The Caldwell Objects

is about 7' in length, and nearly 2' in breadth at
the broadest part, near the southern extremity.
With the sweeping power this appears like a
star with a very faint milky ray south
following, the ray gradually spreading in
breadth from the star, and rounded off at the
broader end. But with a higher power it is not a
star with a ray, but a very faint nebula, and the
star is not involved or connected with it: I
should call it a very faint nebula of a long oval
shape, the smaller end towards the star; this is
easily resolvable into extremely minute points
or stars, but I cannot discover the slightest
indications of attraction or condensation
towards any part of it. I certainly had not the
least suspicion of this object being resolvable
when I discovered it with the sweeping power,
nor even when I examined it a second time; it is
a beautiful object, of a uniform faint light.
To honor Dunlop for his achievements, John
Herschel, on behalf of the Royal Society,
presented him with a gold medal on February 8,
1828. Herschel praised the man for his
observational prowess and his work, describing
the latter as "zealous, active, ready, but above all,
industrious and methodical." Ironically, when
Herschel made his own expedition to survey the
southern skies in 1834, he discovered that about
two-thirds of the objects in Dunlop's catalog
could not be reidentified. " want A of sufficient
light or defining power in the instrument used by
Mr. Dunlop has been the cause of his setting
down objects as nebulae where none really exist."
Of course, NGC 4372 is not one of those
nonexistent objects, as Herschel learned
firsthand.
NGC 4372 is indeed a very open globular
cluster, though, as is the case with its neighbor,
NGC 4833, astronomers do not agree on its size
or magnitude. The Deep Sky Field Guide lists its
magnitude as 7.3 and its diameter as
The Caldwell Objects
105 & 108
18.6'. Robert Burnham Jr. lists its magnitude as 8
and its diameter as 18'. Hartung doesn't offer a
magnitude but measures the cluster out to 10',
and Skiff asserts values of 7.2 and 5', respectivel
Archinal summarizes the problem:
This cluster is open enough, and has a bright
enough star in the field, that it's no wonder the
diameters and total magnitudes are discordant.
The Skiff value may correctly indicate that the
25 magnitude per square arcsecond profile [a
standard surface-brightness threshold] lies at a
diameter of 5', but this does not seem to
represent the overall size of the cluster very
well. The half light diameter of 7.79' matches
the obvious size of the cluster on the Digital
Sky Survey, and there are outliers to at least
10'. On the other hand, the Deep Sky Field Guide
size of 18.6' would appear to be far too large.
So as usual with deep-sky information (as
in the cases with NGC 4833 and 4372), it
apparently just depends on what your defi-
nition of "diameter" and "total magnitude" is.
The diameter is going to depend quite a lot on
the limiting magnitude and how one
determines where the cluster merges into the
background stars. And then given various
diameters, of course the total magnitude
enclosed varies quite a bit. So there appears to
be no definitive sizes and magni-
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