Power Stars (not to be confused with Invincibility Stars are the main collectibles in the 3D Mario games, being found in three of the six titles (excluding Super Mario 64 DS). They are found in every level of Super Mario 64, Super Mario Galaxy, and Super Mario Galaxy 2. There are 634 Power Stars in all three games combined.
In Super Mario 64 and Super Mario 64 DS Edit
In these two games, Bowser scatters the Power Stars throughout several paintings in Peach's Castle. After finding 70 of them (80 in the DS remake), Bowser can be fought in the sky. There are 120 Power Stars total in the original while there are 150 in the DS version due to new Secret Stars and additional course stars. There are six (seven in the DS remake) Power Stars to find in each painting.
In Super Mario Galaxy Edit
In this game, there are 121 Power Stars each for Mario and Luigi to collect, meaning there are 242 in total. There
are many kinds of Power Stars in this game. Of course, normal Power Stars but Green Stars also make an appearance. Green Stars unlock the Trial Galaxies, which are particularly hard levels found in the game.
The new Red Star gives Mario the ability to fly in a single level, Gateway's Purple Coins. There are also seven Grand Stars to collect in each castle, with only one of them being found in Gateway Galaxy.
In Super Mario Galaxy 2 Edit
While there are still 242 Power Stars to collect in this game, Green Stars serve a bit of a different role. After collecting a certain amount of Power Stars, there will be 3 Green Stars in each world to find.
A new type of stars, Bronze Stars, also appear. They only appear if the player uses the Cosmic Guide function which allows the Cosmic Spirit to temporarily take over the character's body and completes the level for them.