To play as Luigi in Super Mario Galaxy, beat the game with Mario. After beating it, get all 120 stars. When you have 120, ask Rosalina if you can fight Bowser again. Beating him will unlock Luigi. Luigi can run faster and jump higher than Mario, but has less traction and will slip.
121st star
To get the 121st star, unlock Luigi and get all 120 stars with him. Defeat Bowser, and you will unlock the Grand Finale Galaxy. Collect the 100 purple coins here to get the star.
Turn coconuts into watermelons
To turn the coconuts in the game into watermelons, collect 9999 star bits. There is no difference in gameplay even after they become watermelons.
