Grand Star

The Grand Stars are seven very large and powerful Stars in Super Mario Galaxy and Super Mario Galaxy 2.

Super Mario Galaxy
As Mario gets these Grand Stars back, he gains access to different parts of Rosalina's observatory. When one is found, a path to the next dome is created. The Grand Stars are the power source of the Comet Observatory's beacon. The beacon is the main source of power of the observatory, but the Grand Stars are needed to power it. Bowser has taken all of the Grand Stars and is using them for his own evil intentions.

At the game's beginning, Rosalina provides Mario with a young Luma who is going to assist him in his quest to save the Power Stars. Mario then sets out through the Gateway Galaxy, eventually finding a Grand Star inside a metal planet. A Luma tells him that the "Bad Guys" are using the Grand Star to power "Something". Mario must then complete a simple puzzle to free it. Mario receives the second Grand Star after defeating Megaleg. The third one is being used as a power source for Bowser's Star Reactor; Mario will have to defeat Bowser to get it. The fourth is being held captive by Bowser Jr.. The fifth is found in Bowser's Dark Matter Plant. The sixth is obtained by defeating King Kaliente in Bowser Jr.'s Lava Reactor. When 60 Stars have been found, Mario will able to fly to the Center of the Universe to confront Bowser for the final battle, where the seventh and final Grand Star is obtained. The Grand Stars have the same goal as the Keys from Luigi's Mansion.

Mario Kart Wii
In Mario Kart Wii one of the Grand Stars appears as Rosalina's emblem.

Super Mario Galaxy 2
The Grand Stars re-appear in Super Mario Galaxy 2, once again under the possession of Bowser and Bowser Jr. This time, Grand Stars give power to Starship Mario and open up the portal to the next world.

In this game, the Power Stars are apparently the power source for Bowser Jr.'s machines and Bowser's huge size. After defeating a machine, it will explode and release a Power Star. After defeating Bowser, he'll cough up a Power Star(literally, it comes from his mouth!).

However, they're not near as big a part of the story as the first game.

Trivia

 * A Star obtained after defeating Bowser at the end of Super Mario 64 is similar to a Grand Star as it is much bigger than the other Stars. However, it does not have orbs on the end of its points and may simply be a giant Star.
 * It is possible to only have 5 out of the 7 Grand Stars collected to reach the final battle, since Bowser's Galaxy Reactor, where the final battle is, is unlocked after completing the fourth dome.
 * The orbs on the ends of the Grand Star's "arms" make them slightly resemble Shine Sprites.