The photographer who took this photo openly said "I've been borrowing nebulae from nasa
hope they dont mind! This was originally shot in broad daylight" on his Flickr page.
This means that he changed the background from day to night using gimp. The photo itself is really cool - except for the shadows of the trees shown on the trail. Those shouldn't be there, right? Trees don't cast shadow - at least not like that at night. What could you have done of you were making this edit to remove those shadows from the ground? Well, you probably could use an overlay to make the specific parts of the ground darker, or simply take this photo at noon, when no shadow is cast
What would help make selecting the sky to replace if with the starry sky easier? Hands down, shooting the photo in RAW
would have helped, because that would give you the chance to create a very accurate layer mask to keep as many of your original tree branches as possible.
One other thing to note about this photo is the interesting color change that was applied to the top of the tree. I like it personally, but how would you add those to the tree? I'd probably mask off the tips of my tree so that my darken layers wouldn't make them dark, and then add a few dark pink overlays on the trees to give them that color.