Bowser's Galaxy Reactor is accessible once you have 60 stars and most of the Grand Stars. To play it, visit Rosalina in the middle of the Comet Observatory.
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, and avoid the Dry Bones and Fire Pillars. The next part is a fire planet similar to a planetoid from the Melty Molten Galaxy. Here you must traverse the platforms before they fall into the lava. Avoid the fireballs and the jets of steam, and don't fall into the lava! The third part has ice blocks that make a path similar to the platforms in Space Junk Galaxy. Unlike those platforms, after a short time each one will collapse. Precision is the key here; one wrong move will drop you in the ice. The fourth part is a sand planet similar to a planetoid from the Dusty Dune Galaxy. Traverse the platforms while avoiding the Fire Pillars. Note the sand is one hit K.O. sand. The fifth part has cosmic platforms similar to ice blocks on the third planet, but this time you need to avoid the Fire Pillars, unlike the third planet. The sixth part has a Fire Ring planet from the Melty Molten Galaxy. Traverse the platforms while avoiding the Bullet Bills, Banzai Bills and lava. Dodge them all to reach a Launch Star to the planet near the Sun. You will then see Bowser Jr., who has Princess Peach behind him on his airship. Bowser is also here. Grab the Life Mushroom and 1-up Mushroom while climbing the stairs.
You will battle Bowser on three different Planets while Bowser Jr. watches the fight from his airship. On the first planet, Bowser will jump 3 times, creating 3 shockwaves. He will then turn into a boulder and roll at you. He looks similar to the boulders with the red centres, which gives you a hint on how to attack him. Just as those boulders can be destroyed by spin attacking them in the red centre, here you have to spin attack him in his face. He moves relatively slowly so this isn't too hard. He will start spinning, so spin attack him, then spin attack him again, then repeat once he stops spinning. On the second Planet closer to the Sun, Bowser will shoot fireballs in a line. The green plants will protect you from his fireballs. He will then roll using two of his shells. Here he looks similar to a Spiny plant, which again gives you a hint on how to attack him. You have to make a green plant hit him by spin attacking it, then spin attack him once he starts spinning. On the third planet, he will shoot fireballs, roll between two shells, and create 5 shockwaves at a time. Burn his tail by luring Bowser to ground pound the glass of Star energy, then spin his tail. He will start spinning, so spin attack him. Repeat this, and eventually you will 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, while all the Lumas jump into the black hole to stop it. Mario, Peach, and Bowser all end up at Princess Peach's castle. After the credits, the Purple Comet is unlocked which has Purple Coins (100 of them give you a star) and the last chapter in Rosalina's storybook is unlocked.