I don't consider this much of a spoiler myself, since most people probably wouldn't find it without knowing. Others might see it differently though, so reader beware!!1!eleven.
The official strategy guide for Super Mario Galaxy reveals a cool tip allowing you to play as Luigi, but only after you've thoroughly beaten the game using Mario.
Firstly you need to have done all the purple coin missions and collected all 120 stars.
Luigi is said to handle a little differently to Mario, he can jump higher and run faster, but is harder to stop (like in the Lost Levels).
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