beryllium732 wrote: ↑Tue Jun 07, 2022 11:07 am
Very cool! How come the pictures are so different? If we would be much nearer the nebula what would it look then? Would it be as colorful as the bluish orange picture?
I guess HD 150136 would have a very short life compared to our sun? Is HD 150136 the sole star the brighens up this nebula and gives it that particular look?
Let's begin by taking a look at the ESO/Robert Gendler image:
In my opinion, this picture was probably made using
R,
G and
B filters. These filters react to light in ways that are somewhat similar to the color-sensitive
R,
G and
B cones in our retinas. They can therefore be used to produce "natural" color images. However, the ESO/Gendler image undoubtedly also makes use of a hydrogen alpha filter, which reacts to a wavelength of 656 nm. To our eyes, this wavelength just looks red, but it is a specific wavelength that is produced as ultraviolet light from hot stars ionizes electrons. The Hα light of 656 nm is actually emitted as the electron rids itself of the extra energy it received and falls back to its ground state again.
If you don't use an Hα filter when you produce images of emission nebulas, you won't detect as much red light (because the Hα light is often somewhat faint). But to summarize, the ESO/Gendler image has almost certainly been produced using RGB filters, to create "natural" (but saturated) colors, and the Hα filter has been used to bring out a lot of red light in the nebulas.
Let's look at the image by Joseph M. Drudis:
Joseph Drudis may or may not have used RGB filters (I think he has), but more importantly, he has definitely used an Hα filter and almost certainly an OIII filter. An OIII filter detects light at 501 nm, (which is this color
███), and in a manner similar to Hα light, it is emitted where there is (extra rarefied) oxygen gas in the presence of hot stars. OIII represents a higher level of ionisation than Hα.
As you can see, the color of OIII is green. But it is often mapped as bluish cyan or blue, which is the case here. So in Joseph Drudis' image we see red Hα mixed with dust and bluish OIII closer to the hot stars in a much "clearer", much less dusty part of this nebula. And Drudis has "tilted" the nebula to make it look like a steep hill.
Let's look at Rolf Wahl Olsen's image:
I'd say that this is an OIII+Hα+SII image, perhaps with some RGB to bring out the color of the stars. The OIII has been mapped as royal blue. Hα is probably mapped as green, and SII is probably mapped as red (well, SII really is red). The orange color in the nebula may be a mixture of Hα, mapped as green, and SII, mapped as red. (Or else it's just the Hα that has been mapped as orange.)
What would these nebulas look like up close? Well, I believe that the "surface brightness" of these nebulas is very low, which is to say that we wouldn't see (much) color in them even up close. At best, we would see some green from the OIII.
As for HD 150136, it is a multiple star, and I believe that there are no less than four O-type components there. O-type (main sequence) stars are always very massive and very hot. They are like the Sun but much, much hotter, brighter and more massive. And yes, they are short-lived. The are not expected to live more than a few million years.
Ann
NGC 6188: Dragons of Ara
but I prefer stars! 
