It's a rather easy process but to write it out is rather long.
You will need to work in Layers. One layer for each moon.
You can rename each layer if you wish, Moon 1, Moon 2. ect.
Bring your first moon into CS. Now to open the next. Now you can do this one or two ways. Drag the next Moon up to the title bar and open it that way of just drag the the New Moon onto the existing Moon. You will see then New Moon is a new Layer.
You will need to select the moon, in the new layer, I would use the magic wand. Then invert you selection and cut away the black ground.
If you opened it as a new image then you can cut and past your moon onto the original image. If you dragged it on the image it will good to go. You will need to do this for each moon.
If you dragged the New Moon on top of the old Moon, you can re-size it by putting your cursor on one of the outer handles and dragging it into or away from the center, this will allow you to adjust the size of the Moon.
Note you can also adjust the Hue/Saturation of each layer by clicking on the Layer. You can also turn off each Layer by clicking on the eye ball in the Layer Selection.
You can also move each Moon in the Layer sequence by dragging the layer up or down in the layer selection.
If you look above the layers, to the right you will see a box that says Opacity, Click on it. You can use the slider change the Opacity of each layer.
Should you decide that the first Moon you brought into CS needs to be moved you will need to Delete the Lock on it. Put the cursor on the Lock and drag it down to the garbage can in the lower right corner.
Good Luck. And play with it before you come to any decision.
You can always start over.
Layers explained:
http://www.youtube.com/watch?v=yEFgOkGgcjkComposite:
http://www.youtube.com/watch?v=MmwrWCMdYqI