When you get the 60th you are presented in the middle of the observatory and Rosalinda asks you if you want to fight Bowser (save the one that you love - Princess Peach), if you select YES then the observatory can fly to the center of the universe and you go to the showdown with Bowser. A little hard at first to reach him, due that his stage is kind of hard, but after many tries you step up the ladders to meet him. Nice fight
When you beat Bowser and everything runs down. The credits are presented and then you get THE END (so you really beat the game) even thou there are 59 extra stars to get if you wish. Bowser's star is the 61th