It needs more colours. Also, there's too much black space (background). Since this is a practice, go to Greg Martin's website and look under Tutorial for "making the riven" to get ideas, maybe even steal some. Also, try to add more layers because from what I can see you have about 3-5 layers for the nebula. To have a realistic nebula, you need something like 10-30 layers (well it's just my opinions). OTher than that, it's really good.