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, away from the stage, yell/scream/etc., and eventually disappear as a star (there is a random chance of a screen KO instead, where characters hit the camera, which is rarer). This takes the longest duration out of any KOs.
Any conditions on the characters, such as if the characters are KO'd while frozen or have a Smash Ball, are seen when they are star KO'd. In Flat Zone 2 and Hanenbow, conditions are not aligned with the character (i.e. the frozen block and character fly off separately).
Giant/tiny characters who are star KO'd remain giant/tiny while falling on the background (their "falling voice" remains deep/tiny as well). On the other hand, star KO'd metal characters appear in normal conditions on the background (including their voice). The only exceptions are the Metal opponents in Melee's 1-player modes. In Brawl, when giant/tiny characters gets star KO'd, they will return to normal size, unless the characters are playing in Giant/Tiny Brawl. When a character is frozen, they will appear frozen when they are star KO'd.
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.
Members of Fighting Enemy Teams, such as the Fighting Wire Frames and Fighting Alloy Team, do not experience star KOs, instead undergoing the usual colorful explosion (with the exception of the oversized Wire Frames that appear in Event Match 37). Team battles in the Classic and Adventure Mode also feature playable characters that cannot be Star or Screen KO'd. In Super Smash Bros., members of a team (such as the Fighting Polygon Team) can be Star KO'd. Common enemies in the Subspace Emissary mode cannot be Star or Screen KO'd (with the exception of playable characters).