In Super Mario Galaxy, 60 Power Stars are needed to unlock Bowser’s Galaxy Reactor.
Bowser’s Galaxy Reactor is the final major galaxy in the Comet Observatory’s Engine Room. Reaching it requires Mario to collect at least 60 Power Stars, after which Rosalina can send him to Bowser’s domain for the game’s climactic confrontation.
The level, titled “The Fate of the Universe,” combines dangerous environments and demanding platforming before leading to the final battle. Completing it allows Mario to rescue Princess Peach and finish the main story. The game’s ending is not the absolute end of star collection, however.
The number is often confused with 120, because Super Mario Galaxy contains 120 regular Power Stars for Mario. Collecting all 120 unlocks Luigi, who can repeat the adventure and collect another 120. The 60-Star requirement is specifically the threshold for entering Bowser’s Galaxy Reactor during the first playthrough.