The Caldwell Objects

82
82
NGC 6193
Type: Open Cluster
Con: Ara
RA: 16 h 41.3 m
Dec: -48° 46'
Mag: 5.2; 4.8 (O'Meara)
Diam: 14.0'
Dist: 4,300 light-years
Disc: James Dunlop, included in his catalog of 1827
J. H ERSCHEL: ClusterVIII; consists of
about a dozen stars [of 10th to 11th]
magnitude, and perhaps as many
[fainter], with stragglers, which fill
field. In its preceding [western] part is
a fine double star. . . and yet more
preceding [farther west] is a very large, faint nebula, in
which the preceding [western] part of the cluster is
involved, (h 3642)
GC: Cluster, bright, considerably large, pretty rich,
G ENTLY STIR CREAM INTO COFFEE AND THE
cream will form a spiral pattern before it blends.
Can the spiral structure of a galaxy be viewed in
much the same way? Only up to a point. As
compression waves course through a disk
galaxy's plane, matter piles up at the crests of the
waves. These density enhancements become the
spiral arms as they are curved backward by
radial differences in the galaxy's angular rotation
speed. But the waves don't blend themselves out
of existence, as the coffee-cup analogy would
suggest. In the Milky Way and spirals like it, the
disk's stars, gas, and dust catch up with these socalled
density waves from behind. When a dense
interstellar cloud encounters a density wave, it
compresses, triggering star formation. Things get
really
328
stars of magnitude 10 to 13.
NGC: Cluster, very large, little rich, little compressed, well
resolved, clearly consisting of stars, faint nebula involved.
interesting when the compressed cloud's selfgravity
becomes a dominant force. Once this
happens, the cloud splinters into swirling ribbons,
fingers, and globs of cold, opaque gas that
can manifest themselves as dark animate shapes;
the Horsehead Nebula in Orion and the Eagle
Nebula (Ml6) in Serpens are excellent examples.
As the cloud continues to collapse, sections
squeeze together so tightly that their interiors
start to heat up. Then, suddenly nuclear reactions
fire up within the densest, hottest concentrations.
Radiant energy pours out from each newborn
star. Some of that radiant energy scatters off the
surrounding dust particles, which we can see as
icy blue reflection nebulae. The energy also can
boil away other sections of the cloud, causing it
to glow
Deep-Sky Companions: The Caldwell Objects