In Super Mario Galaxy, there are 120 Power Stars for you to find. You’ll discover most of them just by completing each of the game’s levels and following the directions to find each;
There are also 51 hidden or secret Power Stars to collect throughout, too, and Polygon’s Super Mario Galaxy guides will help you find them all.
Hidden Stars are, like the name suggests, hidden inside normal levels. Instead of completing the level as designed, you’ll have to do something different, like going in the wrong direction or feeding an out-of-the-way Hungry Luma. There are 19 Hidden Stars (including the Hungry Lumas that appear next to the game’s six Domes) in Super Mario Galaxy.
There are also 24 Power Stars that you’ll earn by completing Prankster Comet levels. While a Prankster Comet is in orbit around a galaxy, they’ll create a special version of one of that galaxy’s levels.
After you rescue Luigi during Luigi and the Haunted Mansion, he’ll head off on his own to find some Power Stars. He, being Luigi, gets stuck every time he wanders off. He’ll mail you a photo, and you’ll have to go off to find him. There are three places he’ll go, and we’ve got a guide for all three.
Finally, there are three Green Stars in the game. Once you’ve collected all three, there will be a new, green Launch Star in the Observatory. That will send you to a three-lobed planet where you can tackle the three Trial Galaxies. These are challenging levels using mechanics you’ve encountered elsewhere in the game.