Super Mario Galaxy Easter Egg - Play as Luigi & get the 121st star

Get all 120 stars & return to Princess Rosalina (she is in the Comet Observatory) & request to fight Bowser. Defeat him (Ofcourse)& beat the game. Let the credits roll & after that, you will get a message saying the the 2nd brother (or the plumber in green), Luigi, is now playable. You can activate him by choosing your save file & use the option that changes Mario into Luigi. For the 121st star, choose Luigi & beat Bowser & you will get to a secret galaxy (also known as the Grand Finale Galaxy). Beat that one, & you will get the 121st star.