The Garden Observatory is the last game you unlock and the only one that doesn't have a boss galaxy. A 1-Up Mushroom is tucked in the observatory spire above the Garden and you can reach it with a Red Star.
Deep Dark Galaxy
You need to have beaten Bowser Jr.'s Lava Reactor and gathered 46 stars to access this haunted galaxy. There's also a beach where you'll meet Guppy again.
The Underground Ghost Ship
Fight Kamella who will attack you with red magic that turns into a fireball when it lands on the ship and green magic that transforms into a Green Shell. Throw the Green Shells back to damage her and cause her to start moving around her ship.
When you hit her a second time she'll head to the crow's nest and summon a pair of Magickoopas. Throw two more Green Shells back at her to win the fight and get the Power Star.
Ground pound a trio of stumps on the Cheep Cheep Gambol Planet to turn on a water cannon that will launch you to the Marble Planet. Avoid a Ball Beamer and ground pound the large tennis balls to expand the watermelon inside until it shatters the casing and exposes the Power Star.
Guppy and the Underground Lake
Guppy will challenge you to another race through eight rings, all of which are either within an obstacle or a group of enemies. Use Green Shells to move through them before they disappear and you'll get the Power Star.
Ghost Ship Daredevil Run
Repeat the fight with Kamela without taking a single point of damage to get another Power Star. Her tactics remain the same.
Plunder the Purple Coins
You need to collect 100 Purple Coins scattered on the ghost ship and the surrounding area including underwater behind your starting position, under the ship, and the ledges near the ship. Get them all and the Power Star will appear on the ship.
Boo in a Box
Make your way to the bottom of the underwater cave and grab the Green Shell. Toss it at one of the mines around the broken ship mast to cause it to explode and reveal a Launch Star that will send you inside a floating box.
Flip the arrow to change gravity until you're standing on the right wall and you'll find a hole blocked by a crystal. Shatter it to let in light that lures the Boo over to destroy it. It will drop the Power Star.
You need 48 stars to access this galaxy, which reuses some elements of the Battlerock Galaxy but centers around a massive spaceship.
Infiltrating the Dreadnought
Take a Warp Pipe to a Tower Plant where you can use the Topmen as springs to climb a rotating tower. Wall jump up two walls to get to a Launch Star that will take you to the next planet.
Get to the Sling Star on the opposite side to get to another planet where you'll need to taunt a Bullet Bill to blow up a cage holding back water. The planet will fill with water and you'll need to navigate around some searching Eye Beamers to get to the Power Star.
Dreadnought's Colossal Cannons
Avoid cannonballs being fired from the Dreadnought and step onto a platform to get it to start moving. Navigate the platforms, some of which change the flow of gravity when you're on them, and avoid the enemies and obstacles. The Power Star is above a Jump Beamer at the end.
Revenge of the Topman Tribe
Fight Topmaniac again using the same method to defeat him: jump on him to retract his blades and spin him into the electric rail. This time he'll have Spiky Topmen at the start of the fight. Shock him three times to get the Power Star.
Topman Tribe Speed Run
Complete the Revenge of the Topman Tribe speed mission in six minutes to clam the Power Star. Many of the Topmen are now Spiky Topmen, so you'll need to somersault to reach higher objects. There are Spiky Topmen where there previously weren't any enemies.
Battlestation's Purple Coins
Gather 99 coins along the side of the Dreadnought where you did the Dreadnought's Colossal Cannon mission, and grab the last one in front of a Launch Star at the end. Then take a Sling Star to a Gearmo who will award you with a Power Star if you got them all.
Dreadnought's Garbage Dump
While you're playing the Revenge of the Topman Tribe mission take a Warp Pipe under the UFO you start on to get to a Gearmo and some piles of trash. Talk to him to accept a challenge to use Bob-ombs to blow up all the garbage in 30 seconds. Use the Coin Generators to determine where to throw the Bob-ombs. Succeed and you'll get the Power Star.
Melty Molten Galaxy
You need 52 stars to unlock this galaxy made of lave and rock, which is the last major dome galaxy of the game.
The Sinking Lava Spire
Use a Sling Star to get to a lava rock where you'll need to collect five Star Chips to create a Launch Star to get to a series of moving platforms. The Lava Spire will start sinking once you step on it so you need to reach the top by jumping over obstacles and avoiding enemies. Break the crystal at the top to get the Power Star.
Through the Meteor Storm
Once you're on the U Tower Planet lure Bullet Bill to the starting area to destroy a cage containing a Launch Star. You'll need to use a Star Ball to roll over lava platforms and through meteors. Large turning lava wheels will impact the ball's spin. If you can get it into a hole at the end of the course, you'll receive the Power Star.
Fiery Dino Piranha
The Fiery Dino Piranha uses the same tactics as the Dino Piranha but will burn you if you touch it and leaves a trail of fire behind when it moves around. Wait for its tail to be extinguished then spin into it to knock it into the eggshell and shatter it. The boss will then start chasing you.
When the tail is extinguished again, spin into it to hit it into the boss' head. Once it's been damage three times it will start shooting fireballs. Each hit also shortens the amount of time when the tail isn't on fire. Hit it one more time and you'll get the Power Star.
Lava Spire Daredevil Run
Complete the entire Sinking Lava Spire with just one health. If you take and damage, you'll have to start the mission from the beginning.
Red-Hot Purple Coins
Collect 100 coins on the galaxy's starting planet, must of which are found grouped together over lava or under sinking platforms. The Power Star will spawn on the highest platform once you have them all.
Stick to lighter ground areas on the Lava Sea Planet to avoid the rising lava tide as you gather five Silver Stars to make the Power Star appear.
Matter Splatter Galaxy
Navigate a maze, falling or long jumping back to the right path if you hit a dead end. Gather Star Bits guarded by Magikoopa. Head all the way to the right and then Long Jump to the left path to reach the Power Star.
Snow Cap Galaxy
You'll need to catch three Star Bunnies in three minutes and 30 seconds. One is in a hole buried under the snow and one is hiding in a treasure chest. Step on all the hidden flipswitch panels to reveal a Fire Flower which you can use to shoot fireballs at snowmen and find the last bunny. They'll then give you the Power 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.