Luma

Lumas are star-like creatures that first appear in Super Mario Galaxy. Unlike most Stars, they have a tear drop-like shape, and come in many colors. They are much like Power Stars, but they are more circular and are able to talk.

Lumas boast the ability to transform, from minor planetoids to entire galaxies. They seem to acknowledge Rosalina as their "mama". According to Rosalina, some can transform into Power Stars. Most of them make their home in the Comet Observatory, while others live on Starship Mario.

Only one cream-colored Luma is seen, which is Mario's Luma companion, although the Luma that transformed into the comet that Rosalina and the Lumas travel on appears to be cream-coloured in Rosalina's storybook's illustration. He is the Luma that Rosalina gives Mario in the beginning of the game. Luma grants Mario the power to do spins. Yellow Lumas transform into Sling Stars and Launch Stars (and possibly Power Stars) and are the most common variety. Red Lumas guard Red Stars. Green Lumas are the guardians of the Trial Galaxies and transform into green Launch Stars and Green Stars. Blue Lumas transform into Pull Stars. Pink Lumas transform into pink Launch Stars to get to hungry Luma galaxies. Lumalee, who is a large, light-blue Luma, hosts the Luma Shop. There is also another type of Luma that is pink, known as the Hungry Luma. The last type of Luma is the Prankster Comet Luma, which has the ability to move Prankster Comets.

In the end, all the Lumas (including Mario's Luma) throw themselves into the massive black hole created by Bowser's dying sun to neutralize it. However, Luma is reborn and living on a small planet while it is hinted that the other Lumas will be reborn later. Indeed, they appear in the sequel.

Mario Kart Wii
In Mario Kart Wii, a Yellow Luma revolves Rosalina at all times. It makes tiny noises whenever Rosalina does a trick in the air. It has no effect on gameplay.

Super Mario Galaxy 2
Lumas also appear in Super Mario Galaxy 2. Some time before the beginning of the game, Bowser attacked Lubba's ship, and many of the Lumas on board fell down to the Mushroom World. A large number of Lumas are shown to reside in Starship Mario, and their leader is Lubba. Hungry Lumas also appear; this time, they ask to be fed both Star Bits and Coins. Like in the last game, Lumas enjoy being fed Star Bits, and may thank the player if he or she decides to shoot star bits at them. There are exceptionally large species of Luma in the Supermassive Galaxy, called Giant Lumas. Additionally, Mario's companion Luma returns in this game; however, Mario doesn't seem to remember him, and he found the baby Luma when he crashed onto the Mushroom World.

Mario & Sonic at the London 2012 Olympic Games
Yellow Lumas appear in Mario & Sonic at the London 2012 Olympic Games in the Dream Spacewalk event. Here, they aid the athletes in defeating Dino Piranha. They first transform into Launch Stars that send the characters into space. Following this, they periodically appear to create rings that the athletes must all pass through in order to damage Dino Piranha.

Fortune Street
Lumas appear in the unlockable Good Egg Galaxy board.

''Mario Tennis Open
A Luma will appear in Mario Tennis Open. It appears in the Galaxy Rally bonus game. It will also be a playable character. 

Colors
Lumas come in a variety of different colors, and they have different abilities depending on which color they are:

Yellow Lumas
Transform into Launch Stars or Sling Stars.

Blue Lumas
Transform into Pull Stars.

Hungry Lumas (Pink Lumas)
Transform into new planets or galaxies and pink Launch Stars.

Green Lumas
Guard the Trial Galaxies. As stated by Polari, they will transform into Green Power Stars someday.

Red Luma
Allows Mario to use the power of the Red Star onboard the Comet Observatory.

Purple Hungry Luma
Gives Mario information about Prankster Comets and shuffles them between galaxies.

Orange Luma
The Co-Star Luma, which the second player controls in Super Mario Galaxy 2.

Trivia

 * Paper Mario featured a similar species, called Star Kids. Like the Lumas, the Star Kids were young stars, who were raised on a special place to become real stars someday. They also possessed magical abilities, like the Lumas, and a very similar, childish behavior.
 * The name Luma comes from the Latin lumin, which means "light." It is a reference to their similarity to the Power Stars and their future as stars.
 * Blue Lumas look similar to an item in Paper Mario: The Thousand-Year Door called Mini Mr. Mini.