Hello again. I'm assuming that, if you're reading this, you've already drained all the syrupy goodness from parts one and two of this star-hording extravaganza, and are now ready for the final push against that evil sod Bowser. I hope so, because things get trickier from now own.
There's now just the Engine Room to complete for the final Grand Star (plus the crisp bonus taste of the Garden), and the big showdown with Bowser. If you've been doing as you're told, you should only be a few stars shy of the 60 required to complete the game, but that doesn't mean we're going to slack off now, as there's plenty more than that to be had.
So, come on. Head up to the walkway between the Library and Kitchen and shimmy up that starbeam to the Engine Room. We've still got a Princess to save.
The Engine Room
Gold Leaf Galaxy
Star Bunnies On The Hunt
Dead easy, this one. First of all, find the five pull star pieces. It's quicker if you grab the Bee Mario power up, so you fly to them. There's one on top of the plateau that the boulders are emerging from, surrounded by piranha plants. There's another on the left of the waterfall, that you can reach when you stomp all three stumps. Another launch star takes you up to a higher area with a fountain, which you can use to boost you up to the third piece. The fourth is in the crate behind the waterfall, while the fifth is in the far corner opposite the waterfall above the spring-loaded plunger (fly there, or use the flower stem). Use the new pull stars to launch to the next planet where - surprise, surprise - Mario is indulging his animal-chasing fetish again. That rabbit has your star, but you can even the odds by stomping the red plunger to create some walls that'll slow him down.
Cataquack To The Skies
This one pretty much does what it says in the title. Use the Cataquacks (or "creatures" as my addled brain forgetfully labelled them in Part 2) to keep moving onwards and upwards. If there's a Bee Mario power up nearby, grab it - the additional lift and distance from a quick hover is often vital. Things start to get more complicated when you reach the cube hedge maze, but it's just a case of clearing all the crates and enemies then luring the snoozing Cataquack to the Cataquack logo on the floor. He'll then sproing you to the next area - a fiendishly thin walkway with several Cataquacks roaming the edges. You need to reach the launch star at the far end with a Cataquack bounce, but being bounced earlier is more annoying than fatal. On the next planet, get rid of as many enemies as you can, then use the Cataquack to grab the Bee Mario power up, and then to propel you to the top of that Jenga tower. From there, simply hover your way from flower to flower until you reach the star.
BONUS: The Bell On The Big Tree
Here's another simple secret star to snaffle while in the Cataquack level. From the start, nip round the back of the big tree in front of you and jump in a bubble. As you float up the tree, use the Wiimote to ring the bell. A spiral of notes will curl around the tree like a musical python, so simply bubble your way along the line hitting all the notes to get a star. Shazam, as Beethoven used to say.
When It Rains, It Pours.
This is a very damp level, and that's a problem since the Bee Mario power up - which vanishes in water - is essential. First things first, make your way around the back of the plateau with the trapeze. Wall kick your way up to the top (there's a vine underneath a piranha plant as an alternate route) and jump on top of the cloud to become the bee. You now need to cross that bridge of flowers, but one of them is under a rain cloud. Use the flower stem near the two piranha plants to propel you over the cloud, then jump and hover to the wooden path. Use the trapeze to reach the top of the clouds, and hover your way across them to the next section. Use the spring-plunger to reach the base of the tower, and dodge the rainclouds using the alcoves. Crack that rock at the top to create a vine to the the observation deck and face the mole-powered gun turret. You need to stomp the big button to raise the steps so you can reach the gun, then run over and stomp it. The bubbles won't hurt you, but they can knock you onto the spikes. If you're still Bee Mario then flying over the spikes is safer than jumping, but don't panic if you're plain old Mario - it's not that hard. Keep moving and stomping and you'll have the star in no time.
Sea Slide Galaxy
Going After Guppy
A simple swimming challenge. Talk to the big ugly killer whale and he'll challenge you to swim through eight rings. That's right - you don't have to beat him, just complete the course. Grab a red shell to speed things up, and watch out for obstacles, but this star is more of a formality than anything.
Faster Than A Speeding Penguin
Another underwater challenge, although this time there is a race. Beat the penguins around the course, and the star is yours. It's the same course as Going After Guppy - though you have to go a little bit further to the finish line - so just grab a red shell, avoid the hazards and take advantage of some yellow rings near the end for a speed boost.
The Silver Stars Of Sea Slide
This is a fairly straight forward romp around the scenery, collecting the five silver stars needed to make the power star. Start off by grabbing the Bee Mario power up from the island, then use the top of the palm tree to fly over to the stone archway for the first silver star. Fly from the next palm tree to dry land, then use the cannon to launch yourself at the Toad's spaceship. The pull stars will allow you get the second silver star. Head back to the shoreline and follow the palm trees to the tower - the third star is along the way. Launch to the next island and fly to the big tree for the fourth star then drop down and spin the valve to shift some of that water. Drop into the middle of the planet to get the final silver star. Your power star, helpful as always, sods off back to where you started.
BONUS: Hurry, He's Hungry
Fatten up the hungry Luma on the stone platform with 40 star bits, and they'll provide you with a real challenge. It's another collapsing platform stage, with musical notes on each and every panel. You need to pick them all up to get the star. There's no definitive way of doing this - just keep moving, and think one step ahead so you don't cut yourself off or leave notes out of reach. Pull it off and you've more than earned that power star.
Toy Time Galaxy
Heavy Metal Mecha-Bowser
Welcome to one of the most fantastically entertaining sections of the game. Jump on the train and let it take you to the first launch star. You'll land on a series of colourful blocks. The purple sections have sticky gravity, enabling you to walk up vertical surfaces, but also putting you in danger of falling to your doom. Follow the course around until you find the launch star stuck behind a blue cage. You need to long jump (run, hold down Z, then A) over to the purple section to the right, spin the yellow screw to open the cage then long jump back into the launch star. Now, remember back in part one when I said Bee Mario was just about the best thing ever. I only said that to catch you off guard with the genius of...Spring Mario. Pick up this marvellous power-up, and turn Mario into a demented zebedee. Use the spring to make short work of the conveyor belts, and avoid the lasers. Launch over to the plate, and spring jump to the top of the bottle. Stomp your way in, then launch out to Mecha Bowser himself. Run up the leg on your left, using the green floor which has the same effect as the purple, spin the screw then jump in the pipe. You'll emerge on the other leg, so spin that screw as well. Launch onto the torso and unscrew the panel. Drop down and enter the green pipe, which takes you up to his arm. Clamber up his arm, watching out for bats, and unscrew his shoulder. Quickly catch this launch star and it takes you up to his head. Grab the Spring Mario power up again, and bounce down the green grill, watching out for that Magikoopa and the fireballs from the cannon. Bounce onto his nose, avoid the laser, and bounce again up to the top of his head. Now just spring jump and stomp that stack of rocks to destroy Mecha Bowser and earn the star.
Mario Meets Mario
Jump aboard the train again, and this time the launch star is to your left. This will send you to a giant screw-like planet covered in electrical fields and lasers. You need to grab the five star pieces - one is right where you land, one is over a spring-plunger further up, another is trapped inside a crystal, one is on top of a blue screw, which you need to spin, and the last one is right on the tip. Now launch over to a giant effigy of NES era Mario. Don't get too nostalgic - it's an absolute bastard, since it's made up of disappearing green panels and rotating yellow ones. In other words, you can't stand still and must grab the five silver star pieces without leaving yourself with nowhere to go. The less green panels you touch, the better, since that leaves you more room to manoeuvre on the way back.
BONUS: The Flipswitch Chain
You can't miss the hungry Luma on the giant screw during Mario Meets Mario. Feed it 50 star bits and it'll create a new planet. It's made entirely of flipswitches, and arranged in pillars. As you can guess, you need to flip them all to get the star. Just be methodical, and clear each section before moving to the next.
Bouncing Down Cake Lane
Mmm. Tasty. Launch from the train track to the first cake and use Spring Mario to navigate the tall walls and conveyor belts. You'll reach a huge rotating orange cake, with deadly gaps. Make your way past those, super spring over the forks and hit the next launch star. Climb to the top of the ice cream and make your way across the poles to the lollipops. Watch out for the shockwaves, and jump clear onto the frozen platform when it comes around. Keep making your way across ice and lollipops until you reach a pole. This takes you to a cannon, so fire yourself at those pull stars and use them to reach the next launch star. Make your way up the final cake, and blow out all the candles with a spin. This triggers the boss battle - another crazy mole gunner. Your tactics are even more simple this time. Grab the Spring Mario and use it to stomp the gunner three times.
Kingfin's Fearsome Waters
This is a fun optional boss battle. It's also against a giant shark that's a skellington which makes it quite cool. There are plenty of shells and air bubbles, as well as a health boost and a 1Up in a chest, so there's no excuse for failure really. It's not like your target is small. Just hit him four times with a shell to crack his bones and earn a star. The more you hit him, the angrier he gets, and he'll send smaller Mario-seeking sharkbots after you, but these are easily evaded.
Bowser Jr's Lava Reactor
King Kaliente's Spicy Return
That octopus is back, and this battle is much the same as your first encounter. But before you reach him, you need to lure a Bullet Bill along those winding paths to free the first launch star, then hurry across the sinking green platforms and wallop two coconuts back at the smaller octopi. You're then ready to face Kaliente one last time. As before, you're looking to knock back his coconut attack three times in a row to cause damage. Things are slightly harder this time, what with the sinking platforms, but if you find a spot with three close together you can hop from one to the other without too much trouble. The blue flame creatures shouldn't give you any real bother, so its really only Kaliente's increasingly frantic fireballs you need to keep an eye on. Send him back to the magma depths (how does an octopus live in lava anyway?) and the Grand Star is yours.
BONUS: Sand Spiral Galaxy
Feed the hungry Luma next to the Engine Room with 1000 star bits to open this galaxy.
Choosing A Favourite Snack
This is quick, but tricky. As soon as you start, watch out for the Magikoopa and his fireball. He'll shoot once, then zip away to the next section. Follow him with the launch star, and make your way to the safety of the stone ledges on either side. You've got a choice here - Boo Mario or Bee Mario? Either way, you need to make it through this hazard filled tunnel. Boo Mario isn't troubled by enemies, but must avoid the spotlights. Bee Mario can ignore the spotlights, but reaching safe places with his limited flying range is a challenge. Whichever you choose, get to the end and use the launch star. You're now faced with two giant concentric circles, with the star in the centre. Grab the invincibility power up, so you don't have to worry about those mines, and use the wider areas of the outer ring to jump to the inner ring. From there, leap to the core planet in the centre and pick up the star.
Special Red Power Star
Gateway's Purple Coins
With all Grand Stars collected, the pathway to The Garden opens up. Along the way, you can stop off at the Gate, which leads you back to where you started this whole ker-azy adventure, and earn a fun bonus. Enter the little cottage to start the Gateway level, and then make your way to the top of the castle. Talk to Rosalina, and then accept the Red Luma's challenge. Pick up the red star, and you can now fly! Give a spin while jumping to take flight, and hold down A to hover in mid-air and point yourself in the direction you want to go. Collect all 100 purple coins from the level and not only will you be able to use red stars to swoop around the observatory (that's the secret that the Red Luma on top of the Bedroom keeps blathering about, by the way) but when you complete the game you'll open up the Purple Comet coin-collecting missions for even more stars.
BONUS: Boo's Boneyard Galaxy
Feed the hungry Luma next to the Gate with 1200 star bits to open this galaxy.
Racing The Spooky Speedster
This is a straight forward race, in which Boo Mario takes on the Spooky Speedster. He's naturally faster than you, but you can slip through the crystal walls with a shimmy of the Wiimote and collect question coins to spawn blue fans to blow you into the lead.
NOTE: None of the Garden stages contain a Grand Star, so if you're only interested in beating the game you can skip these. If you want all 120 stars though, you need to finish these as well.
Deep Dark Galaxy
The Underground Ghost Ship
Make straight for that cannon hole and blast yourself over to the planet up and to the left. Aim for the invincibility star - it'll make life a lot easier when you land, surrounded by enemies. There's also a Fire Flower on the opposite side, but you need that to open the gate on the beach. Grab it and quickly use the launch star to return to where you started, lighting the torches on either side of the portcullis. Run inside and dive into the deep water, swimming down as fast you can. There are air bubbles along the way, just in case. Follow the handy star bit arrow and smash the crates to reveal a switch. Open the gate, then swim all the way up the next passage, hitting the switch at the top. Get out of the water and make your way around to the ship, where Kamella, the large Magikoopa, will appear. You know what to do - throw the green shells back at her. Do this twice and the Magikoopa moves up to a higher platform, and is joined by two smaller pals. Shimmy up the pole in pursuit, and hit her two more times for the star. Watch out - her fireballs now split into three when they land, and you've only got limited dodging space.
No time to dawdle - that gunner needs taking out immediately. Stomp the turret three times, using the scenery to avoid bubbles as necessary. Jump into the cannon hole and, just as in the previous mission, you're after the Fire Flower on the enemy-infested nearby planet. Shoot over there, grab it, launch back and quickly light the three torches in the cave. This gets you the Ice Mario power up, allowing you to run across the water and jump up the leaking valves. Grab onto that pole, climb to the ledge, and then the next pole. Make like the Prince of Persia and swing across the gap using those bars and make your way around to the large blue platform with the question coin. The coin produces pull stars to reach the yellow pillars. You're now making a daring ascent using bubbles and poles, with a Magikoopa in hot pursuit. At the top, you're blasted to a new planet where you must stomp all the stumps to activate the bubble machine. Float away to the final planet where you need to leap over the shockwave rings to survive. See those balls rolling around the inside of the planet? Stomp when one is underneath you to send it smashing into the core of the planet which, obviously, is a watermelon. Do this with all the balls and the star is yours.
Guppy and the Underwater Lake
Dive back into that deep lake and talk to Guppy. You need to hit another eight rings, which really isn't hard once you grab a shell for propulsion. Just watch out for the water spouts near the end - it's easy to crash into them if you use the speed boost ring.
Boo In A Box
A secret star. Go back into The Underground Ghost Ship after clearing the three main missions and, at the bottom of the first dive, look to the back of the star bit arrow. Throw the green shell at the mines near the wreckage to create a launch star. This takes you inside another gravity-flipping level where you need to reach the crystal on the wall. Smash that, and light streams in. Lure the Boo into the light to get the star.
Infiltrating The Dreadnought
From the start, dodge the lasers and make your way to the springboard plunger, which can bounce you onto the top of that shockwave generator for a launch star. You're blasted over to the Dreadnought. Defeat the three enemies and enter the pipe. This leads to another gravity-flipping 2D section, where using the springboards and rising pillars is key to reaching the next area. Once outside, bounce on the heads of the Topmen to scale the generator tower. Use the launch star at the top and make your way to the largish planet with the cage and Bullet Bill cannon. As always, clear a path then lure a Bullet Bill to smash the cage. This floods the planet and causes the yellow laser robots to move and turn closer. You now use these as platforms to reach the star.
Dreadnought's Colossal Cannons
Jump onto those big grey nuts, face left and run on them to spin them along the bolt to the other side. You've now got to hit all these sets of flipswitches while dodging a stupid amount of small Chomps, then use two more sets of nuts to proceed. Two octopi will confuse matters towards the end, so whack those coconuts back at them as quickly as you can. There's a Bob-omb machine, but just leap over it and use the launch star to the exterior of the Dreadnought. Now navigate the moving yellow platforms, bedevilled by cannonballs and lasers, and get ready to jump up to the platform above halfway through, so you're upside down. You'll soon flip right side up again - just before you hit some mines - and after another horizontal dash under a Thwomp you start rising upwards. Things continue in this vein for a little while longer - swapping gravity when possible, avoiding the onslaught - until you reach two pink shockwave machines. Jump on the second one, as it's a spring that will bounce you up to the star.
Revenge Of The Topman Tribe
Bounce off the Topman to reach the first launch star, then shunt the two red Topmen into the electrical fence using spins or star bits. This frees the Luma, who launch stars you to a cube planet. Stomp the button to reveal blue pull star pieces, some of which are just out of reach. Use the old Topman bounce effect to get any that are giving you trouble, then sling your way to the next launch star. There's a health boost on the underside of the enormous metal suppository you land on, which will come in handy as you pull star your way through a broadside of cannonballs and mines. If you do take a hit, quickly grab onto the nearest pull star again or you're toast. Take your time, find the gaps in the firing pattern, and it's not so tricky. At the end, grab the pull star on the right, then use momentum from the one in the middle to carry you over to the launch star. Scoot through the shockwave machines, using the white circles to figure out where their waves will stop. It looks like the Venn Diagram from Hell, but it's handy. Launch from the end, and you arrive at the final boss arena. Make your way to the top (ha!) where Topmaniac awaits. Just as before, you need to jump on the button on top, then spin him into the electrical field. This time the field has gaps, and is rotating, but it's still not one of the harder battles. Zap him three times to win the star.
Dreadnought's Garbage Dump
From the start of Revenge Of The Topman Tribe, run to the underside of the spaceship and jump in the pipe. It's another trash clearing challenge, and it's got an even tighter time limit than the one in Battlerock Galaxy. Those yellow coin spots on the ground are a handy indicator of the best places to land the bombs, but you'll still need to move fast.
Melty Molten Galaxy
The Sinking Lava Spire
OK, this section is tough. Fair warning. From the start, you want to head round the lava pond and to the right. Keep moving. As you jump across the lava those platforms sink fast. Slip past the vents and jump up to the stone button. Stomp that to make a section of the mountain slide down, opening up an area just right for wall kicking - just watch out for vents on the way. At the top, you need to hit four launch stars in a row - the third one is rather spectacular - until you reach three spherical planets close together. Remember these as, you guessed it, we'll be back this way again for a hidden star later. For now, use the pull stars to carefully navigate between the miniature suns (there's a health boost on the way) then hit three more launch stars to reach a planet with cool rock and burning lava. You need to find five launch star pieces so you can finally reach the Lava Spire. The platforms crumble as you head towards it, so no hanging around. Once you reach the base, the "sinking" part comes into play. The whole thing starts to submerge in lava, meaning you got to run and jump around and up the outside as fast as your plumbing legs will carry you. There aren't any really nasty surprises on the way, just nimble platforming, although the moment where you have to stomp a button to create a wall kick section may cause unwary gamers to hesitate fatally. Once you reach the summit, spin that crystal two times and snatch the star.
Through The Meteor Storm
Remember those fiery enemies at the start of the last mission? Well, they're pretty useful now. Get them to follow you, and use them to light the torches (that look like jugs). The new launch star takes you to an octagon planet, with Thwomps and meteorites galore. Make you way around the outside edge to the next launch star, picking up an extra life as you go. Find the five launch star pieces, and health boost, on the next area then blast off through a shower of space rock to the bendy tower. Make your way round to the other end, then lure a Bullet Bill all the way back to where you landed to destroy the cage and reveal another launch star. You've now got a quick Monkey Ball section, through that terrifying revolving tunnel. The ball is fireproof, thankfully, but those meteors can knock you for six while the numerous holes in the tunnel mean you should still resist the temptation to rush. Although the rest of this level is frantic, you need to slow right down for this bit, but move fast when required. Drop into the hole at the end to earn the star.
Fiery Dino Piranha
Yet another returning boss awaits you, this time the very first foe you tackled all the way back in the very first level of the game. First though, you've got to reach him. The hub planet is going crazy now, with huge geysers of lava to contend with, so you want to quickly make your way to the green pipe you saw in the intro to the level. It's just around the corner from the lava pond, and across some tiny collapsing platforms. You'll need to use the high back flip jump (crouch, then jump backwards) to reach it. The pipe brings you to a small metal planet, which has a launch star. This sends you a short distance onto the next planet which you may remember from the previous level. Last time you were collecting launch star pieces, this time you're trying to defeat every enemy. Spin them to put out their flames, then spin them again to shatter their stony bodies. If they fall into the lava, they'll reignite. The next planet just requires you to dodge some fireballs on the way to another launch star, which takes you to a rotating section with moving platforms. Track down all the launch star pieces, making sure the tightly packed platforms don't knock you off, then head for the top to blast off. Just one last mad scramble across sinking platforms to go, then it's time to put Dino Piranha down for good. As with all the second round boss fights, it's the same as your first encounter, but with added hazards. Dino's tail now catches fire, so you need to wait for it to go out before you spin into it. Keep slapping him with his own appendage until the star appears.
Make your way back to the three spherical planets from the Sinking Lava Spire, and make your you collect 80 star bits on the way. There's a hungry Luma here, and once its appetite is satiated it'll create a new planet. Nothing too taxing, you just need to collect five silver stars from the pathways. You can clearly see where the lava tide comes to, so find a safe spot and wait for the lava flow to subside before moving on. Get five silver stars, and they turn into one glorious golden power star.
Matter Splatter Galaxy
Watch Your Step
Much like the Matter Splatter Mansion from the Ghostly Galaxy, this bonus level features areas that only become solid for a short time before fading away. Stay with the solid sections and enter the green pipe. It's side-on now, and you can get the Spring Mario power. Normally this is cause for celebration but under these circumstances it can be a real trial, since this bouncy bastard won't stand still. The final section is relatively simple by comparison, since you can use the star bits up ahead to predict where the platforms will be. Even so, do as the title suggests and watch your step if you want to grab the star at the end.
BONUS: Snow Cap Galaxy
Feed the hungry Luma next to the Garden with a whopping 1600 star bits to open this galaxy. Greedy sod.
Star Bunnies In The Snow
Bowser's Galaxy Reactor
NOTE: To reach this final stage, you must talk to Rosalina after collecting at least 60 stars and confirm that you're ready to go kick Bowser's scaly bum.
The Fate Of The Universe
As poodle-permed rockers Europe almost sang, it's the final showdown - der ner ner, der ner nah nah nah. First, we need to reach Bowser and that means risking life and moustache in a full pelt scramble through pretty much every environment you've traversed in this cosmic adventure. Start off by using the spotlights to stick to the walls and make your way around to the tower. Step into the beam and you're whisked away to a lava planet. The platform you land on starts sinking immediately, so get moving, leaping from one unstable surface to another, and dodging the fireballs on the rolling rock. Leap into the next beam and we're off to an ice planet. Platforms will form as you approach, but they soon vanish again, so don't let up the pace. It's a short visit to the desert next, with lots of rotating fireballs, then a brief - but tough - jaunt through a gravity switching 2D section. Finally, there's a panic inducing race down a revolving tunnel of lava, with moving platforms, Chomps and Bullet Bills galore. Passed all that? Then it's time to dodge Bowser Jr, who'll be bombarding you with cannonballs as you rush to face his dad.
Defeating Bowser himself requires three stages. First off, he'll send out those old shockwaves before covering himself in rock and rolling at you. The only way to damage him is spin so you hit his head as he rolls, then spin him again as he careens around in a daze. Get it wrong, and Mario takes the hit. You need to do this twice, then you move to a different planet for the second stage, which is a little easier. Bowser now unleashes a fire attack, but you can clock him with the many stretchy green punchbag pods to set him skidding. Again, do this a couple of times and it's on to the apocalyptic final phase - which, rather disappointingly, is the same as your previous battles with him. You know the routine: trick him into smashing into the glass panels, then spin attack him as he runs around, and again as he skids. Keep doing that until Bowser executes a perfect Gollum dive into the burning heart of the cosmos. Wow. You just completed Super Mario Galaxy!
Well, sort of.
As any Mario fan will tell you, beating the game is only half the fun. There's a whole heap of bonus stars for you to find, plus the ability to replay the game with Luigi. He moves slightly different to Mario and everything! For those completists out there, assuming you've been following our fine instructions to get the hidden missions in each section, here's what you need to do to get all 120 stars.
You have been doing these, right? There are 30 in all, and involve fiendish challenges set in previously beaten galaxies. Here's what to look for:
Speedy Comets (timed speed runs): Dino Piranha (Good Egg Galaxy), Pull Star Path (Space Junk Galaxy), Sand Blast (Dusty Dune Galaxy), Topman Tribe (Dreadnought Galaxy)
Cosmic Comets (races against Cosmic Mario): Honeyhive Cosmic Mario Race (Honeyhive Galaxy), Frosty Cosmic Mario Race (Freezeflame Galaxy), Cosmic Mario Forest Race (Gold Leaf Galaxy), Underwater Cosmic Mario Race (Sea Slide Galaxy)
Fast Foe Comets (enemies and platforms move faster): Fast Foes on the Cyclone Stone (Beach Bowl Galaxy), Fast Foes of Toy Time (Toy Time Galaxy)
Daredevil Comets (minimal health): Topmaniac's Daredevil Run (Battlerock Galaxy), Bouldergeist's Daredevil Run (Ghostly Galaxy), Major Burrows' Daredevil Run (Gusty Garden Galaxy), Ghost Ship Daredevil Run (Deep Dark Galaxy), Lava Spire Daredevil Run (Melty Molten Galaxy)
Purple Comets (coin runs): These unlock after you beat the game with more than 60 stars, and have completed the purple coin mission in the Gateway Galaxy. There are fifteen spread across all the main galaxies. They appear very frequently and, just as in previous 3D Mario games, involve finding 100 coins by any means necessary.
There are three of these - Luigi On The Roof (Good Egg Galaxy), Luigi Under The Saucer (Battlerock Galaxy), Luigi In The Honeyhive Kingdom (Honeyhive Galaxy). The names, and pictures, should give you a pretty clear idea of where to look for him. Luigi Under The Saucer also nets you the third and final green power star, making the Trial Galaxies available. Speaking of which...
As with the Luigi Stars, there are three Trial Galaxies with one based around each of the fun little mini-games. They're pretty hard, but also enormous fun, so it's worth taking the effort to find them. Once the green launch star in the Observatory is available, it'll launch you up to that three-pronged lozenge in the sky (admit it - you were wondering what that was for) where green Lumas are waiting to blast you to your chosen trial. Here's what's in store...
Rolling Gizmo Galaxy: Gizmos, Gears and Gadgets
This another Monkey Ball styled level, and one that requires an absolutely steady hand. There are some agonisingly perilous sections, and some insanely hilarious swoops and jumps as well.
Loopdeswoop Galaxy: The Galaxy's Greatest Wave
This is a ray-surfing level, in which you have to complete the course - including loop the loops - in three minutes or under.
Bubble Blast Galaxy: The Electric Labyrinth
And this is a long bubble blowing level, jammed with mines and electric barriers.