I think you need to merge the layers with the black background And the stars to keep the same appearance. The 'color dodge' has it's effect you see on the nebula only in combination with the stars and infinite black background. I would advice to merge the set, the stars and the black background and save it with another name and keep working on that. If you want to change, you can always go back. If you do not mind having more layers, ad a layer above the ones you want to merge and alt-'merge layers'. The merged layers will be on the new layer and you can work on top of that. If you want to change something, make the changes and do it again. Good luck!