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, see below), they fly through the background and away from the stage while usually screaming or yelling, and eventually disappear as a star (there is a random chance of a Screen KO instead, where characters hit the camera). In the first three Smash games, this has the longest duration out of any KOs.
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 (unless while playing in Super Smash Bros. Melee's single player modes), characters in Super Smash Bros. Brawl who are giant or tiny due to Super or Poison Mushrooms, 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 a normal KO explosion or a Star/Screen KO at random when crossed. Players launched at extremely high speeds will also prevent Star KOs from occurring. In addition, presumably to solve balance issues, Star KOs no longer occur near the end of a timed match. 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.
Note that all of these exceptions also apply to Screen KOs.
In Smash 64