this is a wonderful picture. I love the redness of the nebulas. the planet, asteroids and the stars looks fantastic. I've tried looking for a you tube tutorials on how to make nebulas but i havent found any that can compare to the life-likeness of yours. its stunning!
The nebula is a combination of a fractal nebula and cloud textures. I often create several nebulae and save them for later use - kind of like my own stock images. One thing though, fractal nebulae often have bright lines running though them. Most times I'll copy the nebula layer and blur the heck out of it and I'll experiment with the blend modes till I find the perfect effect. Using more than one nebula gives the nebula depth. 'Star Nursery' is a good example. [link] The stars in this image are sampled stars from several NASA images. I usually use special brushes as well.