Grandmaster Galaxy

Grandmaster Galaxy is the final galaxy that appears in Super Mario Galaxy 2. The galaxy is located in World S. This galaxy is unlocked when the player gets all 240 stars and is an obstacle course with puzzles, enemies and Power Stars at the end. This galaxy contains two extra Stars: one for completing the level and another one for doing it again, but with only one hit point. In the last case the player gets the star from Rosalina.

Flower Grapple Planet
The first planet is where Mario must use Yoshi to swing on the flower grapples to reach the Launch Star. There are several obstacles along the way, such as Mines and Bullet Bills. Paragoombas also appear, but are replaced by Choppahs in the second mission.

Flipswitch Planet
After using the Launch Star from the first planet, Mario and Yoshi lands on a planet with Flipswitches, with a backdrop showing moving sprites of items from Super Mario Bros.. The player must touch all the Flipswitches (while avoiding laser-firing robots) to reveal the Launch Star.

Electric Maze Planet
After landing, Mario/Luigi will find a Cloud Flower and a fan making an upward gust. Mario/Luigi must use the cloud flower to make it to the Launch Star at the end of the maze. More flowers are supplied along the way. In The Perfect Run, Mario/Luigi is stuck with this status for the rest of the level.

Platform Planet
A planet that has many platforms that have electric walls moving through them, making it tedious. Also, platforms that shrink, and platforms that move when Mario spins appear here too, along with the Electrogoombas.

Pull Star Planet
A planet with a lot of electricity. In the beginning, there are some blocks with electricity on them. Later, there is a long segment of Pull Stars where the player must avoid the electricity and Paragoombas to get to the end, where he finds a Launch Star.

Hammer Bro Planet
This planet is highly infested with Hammer Bros., and Mario has to get past them to reach the other side. Flimps are also present in this area.

The end area has three Boomerang Bros. that Mario must take out. Defeating them reveals a Power Star (or a Launch Star in the Daredevil run).

The Comet Medal for this galaxy is on the far right side of this planet, next to some platforms.

The Gate Planet
This planet is only accessed during the Daredevil run. It's basically a small planet with a small house built on it. Mario also meets Rosalina on this planet, which strongly resembles the Gate in its prequel, Super Mario Galaxy.

The Perfect Run
The Prankster Comet for this galaxy is unlocked when the player finds this galaxy's Comet Medal, and gives Banktoad 9999 Star Bits.

Basically, the player redos the "The Ultimate Test" star, but under the Daredevil conditions. There are no checkpoint flags, and some of the Paragoombas are replaced with Choppahs.

If the player survives all the obstacles, he/she will run into Rosalina, who will thank the player. The Perfect Run is considered the hardest Power Star to collect in the entire game.

Music from the original Super Mario Galaxy is played as well. The music that's played is from Gusty Garden Galaxy, Good Egg Galaxy, Bowser's Galaxy Reactor, and Comet Observatory, in that exact order.

Trivia

 * This is the only galaxy in Super Mario Galaxy 2 to not encounter the green stars. This is primarily because the player had to collect all the green stars to unlock this galaxy.
 * This is the only galaxy in Super Mario Galaxy 2 that requires the player to collect Star Bits in addition to collecting the Comet Medal for a Prankster Comet to appear.
 * Oddly enough, the game does absolutely nothing to either indicate or hint to the player that he/she is required to collect 9,999 Star Bits in order to uncover the last star in the game. However, Lubba does show some curiosity regarding what happens when 9,999 Star Bits are collected.