This is phenomenal and whimsical. I don't know Photoshop that well so I can't fairly say anything about your technique. The color choice is interesting, I think of the ocean instead of space until I see the red giants dotting the lower left corner. This piece makes me feel so small but at the same time I feel like I could reach our and disperse the nebula like mist with a wave of my hand. The transition between the darkest corner bottom left could transition a bit more subtly into the light of the nebula. Unless that is how such space formations are for I have little knowledge on the subject and possibly no say in how the light ought to transition into darkness.
Fair critique, but look at some pictures of actual nebulae and you will find that this painting is extremely accurate, including the sudden transitions between light and shadow. The color choice is also quite normal for a picture of a nebulae, except that it is lacking the pink that is usually chosen to represent hydrogen gas in scientific images. I believe this is an artistic choice rather than a technical one though. Even in reality, the colors you see in professional pictures of space are artificial, since the photos are often taking in spectrums outside of visible light. In order for the data to mean anything to the human eye they have to choose colors to represent the things the telescope picks up that we can't see.
Here are some references from the Hubble Space Telescope's website: [link]
Keeping this in mind, I am EXTREMELY impressed with this painting. Great job! I really thought it was an actual picture, just as many of the other commenters did. I think the choice of colors is beautiful, but I may be biased because they're my favorites.
Yeah I probably should have looked at some reference pics before I critiqued. I just had to get to class but felt I had to say something about this picture that made my day.