In Star World 5 there is a key hidden towards the end of the level. You must have flying power to reach it. If you have found all four switches then the key will be much easier to get to. After you get to an area where there are 4 or 5 upsidedown pipes kill all of the enimies and fly off of the ground to the left. Go straight up there should be a small hole that you can fly through the key is just to the right of the hole.