An issue in such a kind of modelling could be that of the form to be assigned to the meshes for best fitting their complex shapes. One of the best solutions, imho, is that of using math softwares through the use of parametric equations. There are several, powerful softwares which does that, but mostly not free and expensive. My suggestion here is toward K3DSurf, a software that Linux users knows well. It is free, open source and multi platform. Moreover it export the math forms in OBJ format.
http://k3dsurf.sourceforge.net/
The program is shipped with lots of mathematical examples for the "classic" curves and surfaces. Here is used the parametric functions with spherical coordinates ("Parametric" tab). Playing a bit with sine and cosine is easy to achieve the own favorite meshes. Raising to the powers does stretch and ondulate the forms, good for central ejecta. Non powers makes "blobbish" shapes, good for gas coverages. And so on. A slide control increase or decrease their smoothingness. Isn't important to smooth alot the mesh, 1, 2k polys is enough. This will light up the files even for complex nebulae made of several meshes.
TEXTURING
This step is using GIMP as reference.
- make a new 256x256 or 512x512 empty (transparent) canvas (2:1 textures are good too)
- choose "Filters" > "Render" > "Lava" and apply the effect
- choose "Filters" > "Map" > "Make Seamless" ("Make Seamless" isn't strictly necessary, anyhow)
- choose "Colors" > "Color to Alpha" and set the "black" as color to alpha
Repeat the procedure above with other gradients and seeds, and overlay the layers in order to mix colors and save the images as PNG with different custom names for each one of the models.
Note that "Lava" is good for pillars, remnants and clusters, while for gases maybe is better the "Cloud" filter, which is in the same menu. Even for "Clouds" is preferable to do the "Color to Alpha" procedure and then "Colorify". Nonetheless, "Cloud" is more tricky. The "Alpha" procedure allows the CMODs, which must be transparents, to be transparents definitely in such parts. These are peer-user setting, always changeable through the CMODview and/or by assigning more transparency to each texture.
THE CMODs
Once the meshes have been exported in OBJ form from K3DSurf (tab "Export"), must be open within a 3d modeler in order to assign to each one mesh the spherical UV map projection and its texture. Even the cylindric and the plane projection can be used under certain circumstances. Do not forget the placeholder construction, which can be simply a bounding cube as well as points placed on quadrants, like the compass. The final nebula model have to be built within the modeler, since the axes of the meshes couldn't be centered respect to world scale, and whether one would wish to add a point sprite as nucleii instead of another mesh, it should be out of center (and the camera, too).
At this point, do save the model in 3DS format and open it within the CMODview. In there, do assign roughly:
- emissive 1 1 1 (255 255 255, from colors' panel)
- opacity 0.3 to the inner meshes, 0.2 to the outer meshes, and so on.
- shineness 100 (not require)
Save as CMOD and make the .DSC file.
Two screengrabs of the same nebula from two different points of view. The nebula is made of just two meshes. More meshes, more colors, more transparency levels, etc... and the nebula can be more dimmer, which look best. Sprites can improve the look, although expensive for performances.
Test zip: [url] Nebula is named Forum nebula. Extract in ..\extras folder and make sure that nebula render be active.
http://marauder.webng.com/files/forum_nebula.zip 1 Mb mostly textures
) i just feel like a cowboy inside a SF-movie... 
. Provided it's a feasible feature in Celestia? 