The interstellar medium is plenty of huge mass of gas and dust. One way to understand its composition and dynamics consist in the capture of images through narrowband filters. These filters only allow to pass through light of certain wavelengths corresponding to the emission of certain ionized (SII and OIII) or excited (Ha) elements. There are several filter versions with different bandwidths (13, 6 and 3 nm).
The image at right show the Andromeda galaxy, a spiral galaxy similar to our own Galaxy. The image, taken through narrowband filters (HOS), show a detail of the spiral arms, where Ha (in red) and OIII (in green) formations can be seen. It is precisely in the spiral arms of our Galaxy that the diffuse nebulae exist, some of them shown bellow.
For a better comprehension of the images, some understanding on how colours are mixed is needed:
Red + Green = Yellow
Red + Blue = Magenta
Green + Blue = Cyan
A basic colour is attributed to each narrowband filter by the order Red, Green and Blue.
So, in a SHO image, the yellowish zones reveal a mixture of Sulphur and Hydrogen, the green zones Hydrogen, the cyan zones Hydrogen and Oxygen , and the dark zones, dust, of course.