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The Cobalt Star is the power source for Professor E. Gadd's time machine in Mario & Luigi: Partners in Time. The Cobalt Star was shattered into multiple shards by Princess Peach after she used it to trap Elder Princess Shroob, the leader of the Shroobs that were invading the Mushroom Kingdom of the past. However, her sister, the younger Princess Shroob kidnapped Princess Peach in the castle of the past, which is later turned into the Shroob Castle. Destroying the star also caused Time Holes to appear between the past and present, and with the help of their infant selves, Mario and Luigi go about retrieving the shards and reassembling the Cobalt Star to fix the fabric of time and save Peach from the aliens. Later in the game, at Star Hill, the "spirit of the Cobalt Star", which is the Elder Princess Shroob, sends the heroes to the Star Shrine, where there's another Cobalt Star shard.
Baby Luigi and Kylie Koopa both realize something is suspicious about the Cobalt Star, but the full truth is not revealed until it is too late: all of the shards are assembled at the Shroob Castle when Baby Bowser steals the last one from Princess Peach. Baby Bowser is transformed into a Shroob Mushroom, and the Elder Princess Shroob then uses her regained power to break free of her prison and attack the heroes.
It appears that the Cobalt Star does not exist anymore in the Mario universe, as Elder Princess Shroob states that the star "is no more".
Cobalt Star shard locations
Below is a list of where the six shards of the Cobalt Star are located.
Names in other languages
Cobalt Star shard