Chris Peterson wrote:
Ann wrote:And all those "brownish" tendrils of gas filling so much of the scene get their color from hydrogen at low levels of ionization. Almost the entire skyscape here is filled with glowing hydrogen.
It seems to me there are at least three possibilities, and nothing obvious in this image to sort them out. The brown areas could be hydrogen regions only weakly ionized. They could be highly ionized, but low density, and therefore relatively low intensity. They could be dust tendrils, just reflecting light from surrounding hydrogen regions.
Obviously I can't really argue with you, Chris. Nevertheless, I would like to explain why I thought of low-level ionization here. I remember when this picture
first appeared, from which astronomers concluded that there are low levels of ionized hydrogen almost everywhere in our galaxy.
As for today's APOD, it seemed to me that the area between the Horsehead Nebula and M42 is the perfect place to look for gas at relatively low levels of ionization. There are many hot bright stars in the region, whose stray ultraviolet photons could ionize some of the hydrogen atoms some distance away from the obvious emission nebulae.
If you look at this picture of Orion
by Rogelio Bernal Andreo, you can see that the region between the Horsehead and M42 looks reddish brown. I think it is more likely that we are looking at undulating gas at low levels of ionization than that we are seeing a huge rippling sheet of dust. Compare the appearance of the area between the Horsehead and M42 with T.V. Davis image of the Iris Nebula region
. Note that HD 200775, the bright blue star inside the Iris Nebula itself, is the only star in the vicinity that is hot enough to do an appreciable amount of hydrogen ionization in the first place - and yet, the nebula surrounding this star is primarily a reflection nebula, with only some small hints of pink emission nebulosity near the star itself. It is very hard to believe that HD 200775 packs an ultraviolet punch that can "reach out" to hydrogen far away from itself and kick such distant electrons into a "higher orbit".
But look at the dark dust near the Iris Nebula! The dust is very clumpy, and some of it is very dark. Compare it with the undulating structures between the Horsehead and M42. Even though Roberto Colombari & Federico Pelliccia's mosaic isn't strikingly Ha-bright, it is clear that the color of the tendrils is different from the color of the clumps of dust near the Iris Nebula. Also, there are no very dark or strikingly clumpy features among the undulating structures in Orion. That is why I don't believe that the structures that we see are primarily clumps or strings of dust.
In any case, this is a picture of Orion in Ha light only
. The area between the Horsehead and M42 clearly shows up is Ha.
As for the suggestion that the tendrils that we see could just reflect the color of the bright red emission nebulae in the area, I don't believe that that is even remotely possible. Bear in mind that dust typically reflects blue light better than red light, and with all the bright blue stars in Orion, there is no doubt that there is more blue than red light available here for the dust to reflect.
But what about the dust structures near the Iris Nebula? They are clearly reflecting some kind of light, but not red Ha light and certainly not blue light from HD 200775. So what light do they reflect? I believe they reflect the combined light of the Milky Way and the various photons that are travelling this way and that inside our galaxy. Our own galaxy is likely somewhat "red" in color, with its large bulge and thick central dust lane. And the dust grains in themselves are also likely somewhat reddish in color, which in combination with the yellowish light from the Milky Way would explain the brown color of the dust clouds.
So in my opinion, the faintly wine-colored tendrils that we see in today's APOD are not dust structures. But isn't it possible that they are very low density gas tendrils that are highly ionized, although there is very little there to ionize? Yes, I suppose that is possible, Chris.