When aiming to simulate nebulae and other fluid interstellar bodies, the essential information need to be extracted from various scientifical articles and papers available. The formation and workings of a nebula need to be understood, in order to be able to search for correct looking formations and lighting while simulating fluids. There are three basic types of nebulae, which all bear different characteristics in terms of lighting the simulation volume:
The nebulaes are called stellar nurseries or HII-regions (called after the large amounts of ionized hydrogen in the nebulae), for new stars are born in them. Star formation in a nebula can be triggered by the nebula collapsing under it’s own gravity, thus compressing the matter in it and this way attracting further matter. After time the clumps of matter are dense enough to trigger star formation. Or an occurring supernova nearby can also begin star formation. In the supernova explosion, a dying star plunges out it’s outer layers of gas, and this explosion drives a shockwave through the cosmos. While interacting with the nebula, the shockwave compresses the matter on it’s wake and the resulting larger inequalities in the matter density may eventually attract matter dense enough to trigger star formation. When the newly formed stars begin to emit radiation, they excite the particles of the surrounding nebula. The particles are accelerated to higher energy-levels, and upon returning to their normal-state, they release the energy as photons, effectively making the nebulae self-illuminating, or as called an emission nebula. Eventually the radiation pressure from the stars will disperse the nebula around them, effectively destroying their place of birth.
One should not try to emulate the effects of self-illuminating nebula by positioning light source for every star, because it would exponentially increase the lighting calculations to amounts where any amount of processing power would be rendered obsolete. So a good way to approach on lighting these types of nebulae is to place omnidirectional lights inside the volume, with different radiuses and intensities to represent regions of star formation illuminating the fluid from inside, thus bringing out the details of the internal structure. One should not try to cover the whole internal volume of the fluid, for areas receiving no internal illumination between the illuminated areas simulates the dark dust which is present in nearly all emission nebulae and this way increases realism. Also some lights from multiple directions lighting the outside features of the volume is recommendable.
The energy from the stars inside the dust or nearby it are insufficient to ionize the gas to make it self-illuminating, but for creating dispersion from the particles of the gas to make it visible, reflecting the nearby starlight away. These are called reflection nebulae.
Good way of approaching at this matter is to light the volume with singular or multiple lights from outside to volume the achieve the effect of reflected starlight.
Not producing or reflecting any light, diffuse nebulaes are often dark forms discernable over the features of the background space. For example dark diffuse nebula can be seen when it is viewed as being in front of a more brighter nebula.
Good way of approaching this, is maybe by lighting the nebula from behind, so that the nebula is between the light source and the camera, and place it against brighter background.