i never use cycles, only the normal internal render but i can advise that the simplest way of forming a basic space background is to map a seamless texture onto a huge sphere around the scene. it's good for animating with but does not allow a flying past stars effect as you would see in some sci fi when a ship goes FTL.
this tutorial of mine has a section on step 21, under the sub heading "a trick for space scenes". http://www.instructables.com/id/Sketchup-3D-modelling-file-conversion-and-advice-/?ALLSTEPS
I want to create a small sphere or series of spheres with a red nebular/starfield effect inside something actually, and I'd like to do it with cycles since it has some good materials for what I'm working on.
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this tutorial of mine has a section on step 21, under the sub heading "a trick for space scenes".
http://www.instructables.com/id/Sketchup-3D-modelling-file-conversion-and-advice-/?ALLSTEPS