The Startree is a Dyson sphere, a hollow shell completely encircling a star. The sphere itself is composed of intertwining arrays of tree trunks, branches, leaves and roots, each tree being many kilometers in length and width. The trees are nourished by sunlight and watered by passing comets, which are taken from the Oort Cloud and carefully directed through gaps in the foliage as they pass in and out of the sphere.
The trees themselves have been genetically engineered to produce a wide range of structures, including habitation modules, mass-transit conduits, and solar-reflecting mirror dishes made out of specialized leaves. Ergs incorporated into the trees produce containment fields to hold in a breathable atmosphere and magnetic fields for EM levitation. This allows ground-based and flying organisms to live within the branches of the Startree; aquatic organisms also live amongst its branches, swimming within clouds of cometary water vapor thick enough for breathing and moisturization.