Basics of Simulating Nebulae
When aiming to simulate nebulae and other fluid interstellar bodies, the essential information neesd 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 various types of nebulae, and different nebula-types bear different characteristics in terms of simulating and lighting them.
Emission 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.
Bipolar Outflow Nebula
Bipolar outflow nebulae are born in newly forming stars. The star under formation has an accretion disk surrounding it, from where matter falls onto the star. The excess angular momentum is relieved as bipolar outflows along the poles of the star, usually shapes as cones due to the magnetic fields of the star.
Due to their nature, bipolar outflow nebulae are a bit more difficult to simulate. The fluid volume needs to be driven into the correct shape by using geometry and force modifiers. For example a bomb modifier placed to the center, resembling the place of the star will help the fluid to be driven outward from the star. To inhibit the fluid volume from spreading cahotically into all directions, it can be contained into a cone-shaped geometry object. It is recommended to light the nebula from outside lights and with the inner lights in effort to bring out the inner structures of the bipolar outflow nebula.
Supernova remnants are nebulae left from the violent death of a star as an supernova explosion. The matter from the star is directed outwards from the star and is illuminated by the stars radiation.
Recommended way of simulating this is placing a bomb modifier to the center of the fluid volume to resemble the exploding star. Also this can be simulated by animating a geometry object such as a hollow sphere expanding outward from the star, resembling the expanding shell of matter ejected from the star after the supernova explosion. The simulation source should be animated to turn off the production of new simulation volume, to simulate the the shell of gas ejected from the star. Lighting the nebula is recommended to do with outer lights and inner lights
Planetary nebulae are produced by massive stars late in their lives. Massive stars do not die as a supernova like lower-mass stars do, but gradually lose out their matter into the space. Giant stars can lose their matter in pulsations to the surrounding space, often creating ring shaped symmetric structures.
Simulating planetary nebulae is recommended by animating different kinds of circles expanding outward from the star. The simulation source should be animated to turn off the production of new simulation volume, to simulate the the shells of gas ejected from the star. Lighting the nebula is recommended to do with outer lights and inner lights