Cause the 9 is the only one in the kit!
Just as an FYI, here's upload of the original, un-doctored raw file (dust spots and all). I converted this straight to jpg in ACR6.5.
As you can see, there was a luminance tremendous amount of noise in the image even with a 5 min exposure (it was way past twilight when I took this). By pushing that part of the image to black, it essentially reduces (eliminates) the amount of noise in that part of the image. You can try this on your own. Under expose an image by a couple of stops and then push it back up those two stops. You'll see the noise level really increase in the shadow areas.
I could have pushed the exposure even longer to reduce the noise, but I would have burned the sky and lost the detail there. Given that this image is pretty much an abstract at this point, a took a fair amount of liberty with the processing.
