Nebulae are clouds of dust and gas that emmit light (emmision nebulae) or reflect starlight (reflection nebulae)
In emission nebuale the gas is heated by the radiation of bright hot stars in the vicinity. As a consequece the heated gas emits light at very specific wavelengths. For three of these wavelengths I have filters to capture that light. these are ionized Hydrogen (Ha) at a wavelength of 656 nm, singely ionized Sulfur (Sii) at 672 nm and double ionized Oxygen (Oiii) at 500 nm.
These 3 channels can be combined to form an RGB color image. Most of the time the Hubble SHO color pallete is used for that, with Sulfur mapped to red, Hydrogen to green and oxygen to blue.
The Crescent nebula in monochrome Ha recording. The central star in the nebula is a very massive Wolf Rayet star that has shed most of its outer layers. The spectrum (taken by me with a star analyzer diffraction grating) allready show materials deeper inside the star. These massive stars are certain to undergo a core collapse in the future when in the core fusion into Iron starts. The core implodes and a Supernova follows. These stars are very rare. Only 200 are known in the Milky Way. That is because massive stars are rare in itself, and the Wolf Rayet phase is short, in an astronomical sense.
in reflection nebulae star light is reflected of dust clouds surrounding the stars. There ar bright and dark nebulae. The dark nebulae are particularly hard to photograph, as the level of light is very low.
The Iris nebula below is an example of this, whith its brownish dust clouds. The Infrared light recordings show a bit more of the stars that are in and behind the dust clouds. In the pictures without infrared light mixed in there are less stars visible in the clouds.