Cast your mind back to Mario 64, gaining 120 stars was indeed a very self satisfying moment, but all you got from it was a bunch of lives from Yoshi, big wow.
Super Mario Galaxy has done something better than that though, thank god, collecting all those stars actually unlocks something as reported by The Tanooki earlier this week.
When you finish the game, you’ll be able to play through what’s essentially Super Luigi Galaxy. The eternal understudy is finally back and playable in a Mario game! You play through the entirety of Super Mario Galaxy as Luigi, which might not seem like much, but it becomes a very different game. As usual, Luigi can jump higher and farther, and he runs faster but with less traction. The game is supposedly much harder with Luigi, so you’re not only getting a playable platformer starring Luigi, but you’re getting a “hard mode” at the same time!