Melty Molten Galaxy

The Melty Molten Galaxy is a galaxy in Super Mario Galaxy, and the last major galaxy to be unlocked in a dome. It is quite a large galaxy, made completely of lava and rock. It is situated between two stars, a red one and a light purple one, with large streams of plasma pouring from the former to the latter. There are many Launch Stars here, allowing Mario to traverse the massive planet. There are also some small planetoids around it. This is the only galaxy that has Star Chips in all three missions and the comet mission.

The theme to this song is in the key of D, which is used to produce melancholic and dark sounding compositions.

Planets/Areas
Note: Unless otherwise stated, all planet names are unofficial.

Starting Planet
This area is a large disk of lava connected to the main planet. It contains a small volcano with two large platforms on either side of it. Both have various obstacles and enemies. Small steel platforms make a path between the two platforms. The volcano has a switch inside that makes the lava rise and release Star Bits.

Volcano
The Volcano is a volcanic area found in the Melty Molten Galaxy. There is a Launch Star above it, but if Mario drops into the volcano, there is a ground-pound switch on a rock floor at the bottom, which causes star bits to appear. However, the volcano will erupt fifteen seconds after Mario activates the switch, so he will have to use a Sling Star at the bottom to get out. In the Purple Coin mission (Red Hot Purple Coins), there are purple coins down the sides of and inside of the Volcano Area. One coin in this mission is placed on top of a small lava ring inside the volcano, requiring a health slice to reach it, unless the player uses a good timed spin or a Long Jump.

Small Glass and Lava Planets
These are a collection of six small lava planets and three small glass planets. Mario must navigate his way through these planets via Pull Stars avoiding solar arcs that will knock him off the Pull Star path and take him into their gravity fields causing Mario to bounce across the lava surface. In the Daredevil Run, Mario needs to wait for these arcs to pass or he will be hit and lose a life.

Hardening Lava Planets
These are two hardening planets made of magma and cooled lava. One spins and the other does not. They are home to several Li'l Cinders. Beside them lies a crashed Topman saucer that serves as a starting platform; another one, tipped over and scorched, is seen later, but not used as a platform. These planets have their own gravity even though they are embedded in the main planet's surface. These saucers are a reference to an earlier version of the game. Topmaniac was originally planned to be fought in this galaxy, but the idea was eventually scrapped.

Lava Spire
The Formation Planets are a large rock formation surrounded by other large rock formations that are intertwined. The spire is also home to many Li'l Cinders. On top of the spire, a Power Star is incased in a large ice crystal that Mario must get to. When Mario steps on the spire, it begins to sink, so he must run up the spire or be engulfed by the lava. There is a shortcut at the beginning of the spire that leads Mario halfway up and gets him a 1-Up Mushroom.

Thwomp Planet
This planet is a large stone octagon that is covered by three Thwomps, which has a 1-Up Mushroom under the third Thwomp. The core of this planet is a black hole that Mario must avoid.

U Tower Planet
This planet is a large U-shaped planet of cooled magma with stone built onto it to make it like a tower from a castle. The inside of the planet is made out of hot magma. On one side of the planet, there is a Launch Star that leads to the next planet. On the other side of the planet, there is a double-sided Bill Blaster that will try to stop Mario from continuing.

Cooled Lava Area
This is a small network of cooled lava platforms that is home to several Lava Bubbles. Mario must find five Yellow Star Chips to continue.

Lava Ball Rolling Area
This area is made up of many lava panels, four stone panels, and a humoungus tunnel of lava. The tunnel has many holes in it that Mario can fall through to his death. Mario must roll on the Rolling Ball to the finish line so he could receive the Power Star. There is a shortcut Mario can take to avoid the moving lava platforms: he can jump to the stone platform with a 1-Up Mushroom. Jumping on this platform will cause the ball to slow down, so the player must roll the ball near the back edge of the platform, build up momentum, and jump. It saves the time of going over the moving lava platforms.

Steel Platform Area
This is a small area with many steel platforms and sling stars. Mario must use the Sling Stars to avoid falling into the lava and dodge the Lava Geysers. At the end of this area, there is Lumalee that Mario could decide to buy either a 1-Up Mushroom or a Life Mushroom for the battle against Fiery Dino Piranha.

Disk Planet
This is a small disk with a spherical lava core. It has several platforms that rotate around the core that Mario must ride and collect five yellow Star Chips to continue to the next planet.

Fiery Dino Piranha Planet
This planet is the home to Fiery Dino Piranha. Mario first arrives on the planet as he lands on the Fiery Dino Piranha egg and cracks it, which makes a slight crack on the egg. After Mario spins the ball at the end of the tail, the Fiery Dino Piranha breaks out of its egg and tries to attack Mario. Mario is rewarded with a Power Star after he defeats Fiery Dino Piranha.

Lava Sea Planet
This is a small rock planet with a sea of lava around it, created by feeding a Hungry Luma 80 Star Bits. The lava has a tide that reveals a network of paths when low, which Mario can advance through, but the lava covers up the path when it is a high tide. Mario must scuttle across the planet, avoiding high tides and Burn Bits to retrieve five Silver Stars and get the Power Star.

Prima Official Game Guide description

 * "Magmaws and Magmaarghs erupt from the lava-soaked worlds of this galaxy, splashing the skies with molten earth in hopes of keeping you from collecting the Power Stars."

Trivia

 * This is the only galaxy to feature the Star Ball but not have the word "Rolling" in its name, due to the Star Ball being a minor part of the galaxy, rather than the central focus.
 * This is one of only two galaxies to feature the Speedy Comet theme in a non-comet mission, the other being Dusty Dune Galaxy.