The solar body that lit the Astral Exclave - or any of them - was not actually right outside the bubble that made up its border. Sufficiently sized 'windows' of Spatial Element had been arranged around an artificially maintained star in an unknown location by the first Divinity, similar to the concept of a Dyson Sphere. All of it was what allowed the spinning orbs of planetary detritus that met the furthest reaches of of space, forcefully rearranged near the cosmic core into the Exclaves, to have their 'normal' feeling diurnal cycles.