Hatsune Miku has recorded countless hit songs, appeared in many top-selling video games, and helped launch a social media service built entirely around her popularity. She appears on train liveries, in taxi cabs, and in commercials for Domino’s pizza. And you've probably never heard of her.

That's okay, because Hatsune Miku is not a real person — not in the traditional sense of reality. Miku is a piece of software, a Vocaloid, who can be manipulated and modified to sing the songs of her music production team, but also the music created by her millions of fans. She is the closest the world has to an open source celebrity.

Earlier this year, I put together an introductory video for people new to the Miku phenomenon. With the virtual star appearing in American pop songs, cross-country tours, and now Late Show with David Letterman here audience is about to rapidly expand. Up top is our explainer and below is Miku's appearance on Letterman.