A class A stellar engine is a stellar propulsion system, consisting of an enormous mirror/light sail — actually a massive type of solar statite large enough to classify as a megastructure, probably by an order of magnitude.
A class B stellar engine is a Dyson sphere (item 3), which uses the difference in temperature between the star and the interstellar medium to extract usable energy from the system.
A class C stellar engine combines the two other classes, employing both the propulsive aspects of the Shkadov thruster, and the energy generating aspects of a Class B engine.
Matrioshka brain. This structure would be composed of one or more (typically more) Dyson spheres built around a star, and nested one inside another. A significant percentage of the shells would be composed of nanoscale computers. These computers would be at least partly powered by the energy exchange between the star and interstellar space. A shell (or component, should a Dyson swarm be the design model used) would absorb energy radiated onto its inner surface, utilize that energy to power its computer systems, and re-radiate the energy outwards.
The ideal mechanism for extracting usable energy as it passes “through” a shell or component, the number of shells (or orbital levels) that could be supported in such a manner, the ideal size of the shells to be constructed, and other details, are all issues of speculation.
The idea of the matrioshka brain violates none of the currently known laws of physics, although the engineering details of building such a structure would be staggering, as such a project would require the “disassembly” of significant portions (if not all) of the planetary system of the star for construction materials.
Dyson Sphere. A Dyson sphere is a hypothetical megastructure originally described by Freeman Dyson. Such a “sphere” would be a system of orbiting solar power satellites meant to completely encompass a star and capture most or all of its energy output. Since then, other variant designs involving building an artificial structure — or a series of structures — to encompass a star have been proposed in exploratory engineering or described in science fiction under the name “Dyson sphere”. These later proposals have not been limited to solar power stations — many involve habitation or industrial elements. Most fictional depictions describe a solid shell of matter enclosing a star, which is considered the least plausible variant of the idea.
Alderson disk. The Alderson disk (named after Dan Alderson, its originator) is an artificial astronomical megastructure, like Niven’s Ringworld or a Dyson sphere. The disk is a giant platter, like a CD or phonograph record, with a thickness of several thousand miles. The sun rests in the hole at the center of the disk. The outer radius of an Alderson disk would be roughly equivalent to the orbit of Mars or Jupiter. According to the proposal, a sufficiently massive disk would have a larger mass than its sun. One drawback to a disk is that the sun remains stationary. There is no day/night cycle, only a perpetual twilight. This could be solved by forcing the sun to bob up and down within the disk, lighting first one side then the other.
Ringworld. Finally, the Ringworld, It is an artificial ring with a radius roughly equal to the radius of the Earth’s orbit. A star is present in the center and the ring spins to provide artificial gravity.
The Ringworld is described as having a mass approximately equal to the sum of all the planets in our solar system. The adventurers surmised that its construction consumed literally all the planets in that original system, down to the last asteroid and/or moon, as the Ringworld star has no other bodies in orbit. In Ringworld’s Children it is additionally explained that it took the reaction mass of roughly 20 Jupiter masses to spin up the ring; thus either the combined mass of the planets of the original system was that much larger than our solar system’s, or there was other source material.
The construction of a ringworld remains firmly in the area of speculation. If such a structure were built it could indeed provide a huge habitable inner surface, but the energy required to construct it, set it rotating, and keep it stabilized is so significant (several centuries’ worth of the total energy output from the Sun) that without as-yet unimagined energy sources becoming available, it is hard to see how this construction could ever be possible in a time frame acceptable to humans.
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