Bowser's Star Reactor is a level located in Super Mario Galaxy and the first level where Mario fights Bowser. Before Mario fights Bowser, he has to travel through a short level that resembles one of the Bowser levels from Super Mario 64, which is also where the theme music for this level comes from. After Mario reaches a green staircase at the level's end, Bowser will appear and start breathing fiery rocks from his mouth 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.
Mission 1: 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 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.
The entire level is on a space station.
This area resembles a pipe. Two Lumae give advice on what could go on within the area.
Mario heads around a giant loop that has outward Gravity. There is a 1-UP here.