This cozy space will open up once you beat Bowser's Star Reactor. It will give you access to two major galaxies, two small galaxies, and the chance to face off against Bowser Jr.
Beach Bowl Galaxy
True to its name, this galaxy is shaped like a bowl and filled with penguins. You'll need 16 stars to access it.
Swim around and collect five star pieces to shoot to the top of the island where you can ground pound switches until you get to the Power Star.
Passing the Swim Test
Coach Penguin is going to test your skills by having you spin around in the water while trying to chase down some penguins. Nab the golden shell then head back to the surface and give it to the Coach to receive the Star.
The Secret Undersea Cavern
Grab a shell while underwater and use it to break a wall and reveal a cavern where you can flip a switch. Do some platforming and dodge some blocks that will one-shot you. You'll find the Power Star at the end of the path.
Fast Foes on the Cyclone Stone
Get under the Thwomp and hed straight while avoiding Tox Boxes. Jump across the water using the moving platforms. You'll have to deal with more Tox Boxes on the other side. Get past the last Thwomp and spin attack the crystal twice so it will release the Star.
Wall Jumping Up Waterfalls
Get the golden shell from the swim test but instead of giving it to Coach Penguin use it to smash open the treasure chest on the beach to unlock a Launch Star. Take it to another island where you can get the Cataquack to hit you while you're standing under a ? coin. This will fling you into it, making an Ice Flower appear.
Use the flower to don a special suit that lets you treat the waterfalls like walls which you can jump up to reach the top of the island. Once you find the Power Star, pull the same trick with the Cataquack to reach it.
You'll need 20 stars to access this haunted galaxy, which is also where you'll encounter Luigi for the first time and unlock his missions in previous galaxies.
Luigi and the Haunted Mansion
Flip the switch by the door to light the place up and then lure the Boos into the room. One will drop the key to the next room where you can collect Launch Star pieces which will get you to Luigi. Use a Pull Star to get the key and you'll also get a Boo Suit that lets you levitate and phase through bars. Use that to get into Luigi's cell, use the light to become regular Mario again, and talk to him to get the Power Star.
A Very Spooky Sprint
Use Pull Stars until you get to a ghost who challenges you to a Pull Star race. You can use a Launch Star as a shortcut if you lock unto one of the Pull Stars near where you land to avoid falling. If you beat it to the planet, the ghost will give you the Power Star.
Beware of Bouldergeist
To defeat this boss you need to wait until it shoots rocks at you. The black rocks will transform into Bomb Boos, which you'll need to fling back at it three times. This will make it change forms and start floating around the room. Chuck another Bomb Boo at it to make it reform.
This time it has hands that will attack you independently so avoid those as you through three more Bomb Boos at its body. It will go into essence form again and then you just need to hit it one more time to receive the Star.
Bouldergeist's Daredevil Run
You need to complete the same boss fight without taking any damage to get this comet star.
Purple Coins in the Bone Pen
You have one minute to gather 100 of the 150 coins in this level using Pull Stars to fling Mario around. The Star will appear in the location where the life Mushroom is so rush to it as soon as it spawns since the timer won't stop until you have it.
Matter Splatter Mansion
After launching from the Starshroom at the beginning of the level, go towards the bats and spiders and spin attack the rock spire. You'll gain access to a series of moving platforms that you'll need to navigate to get to a set of locked doors. The last key you find will unlock the Power Star.
Bubble Breeze Galaxy
Find a bubble and hop into it, and it will carry you above the poisonous purple water that will kill you if you fall into it. Avoid mines that will pop the bubble, though you don't have to worry about hitting walls. Follow the arrows, flying next to wooden walls to move them until you find the Star on an island to the south.
Bouy Base Galaxy
You'll need to have completed the Sunken Treasure level and gathered 30 Stars to get to this small galaxy.
The Floating Fortress
Collect five blue Star Chips scattered around an area with moving platforms then climb to the top of the fortress where a Pull Star will appear. Use it to get to the water sphere planet then get on top of the Ball Beamer and spin to get a gold screw out. This will reveal a Power Star submerged in the center of the planet. Dive in and get it.
The Secret of Bouy Base
Dive into the pool of water and you'll see a caged green pipe. Open it by luring over a Torpedo Ted. Take the pipe outside the planet where you'll need to lure Bullet Bills into a cage holding the Power Star.
Bowser Jr.'s Airship Armada
While Bowser Jr. shoots cannonballs and Bullet Bills at you from his airship, you'll need to jump on the Koopa Troopas on Mario's airship to grab their shells and throw them at the boss. Hit him twice and Magikoopas will start to spawn. After hitting the airship three times, Bowser Jr. will start throwing fireballs. Hit the ship twice more and he'll abandon it, leaving the Grand Star behind.
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Samantha Nelson writes about gaming and electronics for iMore, Windows Central and Android Central while also covering nerd culture for publications including IGN and Polygon. She loves superheroes, RPGs, cooking, and spending time outside with her dog. You can follow her on Twitter @samanthanelson1.