From the Super Mario Wiki
A galaxy is a system consisting of gases, stars, planets, and comets. Mario or Luigi visit many galaxies in their adventures through Super Mario Galaxy and Super Mario Galaxy 2. There are currently a total of 91 different galaxies between the two games.
Most of the galaxies in Super Mario Galaxy are notably much smaller than real galaxies, often being comprised of nothing more than a few small planetoids, most of which come in many different shapes and sizes. In World 4 however, a galaxy proportionate to a real galaxy appears in the background, while in World 5, there is a galaxy supercluster. Smaller galaxies in the games are more comparable to systems such as the Solar System. There are solar systems that can be seen to make up other galaxies that determine whether it is a large or small one.
In Super Mario Galaxy, there is clearly a size difference between galaxies. Some of them contain six or seven Power Stars, while others possess only one (two in the Buoy Base Galaxy and the Gateway Galaxy). The "main" galaxies with six to seven Power Stars are usually much larger than the "bonus" galaxies with one Power Star, which may contain only one or two planetoids. In Super Mario Galaxy 2, however, this difference isn't as prevalent, as each galaxy contains either two or three regular Power Stars.
In Super Mario Galaxy, most galaxies are located within six different domes onboard the Comet Observatory. These domes show the player up to five galaxies and give them the option to fly to those galaxies in order to search for Power Stars. Just outside of each dome is a Hungry Luma, which transforms into an additional galaxy after the player feeds it a certain number of Star Bits. The remaining galaxies in the game are accessed via other areas on or around the Comet Observatory, including the Planet of Trials and the Gate. In Super Mario Galaxy 2, galaxies are grouped into seven different Worlds, each of which contains a total of seven galaxies. One Hungry Luma is encountered in each World, which transforms into a new galaxy when called to Starship Mario and fed the required amount of Star Bits.
Many galaxies in both games contain Black Holes, which Mario and Luigi run the risk of getting sucked into if they fall off the edge of some planets or areas.
The Center of the Universe
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