Luma

From the Super Mario Wiki

“That's the spirit, mushroom man!”
—Luma, Super Mario Galaxy

A Yellow Luma, the most common type.

Lumas are starlike creatures that 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 able to talk. Lumas boast the ability to transform, from minor objects to entire galaxies. They seem to acknowledge Rosalina as their "mama". Most of them make their home in the Comet Observatory.

Only one Apricot Luma is seen in the game; the Luma that Rosalina gives Mario in the begining of the game. The Apricot Luma grants Mario the power to do Star Spins. Yellow Lumas transform into Sling Stars and are the most common in the game. A few yellow Lumas can transform into Launch Stars as well. Red Lumas guard Red Stars. Green Lumas are the guardians of the Trial Galaxies and transform into green Launch Stars. Blue Lumas transform into Pull Stars, and a large Light Blue Luma hosts the Luma Shop. There is also another type of Luma known as Hungry Luma. The last type of Luma is a purple Luma that holds the power to move Prankster Comets.

Spoiler warning: Plot and/or ending details follow.

In the end, all the Lumas (even the Apricot Luma) were sucked into a black hole created by Bowser's sun. It turns out that the Apricot Luma is still alive on a small planet while the other Lumas will be reborn later.

Spoilers end here.

In Mario Kart Wii, a Yellow Luma revolves around Rosalina at all times. It has no effect on gameplay.

[edit] Trivia

The Apricot Luma was to originally have the name "Tyke" - however, it was given no name in the final version of the game and is simply known as "Luma".
The word Luma could possibly be derived from the Latin word "lumen", meaning "light".
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 possesed magical abilities, like the Lumas, and a very similar, childish behavior.