firstname.lastname@example.org wrote: ↑Sun Oct 14, 2018 8:41 pm
Why does the blue in the uper part of the picture differ from the blue green below and to the right of it.
The blue green is identified as Hydrogen and Sulfur (SII+) which doesn't make sense to me (but I don't have access to the spectra.
Orion Constellation. Credit: Zdeněk Bardon/ESO
Take a look at the picture at left, where the colors are not nearly as saturated as they are in the APOD. You can see, nevertheless, that there is a lot of low-level (almost brownish-looking) hydrogen alpha light surrounding the Ortion Nebula (at left). You can also see that the brightest part of the Orion Nebula itself looks white, and the somewhat less bright part of it (at left) looks red, and even farther to the left is a blue border. (The bluish blotch to the right of the Orion Nebula is the Running Man nebula
The blue border surrounding the Orion Nebula would be blue reflection nebulosity, where light from the hot blue stars in the Orion Nebula is reflected in the dust in the outskirts of it. Why don't we see reflected blue light closer to the Trapezium, the innermost part of the Orion Nebula? Probably because the blue light there is swamped by brilliant Hα and OIII emission from ionized hydrogen and oxygen gas.
But there is red light everywhere around the Orion Nebula. It seems reasonable that the blue light surrounding the Orion Nebula is slightly "contaminated" by the red light surrounding it.
Now take a look at Orion's Belt and Alnilam. Can you see that the otherwise nearly ubiquitous reddish Hα background seems to come to a stop near Alnilam, the middle star of Orion's Belt? And can you see, too, that there seems to be very many small blue stars surrounding Alnilam?
Parallax measurements suggest that Alnilam is farther away from us than the other two stars of Orion's Belt. It may or may not be slightly older than the other Belt stars, too, but in any case it seems to be the central star of a fairly rich young association of blue stars. Together, these numerous blue stars may have blown away the gases around them, so that there is no hydrogen gas left in the vicinity of Alnilam. If there were, Alnilam would produce its own red nebula, because Alnilam is hot enough to make hydrogen glow red.
So there is no hydrogen sufficiently close to Alnilam for Alnilam to ionize it and make it glow red. But Alnilam itself produces copious amounts of blue light, and all the other small stars surrounding it also produce blue light. There are some astrophotos that show that dust along our line of sight to Alnilam and its association is lit up faintly by the light of Alnilam and the other blue stars and is made to glow faintly blue, as in this photo
by Mohammad Noaroozi.
And by the way: There is no obvious OIII light in the ESO picture that you can see in my post, and my impression of the APOD is that there is not too much OIII in that picture, either. That is because OIII light is typically seen very close to very hot stars, not farther away from the hot stars than the red Hα light.