Sh2-101: This is an emission nebula, a cloud of hydrogen gas in which stars are being formed; the energetic,
young stars in the nebula ionize the hydrogen, which causes the characteristic red color of the nebula. Sulfur aand oxygen atoms also are being ionized (others, too, I suspect); the "Hubble
palette) versions I show were created photographing only sulfur, hydrogen and oxygen emissions. Perhaps the star causing the most ionizatino is HD 227018, the very bright, blue star fairly near
the center of the image, partially surrounded by a diaphanous bluish bubble (oxygen atoms being ionized by the star).
Of special note is the presence of a shock wave created by a black hole in the field. If you look carefully in the upper-right corner, there's a bluish arch; this is formed from gas being ejected
from a binary star system consisting of a supermassive star and a black hole (Cygnus X-1, the first celestial x-ray source confirmed to be black hole). The bright star of the binary system is
the brightest white star at the right edge of my photo, about a third of the way down from the corner.
The nebula is about 6,000 light years from Earth. The photographed nebula is roughly 34 by 51 light years in dimension.
I have presented this in six (yes, 6!) different formats (each image is labeled in the lower left corner); this is the order in which they appear as you cycle through (by repeatedly clicking on the photo,
waiting for each to download):
(i) A version (the top photo in the stack, labeled "LRGBNB" in the lower left corner)), with the color created by imaging through red, green and blue filters (with a significant amount of Ha and OIII data blended in to both the color layer and the luminance layer); the effect of weaving the Ha and OIII data into the broadband data (L, R, G and B) is to enhance the red emissions from the Ha and blue/green emissions from the OIII;
(ii) A bi-color version (almost true-color version, labeled "HOO"; the second photo in the stack), in which "red" is ionized hydrogen emissions (Ha), and green and blue are doubly-ionized oxygen emissions (OIII); this works (giving fairly accurate colors) in this nebula because the vast majority of the emissions are in Ha or OIII, and Ha is in the red part of the spectrum, while OIII is blue-green; I used the luminance (shape/detail) layer from the pure-Ha image in this image, also;
(iii) A true-color version (labeled "LRGB"), with the color created by imaging through red, green and blue filters (with no narrow band data included);
(iv) A version in the Hubble palette (a lot of the Hubble photos, including and especially the famous "Pillars of Creation," are made with this set of filters, since it's a useful set for scientists to see what's actually happening), which shows SII emissions as red, Ha emissions as green, and OIII as blue (because Ha is so dominant, the nebula would be much greener were I not to emphasize the OIII; this is why there is relatively little straight green, although you can see a lot of green in the outer parts; I used the luminance (shape/detail) layer from the pure-Ha image in this image, also;
(v) Another version in the Hubble palette, with the green "neutralized" into a tannish color; this is often done because it is thought to be a "prettier" picture; and
(vi) A pure Ha version (grayscale, showing only light in the very narrow Ha band).
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