This is a composite. This is not one that completely changed the scene as it existed, but merely fixed some things that didn't appear in the photo (stars) or had an unusal color cast (the street and sidewalks). Shot as 3 exposures and opened in Adobe Bridge. The 3 photos were loaded into CS5 as layers and then auto alinged and blended. Since the concrete had a greenish cast to it, a gray brush was applied using replace color.

In real life the sky had some stars but in the resulting images none were visible, therefor the sky was removed using the background eraser. I then created a "starry" sky by creating a new layer filled with black, applied a noise filter, gaussain blur, applied a clipping mask, then adjusting the levels to achieve the number of stars I wanted. Since "stars" often are not just white, I also applied a gradiant filter to give them a slight varying color tint..