In the PS exercise, I choose three pictures and combine them together.
The original pictures:
The final picture:
I first set the background color and edit the edges of those pictures. Then I use the magic wand tool to separate the galaxy picture to make it more vivid. But at first the connection between the pictures are too sharp, so I add the mask on the layer to blur the edge. I also use the eraser and set the soft edge to make it more natural.