4e:Spelljammer - Cosmology

The universe of Spelljammer is a strange and unique one that works differently to how you, as a player, might assume. It takes its inspiration from the wild theories of times past, before humanity had the tools and the knowledge to properly discern the nature of the cosmos. As such, there are a number of key differences that should be noted.

Space itself is not an empty void. There is air between the worlds, just far too thin for most creatures to breathe. Only the native inhabitants of deep space find it enough to survive on, like the mighty starwhale. If caught out in space without viable breathing apparatus or magical protection, any living creature will begin to suffocate.^{[DMG, p159]}

All of space is saturated with phlogiston, the liquid fire that each star is made of. The stars radiate phlogiston out into space, providing life-giving heat to the worlds around them and creating a sphere of calmed space around them. Outside of a star's sphere is wildspace, where the phlogiston flows at high speeds in hard-to-predict ways, and can carry spelljamming vessels at remarkable speed. Think of each star's sphere as a safe harbour within a volatile and stormy sea.

The larger the star, the larger the sphere, and the more worlds it can have in orbit around it. Stars come in seven known sizes:

Star Type	Typical Radius	Maximum Sphere Capacity
Dark	Anchorite	3,000 miles
Light	Jennet	6,000 miles
Dark	Auric	9,000 miles
Light	Vizier	12,000 miles
Dark	Matriarch	15,000 miles
Light	Sovereign	20,000 miles
Dark	Titan	50,000 miles