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.
There are three variant classes of this idea.
Class A (Shkadov thruster)
One of the simplest examples of stellar engine is the Shkadov thruster (named after Dr. Leonid 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 × 1026 W and mass 1.99 × 1030 kg, the total thrust produced by reflecting half of the solar output would be 1.28 × 1018 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.
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 heat engines or photovoltaic cells. Unlike the Shkadov thruster, such a system is not propulsive.
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.
Stellar engines in fiction
In Olaf Stapledon's 1937 science fiction novel Star Maker, some advanced galactic civilizations attempt to use stellar engines to propel their planetary systems across the galaxy in order to physically contact other advanced galactic civilizations. However, it turns out that the stars are life forms with a consciousness of their own, and their consciousnesses are extremely upset by this happening to them, because it violates the canon of the galactic ballet dance the stars feel they are a part of and which the stars feel is the primary focus and most sacred ritual of their lives. So, those stars whose surrounding civilizations attempt to force them to move in a different direction take revenge by committing suicide by exploding as supernovae, thus destroying their attendant worlds. This initiates the War of Stars and Worlds, lasting millions of years, which becomes a pivotal event in the history of the galaxy. The war only ends when the galactic civilizations figure out how to telepathically communicate with the stars and arrange a truce.
The novel Manifold: Space by Stephen Baxter has a Shkadov thruster being built around a neutron star which is destined to collide with another neutron star; the intention is to delay the collision, so that Galactic civilization will not be wiped out.
The novel Bowl of Heaven by Larry Niven and Gregory Benford describes a bowl shaped megastructure that uses magnetic fields to cause its star to emit a plasma jet, which moves the star accompanied by the megastructure.
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- Stapledon, Olaf (1937). "11 : Stars and Vermin". Star Maker. Methuen.
- Niven, Larry (2012). Bowl of Heaven. Tor Books.
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- Solar Travel (Astronomy Today, Exploration Section)
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