Bowser's Galaxy Reactor
From Lumapedia
Bowser's Galaxy Reactor is the final grand star galaxy in Super Mario Galaxy.
This galaxy contains plenty of gravity-bending obstacles and environmental planets. Defeating Bowser at the end rewards the player with a Grand Star that marks the end of the Any% category, and beating it a second time is required to complete the 120 star category.
List of Stars
Routing Info
Bowser's Galaxy Reactor is accessible after the player has collected 60 stars, with one of them being Darkness on the Horizon.
The Fate of the Universe must be completed before any Purple comets will appear.