A Grand Star is an item found in the Wii video game Super Mario Galaxy. In the game, either Mario or Luigi, depending on the character you were playing as, would have to find at least six of seven Grand Stars in order to save Princess Peach from the clutches of the antagonistic Bowser. Once completed, he was then allowed to find the remaining one, or even do so before saving her.
Grand Stars resemble an enlarged Power Star with ball points at the ends (note: hundreds of Power Stars are featured in the game). They're much more powerful and are used to give the Observatory enough power to head to the center of the universe where Bowser was stationed. After defeating him in a climatic battle, Mario or Luigi would save Princess Peach and head back to the Mushroom Kingdom, though the player would be allowed to find the remaining Power and Grand Stars if they wished.
List of Grand Stars
One Grand Star is found in the very beginning of the game in the Gateway Galaxy.
After defeating Megaleg, either of the brothers will get a Grand Star.