Here is the full list of all 20 A Sky Full of Stars achievements.
Start the game.
Watch the first opening movie.
Watch the second opening movie.
Watch the ending movie.
Open the gallery.
Open sound screen.
Complete the first common route.
Complete the second common route.
Complete the game for the first time.
Complete the Hikari route.
Complete the Saya route.
Complete the Orihime route.
Complete the Korona route.
Complete all routes.
Watch a cutscene for the first time.
Watch 20% of the cutscenes.
Watch 40% of the cutscenes.
Watch 60% of the cutscenes.
Watch 80% of the cutscenes.
Watch all of the cutscenes.