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Star lifting is any of several hypothetical processes by which a highly advanced civilization (at least Kardashev-II) could remove a substantial portion of a star's matter in a controlled manner for other uses. The term appears to have been coined by David Criswell.
Stars already lose a small flow of mass via solar wind, coronal mass ejections, and other natural processes. Over the course of a star's life on the main sequence this loss is usually negligible compared to the star's total mass; only at the end of a star's life when it becomes a red giant or a supernova is a large amount of material ejected. The star lifting techniques that have been proposed would operate by increasing this natural plasma flow and manipulating it with magnetic fields.
Stars have deep gravity wells, so the energy required for such operations is large. For example, lifting solar material from the surface of the Sun to infinity requires 2.1 × 1011 J/kg. This energy could be supplied by the star itself, collected by a Dyson sphere; using only 10% of the Sun's total power output would allow 5.9 × 1021 kilograms of matter to be lifted per year (0.0000003% of the Sun's total mass). The Dyson sphere would need to be designed to allow the lifted material to egress through it.
Methods for lifting material 
Thermal-driven outflow 
The simplest system for star lifting would increase the rate of solar wind outflow by directly heating small regions of the star's atmosphere, using any of a number of different means to deliver energy such as microwave beams, lasers, or particle beams – whatever proved to be most efficient for the engineers of the system. This would produce a large and sustained eruption similar to a solar flare at the target location, feeding the solar wind.
The resulting outflow would be collected by using a ring current around the star's equator to generate a powerful toroidal magnetic field with its dipoles over the star's rotational poles. This would deflect the star's solar wind into a pair of jets aligned along its rotational axis passing through a pair of magnetic rocket nozzles. The magnetic nozzles would convert some of the plasma's thermal energy into outward velocity, helping to cool the outflow. The ring current required to generate this magnetic field would be generated by a ring of particle accelerator space stations in close orbit around the star's equator. These accelerators would be physically separate from each other but would exchange two counterdirected beams of oppositely charged ions with their neighbor on each side, forming a complete circuit around the star.
Criswell proposed a modification to the polar jet system in which the magnetic field could be used to increase solar wind outflow directly, without requiring additional heating of the star's surface. He dubbed it the "Huff-n-Puff" method, inspired from the Big Bad Wolf's threats in the fairy tale of Three Little Pigs.
In this system the ring of particle accelerators would not be in orbit, instead depending on the outward force of the magnetic field itself for support against the star's gravity. To inject energy into the star's atmosphere the ring current would first be temporarily shut down, allowing the particle accelerator stations to begin falling freely toward the star's surface. Once the stations had developed sufficient inward velocity the ring current would be reactivated and the resulting magnetic field would be used to reverse the stations' fall. This would "squeeze" the star, propelling stellar atmosphere through the polar magnetic nozzles. The ring current would be shut down again before the ring stations achieved enough outward velocity to throw them too far away from the star, and the star's gravity would be allowed to pull them back inward to repeat the cycle.
A single set of ring stations would result in a very intermittent flow. It is possible to smooth this flow out by using multiple sets of ring stations, with each set operating in a different stage of the Huff-n-Puff cycle at any given moment so that there is always one ring "squeezing". This would also smooth out the power requirements of the system over time.
Centrifugal acceleration 
An alternative to the Huff-n-Puff method for using the toroidal magnetic field to increase solar wind outflow involves placing the ring stations in a polar orbit rather than an equatorial one. The two magnetic nozzles would then be located on the star's equator. To increase the rate of outflow through these two equatorial jets, the ring system would be rotated around the star at a rate significantly faster than the star's natural rotation. This would cause the stellar atmosphere swept up by the magnetic field to be flung outward.
This method suffers from a number of significant complications compared to the others. Rotating the ring in this manner would require the ring stations to use powerful rocket thrust, requiring both large rocket systems and a large amount of reaction mass. This reaction mass can be "recycled" by directing the rockets' exhausts so that it impacts the star's surface, but harvesting fresh reaction mass from the star's outflow and delivering it to the ring stations in sufficient quantity adds still more complexity to the system. Finally, the resulting jets would spiral outward from the star's equator rather than emerging straight from the poles; this could complicate harvesting it, as well as the arrangement of the Dyson sphere powering the system.
Harvesting lifted mass 
The material lifted from a star will emerge in the form of plasma jets hundreds or thousands of astronomical units long, primarily composed of hydrogen and helium and highly diffuse by current engineering standards. The details of extracting useful materials from this stream and storing the vast quantities that would result have not been extensively explored. One possible approach is to purify useful elements from the jets using extremely large-scale mass spectrometry, cool them by laser cooling, and condense them on particles of dust for collection. Small artificial gas giant planets could be constructed from excess hydrogen and helium to store it for future use.
Stellar husbandry 
The lifespan of a star is determined by the size of its supply of nuclear "fuel" and the rate at which it uses up that fuel in fusion reactions in its core. Larger stars have a larger supply of fuel, but the increased core pressure resulting from that additional mass increases the reaction rate even more; large stars have a significantly shorter lifespan than small ones. Current theories of stellar dynamics also suggest that there is very little mixing between the bulk of a star's atmosphere and the material of its core, where fusion takes place, so most of a large star's fuel will never be used naturally.
As a star's mass is reduced by star lifting its rate of nuclear fusion will decrease, reducing the amount of energy available to the star lifting process but also reducing the gravity that needs to be overcome. Theoretically, it would be possible to remove an arbitrarily large portion of a star's total mass given sufficient time. In this manner a civilization could control the rate at which its star uses fuel, optimizing the star's power output and lifespan to its needs. The hydrogen and helium extracted in the process could also be used as fusion reactor fuel. Current fusion generators (and fusion bombs) use deuterium and tritium. Most of a star’s mass consists of light hydrogen and helium. A technologically advanced civilization may develop fusion reactors capable of using ordinary hydrogen by currently unknown means. Alternatively, the material could be assembled into additional smaller stars, to improve the efficiency of its use. Theoretically all the energy of the matter lifted from a star could be harvested if it is made into small black holes, via the mechanism of Hawking Radiation.
In fiction 
- In the Star Wars franchise of Knights of the Old Republic, the Star Forge is capable of star lifting.
- In Lexx, the Mantrid Drones eventually became capable of star lifting simply by using their combined gravity to draw material away.
- The novel Star Trek: Voyager – The Murdered Sun featured a reptilian race using the material from a star to sustain the opening of a wormhole. However, the novel depicted the process as shortening the star's lifespan precipitously rather than extending it.
- In The Night's Dawn Trilogy by Peter F. Hamilton, the alien species the Kiint created an arc of custom made planets around their sun from mass extracted from their star.
- In The Time Ships by Steven Baxter, a Dyson sphere is constructed by material harvested from the sun through star-lifting.
- In the web-based collaborative fiction site "Orion's Arm" the star Polaris is subject to starlifting (with image).
- Interstellar Migration and the Human Experience, Chapter 4: Solar System Industrialization, by David R. Criswell