Now we have a neutralized background, we can start pulling out that faint nebulosity. We use the Photoshop Curves tool for this. The image is originally captured by the camera using a linear response, but our eyes don't really see light that way. The Curves tool allows us to enhance the dark, faint stuff while leaving the brighter stars cores intact.
In this screen shot, you can see the Curves tool with two anchor points. If we increase the slope between these points, we effectively increase the contrast between those two points in the histogram. We do this several times to slowly increase the dark contrast.