Bowser's Galaxy Reactor is accessible from the Garden observation dome. You need 60 Stars to enter.
Star 1: The Fate of the Universe
This is it – the final level. This is the 60th star, so when you go for 120, you have to play this level again. The first part is a castle wall with spotlights on them. Stay on the spotlights without losing it. The next part is a fire planet from Freezeflame Galaxy. The third part has ice blocks that make a path from the Space Junk Galaxy, the fourth part is a sand planet from the Dusty Dune Galaxy. The fifth part has ice blocks like the third planet, but this time they disappear after a few seconds. The sixth part has a Fire Ring planet from the Melty Molten Galaxy. There are Bowser Statues, Bullet Bills, and Banzai Bills. Dodge them all to reach a Launch Star to the planet near the Sun. Bowser Jr. has Princess Peach behind him on his airship. Bowser is also here.
You will battle Bowser on three different Planets while Bowser Jr. watches the fight from his airship. On the first planet, Bowser will turn into a boulder and roll at you. Make a green plant hit him in the face. On the second Planet closer to the Sun, Bowser will roll using two of his shells. You can't spin him. Make a green plant hit him again. On the third planet, it is inside the Sun! Burn his tail by luring Bowser to ground pound the glass of Star energy, then spin his tail three times to make him fall off the gravitational pull and into the Sun.
The Ending
Mario flies Princess Peach back to his starting position after receiving the last Grand Star. The spotlights are gone, then something mysterious happens – the Sun is sucking rocks. Bowser is on one of them, then the Sun near his Galaxy Reactor turns into a huge black hole sucking in Mario, Peach, and the Comet Observatory. Then one of Mario's Lumas waves and zooms into the black hole, reversing the gravity. Mario, Peach, and Bowser all end up at Princess Peach's castle. Then the Purple Comet is unlocked which has Purple Coins (100 of them give you a star). You can then begin the Other Half Quest.