Stellar engines are a class of hypothetical megastructures which use a star's radiation to create usable energy. Some variants use this energy to produce thrust, and thus accelerate a star, and anything orbiting it, in a given direction. The creation of such a system would make its builders a Type-II civilization on the Kardashev scale.
One of the simplest examples of stellar engine is the Shkadov thruster (named after Dr. Leonid Mikhailovich Shkadov who first proposed it), or a Class A stellar engine. Such an 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—which would balance gravitational attraction towards and radiation pressure away from the star. Since the radiation pressure of the star would now be asymmetrical, i.e. more radiation is being emitted in one direction as compared to another, the 'excess' radiation pressure acts as net thrust, accelerating the star in the direction of the hovering statite. Such thrust and acceleration would be very slight, but such a system could be stable for millennia. Any planetary system attached to the star would be 'dragged' along by its parent star. For a star such as the Sun, with luminosity 3.85 × 10^26 W and mass 1.99 × 10^30 kg, the total thrust produced by reflecting half of the solar output would be 1.28 × 10^18 N. After a period of one million years this would yield an imparted speed of 20 m/s, with a displacement from the original position of 0.03 light-years. After one billion years, the speed would be 20 km/s and the displacement 34,000 light-years, a little over a third of the estimated width of the Milky Way galaxy.
Class B Stellar engine is a Dyson Sphere
A Class B stellar engine is a Dyson sphere—of whichever variant—built around the star, which uses the difference in temperature between the star and the interstellar medium to extract usable energy from the system, possibly using the phenomenon of thermoelectricity (heat engines, or thermal diodes). Unlike the Shkadov thruster, such a system is not propulsive. The Matrioshka brain concept is based on this stellar engine concept, extracting energy for a specific purpose: data processing.
Class C Stellar Engine
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.
A Dyson shell with an inner surface partly covered by a mirror would be one incarnation of such a system (although it still suffers from the stabilization problems as a non-propulsive shell does), as would be a Dyson swarm with a large statite mirror (see image above). A Dyson bubble variant is already a Shkadov thruster (provided that the arrangement of statite components is asymmetrical); adding energy extraction capability to the components seems an almost trivial extension.
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