On certain places in Braid there will be extremely hidden puzzles for Tim to solve. Each one of them will reveal a star to pick up. In total, there are eight of them. Up in the sky outside Tim's house, there is a constellation showing you which stars you've collected so far. Collecting all eight will reveal the game's true ending.
2-2 The Cloud Bridge
While this first star is not at all hard to get, it will probably test your patience. Be prepared to wait a good two hours in-game to get it. It's going to be easier to get it after picking up either 
Puzzle Piece 2-3-1 or 
Puzzle Piece 2-4-3, but it is obtainable without either.
Begin the level by entering it from the door in 
2-3 Hunt!. It will be easier to get the star going backwards through the level as you do not need to open the gate then, and can use it as a jumping platform. Above 
Puzzle Piece 2-2-4 you can see a lone cloud hovering. In fact it's moving ever so slowly to the left, and you need to get on it and hitch a ride with it over to the star. Go left and the puzzle canvas will be the first obstacle in your way. Use the technique outlined in 2-2 The Cloud Bridge to jump on the applicable puzzle pieces all the way up to the monstar. Up there, continue going left and cross the cloud bridges until you get to the gate. Jump up on it and wait for a good while. It'll take roughly 1 hour and 30 minutes for the cloud to get to the gate from the moment you entered the level. If you play on a PC and + out of the game, that time will be heavily increased though, so don't try it. Let the game run and go do something else for a while.
When the cloud finally does come by, jump up on it and wait about 15 minutes more. But don't wait too long, when the cloud hits the wall on the left of the level, it disintegrates. When close enough to the left side wall, jump up on the shelf at the top of it and the star will become visible. Now you can simply go pick it up.
Tim's house
This star requires you to not have put together the puzzle pieces found in World 3 as you need to use them to build the star yourself. When you have collected at least 
Puzzle Piece 3-3-1 and 
Puzzle Piece 3-6-2 as well as either Puzzle Piece 2-3-1 or Puzzle Piece 2-4-3 you can use these to get your star. Don't worry if you assembled the pieces from World 2, it's only the ones from World 3 that need to be separate.
When at Tim's house, you can see a black rectangle wall acting as a separator between the room leading to World 2 and the one leading to World 3. You can probably see that the wall between these two rooms—unlike the walls between the others—have an opening up top. You need to get into that opening if you want to grab the star here. If you've already gotten the puzzle pieces in 
2-2 The Cloud Bridge, you probably know what to do. Position either Puzzle Piece 2-3-1 or Puzzle Piece 2-4-3 at the bottom right corner of the canvas, get up on its platform, jump up, open the puzzle menu, move the piece a bit up, and repeat until you're all the way up. Stay there for a while, then jump down.
Now go over to the canvas for World 3. If you look very carefully, you might notice a cone in the left window above the canvas, and also a few cracks in the wall immediately below it. Together they kind of look like the beginning to a star shape. Find the two puzzle pieces you need on this canvas and see that they too have shapes resembling parts of a star on them. Try to position these two parts of the star shape at the top left corner of the canvas, so that together with the star shape in the window they form a complete star. This is very fidgety and will probably require many tries. When you hit the sweet spot a sound will play and the star will appear. Rewind time to get back up to the opening you jumped up to before. From here you can simply walk into the star to pick it up.
4-5 Movement by Degrees
To begin with, get back up to the place where 
Puzzle Piece 4-5-2 was located. Find out how to get up there by following the guide for 
4-5 Movement by Degrees. Beyond the three claws, above the monstar cannon is a small platform with the star on it. You need to get past all the claws to get to it, which is rather difficult considering they only ever move when Tim does.
When up on the same platform as the puzzle piece, do like you did when you got up to it. Jump down to below the claws and lead a monstar through the claws to the platform with the puzzle piece on it. You'll have to rewind time to get it past the last claw, just like when getting the puzzle piece. When both Tim and the monstar are on the platform with the piece, bounce off the monstar to get onto the platform even higher up; the one forming a roof for the puzzle piece.
Up here, do the same procedure again. Jump down and get another monstar through the claw path. When he's pacing the platform with the puzzle piece on it, wait for him to get to the edge of it and jump on him, bouncing Tim over the nearest claw. That's one of them down, just two more to pass.
The next claw is going to be a bit tricky too. Find a position where the second claw is completely down its pipe and wait there for a bit. Then run and try to jump over it. You will not make it and Tim will die. The momentum of Tim's corpse will however still make the claws move. While falling from being dead, you can still control the three claws by rewinding time. Try to keep Tim in a position where the last of the claws you need to pass is down its pipe and let a monstar pass by it. Right when he's past, rewind time back to the point where the second claw was down its pipe and let the monstar pass it to. Quickly jump on the monstar and bounce over the second claw. Two down, one to go.
Just getting past the third claw is no problem, as it's mostly down when Tim pass it, but getting up to the star is trickier. It's too high up for Tim to jump. You will need to go back to where the third claw was all the way down its pipe. Wait for a monstar to pass by it and jump on it, bouncing over the claw. There should be just enough time before you land for another monstar to pop out of the cannon. Land on him to get a large enough boost to take you up to the platform with the star.
4-7 Fickle Companion
Even though it might not be the hardest star to find, this is probably going to be the hardest star of them all to actually get. Begin the level by grabbing the key and climbing up to the monstar cannon at the top of the level. What you might not know is that it actually isn't the top of the level. There's a whole other structure above, just out of camera-view. You'll have to jump a monstar as soon as it shoots out of the cannon to reach that further up place. In fact, you'll have to bounce two of them...
A new monstar won't pop out of the cannon until the one wondering around the bottom of the level is killed, so jump down and land on it. Rewind time and get back to the cannon, wait for a monstar to come out, and jump on it mid-air. The bounce won't be big enough though, so keep Tim hovering mid-bounce by controlling the rewind. When a new monstar shoots out of the green-glowing cannon, be sure to land on it mid-air too to get a high enough bounce to get up to the structure above.
Hopefully you haven't dropped the key on your way up here. Remember, you can't move left up here or you will lose your key. Instead, move to the right and jump down to the area below. Moving left again (but not too far, don't go as far as to the cannon), the key will move on up to the newly found part of the level. Remember seeing a few gates up there? You will have to use the key on them, but you can't move left up there without losing the key. The green-glowing monstars can however take the key to the left, so get one up there. Jump down to the new monstar below and kill it. Quickly get back up to the right side of the cannon, either by rewinding time or climbing the ladder. Jump down again just as a monstar is about to pop out and make him land on you and he'll bounce up, just like Tim did before. Rewind time back to solid ground and make the monstar land on Tim again, because the monstar also needs a double bounce to reach all the way up.
With the monstar up there, wait until you hear it grab the key. Then rewind all the way back to when Tim was up where the monstar now is. Make your way to the ladder leading to the gate. Be careful to not kill the monstar or you'll have to get a new one up here. It's better to let the monstar kill you and then rewind Tim back to life than to try to jump over it, as you might accidentally kill it. Grab hold of the ladder and wait for the monstar to get close. It's possible to steal the key from the monstar without killing it, so wait on the bottom on the ladder until Tim touches the key the monstar is carrying and quickly climb up the ladder as soon as you got it.
Open the gate with the key and flip the lever behind it. Run all the way to the right of the platform you're on. Because you flipped the lever a barrier will open up down below and the monstar can walk on further down the path toward the star. Make sure the monstar isn't behind the barrier but somewhere under Tim and run all the way left this time. The key will pop down to where the monstar is and he'll pick it up. Once he does, rewind time to when the barrier was wide open and let him walk through it. Then run through the gate again to close the barrier, sealing the monstar on the other side of the barrier.
Jump down to the monstar and climb up the ladder a bit, but be careful again to not kill the monstar. Like on the previous ladder, steal the key from the monstar again. As soon as you have it, you can go up and unlock the top gate, which will open up the home stretch to the star.
5-4 Crossing the Gap
Getting a hold of this star requires you to master the art of juggling monstars. Begin by climbing up to the lever and pulling it. The moving platform needs to be at the left side to begin with. When it's in position, jump down and kill the monstar down below. Rewind time and let the doppelganger kill it off again. Now, jump down to the wooden platform below the lever. Positon Tim so that the new monstar coming lands on him and bounces up. Rewind time and let it bounce on the shadow clone instead. Now position Tim so that the bouncing monstar will land on him again. When it does and bounces up, rewind time a bit and move away, so that it bounces on the shadow doppelganger instead. Continue to bounce the monstar in the same fashion all the way to the platform with 
Puzzle Piece 5-4-1 on it (not the one with 
Puzzle Piece 5-4-3). You'll have to make the monstar change his direction at some point. To do this you'll have to make him bounce into the side of the platform with Puzzle Piece 5-4-3 on it. Then he'll bounce bounce back in the opposite direction.
When he's up on the platform he needs to be, a new monstar will pop out of the cannon. Pull the lever and let it fall down to the area below. If it managed to wander off to the area by the exit door, go kill it and let a new one fall down to the area below the cannon instead.
Now do like you did in 
5-4 Crossing the Gap when you got Tim up to Puzzle Piece 5-4-1. Jump over to the cannon and then jump down to kill the monster below, rewind time and let the shadow clone do the same, jump on the new monstar as he pops out the cannon and bounce up to the platform above.
Since you've carefully guided a monstar to the platform up here, you can now use it to bounce on. When you do, you'll take Tim to a hidden set of platforms above, where the star is located. Getting to it is as simple as jumping up a few steps.
6-5 Impassable Foliage
To get this star, begin by going to the place where 
Puzzle Piece 6-5-2 is located. You can read how to get there at 
6-5 Impassable Foliage. Position Tim at the left side edge of the platform with the puzzle piece on it; close to the claws.
Like when getting the star in 4-5 Movement by Degrees, there is a row of claws killing off an everlasting barrage of monstars being shot into their path. And also—like in that level—you need to get on top of the roof above the puzzle piece. So start by rewinding time to the point where all claws are down their pipes and pause time. Let two monstars pass all of the claws—because you will need to bounce twice to get all the way up to the roof—and wait for the first one to come over to Tim. Jump on it and bounce up. Now keep Tim hovering mid-bounce by controlling time and wait for the other one to come by. Land on it and bounce all the way up to where you need to be.
Go left and you should soon see the star. Jump up on the scaffolding-like platform above the star and move over to the left side of it. At some place here—above the path the monstars take out of the cannon—you need to place a ring. The approximate placement is important, but there is no exact mark where it needs to be, and there's also no in-game indication for where to put it. Check the video for a generally good placement.
With the ring placed, jump down onto the cannon. Wait until a monstar is about to pop out and jump down on it. You need to land on the monstar mid-air, and bounce upward toward the center of the ring. Tim will not make it up to the platform above but you should be able to keep him afloat until another monstar pops out. Steer Tim toward it and land on it like you did the first one. This time the boost should be enough to propel Tim to the platform above, and the path to the star is open.
6-6 Elevator Action
Here's a star that require you to be quick on the buttons to grab hold on. Begin by setting the level up so that you could get to 
Puzzle Piece 6-6-2. You need the small platform on the left of the big one and you need the ladder leading to the piece down. Read how to get to set this up at 
6-6 Elevator Action.
When you get to the point where you could technically get Puzzle Piece 6-6-2, climb up the ladder which was dropped down by pulling the lever and get off onto the small platform which makes Tim not affected by time manipulation. Rewind time to a point where the ladder is almost about to fall down, all while standing on the green glow, just like you would if you were to grab the puzzle piece. As soon as the ladder is falling and within reach of Tim, very quickly jump on it and rewind time as quickly as possible. This should bring the ladder all the way up to the top, before it started to fall. Stop rewinding time. Now you can climb to the top of the ladder and get off it to the left.
If you continue to walk left you'll see the star on a platform as well as a basin of green glow which transfer over to Tim. Down below is the lever operated elevator and the fireball cannons from when getting 
Puzzle Piece 6-6-3. Only the elevator will be interesting now, though. Stand on the green glow for a bit, jump down and pull the lever, and ride the elevator down.
If you remember, down here you will be attacked by a mimic. Lure the creature onto the elevator and pull the lever down here. You need to get the mimic riding the elevator up to the area above. When it's up there, begin rewinding time and it'll ride down again. Continue to rewind and Tim'll go up the elevator too and jump up to the green glow. Continue to hold the button for rewinding but change the direction of time flow, so that it moves forward. Tim is unaffected by time manipulation since he's standing on the green glow, but all the events that happened before unfold again: The elevator will go down, the mimic will jump on it and ride up with it.
Now you have the mimic up here and you could use it to bounce up to the platform with the star. Unfortunately, the star is too far up for a simple jump on it. You need to jump on the mimic while it's doing its strike, because it's closer to the star then, and Tim's bounce will be enough. Jump down and place a ring somewhere slightly to the right of the lever. Get the mimic to attack inside the ring's area of effect, but dodge the attack. When you feel that you have a jump from the mimic that you'll be able to hit mid-air form the platform above, rewind time to get back to the green glow. Continue to hold the button for rewinding time but make time move forward again, like before. Now the ring should appear again and the mimic will begin to attack. Pause time at a moment you feel the mimic is at a good mid-air position for a bounce over to the star, then quickly let time flow and jump for it. If you hit the mimic at its strike's apex you should bounce all the way over to the star on the platform.
1-1 Braid
The star on this final level will only unlock if you have already collected the other seven stars beforehand. If you have not yet collected all of them, you must do so before tackling this one.
The level will begin and mostly play out as usual. If you need help to just get through it, consult the guide at 
1-1 Braid. When having collected the seven other stars from the other worlds of Braid, some important changes will happen to World 1-1. At two different places, regular levers has been replaced with green-glowing ones. Both of them are operated by Tim, and control barriers blocking off the path for the princess.
To begin with, rush through the level as you normally would. The first green-glowing lever should soon be visible to you. Go up the ladder and flip it, then continue on as usual. The second green-glowing lever will be the one you bounce up on a monstar to; flip it too. With both these flipped it's now safe to rewind time. Rewind a good bit back, so far that the princess has been totally unencumbered by both barriers the levers control. Since these are green-glowing, they will now remain flipped, so the princess has a clear path in front of her. She no longer have to wait for Tim to open up the passages for her, and can therefore run ahead and open up a free path for Tim. Together they can sprint through the level with ease.
Continue time and run through the level as fast as you can, until you get to the wine cellar. This time you will have arrived before the princess, and before the chandelier has rewound itself up to the ceiling. Jump up the small step on the right and wait for the princess to arrive upstairs. When the chandelier begins to assemble itself and move upward, jump up on it and hitch a ride up to the princess. Be ready to catch a hold of the trellis up top.
The princess will explode and Tim will emerge alone. But now he can climb up and enter the princess' house. Above her bed will be the eighth and final star in the game. Grab it and walk in the princess' footsteps all the way back to the beginning of the level. Exit through the door and you'll be taken to the epilogue.
Continue through the epilogue at your own pace. Collecting all stars will have no effect on it. When you go through the exit door you will be taken back to outside Tim's house as usual, but this time the constellation outside his house will show itself in full beauty. An image of Andromeda will be superimposed on it, giving further hints of the story pertaining to Braid.