Melty Molten Galaxy is a galaxy found in Super Mario Galaxy. This galaxy is the final "main" galaxy in the game, and the final galaxy to be orbited by a Daredevil and Purple Comet. As the name implies, this galaxy mostly focuses on lava.
The Sinking Lava Spire
Through the Meteor Storm
Fiery Dino Piranha
Lava Spire Daredevil Run
Red-Hot Purple Coins
This is the only "main" galaxy to have Star Ball Rolling.
If the player falls into the volcano, they can ground pound on a switch to make Star Bits appear. But in the Purple Coin Mission of this galaxy, the switch is missing, most likely because 5 purple coins exist in there.
This galaxy has the highest amount of Star Bits needed to access the hidden star: 80. Interestingly, this is the only galaxy that needs 80 Star Bits to access the hidden star.
For some unknown reason, if the player uses the first Pull Star in the first mission, the player can't get the gravity to pull him/her back to the three ball-shaped planets: the gravity instead pulls them to a lava planet.