HII Region Sh2-112
Star BD +45 3216
This small, oft-neglected HII region in Cygnus is more interesting than at first glance. HII regions are formed when massive and hot Type O and (some) Type B stars ionize the surrounding gas. This ionization and the attendant stellar winds then lead to the further evolution of nearby giant molecular clouds (GMC) that triggers star formation. Although this is a simplistic view, I want to talk about one particular aspect of this process.
As it turns out, this emission nebula is located near the edge of a GMC. This location is important not only for scientists studying stellar evolution, but for us, as imagers.
Imagine for a moment a vast expanse of gas and dust. Within that expanse, near the outer boundary, a Type O or hot Type B star evolves. It "does its thing" and begins ionizing nearby hydrogen. The stellar convection creates shockwaves that propagate outward from the star. Within that expanse the shockwave bumps up against dense gases and dust, forming bok globules and other structures where other stars may be born, but on the very outer margins the intense pressure of the shockwave finds little resistance from the thin, now-ionized boundary layer of the GMC...
And so, the shockwave breaks through into the much less dense Interstellar Medium (ISM) beyond. This is called a "Champagne Flow", the analogy being the pressurized contents of a champagne bottle blowing off a cork and spewing forth the pressurized champagne. The "hole" created in the GMC gives a view into the HII region, which is referred to as a 'blister' and so we end up with "blister HII regions".
The vast majority of emission nebula are actually these blister HII regions. Since star-forming regions can vary in shape and size, have multiple ionizing stars. and gas densities can vary, these various conditions exert quite a dramatic effect on the appearance of a blister-HII region, you just have to be able to adjust your perspective to see them that way...but they all have that blister resulting from a champagne flow from the GMC to the ISM.
From an imaging perspective this means that all these various "flavors" of Champagne Flow Blister nebulae will appear somewhat like a hole blown in a wall that is situated between the ionizing star(s) and us. Sh2-112 is a perfect example of this kind of nebula and is a great example of how understanding our target can better inform our processing. Knowing that we are dealing with a flow and hole, we can be sensitive to the details already inherent in the image to illustrate the 3-D reality of the object. I hope this picture captures that for you.
Incidentally, the ionizing star in S-112 is listed as a double or multiple star (in Simbad) called BD+45 3216. Sources conflict (of course they do) as to its exact classification, but we'll call the main partner a O5V((f))z star (per Simbad):
O = Type 0
5 = Mid-range effective temperature for O stars
V = Main sequence
((f)) = Strong HeII absorption lines, weak NIII emissions
z = oddly strong ionized Helium
These data were taken over four nights last August. I struggled a bit with the image back then (using PS) and set it on the shelf, only to return to it recently and processing it in PI using only global actions.
Thank you for stopping by, having a look and reading my little article.
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If you're interested in a bit of (very) technical reading on this subject, consider these two articles from among the many available:
Star Formation and Evolution of Blister-type H ii Region Sh2-112
https://iopscience.iop.org/article/10.3847/1538-4357/abc42eEvolution of Blister-Type HII Regions in a Magnetized Medium
https://iopscience.iop.org/article/10.1088/0004-637X/745/2/158/pdf********
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