Dyson Spheres are shells wrapped around a star, soaking up its entire energy output to provide the energy requirements of an advanced civilisation. For decades, scientists and science fiction authors have imagined these behemoths circling Sun-like stars, about the size of Earth’s orbit. Such alien megastructures would emit waste heat in characteristic infrared radiation that could be spotted over interstellar distances, and we may have found some of them. Sun-like stars live for about 10 billion years, while cooler, smaller stars can burn longer, in excess of 100 billion years, before the expand into Red Giants and blow up in supernova explosions. The death of a small star leaves behind a remnant core, called a White Dwarf star, which can last essentially forever, slowly dimming over trillions of years, a decommissioned nuclear furnace radiating leftover heat.
White Dwarf stars are about the size of the Earth, and are incredibly dense, with extreme gravity. While larger Dyson Spheres would require some kind of means of generating artificial gravity, the Dyson Spheres around Whit Dwarf stars would have Earth-like gravity across the entire structure. The inner surfaces, basking in the dim, steady glow of the white dwarf can be engineered to be within the Goldilocks zone, where water can exist in liquid form. Such a Dyson Sphere with a radius of three million km and a shell about a metre thick would weigh less than the Moon. This allows for a practical, lower cost megastructure that does not require the demolition of entire planets. A savvy civilisation could pluck the debris surrounding a dead star, and still have plenty of spare material.
White Dwarf stars are not very bright, and a Dyson Sphere surrounding one would not radiate too much waste heat. These Dyson Spheres would be far more challenging to detect over astronomical distances. If a civilisation intends to be stealthy, constructing a Dyson Sphere around a White Dwarf star is a sensible option. The intense gravity would spell trouble for a rigid shell, pushing the builders towards Dyson Swarms or Rings. The upside is that the structure has an in-built waste to energy system at the core. The civilisation could just chuck stuff like old refrigerators into the host star, that provides more fuel for it to glow. This is a closed-loop dream for a tidy civilisation. A stable gravity, lower material costs, stealth, and easy waste disposal are all advantages of a Dyson Sphere around a White Dwarf star.
There are some White Dwarf stars discovered in Spitzer, Wide-field Infrared Survey Explorer (WISE), Transiting Exoplanet Survey Satellite (TESS) and Kepler surveys that are glowing in excess infrared radiation. The spectral features suggest that the White Dwarf is surrounded by material or an object that is cooler, and is radiating absorbed infrared light. These may be orbiting dust or debris disks, cool Brown Dwarf or White Dwarf binary companions, or alien megastructures such as Dyson Swarms or Rings. Astronomers estimate that about 25 per cent of the White Dwarf stars with excess infrared radiation could be surrounded by an artificial alien megastructure. Follow-up observations that can eliminate dust or binary companions as the source of the excess infrared radiation may be able to confirm the presence of Dyson Spheres around White Dwarf Stars.
Sources:
Dyson Spheres around White Dwarfs
Infrared Excesses around Bright White Dwarfs from Gaia and unWISE. I.
Infrared and optical detectability of Dyson spheres at white dwarf stars
Excess infrared radiation from a white dwarf—an orbiting brown dwarf?
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