These devices are also sometimes called star movers or stellar engines. The first use of these in science fiction appeared in the 1937 novel Star Maker by Olaf Stapledon. They have been seen occasionally in various other works since.
Sun Movers would employ energies and capabilities that would be difficult for us in the twenty-first century to easily comprehend. Moving a star is an ability that lies many centuries if not millennia in our future, if it can ever be done at all. As such, they fit squarely in the upper end of the Tech Level scale.
These engines can alter the trajectory of a sun, but only very, very slowly. Sun Movers would be employed not only by the most powerfully advanced civilizations, but also only by those with the ability to execute extremely long-range planning.
There could be many reasons why a civilization would want to alter the course of a star. Moving a star also moves its planetary system, which is bound to it gravitationally. One assumes that if a culture can create a Sun Mover, they also can adjust the orbits of planets that may be distorted by the move.
Sun movers may thus a means to protect inhabited star systems, particularly those that may contain delicate life-bearing planets, from long-term dangers. For example, if two galaxies are relatively close to colliding (as the Milky Way and Andromeda galaxies are expected to do in about 2 billion years), advanced civilizations may move their home star systems out into the galactic rim, where they would be safe from the waves of deadly radiation that would result from the galactic cores chaotically smashing together and merging.
Races that may not want to leave or travel far from their home planets, for whatever reason, may also employ sun movers as a way to very gradually explore or colonize other points in the galaxy while remaining comfortably at home.
Suns may also be moved as part of another, larger mega project. For example, small stars may be moved into a close klemperer rosette around a large Type II Dyson Sphere, so its outer surface could warmed enough to be terraformed and colonized. Two or more stars could also be smashed together at high relative speed to create the equivalent of supernovae, perhaps in sequence, in order to function as an intergalactic beacon to unknown far flung alien civilizations.
The Sun Movers may also maneuver their stars close to their neighbors, and thus use slingshot trajectories close to other stellar objects to impart more velocity to their subject. If calculated properly, the star could do this to many of its neighbors in sequence along a desired path with only a single initial push and minor course adjustments once underway. This in fact may be the real means by which the building civilization may mean to move their star around, and the Sun Movers are only there to act as the initial metaphoric pool cue in a cosmic game of billiards.
Some straight forward brute force methods of moving a star are available to civilizations with the proper resources. For example, a society may mobilize all the minor objects in their star system (comets, asteroids, and even small moons,) outfit them with independent means of propulsion, and use them as gravity tractors on the star. Any one such object would have a very miniscule effect on the star, but multiplied by the billions or even trillions over many millennia, they would begin nudging it ever so slowly along. More advanced societies could also do this with actual planets, if they had the means of moving them independently of the primary's gravity.
Societies desperate enough could also smash large objects such as planets into a star at high relative speeds to move them along, but the collateral damage of such an explosive impact (especially to the surrounding system) may not make such tactics worth it. Plus, stars are not exactly solid objects like a terrestrial planet is, and while one or more of these impacts would begin pushing the star along, they would do so very inefficiently.
Some uber-tech mega weapons, such as immense antimatter converters or singularity generators, could also create enough of an explosion or high energy impact to start a sun moving.
Some exotic means of transport could be scaled up by an uber-tech society to move a sun. For example, artifical gravity technology such as repulsor or tractor beams could be created with power levels on an unheard-of scale, making a ship (or a fleet of them) into what is essentially a stellar tug. In the novel Ring by Stephen Baxter, a wormhole at the center of a naked singularity was pulled wide open enough to allow entire star systems to pass through into other universes.
This concept was originated by Leonid Mikhailovich Shkadov in 1987. It has also been called a Type A Stellar Engine.
A Shkadov Thruster consists of a single gigantic megastructure statite that acts as a solar radiation mirror. When this mirror is precisely aligned at the proper angle and distance, it causes an asymmetric radiation pressure in the star, pushing it in the direction of the center of the mirror.
Since the mirror is a statite and is precisely balanced in its position relative the star by both light pressure and gravity, the mirror moves with the star while maintaining its relative position to its primary. Thus the system remains stable.
To understand the principle behind the system's thrust, it is important to picture the precisely-aligned mirror continually returning the same amount of radiation to the solar surface that the star has lost from that side. There is zero net energy loss, so itís the equivalent of star radiating nothing on that side. That hemisphere is essentially "capped." Or tamped, if one wants to metaphorically visualize a sun as continual explosion.
Meanwhile, its other end continues to radiate as normal. This asymmetry--one side radiating much more than the other--creates a small amount of thrust that slowly moves the star. Very much like an exploding firecracker tied under a tin can, the star pushes the whole assembly in the direction of the mirror. Only in this case, the metaphoric 'explosion' of the star never ends.
Progress would be excruciatingly slow by human standards. It would take over one million years for the system to reach a velocity of 20 meters per second. In that time, the star would have moved about three-hundredths of a light year from its original trajectory through the galaxy. However, after about one billion years, continual acceleration would have pushed the velocity up to about 20 kilometers per second and the star would have traveled over 34,000 light-years.
The Shkadov Thruster is sometimes mentioned as being used with other types of megastructures, such as stellar rings or partial dyson spheres or ringworlds, where the second megastructure is used primarily for power production. These are called Type C Stellar Engines (where the Type B would be a single static stellar megastructure dedicated primarily to energy gathering).
Seen in a science fiction novel years ago whose name escapes the author of this article.
This is a megascale magnetic field generator array built around a star. It can be made of many individual components in independent orbits around the sun. Alternately, it can also be a single or a handful of megastructures, such as ringworlds or even a partial Type II dyson sphere.
The array generates massive magnetic fields which in turn manipulates the magnetic field of the star. This in turn is used to draw out massive, continuous solar flares from any one point on the star, most typically one of its poles. The flares eventually merge into a single gargantuan emission jet, with an 'exhaust' flame that can reach many millions of miles. This flare jet very slowly begins moving the star, in essence turning it into a stellar-scale rocket engine. Manipulating this jet by deflecting its angle a few degrees, allows the sun to be very slowly maneuvered into a desired direction.
The generator array would have to be constructed fairly close to the star, probably within a few million miles of the surface at most, to allow the device to properly manipulate the sun's magnetic field. At this Tech level (23) materials which can withstand such conditions for long periods of time are thought to be readily available. The generator array would also of course have to be open in the area where the flare jet is expected to emerge.
The generator is thought to be able to draw the energy needed for its operation from the star itself, converting the radiated sunlight into power.
Even though the initial movement of the star would be excruciatingly slow, it would build up velocity much faster (relatively) than the Shkadov Thruster, theoretically being able to eventually achieve a small percentage of lightspeed. The actual acceleration would depend on the size of the star (the mass and power source) and the efficiency of the generator in creating a sufficiently powerful jet. In the novel this concept came from, the flare jet generator was expected to move a star through a local neighborhood of stars (a dozen or so light years) in only a few dozen millennia.
The fate of a life-bearing world in a system with an active Stellar Flare Jet Generator may not be very pleasant. Though the star would drag along its family of planets gravitationally as would one with Shkadov thruster, the flare jet would create an additional major radiation source. Though most of the radiation would be in the direction of the flare jet exhaust, enough could leak to threaten an Earth-like world's fragile ecosystem.
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