Secrets
Unlock Luigi
Collect all 120 Power Stars and defeat Bowser to play through the game again...except as Luigi! Luigi controls slightly differently than Mario; in fact, the differences mirror those in Mario All-Stars' Lost Levels. In short, he jumps much higher (making some puzzles easier), but he has less traction, so he slides around a lot more.
Cosmic Races - Boost Start
Remember the boost starts of Mario Kart fame? Well, they're back in Galaxy's footraces! To get a boost, you'll have to press several buttons in sequence during the "3, 2, 1" countdown before the race. When the countdown begins, hold forward, then press "Z" when the countdown reaches 2, then press "A" when "GO!" appears. This will give you a huge boost!
Super Ground Pound
Try performing a ground pound during a spin attack. This will cause Mario to spin around in the air, then home-in on any nearby enemies with a ground pound -- sweet!
121st Star
This star is the 121st and true final star in the game, found in the Grand Finale Galaxy. To unlock it, you must get all 120 stars and defeat Bowser with both Mario AND Luigi. Once you do, you can access this galaxy from the Planet of Trials, in the center. Just speak to the Luma to launch off to the galaxy. As for the mission itself, see the Grand Finale Galaxy page for details.