Bowser's Star Reactor
From Lumapedia
Bowser's Star Reactor is the fifth and final accessible galaxy in the Fountain.
This galaxy is the first in which Mario/Luigi faces Bowser, after traversing many planets filled with fire bars and other traps. Once defeated, Bowser drops a Grand Star that unlocks the Kitchen.
List of Stars
Routing Info
Bowser's Star Reactor is accessible after the player has collected 15 stars.
The Fiery Stronghold is a required star.