Yes, an image is always helpful.
But it sounds like a masking job in general... This video gives good info on masking:
Try "Color Range...". It's in the Select menu, and it can be accessed in Properties panel when a mask is targeted.
With nothing selected, add a mask to the image - it'll be white so everything shows. In Properties panel, click "Color Range...". In the CR window, set the dropper to "+" mode and tick the invert box. Click a few places in the sky until the sky is blacked out in the mask. Experiment with the Fuzziness slider.
The mask will need some editing and feathering of edges, but CR will do most of the work.
This took a few seconds and obviously is not good enough. You won't want gray patches in the mask causing semi-transparency.
Quick Selection Tool applied to trees then either:
a) Add Mask followed by "Refine Mask...", or
b) "Refine Edge..." followed by Add Mask.
You'll want to invert the mask, of course, to hide the trees.
If working in 16-bit per channel mode, "a)" can create finer gradation in the mask, but it wouldn't be a significant difference in this particular case.
That looks like a "Close Encounters of the Third Kind" image
A dead calm night, suddenly the stoplights are swinging wildly and flashing randomly, Then a sudden shoosh and something shoots across the sky!
I missed a trick - I should have animated a saucer flying across it.
There's a galaxy (M104, the "Sombrero Galaxy") that when made small looks a bit like a flying saucer. I animated that once for use as an avatar on an astronomy forum...
My favorite galaxy is M31, the Andromeda Galaxy.
I remember one time when I was a kid and drowsing in church when my father the minister mentioned "the Great Galaxy of Andromeda" during a sermon.
Whoa, did I snap to attention! I didn't even know he knew anything about space & astronomy.
Oh well, it's our fate to collide in 4.5 Billion years.