NGC 6822 in Sagittarius
NGC 6822 in the north-eastern corner of Sagittarius is an irregular dwarf galaxy of our Local Group at approximately two thirds the distance to M31, the Andromeda Galaxy. The dwarf galaxy does not display distinct structures as for instance spiral arms. It resembles a bit the Magellanic Clouds, but is smaller and appears in part due to its distance much dimmer.
NGC 6822 was discovered in 1884 by Barnard using a 5 inch refractor and was described as an extremely dim nebula. 1925, Edwin Hubble examined the galaxy more in detail and described several diffuse emission nebula, huge HII regions, within NGC 6822. Furthermore, he investigated several variable stars, Cepheids, by which he was able to determine the distance of the galaxy.
From our northern latitude, a visual observation of Barnard's Galaxy is always hampered by its low altitude in Sagittarius in the only short summer nights. The low surface brightness of the galaxy and the star-rich environment at the edge of the summer Milky Way further adds to its difficulty. Besides a dark sky, a night with exceptional transparency down to the horizon are requirements for a successful observation. Given that these conditions are met, the central part of the galaxy can be glimpsed as a weak glow that is slightly elongated and that extends over around 10 arc minutes. As Barnard discovered this galaxy with a 5 inch telescope, a large aperture telescope is not a strict requirement for a successful observation. The sky conditions are much more important.
I had observed this galaxy during two nights in June and July 2006 with very good transparency in the Black Forest nearby Freiburg using my 22 inch Dob at 150x and 250x, corresponding to 3.7 and 2.2 mm exit pupil. The galaxy appeared as a weak, but distinctly brighter patch that was elongated in direction north-south. Orientation and size could be compared with a DSS print of the galaxy. Using field sweeping made it easier to distinguish the diffuse brightening of the galaxy. Interestingly, the glow of the galaxy did not appear to be smooth, but slightly "noisy". The brightest stars in NGC 6822 reach mag 15 to 16 and the grainy impression could be due to the brightest start close to being resolved.
Even without nebula filter, several of the bright HII regions in Barnard's Galaxy can be discerned, which are grouped in the halo around the central bar of the galaxy. In particular at the northern edge of the galaxy extending toward west is a group of HII regions, that can be clearly identified using a narrow band filter. Several of these HII regions appear more prominent than the galaxy itself, which lead historically to a series of misunderstandings about the nature of NGC 6822. With some of the HII regions, intrinsic detail can be seen at higher magnification. 10 putative HII regions had bee catalogued by Hubble in his classical paper of 1925 and bear Roman numerals, Hubble I to Hubble X.
Barnard's Galaxy is clearly an exceptional galaxy and should be on any summer observing list. By the way, nearby is NGC 6818, a nice planetary with a bright disk.
(click for larger image)
Images: UK Schmidt Survey, Equatorial Red Plates (DSS copyright). The Roman numerals indicate several of the HII regions catalogued by Hubble.
Barnard's Galaxy, NGC 6822, RA 19h 45, DEK -14°48'
Edwin Hubble's original paper (Hubble E (1925) NGC 6822, a remote stellar system. Astrophysical Journal 62:409)
Hodge et al. 1988PASP..100..917H
A nice article about Barnard's Galaxy by Rich Jakiel is here, together with more data to the HII regions in NGC 6822
Another observing report by Peter Surma with more background information is here
Image by Leonardo Orazi with further HII regions.