eburacum45's rendering of a proposed Fractal Dyson Sphere
A Dyson sphere is a hypothetical megastructure originally described by Freeman Dyson. Such a "sphere" would be a system of orbiting solar-power satellites meant to completely encompass a star and capture most or all of its energy output.[1]
The Ringworld is a similar if much less ambitious megastructure, that harnesses much more energy from its star than a normal planet is capable of.