Star Sphere

From the Super Mario Wiki
Not to be confused with Star Ball.
“The clear ball you see in front of you is a Star Sphere. When you get all 5 Silver Stars, the Power Star will appear inside the Star Sphere.”
Signs about the Star Sphere, Super Mario 64 DS
A Power Star in a Star Sphere in Super Mario 64 DS

A Star Sphere is a transparent sphere that holds Stars. They appear in Super Mario 64 DS, Super Mario Galaxy and Super Mario Galaxy 2.

In Super Mario 64 DS, if a player obtains five Silver Stars or presses a Star Switch, then a Power Star appears in the crystal Sphere. If a Star Switch is pressed, the Star remains in the Sphere for a short time. When the Sphere is touched with a Star inside, it will break and the player will get the Power Star. In VS Mode, most of the Power Stars are found in Star Spheres, except for the ones in The Princess's Secret Slide.

A Star Sphere, containing a Silver Star, from Super Mario Galaxy

The Star Sphere reappears in Super Mario Galaxy and Super Mario Galaxy 2 containing the Silver Stars, though its appearance is now more bubble-like.