Not wanting to confuse you even more, I do it a clunky roundabout way, more steps than Drac, but the same result. This illustration was to get a transparent background, so just pretend that your starfield is the bottom layer. so.... bottom (background) layer is the starfield, then the object you want visible, then the Object Mask. Select the white part of the mask, and Edit>Clear. Then Select inverse, and clear. Make the next layer down (the object you want to be visible) the active layer, and hit Edit>Clear again, which removes everything round your object. You can now get rid of your mask layer, and merge the other layers down. I never use the Photoshop maskd, because I can't get my head around them.