Alex_g444 wrote: ↑Fri Aug 30, 2019 7:43 am
So the physics of emission nebulae is close to what we see on Earth when sun light photons diffuse in the atmosphere, with the exception that in emission nebulae the diffusive medium is dust. Does that mean that, if could travel to NGC 7129 with a space ship, we would see on arrival there an immense blue light like in our sky ?
Rayleigh scattering in the Earth's atmosphere.
Basically the atmosphere of the Earth contains molecules of just the right size to preferentially scatter short-wave light photons over long-wave light photons, which is why the Earth's sky looks blue to us. This is called
Rayleigh scattering.
So in the Earth's atmosphere, it is the gaseous molecules that scatter blue light and make the Earth's sky look blue. In space, it is dust grains of the right size that preferentially scatter blue light and create the typical blue reflection nebulas.
The Rho Ophiuchi cloud complex.
Photo: Takayuki Yoshida
Consider the colorful Rho Ophiuchi cloud complex. Rho Ophiuchi is the multiple star at upper right surrounded by a large
blue reflection nebula. All the three stars that you can see clumped together are hot B-type stars, which produce lots and lots of blue-light photons. These are scattered by the dusty nebula that is particularly obvious in the upper part of the picture. In turn, this gives rise to a large and relatively bright blue reflection nebula. There is also another blue reflection nebula in the picture, at center left, surrounding another B-type star, 22 Scorpii.
But what about the huge
yellowish patch of nebulosity in the middle of the picture?
There are several components that give this yellow nebula its color, among them dust reddening that makes the nebula extra orange-brown in its upper parts. Primarily, though, this yellow patch is a yellow reflection nebula, lit up by the light of red supergiant Antares (below center in the image). Antares is so cool that it hardly produces any blue light at all, which is why Antares can never give rise to a blue reflection nebula. But Antares is extremely bright, and some of its yellow light is scattered by the dust around it.
So what about the patches of
pink nebulosity at right and at lower left? They have nothing to do with dust, but are caused by the ionization of hydrogen.
Hydrogen, as you know, is the simplest and lightest element in nature. It consists of a proton and an electron that is in orbit around the proton. The electron is normally found in its lowest electron shell, the shell that is called 1n in the picture at right.
There are blue stars located right next to or inside the pink nebulas in the picture at left. Consider the blue star at middle right and the one at lower left. These are hot stars, the one at right is spectral class B1III+B1V, and the other one is spectral class B0V. They produce enough ultraviolet photons to knock electrons in hydrogen atoms from one electron shell to another. When an electron is hit by an ultraviolet photon and knocked from electron shell one to electron shell two, it will soon fall back to electron shell one again. And when it does so, it emits a photon of red light at 656 nm. Occasionally an electron is knocked into shell number three, and when it falls back to shell number one it emits a blue-green photon of light at 486 nm. Together the red light at 656 nm and the blue-green light of 486 nm creates the pink hydrogen emission nebulas that are relatively common in the sky.
What about Rho Ophiuchi, the multiple blue star that has given this cloud complex its name? The blue stars of Rho Ophiuchi are just a tad too cool to fail to ionize hydrogen. Their blue light is scattered by dust, but they fail to knock many electrons out of their electron shells. That's why the nebula at top of the picture is blue, not pink.
So if we could magically travel to Rho Ophiuchi, would we see the large blue reflection nebula surrounding Rho Ophiuchi itself, or the yellow nebula surrounding Antares? Would we be able to see the pink color of emission nebulas for ourselves?
I would say no. These nebulas are faint, and the clouds of gas and dust that scatter or emit their colors are extremely thin. Compared with the thickness of the Earth's atmosphere, most nebulas in space can be considered hard vacuums. So we would see nothing but the bright stars themselves if we were able to travel to the Rho Ophiuchi complex, or, at best, we would see the nebulas as a faint grey fog.
Ann
NGC 7129 and NGC 7142
And as always, it prompts my desire to find pictures of "similar" phenomena in order to shed more light on the properties of the objects that are highlighted in the APOD.
Sol lives in the rural areas of space in comparison! 
