Not to be confused with KO Star.
A star knock-out or star finish (星になって, Become a star), abbreviated as star KO, is a type of KO. When characters are knocked beyond the upper blast line (save for a few exceptions), they fly through the background and away from the stage while usually screaming or yelling, and eventually disappear as a star. In Melee and Brawl, this has the longest duration out of any KOs. Along with Star KOs, there is a chance of a Screen KO happening instead, where characters slam into the camera.
Most conditions on the characters, such as the invincibility of the Super Star or standby mode after breaking the Smash Ball, are still visible on the characters while they are Star KO'd. Exceptions include metal characters in Melee and Brawl (unless while playing in Super Smash Bros. Melee's single player modes), giant or tiny characters due to Super or Poison Mushrooms in Super Smash Bros. Brawl, and frozen characters in Super Smash Bros. 4. In Flat Zone 2 and Hanenbow, conditions are not aligned with the character (i.e. the frozen block and character fly off separately).
In Super Smash Bros. 4, Star KOs are no longer a certainty; the upper blast line will cause either a Blast KO, a Star KO, or a Screen KO at random. Players launched at extremely high speeds, however, are not Star KOed or Screen KOed. In addition, presumably as to solve balance issues, Star KOs no longer occur near the end of a timed match (unless an unknown glitch occurs); Screen KOs were increased in length, and as a result, Star KOs actually last shorter than Screen KOs. In Smash 64 and Melee, all playable characters have a voice clip for being Star KO'd (including Samus), while Olimar lacks one in Brawl; SSB4 later introduces more characters that remain completely silent when Star KO'd, with these being Mega Man, the Mii Fighters, Pac-Man, Villager and the wireframe version of Little Mac.
Unless otherwise noted, all of these exceptions also apply to Screen KOs.
In Smash 64
In Super Smash Bros. Melee
In Super Smash Bros. Brawl
In Super Smash Bros. 4
In Smash 64
As getting Star KO'd in Melee yields the Rocket KO bonus, Star KOs are intended to be a reference to the original Pokémon anime series(citation needed), where the Team Rocket Trio, recurring villains in the show, would often be sent flying away into the sky by Pikachu, leaving behind a star when they vanished from sight. The frequency of this gag led to it becoming a recurring trope in Eastern media, with some of Nintendo's own video games later referencing the gag.