Cut the bottem half of the disk, select the resulting flat bottom polygon if you used a boolean for that. This prevents the problems that are visible when you use transparent object that must look as if they are gas like objects. In the final animation you must constantely keep a look from above the object preventing that you look inside the galaxy. Save this object as galaxy. Do a random prick with a lot of additional points on this object and kill poly's.
Some jitter clears the straight lines.
Perform point2polys and save this as stars1. Rotate the object and save as stars2. Make some clouds, using a large image results in smaller clouds wich gives a better end result. Use twirl to get those arms. Using a large air brush, paint the center black, so in the final image this area won't have transparency at all.