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 six known sizes:

Star Type	Typical Diameter	Maximum Sphere Capacity
Dark	Anchorite	5,000 miles
Light	Mendicant	10,000 miles
Dark	Jeremiad	20,000 miles
Light	Exarch	35,000 miles
Dark	Sovereign	50,000 miles
Light	Titan	100,000 miles

Stars are also categorised by their colour, which determines the qualities and the potency of their light and heat. Some stars are naturally hotter than others, or cooler, or brighter, or have unusual effects on their surrounding sphere.

✦ Auric stars have a soft golden light, and are one of the most common colours of star. The particular radiance of their phlogiston burns the undead.

✦ Crimson stars have volatile phlogiston that emit far more heat than light, making the worlds that orbit them charred and dry. They are known to eject huge stellar flares of phlogiston on occasion, causing sweltering heatwaves across the entire sphere for days at a time.

✦ Ebon stars emit very little light, keeping the worlds around them shrouded in eternal night. They are hard to sight from afar, and as such are often uncharted. Dangerous creatures may lurk within the sphere of a black star, such as naturally-occurring undead.

✦ Mithril stars shine with silver light, their phlogiston charged with arcane power. Though a rare colour of star, they are highly sought after by travelling practitioners of magic, so their ethereal light may assist in spellcasting and matters of research.

✦ Sapphire stars are a bright and brilliant blue. They are young and unstable, filling the worlds in their sphere with wild climates and dangerous weather. However, they are rich in minerals and metals, making them valuable territory for mining concerns.

✦ Verdant stars bolster plant life with their soothing green light, capable of turning entire worlds in their orbit into lush jungles. Even the very space between worlds can be filled with life, like the colossal sunflowers that always drift to face the star they orbit.

Each star is generally described with its colour and size, such as a Mithril Anchorite or a Crimson Titan. Although all stars in the Spelljammer universe are much smaller than their real-life counterparts, the equivalent of the Earth's sun would be an Auric Jeremiad.

Stars form natural archipelagos, called constellations, where their spheres are connected by reliable phlogiston currents through wildspace. These naturally-forming trade routes allow an easy exchange of culture throughout all the spheres in a constellation, for better or worse. Travel between different constellations is harder and usually much slower, but not impossible.