After defeating Boswer Jr's Airship Armada, you'll gain access to the fourth observatory dome. This one has three major galaxies instead of the typical two, plus a big boss fight.
Gusty Garden Galaxy
You need to have beaten Bowser Jr.'s Airship Armada and collected 24 stars to access this galaxy, which is filled with grass- and plant-covered planets navigated by gliding on gusts of wind.
Bunnies in the Wind
Ride Floaty Fluffs to get between planets until you get to a cluster with Piranha Plants and plant buds that will release vines. Use them to climb to one of the higher planets where you'll defeat a Spiny Piranha Plant to release a Launch Star. That will take you to a Puzzle Cube where you need to chase down a Star Bunny. Catch it, and you'll get the Power Star.
Dirty Tricks of Major Burrows
Ground pound near the boss to make him fully emerge. Spin behind him to damage him. Alternate between spinning and ground pounding based on whether he's chasing you or underground and you'll defeat him and get the Star.
Gusty Garden Gravity Scramble
Collect Star Chips to form a Launch Star that will take you to a large structure made up of moving blocks. Change the flow of gravity by spinning into arrows to navigate the structure while avoiding cannonballs and blocks. You'll find the Power Star stuck in a large crystal at the end of the structure.
Major Burrows Daredevil Run
You'll have to defeat Major Burrows again, but Mario can only take one hit.
The Golden Chomp
Collect all the ? coins in the air as you use the Floaty Fluff to move between planets and you'll spawn a Rainbow Star. Use it, then find the Gold Chomp on the next planet and destroy it before the power-up wears off. You'll be rewarded with a Power Star.
You need 26 stars to unlock this galaxy of extreme temperatures, where you'll spend a lot of time relying on both Fire and Ice Flowers.
The Frozen Peak of Baron Brr
Avoid the chunks of ice Baron Brr shoots at you and make your way to a crevice to the left of the platform he's on to grab an Ice Flower. Wall jump up to Brr. When you get on the platform, he'll slam down to create a shockwave that will knock Mario off unless he leaps over it.
Spin near Brr to make him shrink and become defenseless. Do it again to damage him. Hit him again and he'll call in some Ice Bubble reinforcements. Spin into him a third time and he'll shatter, revealing the Power Star.
Freezeflame's Blistering Core
Collect Star Chips to assemble a Launch Star that will take you to the Lava Core Planet. Go up a pole to reach a higher ledge and cross platforms until you find a ? Coin that will spawn a Fire Flower. Head back to the area with the pole and light a torch which will cause a staircase to open that you can take to the next area.
Flip gravity by jumping under the stone platform. You'll find another Fire Flower under a bubble; you'll need it to light two more torches, which will raise a wall. Wall jump until you get to another moving platform area and cross them until you get to another Fire Flower. Ride a moving platform to light two more torches which will open a gate and let you get the Star.
Hot and Cold Collide
Take a Launch Star to a planet featuring big Black Holes and some ice and lava platforms. Use Ice Flowers to create your own platforms over the lava. Navigate the moving platforms until you get to a path featuring just lava platforms. The Power Star is at the end.
Frosty Cosmic Mario Race
Cosmic Mario will challenge you to race across a series of ice platforms where gravity flips. If he wins, you'll lose a life and need to start over. If you win, you'll get the Power Star.
Purple Coins on the Summit
Collect all 100 coins on the planet using Ice Flowers to climb waterfalls and access a secret path that requires a Fire Flower. Once you've got all the coins, head back to the start of the level to get the Star.
Conquering the Summit
Grab the Ice Flower you used when fighting Baron Brr and jump up two sets of water spouts until you get to a Sling Star that will send you to the other side of the planet. Use a Fire Flower to destroy a snowman blocking your path then climb a set of platforms with Bomps on them. Destroy the next snowman to spawn a Sling Star. Take it to an area where a hole in the cliff releases rolling boulders.
Destroy another snowman at the bottom and use a Bomp as a platform to reach the upper ledge. Avoid boulders as you run up a hill, so you can destroy another snowman fire your Fire Flower buff wears off. That will spawn another Sling Star which will take you up the mountain where you'll need to Wall Jump. Shove a wall of Bomps to get to the Power Star.
Dusty Dune Galaxy
You'll need 39 stars to access this desert-themed galaxy filled with sand seas and cacti planets.
Soaring on the Desert Winds
Take a Warp Pipe to a Launch Star that will send you to the other side of the planet. Use tornadoes to gain elevation and the moving blocks as platforms, moving against the sandstreams while avoiding Cluckbooms. Jump up some blocks to move up a sandfall and use a tornado to get to a ? Coin to spawn Sling Stars which will send you to a glass-covered area you navigate through wall jumping. Use a group of blocks going in and out of the sand to reach the Power Star, which is encased in a crystal.
Blasting Through the Sand
Navigate through a planet filled with moving and sinking platforms until you get to a planet packed with stone platforms. Use the Tweetsters to reach higher places, though avoid getting hurt by the surrounding spikes. Ground pound a switch on the side with the lighter-colored stones to spawn platforms on the darker side which can be used to get to the Power Star.
Sandbaked Sand Castle
Defeat a large Pokey using coconuts from a nearby planet and it will spawn a Launch Star that you can use to get to a large stone structure on the bottom of the planet. Break a crate to reveal a Warp Pipe that will take you inside. Spin to break the large block and then rush up the platforms before the structure sinks, crushing Mario.
Sand Blast Speed Run
Complete the Blasting Through the Sand mission in four minutes and 30 seconds to get the Power Star. Some enemies will be missing and there will be more Star Bits to make things a bit easier.
Purple Coins in the Desert
Collect 100 coins scattered around the Maze Planet using the Tweetsters, Thomps, and moving platforms. Collect them all by traveling between the two sides of the planet. The Power Star will spawn on the rising platform on the dark side of the planet.
Bullet Bill On Your Back
Do the Sunbaked Sand Castle mission until you reach the Sand Tide Planet. A stump will appear as the sand lowers. Ground pound it to spawn notes, which you can collect to produce a Launch Star. It will take you to a disk planet with a Bill Blaster shooting two Bullet Bills.
Ground pound a switch to cause the Bill Blaster to rise on a platform as cages appear around it. Lead Bullet Bills to break into the cages to get Star Bits, a Rainbow Star, or the Power Star.
Treasure Of The Pyramid
Go through the Blasting Through the Sand mission until you use the first Launch Star. Feed 20 Star Bits to the Hungry Luma and it will turn into a Pyramid Planet which you can access through a Launch Star. Take the Warp Pipe you land on inside the pyramid and collect five Silver Stars.
When you step over the green line, the sand will move and reveal a path towards more Silver Stars which are trapped in crystals. Collect them all to make the sand disappear, revealing the Power Star at the bottom of the pyramid.
Use the Bee Mushroom to climb up honeycombs on the wall, flying between them when necessary. Use the ? Coin to spawn more moving honeycombs. Take the Launch Star at the end of the first wall to get to a second wall where you need to avoid falling meteors as you climb.
On the third wall, you'll have to avoid Mandibugs who will knock you off as you navigate the moving honeycombs. Get past the rotating large Mandibugs near the top of the wall to get to the Power Star.
Bowser's Dark Matter Reactor
This fight is pretty similar to your last battle with Bowser. You need to get him to try to jump on Mario while he's standing on blue coverings, which will cause him to burn his tail in the lava underneath. While he's running around on fire, spin into his tail, and then spin again once you've knocked him on his back. His speed will increase every time he's knocked over. Hit him twice after flipping him for a third time and you'll receive the Grand Star.
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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.