Chris Peterson wrote: ↑Tue Jan 25, 2022 1:39 pm
Ann wrote: ↑Tue Jan 25, 2022 5:01 am
Chris Peterson wrote: ↑Mon Jan 24, 2022 9:21 pm
I thought I did answer that above. The answer is neither. Or both, if you prefer. Scattering requires a low density dust cloud. A high density cloud is opaque, and there is almost no scattering. Such a cloud reflects light, and is therefore brown (unsaturated red). A low density cloud doesn't reflect much, but is an efficient scatterer. So it appears blue.
I don't see much blue in the Witch Head. Mostly I see reflected light, fairly unsaturated brown (like a warm gray).
That's my point, Chris. There is less blue than usual in the Witch Head Nebula in today's APOD. Usually there is more blue there in pictures - because there is more blue there.
I don't see much blue in other images, either.
You may check out several photos of the Witch Head Nebula here
. I find all the images relatively blue, except the fifth image, which is dominated by a very red background. The red background of that image resembles the background of the APOD, and in my opinion, this emphasis on red hydrogen alpha background light (which is not strong near Rigel) alters the color balance of the entire image and risks making the Witch Head Nebula look almost completely non-blue.
The image above is a favorite of mine. We can clearly see that only some parts of the Witch Head Nebula scatters blue light, while other parts appear gray or brown. Nevertheless, to me this nebula, when it looks the way it looks in the picture above, is clearly more blue than red.
If I know you, you remain perfectly unmoved by my arguments! That's okay, because I'm not really trying to convince you, and instead I'm arguing for the benefit of other members of (and visitors of) Starship Asterisk*.
Bear in mind that I have never said that the Witch Head Nebula is very blue
. Indeed, and as you can see from the picture I posted, it is not even all blue in the first place. I'd describe the Witch Head Nebula as moderately blue, but in my opinion (and in the opinion of more or less everyone who writes about it on the net) the Witch Head Nebula is indeed more blue than red. However, if a photographer emphasizes the red background of this general area, and brings out red light more than blue light, then there is an increasing likelihood that the Witch Head Nebula is hardly going to look blue at all.
And now I've got nothing more to add!