Dyson sphere

A Dyson sphere is a megastructure that completely encompasses a star and captures a large percentage of its power output. It's used by a spacefaring civilization to meet its energy requirements once those requirements exceed what can be generated from the home planet's resources alone. Only a tiny fraction of a star's energy emissions reach the surface of any orbiting planet. Building structures encircling a star would enable a civilization to harvest far more energy.

The concept was popularized by Freeman Dyson who speculated that such structures would be the logical consequence of the escalating energy needs of a technological civilization and would be a necessity for its long-term survival. He proposed that searching for such structures could lead to the detection of advanced, intelligent extraterrestrial life. Different types of Dyson spheres and their energy-harvesting ability would correspond to levels of technological advancement on the Kardashev scale.

This megastructure is a vast artificial structure. Some examples of it are in Halo, Stellaris and Doctor Who.