Deep Future Real Estate

arXiv Preprint [1503.04376]: Dyson Spheres around White Dwarfs.

White dwarf Hab-Zone

Ibrahim Semiz & Salim Ogur, a pair of researchers in Turkey, have posed the possibility of extraterrestrial civilizations building Dyson Spheres around white dwarfs. It’s an astroengineering option for the long-lived civilization that wants a fixer-upper or is looking to renovate their own system after the sun has gone Red Giant.

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.

Such an object would “glow” in the infrared at ~280 K, 9 times the physical size of the Sun, and have a mostly helium spectrum. It’d look like a very odd infrared protostar from afar, compact and opaque to other frequencies.

4 Responses to “Deep Future Real Estate”

  1. Marshall Eubanks says:

    Such Dyson spheres would not only obscure visible light, they would also obscure X rays from material accreting onto white dwarfs — doubly so, both by blocking them, but also by keeping the rate of material accreting onto the white dwarf under control.

    Interesting, the white dwarf X ray emission from some galaxies (the bulge of M31 and some nearby ellipticals) is ~ 60 times smaller than expected. Also interestingly, these are exactly the sort of regions where there might be truly old civilizations. So, some of this real-estate might already be taken, and I have to wonder about the X-ray emissions from our Galactic bulge.

  2. Adam says:

    Marshall that’s almost too coincidental to not be an observable of very old ETIs. Proof positive of our Galactic Seniors…

  3. Marshall Eubanks says:

    Well, as you know I think that there are a number of indicators of old ETI’s on a galactic and near extra-galactic scale. This will definitely go onto the list – and it leads to a further test. If I were running a Kardashev Type II civilization, I would want to expand out into the galaxy ASAP to get a handle on potential sites of GRBs. (Think of that as basic Galactic housekeeping.) That suggests a correlation between WD and GRB – i.e., that lower than expected white dwarf X rays should be associated with a lower than expected GRB event rate. Now, all we have to do determine the WD X ray emission for 10^7 to 10^8 galaxies, and monitor them all for GRB for a few years.