Separate lights held as one figure by those who look. The pattern is real and lives in the seeing, not in the stars.
Late Latin constellatio (a setting with stars), from com- (together) + stella (star); separate lights joined into one figure by whoever is looking, the pattern living in the seeing rather than in the stars themselves.
Latin stella names the star, but the figure drawn between stars belongs to whoever is looking, not to the sky. Two lights held together this way remain, physically, unrelated.