true
cluster members.) The cluster's dominant feature
is a small dipperlike asterism with a short handle
to the east of HD 150136. A 7th-magnitude star
lies about 5' west of the cluster's dominant star,
and a pair of roughly 11th-magnitude suns lie
about 4' due south of that 7th-magnitude sun.
The bright Milky Way wedge dominates the area,
however, and the reflection nebula is
superimposed on it. With a sweeping glance I
noticed that the Milky Way region around that
wedge has a very peculiar shape, and it soon
became apparent that one section of it appears to
follow a long
The Caldwell Objects 329