OK, Photoshop 101:
Here's your original image:
And its histogram (you open the image in photoshop and go to Image-Adjustments-Levels):
What the histogram actually does
, is tell you how much of each color you get in your image. On the far left is black and on the far right is white, with grey (50% Brightness) in the middle. So in your image, the histogram starts out fairly 'low', which means there is not a lot of absolute black, but then it 'spikes' somewhere in the 'dark gray' area.
But the human eye is pretty good in figuring out if somethings dark or really really
dark. And space is really really
dark. So, you gotta change the dark grays into blacks, but without affecting the rest of the colors much. How do we do that, I hear you ask?
You move the 'black' stop to the right, until you reach the histogram 'spike'. That tells Photoshop: "This is where I want you to put the absolute black. Every shade on the left
of this stop, is to be black (RGB: 0,0,0). Oh, and you should move the 'gray' stop, so that it stays in the middle between the two extreme stops."
So, now, you have some perfect black. But you've also messed up the brightness of the rest of the image, because the 'gray' stop has moved as well.
So, we move the 'gray' stop back to the left, just for a bit...
And that will lighten up our nebulae, without affecting our perfectly black backdrop.
And that's it.
Fiddle around with the levels, to get the values that work the best for you.