A simple tool is Starbits Starfield Generator, an old shareware program. You can creates stars, galaxys, nebula clouds, etc.. Saving in .bmp format and apply at your scene as backdrop.
One working link for download it still work, because was designed for Windows 95/98 and apparently the developer loose the interest for the software:

Starbits Starfield Generator 7.3

I hope you enjoy it. Chhers!