How can i put a "hubble telescope" photo of star cluster & nebula into the photo I'm working on? I can't select the objects because they fade into the dark sky background.
Try placing the image at the top of your layers then:
Layer > Layer Style > Blending Options...
Then drag the sliders shown here in the "Blend If" Section from the left side to the right while viewing the image. To split the sliders (for smoother transitions) press the Option/Alt key while dragging.