Dyson Spheres and Swarms around White Dwarfs avoid two out of three major problems with Dyson Spheres

A Dyson Sphere is a hypothetical structure that an advanced civilization might build around a star to intercept all of the star’s light for its energy needs. One usually thinks of it as a spherical shell about one astronomical unit (AU) in radius, and surrounding a more or less Sun-like star; and might be detectable as an infrared point source. Researchers point out that Dyson Spheres could also be built around white dwarfs. This type would avoid the need for artificial gravity technology, in contrast to the AU-scale Dyson Spheres. In fact, we show that parameters can be found to build Dyson Spheres suitable –temperature and gravity-wise– for human habitation. This type would be much harder to detect.

(H/T Adam Crowl at CrowlSpace

The simplest form of the Dyson Sphere, a solid spherical shell, is problematic: It would be subject to unaccetably large stresses and its equilibrium around the star is neutral at best. Therefore, variants were suggested where the “sphere” actually consists of pieces in independent orbits (A “Dyson Swarm”). Another consideration is gravity: If the sphere were built in the Sol system with 1 AU radius, the gravity due to the Sun would be only 5 × 10^−4g, so humans could not live on it without either genetic modification to become compatible with microgravity, or a technology of artificial gravity.

A rigid Dyson Sphere around a white dwarf still could not be built without some extra means of support (some magic material like the fictional Scrith), about which they chose not to speculate here.

In the Arxiv study they have chosen to mostly ignore the mechanical complications associated with the simplest case, the rigid spherical shell, but make the case for a variant where the central object is different: A white dwarf (WD). They show that a smaller Dyson Sphere built around a typical white dwarf can simultaneously satisfy the temperature and gravity requirements for human, and therefore presumably similar, life. It would also require less building materials than an AU-scale Dyson Sphere, still provide one hundred thousand to one million times the living area of a planet, and obviously not require artificial gravity technology. Such a Dyson Sphere would also radiate in the IR, but since it will have white-dwarf power, it would be harder to detect.

Stars with masses of up to approximately 4 solar masses will eventually become white dwarfs.

The temperature and the gravitational field on a Dyson Sphere are both functions of its radius, once the mass and luminosity of the central object are given.

Denizens of the Dyson Sphere would live on the outside of the sphere, using energy collected on the inside surface, e.g. photovoltaically. This means that they will either have to use artificial lighting, or light pipes. Both possibilities will facilitate creation of day/night cycles, if the metabolisms of the denizens requires it, as expected for creatures originating on a rotating planet. They could also use nuclear power (fission or fusion) for extra energy, which will increase the infrared emitted out by the Dyson Sphere slightly.

Properties of some particular suitable white dwarfs and the associated potential Dyson Sphere ranges. The radius, temperature and gravity data are given from the smallest to the largest suitable Dyson Sphere for a given white dwarf.

Dyson Sphere schematic and Dyson Swarms

A one million km-scale Dyson Sphere built around white dwarfs are at least as realistic as the ‘standard’ ones, and possibly more probable. Unfortunately, they would also be harder to detect.

NBF – A Dyson Swarm around the same dimensions seems achievable.

Adam Crowl Provides some more Analysis

Consider a white dwarf that has chilled to 1/2500th of the Sun’s luminosity. It’s habitable zone is at 0.02 AU, but to sustain a clement environment, the Shell has to be a bit further out at 0.04 AU, at the desirable thermal equilibrium temperature of ~280 K. More or less. That’s a radius of 6 million kilometres and a surface gravity of 3.75 m/s2 for a 1 solar mass white-dwarf. The habitable surface is on the outside. The habitat’s total area would be ~887,000 Earths, so it’s a substantial piece of real estate. To sustain a breathable atmosphere at 1 bar pressure a gas mass of 1.2E+25 kg is required – the mass of Neptune, though in the right mix of gases. While oxygen and carbon are fairly easy to source, the nitrogen might be more difficult, thus a heliox mixture might be required. Some nitrogen is still needed to make protein, but most of the atmosphere would be helium for fire suppression and reduction of oxygen toxicity risk.

Due to the intense gravity of a 1 solar mass white-dwarf star, mass falling onto would release ~27 TJ/kg from gravitational energy alone. By trickling mass on the star, very carefully, its luminosity could be sustained at 0.0004 solar for aeons before it ran into the Chandrasekhar Limit at 1.44 solar masses. Exactly how close one could get to that Limit, without triggering a C/O fusion conflagration and a Type Ia Supernova, is an important bit of astrophysics to learn before building the Sphere.

SOURCES -Arxiv, Crowlspace

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