Super Mario Galaxy-
As Mario gets these stars back, he gains access to certain parts of the Comet Observatory, Rosalina's observatory. When one is collected the path to the next dome is unlocked and created. Grand Stars are the power source of the Comet Observatory's beacon. The beacon is the source of power in the Observatory, but Grand Stars are needed to power it. Bowser has taken all the Grand Stars and used them for his evil inventions. At the beginning of the game, Rosalina gives Mario a young Luma who helps him find the Power Stars. Mario then sets out through the Gateway Galaxy eventually finding a Grand Star in a metal planet. A Luma tells him 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 in Bowser's Star Reactor. Mario must defeat Bowser to get it. The fourth is being held by Bowser Jr.. The fifth is found in Bowser's Dark Matter Planet. The sixth is obtained by defeating King Kaliente in Bowser Jr.'s Lava Reactor. When 60 Stars have been found Mario can go to the Center of the Universe and defeat Bowser in his final battle to obtain the seventh Grand Star.
Super Mario Galaxy 2-
The Grand Stars re-appear in Super Mario Galaxy 2. They are, once again, captured and used by Bowser and Bowser Jr.. This time Grand Stars give power to Starship Mario. They open up the portal to the next world. In this game, the Power Stars are the power source for Bowser Jr.'s machines and Bowser's huge size. After defeating the machines releases a Power Star. After defeating Bowser he coughs up a Power Star from his mouth! In the final battle he swallows it again. This leads to the final segment of the battle.