M 42 (OSC, HDR, and all those other acronyms)
The Great Nebula in Orion, NGC 1976
OTA: RASA 11 f/2.2
Camera: QHY268 OSC
Observatory: Insight Observatory (AFIL 22)
Date of Capture:
Date of Processing:
Exposures:
OSC: 24 x 30 sec
OSC: 22 x 60 sec
OSC: 20 x 90 sec
OSC: 18 x 120 sec
Total Exposure time: 1.7 hours (equivalent to 21hrs at f/8)
Image Radius: 1.28 degrees
Processing Tools:
1. Commercial: PixInsight, Topaz, Photo Director 365, Luminar Neo
2. Pixinsight Addons: NoiseXTerminator, BlurXTerminator, StarXTerminator
3. My Scripts: NB_Assistant, AC_Restar, Subframe Weighting Tool (Excel w/ J. Hunt), ColorTweaker, StarTweaker
M 42 Description:
In the simplest view, M 42 displays three zones moving outward from the star-formation area of the Trapezium. The young stars of the Trapezium (the stars may be as young as 10K years), like young stars everywhere, are wont to do, emit intense solar winds when they first ignite and intense UV radiation as they begin life. These stages can affect the parental nebula in at least two ways. First, there is the dynamic effect. The strong winds blow the proximal nebula away from the star nursery. The second is "photoevaporation," where the UV ionizes and pushes the proximal gases outward. Commonly, young stars are buried from view in their nebulae, but in M 42, these two processes have pushed away enough nebular gas and dust for us to see these young stars. This affected area immediately around the young stars is the "Kleinmann–Low Nebula" (or "Orion KL Nebula"). In this region, "Hot stellar winds circulate off large, young, stars in Orion's nebula and heat the surrounding gas." (5) I presume from this description that the relatively neutral tone of this inner area of M 42 results, at least in part, from black-body radiation peaking at a wavelength near green (which we call "white hot.")
The following two regions, in radial order, are the blue region, caused by OIII radiation, a red region, caused by Ha radiation, and then another blue region, again, probably caused by OIII radiation? These nested ionization regions have been likened to a Strömgren Sphere. (1,2,3), and the inner blue plus red regions probably have that origin. The structure in the outer blue region suggests it is heated and ionized by shock fronts from the expansion of the nebula within.
The Strömgren Spheres model presumes the young stars reside at the center of concentric nebulae (
https://www.astrobin.com/29jims/). Where the star(s) provoking the ionization is (are) at the edge of the nebula, as is the case for M 42, the surrounding nebulae have been referred to as "blister" nebulae (1,4): "…a star ionizes surrounding gas, but the star is at the surface or edge of a giant molecular cloud (GMC [Giant Molecular Cloud]), rather than at the center of it. As a result, ionized gas breaks out of the GMC, like a popping blister, which in turn can prompt “champagne flows” [whatever that is] of ionized gas leaching into the surrounding medium." (1) The modeling of the M 42 has suggested it is a planar nebula that we view face-on. (4)
Please notice in this image how the wisps of ionized OIII (blue), which obviously lie exterior to the red (Ha) in the nebula's center, also lie above it—between the nebula and us. Does this counter the argument that the nebula is a planar? The apparently more prevalent OIII nebula around the circumference of the nebula could be a limb effect similar to that seen when observing the sun but the result of light traveling through a medium with less uniform density.
Processing Description:
I rarely process one-shot color images and even rarely process HDR exposures—but here it is. As with most images of the Orion Nebula, getting the Trapezium to show is a big challenge. Supposedly, that is where high-dynamic range (HDR) processing comes in. And applying the technique to the unstretched stacks (using PI's process) worked well. But then stretching occurs, and the Trapezium disappears into the blend of bright clouds. Anything above a luminance of 9.3 or so looks all the same to me, and in utilizing the full zero-one range of brightness, most stretching processes dump both the cloud and the Trapezium into that "all-the-same" luminance range. The one exception is Masked Stretch.
But, even with a Masked Stretch, processing of the starless image tends to push the bright nebula above 0.95, and when the stars are replaced, the Trapezium tends to disappear again. Brighter image or Trapezium? It is a struggle, and this is what I came up with. Some Trapezium and a lot of learning…I will do better more quickly next time!
Just hope you like it this time…
Alex Woronow(1)
https://ay201b.wordpress.com/2013/02/12/the-physical-state-of-interstellar-hydrogen/(2)
https://en.wikipedia.org/wiki/Str%C3%B6mgren_sphere(3)
https://www.astrobin.com/29jims/(4)
https://iopscience.iop.org/article/10.1086/132889/pdf(5)
https://en.wikipedia.org/wiki/Kleinmann%E2%80%93Low_NebulaStudying Nebulae is Tough:
https://adsabs.harvard.edu/full/1938ApJ....87..559S
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