From the Super Mario Wiki
The Comet Observatory (also called Observatory Grounds or just simply Observatory by Super Mario Galaxy's pause menus) is one of the many areas from Super Mario Galaxy for the Wii. It acts as the hub area, similar to Peach's Castle in Super Mario 64 and Super Mario 64 DS or Delfino Plaza in Super Mario Sunshine. The Comet Observatory is controlled by Rosalina, and acts as the home of both her and the Lumas, as well as a single Gearmo.
Super Mario series
Super Mario Galaxy
It was built by Rosalina and Luma after Rosalina agreed to take care of him. Over time, it grew to be much too big for just the two of them. Soon, other Lumas came to live in the Comet Observatory as well, and Luma transformed into the Beacon that powers the Observatory.
The Comet Observatory normally comes into orbit around the Mushroom World every 100 years, and during that time, the people of the Mushroom Kingdom celebrate the Star Festival, thinking that the observatory is simply a comet. During the most recent Star Festival however, Bowser's forces attacked the Comet Observatory, causing it to lose its power source: Grand Stars and Power Stars. They were scattered throughout the cosmos following the attack, causing the Comet Observatory to remain immobilized above the Mushroom Kingdom. He later attacked Toad Town during their celebrations. Only after Mario recovers five out of seven Grand Stars can the Observatory fly like a starship again. When this has been accomplished, Rosalina takes Mario (or Luigi) to the Center of the Universe to rescue Princess Peach.
The Comet Observatory can be accessed through a portal in the Gateway Galaxy. Throughout the game, there are plenty of characters residing here, including Mario, Rosalina, the Lumas, Luigi, and the Toad Brigade. Much like Peach's Castle in Super Mario 64 or Delfino Plaza in Super Mario Sunshine, certain parts of the observatory are unlocked as Mario gradually acquires Grand Stars and Power Stars. This is because the Grand Stars are used to power the large Beacon in the middle of the Comet Observatory, which enables it to transform into a comet and fly across the cosmos. Once five Grand Stars have been acquired, the observatory will have enough power to travel toward the center of the universe. After six Grand Stars have been obtained while the total Stars obtained is less than 60, Polari says that the beacon is the brightest that it can be.
There are six domes located throughout the Comet Observatory, which act as Mario's means of accessing all of the galaxies that he will encounter in the game. In order, the domes are the Terrace, the Fountain, the Kitchen, the Bedroom, the Engine Room, and the Garden. Galaxies are accessed by collecting multiple Power Stars to unlock more and more domes. Additional Galaxies can be unlocked by feeding the seven Hungry Lumas that appear outside each dome (and the Gate). According to the Lumas, the domes are used to view the cosmos. Other areas in the Observatory are the Garage, the Library, and the Gate.
After Bowser's giant sun collapses into a massive black hole following his defeat by Mario in the final battle, the Comet Observatory is pulled toward the black hole. Parts of the observatory were broken off and scattered as Peach's Castle crashed into the side of it. However, the Lumas all sacrificed themselves by jumping into the black hole, creating a large explosion and reforming the entire cosmos. It can also be inferred that this explosion reformed the Comet Observatory, as it is seen intact in the ending sequence after collecting 120 Power Stars and defeating Bowser a second time. Rosalina transforms it into a comet once again and then flies off to parts unknown.
As the game progresses, the waltz-like theme that plays in this area gains more instruments, though the Gateway Galaxy theme plays instead before the player completes the first mission. If Mario or Luigi try to jump off of the Comet Observatory, they will be enveloped in an energy-like bubble and return to safe ground. After completing Bowser's Galaxy Reactor for the first time, a countdown will appear on the tiles underneath Rosalina (talking to her again from that point on will result in her asking if the player wants to complete that level again) showing how many Power Stars are still left to be collected. When all of the stars are finally collected, fulfilling 100% completion, then the countdown will read zero and it will be possible to play as Luigi.
Despite being the game's main hub area, it was actually created rather late in the game's development, after plans to use the Starship Mario (which would be seen in the game's sequel instead) as the hub area were not realized.
Super Mario Galaxy 2
The Comet Observatory is absent from most of the game, while its role as a hub level is now filled by Starship Mario. Once Mario has obtained enough Power Stars to access Bowser's Galaxy Generator, he, Luma, and Lubba witness from the starship a large comet being drawn into Bowser's fortress. This comet resembles the observatory as it was seen flying in the previous game, and Luma's despairing reaction to seeing it heavily implies that they are the one and the same. Lubba speculates that Bowser must be seeking to become even larger, indicating he had been trying to steal power from the observatory and at the same time keep it from aiding Mario on his journey.
Once Mario has defeated Bowser for the final time and returned to Starship Mario with Peach, the two watch as the Comet Observatory flies up to them and Rosalina appears from inside, seeking to finally reunite with her lost Luma. After thanking Mario for his help, Luma takes his hat as a souvenir, so that he may never forget the adventures they went on, before departing in the observatory with his mother.
Super Mario 3D World
The Comet Observatory makes an appearance in the background of World Star-2 (Super Galaxy), which is based off of the Super Mario Galaxy games. The Comet Observatory can also be seen faintly in the top right of Super Mario 3D World's full group artwork, and in the sky orbiting the Mushroom World after the game's credits.
Mario Kart 7
A structure that strongly resembles the Comet Observatory appears on the side of the Rosalina's Ice World course. Instead of being a peach color with purple outlining, it appears in an aqua color, but still includes the stars on the side of the observatory, hinting that this is the Comet Observatory. However, it serves only as a part of the course's background.
Mario Tennis Open
The Comet Observatory first appears in the title screen of Mario Tennis Open. It is seen hovering with the Galaxy Arena above the Mushroom World. It appears prominently as a background for the Star Open's Final cup, the Galaxy Arena. One can clearly see the contents of the observatory by simply switching the camera's angle.
When the game is played in Tournament Mode for the first time and the player beats the World Open, the player receives an invitation from the Comet Observatory stating that the player is now proclaimed as a Star Player and is now able to play in the Star Open.
Mario Party: Island Tour
The Comet Observatory is seen in the background of Rocket Road in Mario Party: Island Tour for the Nintendo 3DS. When a character wins the game, they circle the Observatory in a victory lap. In this game, both the Fountain area of the Observatory and the Beacon are absent.
Mario Golf: World Tour
Mini Mario & Friends: amiibo Challenge
The Comet Observatory makes an appearance in the background of Mini Rosalina's levels in Mini Mario & Friends: amiibo Challenge. The observatory has a few differences, such as the Garden platform and the Engine Room platforms having been replaced with glowing rings. The Fountain is much farther from the observatory then usual, and the front platform is, upon closer examination, missing.
Mario Sports Superstars
Since Mario Sports Superstars reuses Rosalina's golf animations from Mario Golf: World Tour, the Comet Observatory reappears in the same capacity.
Dr. Mario World
The Comet Observatory's background music is titled "Rosalina in the Observatory". It has made multiple reappearances in later games:
Names in other languages