NGC 6822 - Barnard's Galaxy
"Barnard's Galaxy" carries the catalog number NGC 6822 and is an irregular dwarf galaxy about 7,000 to 8,000 light-years across in the constellation
Sagittarius. It was found in August 1884 by Edward Emerson Barnard with a 6" refractor - and received his name. With an apparent brightness of 8.7mag
it is one of the 50 brightest galaxies in the sky, but due to its large size of 15.4'x14.2' arcminutes it has a very low surface brightness of only 14.7mag/'²
and is therefore difficult to observe.
With a distance of 1.6 million lightyears it is even closer than the Andromeda galaxy, but of course it is much smaller and contains only about 10 million
stars - no comparison with the up to 300 billion stars of our own Milky Way. Accordingly low is the absolute magnitude, which is given with M= -16.4mag.
This corresponds to a luminosity of only 310 million suns.
Its structure is similar to that of the Small Magellanic Cloud (SMC) in the southern sky. There are 150 HII regions in NGC 6822, four of them are very
bright. Star formation takes place in areas with dense molecular gas clouds, which are so cold that they collapse under their own gravity and form stars.
A period of active star formation occurred about 3-5 billion years ago, which roughly coincides with the time when NGC 6822 had a close encounter with
our Milky Way.
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When observing, do not be fooled by the total visual brightness of 8.7mag, which sounds like a lot. However, the total brightness is distributed over a very
large area, so NGC 6822 has only a low surface brightness of less than 14mag/'². In addition, the declination is -15°, so that NGC 6822 rises only about 25°
above the horizon here in the south of Germany. So a dark sky and a low magnification are important to have a good view of Barnard's galaxy.
In my 20" f/3 Dobsonian, NGC 6822 shows up as a pale and quite large brightening in the eyepiece at a magnification of 120x. The northern part of the
oval galaxy seems to be slightly brighter than the southern part. Reason for this is a higher stellar density.
It gets interesting when you mount an UHC filter in the eyepiece to observe the many HII regions. I first concentrated on the four brightest ones, which are
all located in the northern part of the galaxy (at the top of the drawing). The three brightest of them were easily seen with averted vision and at higher
magnifications even a star-like condensation was visible in one HII region (V). The fourth and largest HII region (III) was a little bit shy and only visible as
a very faint, extended brightening with an UHC filter.
