Recently been updating my photoshop files and tools and made a nebula as a testbed for an upcoming novel I am working on, was quite happy with it despite not using it for my cover.
Created a black layer, added noise, next I added a guassian blur before adjusting the levels. Lastly added a lens flare.
I then added two layers of clouds using the brush tool.
Then I created a new layer over the top of the prior two layers. I used fill and reduced the opacity until I was happy it had a cold feeling to the nebula.