While editing the image I came across an interesting article by Jens Zippel about the galaxy NGC 4236, especially the color of the HII regions.
Quote:
NGC 4236 - like all galaxies in the M81/M82 group - can be resolved quite well with larger telescopes. Their brightest blue supergiants are at 19 mag. At the ends of the bar some strong H II regions become visible, in blue color - blue - you read that right! The largest in the southern area measures 17 arcseconds, the largest clump in the north measures 21 arcseconds. This amounts to values of up to 1200 ly, which is comparable in size to the Tarantula Nebula in the Large Magellanic Cloud (giant H II region) or to NGC 604 in M 33. Since the star formation regions are very young, the proportion of Wolf-Rayet stars and O stars contained is still very large. This means that the surrounding dust also glows strongly as a reflection nebula. In addition, the proportion of [OIII] emission is very high. Furthermore, H-Beta is generally not negligible; this emission line has about 33% of the intensity of H-Alpha. Because the blue color dominates, we see these HII regions not red, but blue. Only in later years, when the dust has been blown outwards by the hot stars, are the blue stars able to stimulate the nebula with their UV radiation, with the typical red hydrogen emission (H-alpha). As for blue HII regions, there are two other galaxies in the M81/82 group: IC 2574 and NGC 2366. They also present HII regions with a clear blue color.
Source:
https://www.spaceimages.de/astrofotos/galaxien/ngc-4236Nevertheless, I also created and uploaded a version with red HII regions.
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