Object of the Week - October 1, 2017 – IC 5076 aka NGC 6991

[Uwe Glahn]

IC 5076 (LBN 394, Ced 185)

Type: Reflection Nebula
Constellation: Cygnus
RA: 20h 55.5m
DEC: +47° 24.0’
Magnitude: 11.3
Size: 5’x9’

Only 3° northeast of Deneb and so just north of the famous North America Nebula another beautiful but almost unknown or better rare visited nebula can be found.

The reflection nebula IC 5076 was first found on a photographic plate from Isaac Roberts – an English pioneer in astrophotography. With his modern and powerful 20-inch reflector (silvered glass mirror) he found the nebula at September 13, 1895.
But following the star charts an open cluster with the designation of NGC 6991 is also catalogued on the position of IC 5076. Archinal/Hynes refer to two observations from William and John Herschel. This hardly noticeable cluster was also discussed with Steve G., perhaps he can give clarification here. If I understood it right the cluster refers to a 25’ large group of 35 brighter stars east of the 6mag HD 199478.

To go back to IC 5076, only little information and scientific studies are known. The nebula itself seems to be illuminated by the above mentioned bright HD 199478 at the east side of the nebula. This blue supergiant is also known as V2140 Cygni because of its pulsation in brightness of the order of 0.1 magnitudes. The distance is estimated to 5700 light years.

Existing observations of 8-inch to 10-inch suggest the visibility with even smaller aperture. With larger telescopes some structure like a dark structure from the west and the thin ejection to the south could be detected.

DSS: 15'x15' blue
2677

sketch: 27", 172x, NELM 6m5+, Seeing IV
2678


But as always - give it a go and let us know

[Howard B]

Cool object Uwe, and it took me back my curious and confusing notes from 1998 when I wrote about an observation of NGC 6991 (the star cluster) with my old 20 inch Obsession:

"An invisible open cluster! Absolutely in the right spot (IC 5076 and 59 Cygni are where they're supposed to be) - just a smattering of Milky Way stars. 83x."

What's curious is that I didn't make a sketch, or even describe IC 5076 - and why did I mention 59 Cygni instead of HD 199478? Now I'm really interested in re-observing 5076 and 6991 to see what the 28 inch can show me - and this time I'll take more detailed notes and make a sketch. I may have to wait until next summer though because the rainy season is about to begin...

[Ivan Maly]

I have a pretty long writeup from when I was examining this area with 16" and 140x, close to W. Herschel's light gathering and magnification (although my field was 6 times bigger):

NGC 6991. OC N of the prev. object, same distance as to it E of Deneb. Immediately obvious and just framed by the Ethos, although the main concentration of uniform stars of intermediate brightness is much smaller and gravitates from the center of the NGC (it is misplotted in the 1st ed. of Uranometria) toward h2091. I suspect that it is this uniform group of stars that is the h [John Herschel’s object] and it + the few brighter stars around (all Uranometria stars) that is the H [William Herschel’s object]. If I recall correctly, Archinal and Hynes say that the 1st ed. of Uranometria plotted the h object as the NGC. I observe, however, that in the 2nd ed., the h symbol is much smaller than the said star group and positioned at its SW end. There is nothing there visually that could be a distinct object of this size, except the Uranometria star at its exact center. [The symbol on the chart is small and probably not to scale.] More interestingly, the nebulosity IC 5076 around the bright Uranometria star on the E edge of the NGC is clearly visible and actually looks good – it is textured though diffused. There seems to be a connected patch to the WSW beyond the Uranometria symbol; there is no visual suggestion of eccentricity to the E [relative to the “central star”] as in Uranometria. It is muggy, but stars of similar brightness have no halo, and in general the visible nebulosity is very distinct. Very slightly bluish.