From the Super Mario Wiki
Power Stars are collectible items that can grant Mario access to more levels in-game, unlike the regular Stars, which give only limited invincibility. It has been subtly hinted over and over that Power Stars may make Mario stronger. In Super Mario 64, Super Mario Galaxy, and Super Mario Galaxy 2 when the star for any given level has been collected, playing the level again will turn the Star into a transparent version of itself. This Star can be collected, but it will not add to the player's total Star count. In Super Mario Sunshine, they are replaced by Shine Sprites. In all, there are 754 Power Stars in these four games combined.
 Super Mario series
 Super Mario 64 / Super Mario 64 DS
In Super Mario 64 and Super Mario 64 DS, Power Stars are spread throughout the worlds, and Mario (along with Yoshi, Luigi, and Wario in the Nintendo DS remake) is sent to go find them all. There are six Power Stars (seven in the DS remake) to collect in each world, and after finding 70 of them (80 in the DS remake), the final boss, Bowser, can be defeated. There are 120 Power Stars total in Super Mario 64; this was increased to 150 in the DS version due to 15 extra Secret Stars and 15 additional course stars. After a long quest, Mario manages to find all the Power Stars and restore them to Peach's Castle, freeing Princess Peach and the Toads.
 Super Mario Galaxy
There are four types of Power Stars: First, there are regular Power Stars which serve the same purpose in other 3D Mario games. The second kind is the three Green Power Stars that, in addition to acting as normal Power Stars, unlock the Green Launch Star in the Comet Observatory which leads to the Trial Galaxies. The third type is the Red Power Star. There is only one; it appears in the Super Mario Galaxy mission "Gateway's Purple Coins". Last are the seven Grand Stars, the first of which, and the only one obtained without a boss fight, is the first star in Gateway Galaxy. Subsequently, the other six Grand Stars are each collected by beating Bowser or Bowser Jr.. These power up the Comet Observatory and unlock new Domes to explore.
There are also Silver Stars, which aren't as powerful as the regular Power Stars. Five Silver Stars are equal to one Power Star. They can combine to form a Power Star.
 Super Mario Galaxy 2
In Super Mario Galaxy 2, Power Stars play the same role as in Super Mario Galaxy. There are 242 Power Stars in total (116 normal Power Stars, 6 Grand Stars, and 120 Green Stars). If the Cosmic Spirit is used to complete a mission, the star collected will be a Bronze Star instead of a gold one.
 Mario Party series
Stars have since been a recurring theme in Mario games, but in particular Power Stars are used often in the Mario Party series of games. They are most often found at a certain spot on the board, waiting for characters to grab them. Traditionally, twenty coins are required to pay for these Stars, but some of the more recent titles in the series feature more unique ways for acquiring these precious items. All of these games allow an option to retrieve Bonus Stars to increase the chances of becoming the Super Star.
Mario Party 9 is the only game to break this trend. Instead of collecting Stars to become the Super Star, one must collect Mini Stars instead. However, at the end of a party, several Mini Stars combine into one Power Star when determining the Super Star.
 See Also
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