The Caldwell Objects

pressed, stars of magnitude 6 to 11." Why did the
descriptions change, and does the discrepancy
have anything to do with the incompatibility of
the various atlas treatments?
The NGC is based on John Herschel's General
Catalogue of Nebulae (GC, which was published in
1864, and not on William Herschel's original
discovery data (at least not directly). In the GC
John Herschel described NGC 6885 thus:
"Cluster, very bright, very large, rich, little
compressed," with stars ranging from 6th to 11th
magnitude. Wilson suspects that John Herschel
departed from his father's description of NGC
6885 to match what he saw in the sky. Johann
Louis Emil Dreyer retained John Herschel's
description in his 1888 New General Catalogue of
Nebulae and Clusters of Stars (NGC). Neither
William Herschel's description of NGC 6885 nor
the NGC's matches what modern atlases depict
— a tiny cluster of faint stars around 20
Vulpeculae. Also note that William Herschel's
description does not mention the presence of a
6th-magnitude star; that addition was made by
John, who also noted the star's position. When
the position for the 6th-magnitude star is precessed,
we see that he was referring to 20
Vulpeculae. But a survey of John Herschel's
original observing notes reveals that he did not
say the cluster is "centered" on 20 Vulpeculae; he
merely provides a position for the star.
Dreyer undoubtedly used the position of 20
Vulpeculae, however, to represent the "center" of
NGC 6885. But 20 Vulpeculae is probably not the
true heart of NGC 6885. For one thing, a quick
look through a telescope reveals that no obvious
7'-wide cluster is centered on the star. (Of course,
it's easy for anyone to visually force 20
Vulpeculae into the center of a bunch of stars in
this rich Milky Way region, but it isn't immersed
in a compact cluster.) For another, the Hipparcos
satellite has pegged 20
Vulpeculae's distance at 1,140 light-years (plus or
minus 20 percent), most likely making it a
foreground star on the sightline to NGC 6885. In
the September 1978 issue of Sky & Telescope
Walter Scott Houston also placed 20 Vulpeculae
"at the southeast edge" of NGC 6885, not at its
core.
For its part, Volume 3 of the Webb Society
Deep-Sky Observer's Handbook contains no listing
for, or drawing of, NGC 6882, but under the
listing for NGC 6885 contributor Edmund S.
Barker did include this terse note: "Superimposed
on 6882." A further note describes an observation
of NGC 6885 with an 8-inch telescope at 170x:
"the two clusters [lie] 6' apart, the separation
being somewhat arbitrary." The "two clusters"
undoubtedly refer to NGC 6885 and NGC 6882,
but, unfortunately, the book provides no clue as
to where NGC 6882 lies.
Brent Archinal of the U.S. Naval Observatory
has discovered one clue to the NGC 6882-
6885 puzzle, but it raises as many questions as it
answers. In 1930, Robert J. Trumpler cataloged
the two clusters, as Archinal explains:
In the region around 20 Vulpeculae two clus-
terings seem to be superposed: A loose clus-
tering of 20-30 mostly faint stars, about 8' in
diameter, with center at 20h 7m 28s +26° 15.0'
(1900) [the very position (once processed)
given for NGC 6885 in NGC 2000.0] was
identified with NGC 6882. The coarse group of
a few bright stars clustered around 20
Vulpeculae, with a diameter of about 22', was
identified with NGC 6885. In each of these two
clusters a physical relationship of the stars is
indicated by the magnitude spectral class
diagram, but the two clusters are evidently at
different distances.
37
Trumpler went against the grain and made NGC
6882 a small cluster to the northwest of 20
Vulpeculae and NGC 6885 a large cluster around
20 Vulpeculae. The following year Per
The Caldwell Objects 145