ok to do that look at the map where all the stars are. Then look what places you didn't get the stars at. Go to those places and you will get them and pretty soon. You will have. All The Stars
If you beat every mission, it will give you all the stars, of course. Remember, after you get all 120 stars as Mario, you unlock Luigi. After getting all 120 stars as Luigi, you unlock a bonus star...
Bouy base Galaxy- use to underwater bullet bill to break to glass with the pipe and follow the path 2nd Luigi letter Dusty Dune Galaxy - go on underside of planet after 1st Launch Star on 2nd...
get all 120 stars with Mario then luigi will be unlocked do it again with him a new galacy will apear then get all 100 purple coins on it to get the final star.