Sharpless 2-276 (Barnard's Loop)

CCD Image by Steve Mandel

Observing Barnard's Loop

-by Dave Riddle

The part of the Loop north of the Belt of Orion is a well known "H-beta" object. My previous attempts to follow the nebula southward with an H-beta filter revealed nothing and I had pretty much written off the object as "photographic only." When I spotted a large bright glow _without_ the filter in the position of the Southern Loop, I initially was puzzled by what I was seeing and thought it must be one of the weakly illuminated dark nebula that mottle this area of Orion. A follow up observation made this past Saturday evening revealed this bright area to be the southern extension of the Loop and I could trace the entire nebula that curves westward near 53 Orionis (Kappa) and ends just eastward of Rigel. I found the "curve" west of Kappa particularly well defined in the Pronto at 16X with an UHC filter. A broad diffused glow runs north to connect with the part of the Loop that is plotted in the Uranometria. Westward of the curve is a thin wash of nebulosity that terminates in the position of the "nonexistent" nebulae NGC 1927 and NGC 1909.

Did Herschel see the western end of the Loop and assign it two NGC numbers? He suspected the _northern_ end of the Loop during his sweeps and commented "effected with milky nebulosity" and noted it as "area 27" in his list of suspected nebulous regions and it appears he saw the extreme western end of the Loop also. I found "NGC 1909" to been easily visible in all of the small refractors. My 18" (20 mm Nagler with a broadband filter) revealed a weak amorphous glow about a degree across oriented NE to SW. I suspected a much fainter halo in the 5" at 35X. I contend this is the "real" NGC 1909 and it is at the position Herschel noted ( 5 25 54 -8 08 {2000.0}). The question of NGC 1927 is more difficult. I suspected an exceeding faint glow extending south of the double star GSC 5322:1833 with the 18". Alex Langoussis' 24" reflector made the object a bit easier -- appearing as a very faint fan extending about 18 arc minutes southward of the double. The south end of this nebula appeared very slightly brighter at 84X with an UHC and H-beta filter.

How many amateurs are aware that Barnard's Loop has an optical counterpart that lies about 28 degrees west of Sh2-276? One theory concerning this western arc (the optical nebula associated with the "Ori-Eri Bubble") is the western arc expanded at a greater rate than the eastern arc (the Loop) due to the absence of interstellar matter -- Barnard's Loop being slowed in expansion by colliding with interstellar material. The Eridanus arc was catalogued by Sharpless as Sh2-245 and Lynds noted it as LBN 835 and 836. I found this arc to be a surprisingly easy target in my 18" at 58X (40 mm Pentax eyepiece) with an [OIII] filter with a true field a bit over one degree. This arc appeared as a nicely contrasting "stain" about one degree long and 15 arc minutes wide running north to south. I didn't take the time to explore beyond this area although it is possible many more parts of the Eridanus Bubble are visible -- the dimensions of Sh2-245 are 30 by 600 arc minutes! The area I observed is LBN 836 at 4 02 +3 48 (2000).

To sum it up, the entire arc of Barnard's Loop is visible in surprisingly small apertures. While the part north of the Belt of Orion responds to a H-beta filter, the southern arc can be seen without a filter. Apparently, the emission characteristics change over the course of the Loop. NGC 1909 certainly is a real object and visible in a 70 mm refractor and this nebula appears to be associated with the great arc of Barnard's Loop. The Eridanus Bubble has spots of nebulosity that aren't difficult objects as seen from a dark site. Although it required a few evenings to observe and map this cosmic bubble, I received a great deal of satisfaction making the observations.