Barnard's Galaxy or NGC 6822 is a small irregular galaxy only 14 degrees east of the galactic plane in the constellation of Sagittarius. A member of the Local Group of Galaxies, it is quite similar to the Small Magellanic Cloud in composition and structure. Though it has a fairly low surface brightness, NGC 6822 is one of the easiest galaxies to resolve. Numerous H II regions and bright supergiant stars can be resolved with only a 20cm telescope equipped with a CCD camera. Several large H II regions and the brightest stars are visible in large aperture telescopes under dark skies.
Today the galaxy is under intense scrutiny and has been the subject of numerous papers over the last thirty years. However the past observational history has been much more colorful. The low surface brightness of the galaxy itself combined with the relatively bright H II regions has lead to a great deal of confusion over its identity during the late 19th to early 20th centuries. The unique observational history of Barnard's Galaxy will be covered in much more detail in the next section of the paper.
1) Early Work
The early observations of NGC 6822 are quite interesting and rather confused, representative of conditions under which the object was observed. NGC 6822 is a large, low surface brightness object and is best viewed at low magnification for maximum contrast of field. Barnard's early observations certainly reflect the challenging nature of this object. Barnard first observed the galaxy on August 17, 1884 with a 5-inch refractor. He described the object as.. "an excessively faint nebula" and "it is some 2' in diameter, and is very diffuse and even in its light" (Barnard, 1884). He later determined its position with a 6-inch equatorial and noted it was "seen with difficulty", while in the 5-inch at 30x it was "quite distinct". In 1885, he examined it again with a 6-inch refractor and a "comet" eyepiece. The wide field boosted the visibility of the galaxy, and Barnard made a most unusual observation (Hubble, 1925):
It certainly seems much larger and much denser than last year and I certainly think it has increased in density and size since that time...If it had always been as large and bright as now, I cannot conceive how it could have been missed by observers when examining G.C. 4510 (the bright planetary nebula NGC 6818, which is about 40' north preceding). Probably this is a variable nebula.
The problems associated with observing NGC 6822 will plague many observers over the next 40 years. Hubble in 1925 writes: NGC 6822 is fairly conspicuous in a short 4-inch finder...but is barely discernible at the primary focus of the 100-inch (Hubble, 1925; Burnham, 1977). Large refractors with their restrictive field of view were particularly ill-suited for the study of this low contrast object. The 26-inch at Leander McCormick Observatory completely missed the galaxy, instead the two brightest H II regions located on the north end of the galaxy were observed. The brighter of the two diffuse nebulae was assumed to be NGC 6822, while the fainter received the designation of IC 1308 (Hubble, 1925).
Even photography did not quickly resolve the discrepancies between Barnard's original observations and those made with much larger telescopes. Max Wolf at the Heidelberg Observatory photographed the region in 1906 and 1907. Wolf, using the 16-inch Bruce Camera and the 28.5-inch Walz Reflector identified the two largest diffuse nebulae as NGC 6822 and IC 1308. Wolf also noted a dense region of small clusters and nebulae (Wolf, 1907; Hubble, 1925). The "new" object was actually the main bar of the galaxy, now well resolved into stars and nebulae. It received the designation of IC 4895, and once again Barnard's original observations were overlooked. It took 15 years before Perrine (1922) of the Cordoba Observatory, Argentina, finally resolved the discrepancies in identification and interpretation. Perrine correctly identified that the diffuse nebulae and Wolf's group of small clusters were all part of the same system. He was the first to recognize its true nature as an extragalactic system. Perrine properly classified NGC 6822 as a Magellenic Cloud-type object. This helped cleared the way for Edwin Hubble's groundbreaking work in 1925.
2) Hubble's 1925 Paper
Edwin Hubble's 1925 paper: N.G.C. 6822, A Remote Stellar System remains as one of the great classical studies of the early 20th century. The galaxy's resemblance to the Magellenic Clouds led to intensive study by the large reflectors of the time. He conducted a detailed survey of the structure and stellar population of this system. Hubble discovered 15 variable stars (11 Cepheids), measured the distribution of stars down to magnitude 19.4, and described five "diffuse nebulae" (giant H II regions) plus determine the absolute magnitude of the entire system.
Hubble's discovery of eleven Cepheid variables was one of the most important milestones in galactic astronomy. Using the Cepheid Period-Luminosity relationship, Barnard's Galaxy became the first system beyond the Magellenic Clouds to have its distance determined (Hubble will soon follow suit with M31 and M33). He derived a distance of 214,000 parsecs, or over 700,000 light years, far beyond Harlow Shapley's 300,000 light years as the size of known universe. In one paper, Hubble resolved the "Great Debate" of 1920 between Curtis and Shapley over the scale of the universe and the nature of the "spiral nebulae". It soon became clear that all spiral nebulae were galaxies far beyond our own.
1) The Structure - Modern Studies
After Hubble's 1925 paper, detailed research on NGC 6822 stagnated for nearly 50 years. Starting in the late 1960's through the present, considerable research has been conducted on the structure and content of this galaxy. Some of the most important properties of NGC 6822 are summarized in Table One.
Table One: General Properties of NGC 6822
|Position (center of bar)|| (2000) 19hr 44.9m
(2000) -14o 48'
|Hirshfeld and Sinnott (1985)|
|Galaxy Type||IB(s)m||Luginbuhl and Skiff (1989)|
|Size||Overall: 10.2' by 9.5'
Central Bar: 8' by 3'
|Luginbuhl and Skiff (1989)|
|Magnitude||Vtot = 8.18; Btot = 8.97
B = 9.3
Luginbuhl and Skiff (1989)
|Absolute Magnitude||Mv = -16.4||Hodge (1994)|
(1.7x 106 light years)
|Number of OB Associations||16||Hodge (1977)|
|Number of H II Regions||157||Hodge et al (1988)|
|Stars brighter than 18th mag.||363; Mv - 6.5||Hodge et al. (1991)|
|Alternative Designation||DDO 209||Hodge (1994)|
Overall Structure: Barnard's Galaxy consists of a "bright"
central bar embedded in a broad oval halo of lower surface brightness.
The bar has a position angle of 10o 3o
and a mean ellipticity of 0.47 0.18 (Hodge, 1977). There are
two main extensions of luminosity off the ends of the central
bar. The brightest extends towards the west off the north end.
It includes one of the brightest H II regions (= Hubble V) plus
two bright OB associations (Hodge, 1977). The other extension
sweeps to the southeast off the southern portion of the bar. It
is much larger, though fainter and lacks the large, bright H II
regions associated with the northern end of the galaxy. A large
stellar association is located near the southeastern terminus.
The bar is surrounded by a broad outer halo. Based on photographic
photometry, the halo is much larger than most published dimensions,
measuring at least ~ 15' by 10'. Several OB associations and the
bright H II regions of Hubble I+III and II are located in the
northwest section of the halo.
Stars and Stellar Associations: The open structure and relative closeness of NGC 6822 allow for easy resolution into stars. Hodge et al. (1991) conducted a survey of deep Schmidt plates in four colors to determine the stellar population of the galaxy. After removing foreground stars from our galaxy, they found over 360 stars brighter than Mv = - 6.5, or magnitude 18.0. Over one hundred stars have magnitudes in the range of 13.5 to 16.5, well within range of larger amateur telescopes.
Most of the brighter stars are concentrated in the northern portion
of the bar and in the large OB associations. Hodge (1977) described
16 separate OB associations. These are located along the bar and
the two main extensions, plus in other regions of higher star
density. Characteristics of the brighter OB associations are outlined
in Table 2. Many of the larger associations are also integrated
into giant H II regions. All five Hubble H II regions incorporate
a large OB association (Table 2).
Table 2: The Larger OB Associations of NGC 6822
|Hodge Number||Dimensions (")||H II Region|
|3||45 x 32||Hodge 4
|5||99 x 64|
|8||35 x 22||Hodge 9,11|
|9||80 x 67|
|11||64 x 25|
|12||96 x 80|
|13||38 x 32||Hodge 14|
|15||102 x 67|
H II Regions: The H II regions are perhaps the most thoroughly studied component of the galaxy. Hubble (1925) studied the five brightest regions, two of which had been discovered over 30 years earlier and misclassified as IC 1308 and NGC 6822. He was the first to determine their spectral characteristics, luminousities and dimensions. In 1977, Hodge increased the number of known H II regions to 16. Today over 150 are catalogued. Most of these objects are very small and faint and not observable in amateur telescopes. Table 3 summarizes the five brightest objects that are visible large aperture amateur telescopes. Another table (Table 4) summarizing the properties of a few faint nebulae of Killen and Dufour (1982) can be found in appendix 2.
Table 3: The Brightest H II Regions of NGC 6822
(Data from Hubble 1925; Hodge 1977; Hodge et al., 1988)
|Hubble No.||Mag (V)||Size (")||RA(2000)||DEC(2000)||Hodge No.|
|I||~15.01||48 x 32||19hr 44.5'||- 14o 42.1'||2|
|II||16.02||29||19hr 44.6'||- 14o 44.1'||3|
|III||~14.51||53||19hr 44.6'||- 14o 41.7'||4|
|V||~ 14.03||32 x 22||19hr 44.9'||- 14o 43.3'||9|
|X (= IC 1308)||~ 14.03||22||19hr 45.1'||- 14o 43.0'||14|
_1 Visual estimate by the author
_2 Rough estimatebased on H flux from Hodge (1977), flux of Hubble II compared with those of Hubble V and X.
_3 Visual estimates of the author and Steve Gottlieb
Table 4: Other H II Regions In NGC 6822
(After Killen and Dufour, 1982)
2) Making Visual Observations of Barnard's Galaxy
Gradually over the past thirty years Barnard's Galaxy has emerged from relative obscurity to become a target for amateur telescopes. Although not in any popular list of deepsky objects, several good observing handbooks have fairly detailed descriptions of the galaxy. Two Webb Society Deep-Sky Observer's Handbooks (DSOH): Galaxies (Vol. 4) and The Southern Sky (Vol. 7) contains observations of the galaxy, while Galaxies has a detailed chart in appendix. Other good descriptions can be found in Hartung's 1968 classic: Astronomical Objects for Southern Telescopes and Luginbuhl and Skiff (1989) Observing Handbook and Catalogue of Deep-Sky Objects. These provide good, general descriptions of the galaxy and the most prominent features.
In the last section of this paper, I will review some of the
best observations from these guidebooks, plus submissions from
several experienced observers of the Webb Society. I will also
include some of my own observations whenever appropriate. Instruments
used will range from the humble binocular up to large telescopes
of the 50cm class.
One of the biggest surprises I had uncovered was that Barnard's Galaxy is a "binocular object" and a fairly easy one at that. Jay Reynolds Freeman, an experienced observer from the "Bay Area" of California, USA was describing his observations of the galaxy on the Usenet forum sci.astro.amateur. He was using 11 x 80mm and 10 x 50mm binoculars. I was amazed, as it had never occurred to me that NGC 6822 was visible in anything smaller than a 4-inch richfield telescope.
I then contacted Brian Skiff inquiring if he had made similar
observations. He wrote that he was able to detect the galaxy
with only a 7 x 35mm binoculars, while a larger 10 x 50mm model
easily revealed a faint smudge elongated north-south. These observations
were carried out under fairly dark (visual magnitude ~ 6.5) skies.
The relatively bright visual magnitude combined with large size
make this galaxy a good challenge for binoculars. The sharp, high-contrast
image in good quality binoculars has greatly expanded the "list"
of viewable deep-sky objects.
Small, short focal length "richfield" telescopes are
often the instrument of choice for general study of the galaxy.
Wide, low power fields combined with sharp optics are ideal in
revealing the low contrast bar and its orientation. With my 15cm
f/5 reflector at 29x and 43x (Lm (visual) ~ 5.5 in
the area, 6.5 at zenith), I could easily see the a broad, diffuse
oval glow gradually brightening in the area of the central bar.
The galaxy was aligned nearly north-south, and set in a relatively
rich star field. Some slight mottling was visible, but no other
structural details were apparent. Observations with an 8 1/2-inch
in the DSOH #4 and #7 reveal few details other than:..still
very faint but seems to be comprised of 3 distinct but connected
regions. Observations of the various structures in NGC 6822
requires both larger aperture and considerably higher magnifications.
Moderate to Large Aperture Telescopes:
To fully appreciate all the structural diversity that Barnard's
Galaxy has to offer, moderate to large sized (> 30 cm) telescopes
under dark skies are best. I have gathered observations from several
well-known deep-sky observers and have included a few of my own
observations. Jeff Corder, Steve Gottlieb, Tom Polakis and Brian
Skiff have made detailed observations of the H II regions and
the "bar" of the galaxy. My observations are with the
Atlanta Astronomy Club's 50cm f/4.5, and include observations
of the bar, H II regions and a couple OB associations. Of all
the structures in the galaxy, the H II regions are the brightest
and most easily visible.
Observations of the Largest H II Regions:
Hubble I (= Hodge2)
50cm: 175x with UHC filter. A very faint, diffuse patch about
30" in diameter located just NW of larger Hubble 3. Estimated
visual magnitude ~ 15.0.
Hubble II (= Hodge 3) Not yet observed...
Hubble III (= Hodge 4)
33cm: ..a round, 30" diffuse patch around a tiny clump of
stars, not responding at all to the O III filter.
Tom Polakis (1991).
50cm: 175,260x. The largest H II region appears as a low surface
brightness object nearly 1' in diameter. In the center lies a
small group of faint stars (~ 16 mag), this is the OB association
Hubble V (= Hodge 9)
30cm ..very faint and slightly oblate,...north edge the brightest
J. Corder (1987), in DSOH #7 ..The Southern Sky
33cm ..is brighter than Hubble X, and shows some brightening in
Tom Polakis (1991).
44cm ..At 82x and O III filter appears as a very small knot, round
approximately mag 14. A mag 12 star lies 2' SE. Not seen unfiltered.
Steve Gottlieb (personal communication)
50cm..At 175, 260x. An irregular oval, ~ 20" by 15"
that is brightest in the core. The moderate surface brightness
of the nebula is greater than the galaxy itself and allows for
observation without filtration. The UHC filter increased the contrast
and delineated the shape more distinctly.
Hubble X (= IC 1308, Hodge 14).
33cm ..blinks well with the O III filter and showed a round, uniform
disk 15" across at 310x.
Tom Polakis (1992)
44cm ..At 82x and O III filter appears as a very small but clearly
nebulous round knot. Estimated magnitude is 14.
50cm ..At 175, 260x. a diffuse oval patch ~ 20" in diameter,
somewhat fainter than Hubble V. Responds well to the UHC filter.
A modest elongation in P.A. ~ 30 degrees.
Observations of the "Bar"
The central bar of the galaxy is easy to observe, but descriptions of structural details have been elusive. Broad, low contrast variations have been noted but little else have been recorded. Most of the galaxy's brightest stars are concentrated along the bar and there have been several convincing observations of partial resolution. Of the probable +100 hundred galaxy member stars brighter than magnitude 16.5, most are concentrated in the northern end of the bar (Hodge et al., 1991). Some interesting observations of resolution are given here:
25cm. "Many field stars are superimposed on an indefinite
haze of faint stellarings near the threshold of vision.".
Luginbuhl and Skiff (1989)
30cm " The field is rich with faint stars particularly to
Luginbuhl and Skiff (1989)
50cm At 175, 260x. Partial resolution into a swarm of faint stars
of ~ 15 magnitude and fainter; these are mostly concentrated in
the northern end of the central bar. Some obvious clumping visible,
including OB Association Hodge 9 was visible as a diffuse
patch with a few scattered ~16 mag. stars.
Obviously, the key to detailed study of this elusive object is to maximize light gathering efficiency and higher magnification. Low magnification is all too often stressed and although it will reveal the general shape of the galaxy, small structures as the H II regions may be overlooked. Don't be afraid to use higher magnification on the order of 3 to 5x /cm or more! The galaxy may actually fade from view at higher magnifications. Generally, the small, higher surface brightness features will be better seen. Suggested minimum size for detailed study is at least 30 cm, and 40cm or more is recommended for partial resolution of the bar and OB associations.
Many of the fainter H II regions have yet to be observed (Table 4) along with the brighter OB associations. There are also four open clusters discovered by Hubble that have photometric magnitudes brighter than 17.5 (Hodge, 1977). These objects would make an interesting challenge for owners of very large telescopes, or those equipped with CCD setups. Even a modest telescope equipped with a good CCD can easily resolve this galaxy into stars making it practical for nova patrols to be conducted. So after hundred years of observation, Barnard's Galaxy remains an interesting challenge for amateurs and professionals alike.