Fire requires energy, our most abundant source is stars. They last 5 billion years or so and use nuclear fusion.

Fire requires energy, our most abundant source is stars. They last 5 billion years or so and use nuclear fusion.

A "city" built around a star is called a Dyson Sphere (https://en.wikipedia.org/wiki/Dyson_sphere, also Dyson Nets and similar) or a Ringworld (https://en.wikipedia.org/wiki/Ringworld).