Black Hole Sun

Black Hole Sun

Robert Zubrin discusses artificial black holes used as mini-Suns here:
Artificial Singularity Power: A Basis for Developing and Detecting Advanced Spacefaring Civilizations

Artificial black holes powered by their Hawking radiation are, by necessity, very small and require constant energy input at greater than the Eddington Limit. That limit, which equates to 33,000 Solar Luminosities per Solar Mass, or 6.4 watts per kilogram, is the limit at which the radiation from the object exceeds its force of gravity keeping mass attached to it. A force fed black hole would have mass beamed into it via a particle beam and thus can (probably) exceed the Limit.

While Hawking radiation is 100% efficient at turning mass into energy, a black hole has another energy creating process that’s better than the 0.7% of nuclear fusion. If an accretion disk forms, the infalling matter emits up to 5.8% of its rest mass energy. The innermost stable orbit is 3 times the Black Hole’s Schwarzschild radius, thus there’s a long way to fall to the Event Horizon, resulting in this emission. Larger black holes thus can act as energy sources that are better than stars. A hole massing just 3 x 10-5 solar masses (ten Earth masses) would be as bright as the Sun. Wrapping it in an absorbing shell to down-convert the x-ray emissions into visible light would create an artificial Sun which can be trickle fed new mass to keep shining.

How long would such a Black Hole Sun last? Fed at the rate to match the Sun, it would use a Solar mass in 855 billion years. Better than the Sun’s piddling 10 billion. An orbiting planet would need to be accelerated gently to match the gradual increase in orbit velocity needed, else it’d spiral inwards as the hole’s mass increased. It’s a job that would be easily achieved by a giant solar sail acting as a gravity tractor.

If the efficiency is lower, why use a Black Hole Sun rather than a tamed Singularity for its Hawking radiation. For one thing, the Black Hole Sun is ‘safer’ – it doesn’t decay within a few years if not fed. Secondly, a Singularity Sun is for one world, while a Black Hole Sun could sustain thousands. As Sean Raymond has described on his PlanetPlanet blog, orbits aren’t limited to just one planet: The Ultimate Engineered Solar System

In the long term, Hawking Stars might become the main option, but even they will run out of mass. Something longer term is needed to sustain life for trillions of trillions of years.