The Barycenter

A star composes the center of most solar systems, which develop planets from the detritus following the collapse of a nebula or other cosmic shock wave. In a binary star system, the barycenter of a system is likely the center point between the two rotating stars. Erratically shaped systems can shift the barycenter away from the center of gravity, whether by recent cataclysm or proximity to another source of gravity. A supernatural collapse of a solar system's star could even result in a black hole developing at the center of a cluster of doomed planets, entire generations passing in the eternal darkness only to one day be consumed by their dead star. What lies near the barycenter of your galaxy? Does anything reside there, mortal or otherwise? How do the civilizations in your setting view the construct that keeps their worlds in place? There are effectively three options for most settings.

Dyson Sphere: Perhaps a solar system has been enclosed in an artificial structure designed to harness the power of the sun. The residents have long since forgotten, and escaping the sphere is an eventual goal of the campaign.

Geocentric: What if another world was the center of the known universe? Perhaps it's the primary game world, or perhaps it's the campaign's final destination.

Heliocentric: Physics dictates that all planets in a system orbit the sun, and the center of mass of a galaxy orbits a black hole. Some systems might have more than one star.