Bowser's Galaxy Reactor

Bowser's Galaxy Reactor is the final level in Super Mario Galaxy. It is located in the center of the universe. This is where the final Grand Star is located. It features the final battle between Mario and Bowser. Bowser Jr. watches from the sidelines on his airship with Princess Peach. After this battle, the galaxy collapses, transforms into an huge black hole, and begins sucking up the entire Universe. The Lumas throw themselves into the hole and reverse its effect; the Universe is reborn, and Mario, Peach, Bowser, and everybody else land safely back in the Mushroom Kingdom.

Traversing the Galaxy
The galaxy contains seven planets (not including the three planets in which Mario battles Bowser). The first is a planet with spotlights that change the direction of gravity. This planet is reminiscent of the castle. The second is a spherical planet with a lava core and a few rocks. This planet is reminiscent of the flame half of the FreezeFlame Galaxy. The third is the same with an ice core and ice blocks. This planet is reminiscent of the freeze half of the Freezeflame Galaxy. The fourth planet has a sand core with rocks and firebars. This galaxy is reminiscent of the Dusty Dune Galaxy. The fifth one is actually not a planet. As Mario walks along, glowing blocks begin to appear, forming a path. This is reminiscent of the Space Junk Galaxy. The sixth is a cylindrical planet resembling a castle where Mario travels through the lava-filled core. This planet has Bullet Bills and Banzai Bills. The last planet is the pathway toward the battle between Mario and Bowser. There is actually two more unreachable planets of to the side of planets two, three, and four. One is completely made of water and the other of Lava.

The Fate of the Universe
This is the only level in the galaxy.

The Ultimate Pathway
The way Mario has to take to reach the battle with Bowser is not an easy one. He starts on a small rock resembling part of a castle wall. Using gravity spotlights, Mario could climb the walls and arrive at a tower guarded by Dry Bones. At the top a large Gravity Spotlight would carry Mario across to the Fire Planet, the path across this was over rocks that would topple over when stepped on, and sinking platforms. A second large spotlight carries Mario to the ice planet at the end of the pathway. Both the ice planet's surface and the fire planet's surface would cause one point of damage if touched. The Ice planet's pathway is formed by ice blocks that shoot in from space, but these would vanish after a time. The third planet, reached by another spotlight, is made of sinking sand, which is fatal if touched. The planet is traversed over circular platforms which move around but dip beneath the planets surface. There were also fire bars in the way. Yet another spotlight would carry Mario into a space junkyard, platforms would shoot in and shortly vanish, like on the ice planet. However, gravity arrows and fire bars get in the way this time. At the end of the pathway, a Gravity spotlight allows access to the fire tunnel, a hollow planet made entirely of fire and is cylindrical. Bullet bills, sinking platforms, Thowmps and Banzai Bills all get in Mario's way here. At the end, a launch star takes Mario to Bowser's Final Stand.

Jr. Attacks
Bowser Jr. appears on his airship at the bottom of the staircase to where Bowser stands with Princess Peach behind him. He taunts Mario, then flies behind his father and begins shooting down the staircase. When a meteor hits the staircase, the piece will crumble and fall. Along the way there are a 1-Up Mushroom and a Life Shroom. Grabbing them takes up valuable time, however.

Bowser's Showdown
After Bowser explains his plan, the gravity a nearby planetoid made of metal attracts Mario and he. He transforms into a boulder with his head sticking out of the top. If his head is hit by a spiky bush or Marios spin attack, he will fall out of the boulder, then must be spun again to take damage. After he is hurt twice, Mario and Bowser will fly to another planetoid. Here, Bowser transforms into a ball made of two of his shells. He then rolls around the planet. Mario must Star Spin into sling plants to hit Bowser on his back, then the player has to Star Spin him. After doing this twice, Mario and Bowser will fall towards a giant Sun, one of the biggest in the game. It is, however, hollow, with holes, and the two combatants will fall through one of these to land on the reactor at the center.

Final Fight


Here, Mario fights Bowser in a similar way to how he does in Bowser's Star Reactor and Bowser's Dark Matter Plant, except Bowser has the attack he used on the second planetoid, which Mario can only avoid. After hitting Bowser twice, Bowser will fall out of the reactor's gravity field and into the Sun. A Grand Star will then be revealed from inside the reactor, which the player takes.

Final Flight
As Mario flies away with the grand star, Jr.'s airship plummets towards the sun, but Mario rescues Princess Peach and flies her to the area where he first entered the Galaxy.

Burning Bowser
It is then revealed that Bowser has survived his fall into the gigantic Sun, but is stranded on a smaller one, on a platform which is sinking into the planet. As its gravity increases, the Sun sucks in more and more rocks from around it, then collapses on its own weight, becoming a giant Black Hole. It begins to suck up everything around it including Mario, Peach, Bowser's Airship Fleet, Peach's Castle, and the Comet Observatory. It would appear that everything is lost until Mario's (or Luigi's) Luma pops out, waves its arm and zooms toward the black hole. It is soon joined by all of the other Lumas. The Lumas twist the Black Hole into oblivion, but appear to be destroyed in the process.

Trivia

 * On the planetoid made of sand the player can reach the Tweesters by long jumping off the moving platform and Star Spinning at the ending height of Mario's long jump. The Tweesters do nothing good for the player and they occasonaly launch the player into space, making Mario lose a life.
 * This is the most hardest part of Super Mario Galaxy and the last part.