To put it simply, I don't want to just switch black to alpha. I want to make it so that the more black a pixel is, the more alpha it has.
I've got this nebula I made following a tutorial:
The image shown doesn't do it justice; there's a lot of very minor flushes of color around the nebula that I need to keep. As you can see, it's also black.
I want to make it so that the more black an image has, the more alpha it has. How would I do this in Photoshop (CS6)?