Not to be confused with KO Star.
A star knock-out or star finish (星バースト, Star Burst), abbreviated as star KO, is a type of KO that can occur beyond the upper Blast line. They often last about 2 seconds and are the longest type of KO, often leaving their perpetrator safe from the victim.
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 shouting, 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 KO'd or Screen KO'd. In addition, presumably as to solve balance issues, Star KOs no longer occur when there's only five seconds remaining in a timed match and in Sudden Death (unless an unknown glitch occurs or in Trophy Rush); 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. Ultimate adds Isabelle and Piranha Plant.
In Ultimate, the characters now tumble away from the screen, similar to Screen KOs in Smash 64, Melee and Brawl, and seemingly go a much further distance away from the stage (though this is an illusion, since the characters actually start shrinking the further they fly away, as easily seen in the King of Fighters Stadium). Most characters also now use a sideways version of their tumbling animations, with the exceptions of Cloud, Corrin, Incineroar, Inkling, Shulk, and Joker, who spin forwards like their normal tumbling animation, and by extension, similarly to the previous games, alongside Mr. Game & Watch (due to his 2D nature). The star is also now a purple color. Additionally, the Boss Galaga will always cause a Star KO and cause the character to go into their normal Star KO animation seen in the previous games.
It is a recurring trope, especially in Eastern media, for characters and objects to be sent flying into the sky, leaving behind a twinkling star when they vanished from sight. Since Star KO'ing an entire team in Melee yields the Rocket KO bonus, the Star KOs in Smash may be intended as a reference specifically to the trope's use as a running gag in the Pokémon anime, where the Team Rocket trio would often be sent "blasting off again" whenever they were defeated.
Unless otherwise noted, all of these exceptions also apply to Screen KOs.
In Super Smash Bros. Melee
In Super Smash Bros. Brawl
In Super Smash Bros. 4
In Super Smash Bros. Ultimate
In Smash 64