Stellar engines are a class of hypothetical megastructure

Megastructure

A megastructure is a very large manmade object, though the limits of precisely how large this is vary considerably. Some apply the term to any especially large or tall building....

s which use a star's radiation to create usable energy. Some variants use this energy to produce thrust, and thus accelerate a star

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

, 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

Kardashev scale

The Kardashev scale is a method of measuring an advanced civilization's level of technological advancement. The scale is only theoretical and in terms of an actual civilization highly speculative; however, it puts energy consumption of an entire civilization in a cosmic perspective. It was first...

.

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 Mikhailovich Shkadov

Leonid Mikhailovich Shkadov

Leonid Mikhailovich Shkadov was a physicist who came up with the idea of the Shkadov Thruster....

 who first proposed it), or a Class A stellar engine. Such an engine

Engine

An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

 is a stellar propulsion system, consisting of an enormous mirror/light sail—actually a massive type of solar statite

Statite

A statite is a hypothetical type of artificial satellite that employs a solar sail to continuously modify its orbit in ways that gravity alone would not allow. Typically, a statite would use the solar sail to "hover" in a location that would not otherwise be available as a stable geosynchronous...

 large enough to classify as a megastructure

Megastructure

A megastructure is a very large manmade object, though the limits of precisely how large this is vary considerably. Some apply the term to any especially large or tall building....

, probably by an order of magnitude

Order of magnitude

An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...

—which would balance gravitational attraction towards and radiation pressure

Radiation pressure

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...

 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

Thrust

Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

, accelerating the star in the direction of the hovering statite

Statite

A statite is a hypothetical type of artificial satellite that employs a solar sail to continuously modify its orbit in ways that gravity alone would not allow. Typically, a statite would use the solar sail to "hover" in a location that would not otherwise be available as a stable geosynchronous...

. Such thrust and acceleration would be very slight, but such a system could be stable for millennia. Any planetary system

Planetary system

A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...

 attached to the star would be 'dragged' along by its parent star. For a star such as the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, with luminosity 3.85 × 10²⁶ W

Watt

The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

 and mass 1.99 × 10³⁰ kg

Kilogram

The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

, 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-year

Light-year

A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

s. 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

Milky Way

The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 galaxy.

Class B

A Class B stellar engine is a Dyson sphere

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...

—of whichever variant—built around the star, which uses the difference in temperature between the star and the interstellar medium

Interstellar medium

In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

 to extract usable energy from the system, possibly using the phenomenon of thermoelectricity (heat engine

Heat engine

In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...

s, or thermal diode

Thermal diode

The term thermal diode is sometimes used for a device which causes heat to flow preferentially in one direction. Or, the term may be used to describe an electrical diode in reference to a thermal effect or function...

s). Unlike the Shkadov thruster, such a system is not propulsive. The Matrioshka brain

Matrioshka brain

A matrioshka brain is a hypothetical megastructure proposed by Robert Bradbury, based on the Dyson sphere, of immense computational capacity. It is an example of a Class B stellar engine, employing the entire energy output of a star to drive computer systems...

 concept is based on this stellar engine concept, extracting energy for a specific purpose: data processing.

Class C

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

Statite

A statite is a hypothetical type of artificial satellite that employs a solar sail to continuously modify its orbit in ways that gravity alone would not allow. Typically, a statite would use the solar sail to "hover" in a location that would not otherwise be available as a stable geosynchronous...

 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

Olaf Stapledon

William Olaf Stapledon was a British philosopher and author of several influential works of science fiction.-Life:...

's 1937 science fiction

Science fiction

Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

 novel

Novel

A novel is a book of long narrative in literary prose. The genre has historical roots both in the fields of the medieval and early modern romance and in the tradition of the novella. The latter supplied the present generic term in the late 18th century....

 Star Maker

Star Maker

-External links:*...

, some advanced galactic civilizations attempted to use stellar engines to propel their planetary systems across the galaxy in order to physically contact other advanced galactic civilizations. However, it turned out that the star

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s were life forms with a consciousness

Consciousness

Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

 of their own, and their consciousnesses were extremely upset by this happening to them, because it violated the canon of the galactic ballet dance the stars felt they were a part of and which the stars felt was the primary focus of and most sacred ritual of their lives. So, those stars whose surrounding civilizations attempted to force them to move in a different direction took revenge by committing suicide by exploding as supernovae, thus destroying their attendant worlds. This initiated the War of Stars and Worlds, lasting millions of years, which was a pivotal event in the history of the galaxy. The war only ended when the galactic civilizations figured out how to telepathically

Telepathy

Telepathy , is the induction of mental states from one mind to another. The term was coined in 1882 by the classical scholar Fredric W. H. Myers, a founder of the Society for Psychical Research, and has remained more popular than the more-correct expression thought-transference...

communicate with the stars and arrange a truce.