This is a region in Cygnus offering not those prominent standard objects we see relatively often here on AB.

I was looking for a region that is rich of Ha structures and dark nebulae.

I ended up with a field of view offering little more than just that...

The data was gathered in June 2023 and the image was presented on 08/29/2023 as the "Astrophoto of the week"  35 or AdW 35 on Astronomie.de

For this AdW, Peter Riepe created a great write up with plenty of details and enriched with lots of scientific reasearch as well.

With his permission you will find a translated version of his text here in English:

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Week 35 - In the dark area of the Swan

Today we enter a supposedly less spectacular area in the constellation Swan. This area is located about 3.5° southeast of the connecting line Gamma Cygni / NGC 6888. But it is not very spectacular only because it is a little noticed part of the sky. And nevertheless our image author Andreas Kempter has picked out an interesting area, that will be shown in a moment. By the way - Andreas Kempter is new in the circle of AdW astrophotographers, so here officially first a warm welcome! The image field measures 2° x 1° 22' and shows a somewhat unusual orientation: North is at 16 o'clock in the lower right corner, East at 13 o'clock in the upper right corner.

Location was Hünstetten in the Taunuskreis, about 40 km northwest of Frankfurt/Main. Two Takahashi TSA 120 with 120 mm aperture were used as telescopes. The focal length of 900 mm was reduced to 630 mm by a reducer. Cameras were (a) a ZWO ASI2600MM (Pro) and (b) a ZWO ASI2600MC (Pro). Exposure was as follows: RGB with the One Shot Camera 184 x 60 s, Hα 43 x 360 s, and 126 x 120 s, and that's 11 h 34 min total. In addition, an explanation of the image author: "The color images were taken in the night from June 13 to June 14, 2023, the Hα images in the following night with a double setup with one Apo each on the Skywatcher AZ EQ 6 GT and the EQ6 R-Pro. The imaging window was between 23:15 and 03:30. A TS 80 mm guiding scope with a focal length of 328 mm and an ASI 120 Mini Mono and the ASI 174MM served as guidescopes in each case. The guiding error at the beginning of the session was a little over 0.55 arc seconds. The filter used was the Astronomik Hα-CCD filter (Ø 36 mm, HWB 6 nm) in a ZWO EFW 7x36. Everything was taken in continuous nautical twilight at gain 100, temperature -15°C, autoguiding with PHD2, acquisition software N.I.N.A., image processing in PixInsight, further processing in Luminar 4." So far the image author´s comment.

Now we go closer into the AdW. Most striking is that there is no dark zone here, but a "foggy landscape" filled with faint emission nebulae. Once again the author of the image: "The goal was to image a region in the Swan, which is away from the prominent targets, but still has interesting Hα- and dark nebula structures. The region was selected in Stellarium. Of interest in this field are the bipolar planetary nebula Sh2-106 and the 'nebular cloud' around the star 44 Cyg."

But now first a "tooth has to be pulled": Sh2-106 is no PN! This was found out by astrophysicists long ago. Sh2-106, the small multi-part nebula on the lower right, is indeed bipolar. But one speaks in the meantime of a HII region. Reason: In the apex of the two bipolar halves sits a very young star of spectral type O9 to B0. It provides with its immensely high UV energy for the excitation and emission of the surrounding bipolar nebula. A PN, on the other hand, always (!) has an ancient, dying star as its center - usually a white dwarf. The central star of Sh2-106 is hidden in dust and gas, it can only be identified by IR images, because the extinction is extremely high with 13.4 mag absorption in visual light. In such IR images (K-band) also a star cluster was found inside the nebula, to which the central star named IRS4 = 2MASS J20272677+3722477 belongs.

So: Star formation in a young HII region. There is an exciting paper by K.-W. Hodapp and J. Rayner: The S106 Star-Forming Region; Astronom. J. 102, p. 1108 (9/1991). They found that Sh2-106 at an assumed distance of 600 parsecs (about 1960 Ly) is interspersed with a protocluster only 2 light-years across. This cluster has about 160 stars and comprises about 140 solar masses. It is estimated to be only 1 to 2 million years old. Shortly after its "birth" the central star IRS4 formed a bipolar jet within the molecular cloud (from which it originated). And this is currently in the process of blowing all gas out of the internal star cluster. Thus a further star formation is prevented.

In the upper right corner of the image sits a bluish nebula surrounding the star 44 Cygni. It measures 11.2' in diameter here in AdW. This nebula with the catalog number GN 20.29.1 is a reflection nebula. If you download the AdW (see below behind the report), you can go into detail. The star is not blue at all! Also the nebula is rather dirty-grey. Cause: 44 Cygni is of spectral type F8, thus has a yellowish-white intrinsic color, which is clearly expressed here in the AdW. Something is very strange: The database Simbad calls GN 20.29.1 a reflection nebula. But at the same time this nebula is cataloged as LBN 076.41-01.45 or LBN 218. And this object, which is congruent with the reflection nebula, Simbad erroneously calls a HII region! But Ms. Lynds clearly pointed out in her LBN that her 10' x 10' LBN 218 glows blue.

It makes no sense to look for catalog numbers for the individual nebular components in this magnificent nebular image. The fainter these nebulae are, the more it becomes obvious that they are interwoven and show no demarcations. Only for one other bright HII region (for which, however, what I just said also applies) did I find a catalog number. This is about 15' below 44 Cygni (in reality northwest) LBN 076.42-01.19 = LBN 219. From there an HII part arcs beyond Sh2-106 to the left to the central lower edge of the image, to rise again from there to the upper left as a double arc. Such structures inspire!

Notes: It has become normal that stars and nebulae are processed separately. That happened here, too. The additional image 1 shows a variant, where the stars were reduced much more and the nebula could be processed separately. This way some fine structures come out better. I (P.R.) may admit that I do not like this variant so much and therefore had asked for the more starry AdW variant. Everybody has there just so his ideas about the naturalness of the represented sky sections. Thanks again to the author of the image! But I also accept that one or the other wants to emphasize certain parts of the image by "image processing methods", which we could do here. And an additional image 2: It shows a detailed view around Sh2-106, strongly magnified from additional image 1, in comparison to an image of Sh2-106 from the deep sky survey (POSS R). Both subimages are enhanced about the same in contrast and brightness. Regarding the rendition of faint HII regions there is not so much difference.

The AdW impresses with its colorful reproduction of faintest nebulae. Exposure time is just the most important thing concerning deep sky objects!!! And also the star colors match well, although this is a "false color" image. Many thanks to Andreas Kempter and congratulations from the AdW team for the astrophoto of the week.

Peter Riepe

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Translated with www.DeepL.com/Translator (free version)