Pop Star (also sometimes written PopStar or Popstar) is the star-shaped planet that Kirby and friends lives on and is the setting of most of his adventures.
The geography and surrounding solar system of Pop Star are never clearly shown or explained, but there are features that remain fairly consistent. It is always star-shaped, usually with 2-3 rings encircling it. They can be either white, or more sometimes red, green and blue.
Despite its star shape, yellow appearance in space, once through the atmosphere Pop Star has blue oceans and landmasses similar to Earth's. It's possible that something in the atmosphere reflects - or gives off - a yellow light to make it appear the way it does from space.
Pop Star has its own sun and moon (note that the sun orbits around Pop Star instead of the other way around). The plot of the Kirby Super Star subgame Milky Way Wishes begins with the problem of the sun and the moon actually fighting.