The secondary star of Ahch-To, part of a binary star formation, resided within the galaxy's Unknown Regions as a component of the Ahch-To system. Both the planet Ahch-To and its moon were in orbit around it.
The second sun of Ahch-To made its initial appearance in the 2017 cinematic release, Star Wars: Episode VIII The Last Jedi. Its identification was first established within Star Wars: The Last Jedi: Expanded Edition, the 2018 novel adaptation of the movie, penned by Jason Fry.