From the Spelljammer Wikipedia Article.
A crystal sphere is a gigantic spherical shell which contains an entire planetary system. Each sphere varies in size but typically they are twice the diameter of the orbit of the planet that is furthest from the sun or planet at the center of the sphere (the system’s primary).
The surface of the sphere is called the sphere wall and separates the void of wildspace (within the sphere) from the phlogiston (that surrounds and flows outside the sphere). The sphere wall has no gravity and appears to be impossible to damage by any normal or magical means. Openings in the sphere wall called portals allow ships or wildspace creatures to pass through and enter or exit from a crystal sphere. Portals can spontaneously open and close anywhere on the sphere wall. Spells (or items that reproduce their effects) can allow a portal to be located. Other magic can open a new portal or collapse an existing one. Ships or creatures passing through a portal when it closes may be cut in two.
The Akaillian Sphere
The sphere holding the prison world is one without natural portals. It contains only the Prison of the Worm. Only the most seasoned of planetary travelers dare the maelstroms that swirl around the dead sphere and most travelers of wildspace don’t believe it even exists. No known inhabitant of Akallia has ever been known to have breached the Akaillian sphere. The Temple of Thoon is rumored to have been a ship that crashed upon Akaillia during the Aberrant Age. The neogi Flesh Merchants are the only group known to have a reliable means of breaching the dead sphere.