The Caldwell Objects

105 & 108
southeast, like the wings of a butterfly (thus my
nickname for the cluster). The "butterfly" appears
to be resting in a field of stars, which may or may
not be related to the cluster. The drawing below
reflects the view through the 4½-inch finderscope
(at powers of 19x, 75x, and 150x) and through the
Auckland Observatory 20-inch reflector.
The cluster's fainter, roughly 5'-wide outer
halo was not noticeable as such in the 20-inch.
Perhaps if I had seen the cluster at low power
from a dark site, with no Moon, I would have
realized that there was more to the cluster than
first met my eye. In any case, the outer shell is
obvious in photographs, though it is hard to tell
where the cluster ends and the star fields of the
southern Milky Way begin. (No wonder
astronomers have had trouble determining the
cluster's angular size and total apparent
magnitude.)
NGC 4372 is another extremely metal-poor
globular cluster. Its 7th-magnitude form lies 40'
southwest of 4th-magnitude Gamma (γ) Muscae
and 5' southeast of a 6.6-magni-tude star. The
cluster also lies about ¼° due east of the southern
tip of a narrow, 3°-long river of obscuring matter
called the Dark Doodad. The Doodad starts just
1¼° due south
420
of Alpha Muscae and flows southwest from
there. (The presence of this dark cloud also
suggests that the cluster is being dimmed by
obscuring matter, and indeed, Harris's data
suggest that the cluster loses a full magnitude of
its brightness to interstellar extinction.) Telescope
users under dark skies should try to sail "down"
this river, because it is almost free of bright
starlight, though two roughly 9th-magnitude
stars (separated by about 15') lie nearly at the
halfway mark. (The brighter of
these two suns is 168 light-years distant). Can
you see the Dark Doodad with binoculars? With
the naked eye?
The 19th-century "gentleman scientist" James
Dunlop discovered NGC 4372 while surveying
the southern skies from Australia with a 9-inch
f/12 reflector. It is the 67th of the 629 objects he
recorded in A Catalogue of Nebulae and Clusters of
Stars in the Southern Hemisphere, Observed at
Parramatta in New South Wales. Here is his
description:
A star of the 6th magnitude, with a beautiful
well-defined milky ray proceeding from it
south following [to the southeast]; the ray is
conical, and the star appears in the point of the
cone, and the broad or south following
extremity is circular, or rounded off. The ray
Deep-Sky Companions: The Caldwell Objects