I am trying to recreate in Blender some space scene like the one of this image

So far, I wanted to create some "clouds/nebulae" that they are somehow "circular" in a given region of the space (Environment texture), so I am using a mask for that like this (so the noise texture is only applied here)
However, I would like to add some "noise" to the edge of that texture so its not that sharp and it adds some randomness to it. Basically, I would like to get something like this (done in photoshop)

How could I get something similar using Nodes in Blender?





