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Best answer as chosen by Asker / Ratings
If you refer to green stars ,one is in the buoy base(underwater, break the container with a pipe inside with a torpedo and continue your path to the green star), another in the battlerock galaxy (break the conainer where luigi is trapped with a bullet bill, he will thank you with the 2nd green star) and the last one is in the dusty dune galaxy (go to the brown ring with 4 rocks rolling on it and go to the down side of that "planet". You should see a hungry luma. Feed it and grab the last green star on the next planet).
If you refer to all other stars, but playing with luigi, just do the same you did with mario.