Bowser's Star Reactor

Bowser's Star Reactor is a level located in Super Mario Galaxy and the first level where Mario fights Bowser. Before that, he has to travel through a short level that resembles one of the Bowser levels from Super Mario 64 (the music is also the same from those levels).

After Mario reaches a green staircase at the level's end, Bowser will appear and start breathing meteorie like objects(probably to fit in with Galaxy's space theme) at the staircase. This causes the stairs to start breaking; Mario must reach the top before they break completely. Upon reaching the top, Bowser roars and changes the gravity; Mario and Bowser "fall" to a metallic planet above the level.

Mario fights Bowser on this planet. After he defeats Bowser, the king of the Koopas says he has not been defeated yet, teleports away, and leaves a Grand Star behind, which Mario gets.

Loop Planet
This Planet contains Firebars and a 1UP that Mario can get. When the player advances on the platforms, they will go into a loop. This planet also has a Thwomp and a flamethrower, and a platform that takes Mario to the Gravity Planet.

Gravity Planet
Here, Mario must be careful as to not fall off the platforms and into a Black Hole, because the gravity changes back and forth here. There are also more Firebars and a Thwomp here. At the end of this area is a platform leading to the Stairs.

Stairs


This area are stairs that lead to the platform where Mario fights Bowser. There are yet more Firebars, flamethrowers, and Thwomps that make getting to the top harder. After the Firebars, Bowser will appear and then he will try to throw fireballs at Mario. When the player reaches Bowser, they will be taken to the Magma Planet.

Magma Planet


This is the planet where Mario battles Bowser. When Bowser Ground Pounds the blue glass, Magma will erupt and burn his tail. When Bowser is defeated, Mario will recieve a Grand Star, unlocking the Kitchen.

The Fiery Stronghold
Mario starts the level flying through space past Bowser's airship fleet and landing on a platform with a long path bending upwards and a giant warp pipe attached to it. The player will have to walk on the path. The player will have to walk on to find a crystal with a 1-Up Mushroom in it and the main platform, with two firebars, a flamethrower and a Thwomp. Up next is a green platform which rotates 180º to come to an area with shifting gravity. The player will need to go onto a yellow platform to come to a grey platform with four firebars in a cross shape turning counterclockwise. A Thwomp will try to squash Mario as the player came into an area with three flamethrowers, a moving platform, and three groups of firebars. After passing through these obstacles, the player will have to to fight Bowser. On the way he will breath fire at Mario, breaking the area's platforms. The player can obtain a Life Mushroom trapped in a crystal. When Mario gets to the top Bowser swaps the gravity between the two planets. His first attack is a stomp which will make a shockwave move around the planet, and Mario will have to jump over it. Then Bowser will try to ground pound Mario. The player must make Bowser smash through the planet's glass and burn himself on the Star energy underneath. He will run around the planet, and the player must try and Star Spin to attack his tail that will send him spinning on his shell. The player should Star Spin at him again to give damage. He will then start shooting fireballs at Mario. The player will have to repeat this process to defeat Bowser.