Stars are a race of celestial beings who have played major roles throughout the Mario franchise.
Super Mario RPG: Legend of the Seven Stars
In Super Mario RPG: Legend of the Seven Stars, Geno is a Star being from Star Road who was sent by a "higher authority" to aid the Mushroom Kingdom in its hour of need against the Smithy Gang. He decided to join Mario on his quest to save the kingdom when he realized he could not do it alone. Although Geno is a type of Star being, it is unknown if he is directly related to the Star species.
In Paper Mario, Stars are known to live in a city above Shooting Star Summit, called Star Haven where they are led by the Star Spirits. Stars begin their lives as Star Kids in Starborn Valley under the care of Merle and some Ninjis. Twink, a notable Star Kid, aids Princess Peach while she is kidnapped by Bowser.
Mario & Luigi: Bowser's Inside Story / Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey
In Mario & Luigi: Bowser's Inside Story, a species of Star known as Star Sprites appear. Unlike regular stars, Star Sprites have spherical yellow bodies with small feet and have stars perched on their heads. Only four members of this species are shown, the first being Starlow, who appears in the present time-line of the game, and the other three are elderly Star Sprites, who were shown in a flashback and were responsible for sealing away the evil Dark Star.