The Caldwell Objects

57
Barnard's remarks obviously refer to the cluster as a
whole and furnish the description in the [original]
N.G.C. — ["very faint, large, extended, diffused."]
During the next year, 1887, the region was examined
with the 26-inch refractor at Leander McCormick
Observatory. The relatively high magnification and
restricted field conspired to render the cluster
inconspicuous. It was missed entirely by the observer.
The two brightest of the small involved nebulae,
however, were seen and measured. Curiously enough,
they were both attributed to Barnard and the
discrepancy with the N.G.C. description was
overlooked. When the First Index Catalogue was
compiled, the brighter of these two objects was
assumed to be N.G.C. 6822, although the description in
the N.G.C. was not amended, and the fainter object was
listed as a new nebula, "I.C. 1308, eF, eS, IE, gbM, 6822 ρ
12°" [excessively faint, excessively small, little extended,
gradually brighter in the middle, NGC 6822 preceding
toward position angle 12°]. H. A. Howe, with a 20-inch
refractor, found only the brighter of these two small
nebulae and remarked that N.G.C. 6822 was "very
small" instead of "large," as described by Barnard."
The first photographs were made in 1906 and 1907 by
Wolf at Heidelberg with the 16-inch Bruce camera and
the 28.5-inch Walz reflector. He identified the two
small nebulae as N.G.C. 6822 and I.C. 1308, and
announced the cluster, Barnard's original discovery, as
a new object which was duly listed in the Second Index
Catalogue as "I.C. 4895, group of neb., 25' diam." Wolf
described the cluster as an exceptionally dense
Nebelfleckhaufen, or cluster of small nebulae, similar to
those more open clusters to whose study he has
contributed so largely. This remarkable interpretation
existed for fifteen years unchallenged and apparently
unnoticed.
Note that Hubble described the galaxy as a
"cluster."
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In modern-day photographs NGC 6822 is
reminiscent of the Large Magellanic Cloud, but
its metallicity is more similar to that of the Small
Magellanic Cloud. The galaxy seems to lack
symmetry, and it has been classified as an
irregular dwarf system. A prominent bar runs
along its major axis. At the northern end a few
clouds of glowing gas can be seen, while bright,
bluish stars scatter out into a straggling arm on
the other end. The distribution of the galaxy's
neutral (cold atomic) hydrogen is rather complex
and clumpy; the gas, "seen" by radio telescopes,
appears to form a rotating disk. There is also
evidence for some expanding shells. Indeed,
Barnard's Galaxy contains many Wolf-Rayet stars
— very hot, luminous suns with expanding
atmospheres and intense stellar winds. Small
groups of such stars have been found at the
centers of the larger nebulous bubbles in NGC
6822. These larger nebulous structures are most
likely the result of Wolf-Rayet winds colliding
with the galaxy's more or less stationary
interstellar medium. At each collisional interface
the interstellar medium glows, producing a thin,
luminous bubble.
Recent Hubble Space Telescope observations of
individual stars in NGC 6822 reveal that star
formation probably began there about 12 to 15
billion years ago. The star-formation rate has
been close to constant (or declining) in the last
few billion years, though episodes of enhanced
star formation have occurred across the galaxy's
body in the last 100 to 200 million years. The
strength of this enhancement has been somewhat
different from one region of the galaxy to
another, being higher in the bar region than in
the galaxy's outskirts, and higher still along the
bar's northern and southern edges than in its
center. In a 1999 paper in the Publications of the
Astronomical Society of the Pacific, John B.
Hutchings (Dominion Astro-physical
Observatory, Canada) used HST data
Deep-Sky Companions: The Caldwell Objects