Caelum Nostrum

The Caelum Nostrum is the volumetric sphere of reachable, habitable worlds. Cryogenics allows humans to be in transit for hundreds of years, but there is a limit to how long it can last, based on the fuel requirements of their fusion reactors plus the physiological damage long term freezing can do.

At the beginning of Children of Abeona, Abeona is the last habitable world in the Caelum Nostrum, and the colonists have been pushed there by overpopulation on their world (they could have joined another world, perhaps, but they wanted to keep their own government).