A Dyson sphere is a hypothetical structure first described in 1959 by the physicist Freeman Dyson in a short paper published in the journal Science entitled '"Search for Artificial Stellar Sources of Infra-Red Radiation". It is an artificial hollow sphere of matter around a star designed to capture nearly all of the star's radiated energy for industrial use. Although Dyson is credited with being the first to formalize and popularize the concept of the Dyson sphere, Dyson himself got the idea in 1945 from a science fiction novel titled Star Maker[?] by Olaf Stapledon. The original proposal by Dyson did not go into much detail about how a Dyson sphere would be constructed, focusing instead on the more fundamental issue of how an advanced civilization could expand its energy production to the maximum possible for a given solar system. Such a civilization would be classified as a Type II civilization under the Kardashev classification scheme developed by the astronomer Nikolai Kardashev[?].
A star contained within a Dyson sphere would not be directly visible to the outside universe, but the Dyson sphere itself would radiate an equivalent amount of energy in the form of infrared light due to solar heating from within. In addition, since Dyson spheres are composed of solid matter, it would radiate a black body spectrum. Dyson proposed that astronomers search for such giant anomalous "stars" in order to detect advanced alien civilizations, but none have been recorded. Attempts to detect Dyson Spheres using the IRAS[?] (Infrared Astronomical Satellite[?]) sky survey data are currently underway.
There are several basic varieties of hypothetical design for a Dyson sphere. The most realistic of them, and the closest to Dyson's original conception, is the "Dyson swarm". It consists of a large number of independent solar collectors orbiting in a dense formation around the star. The solar collectors could range widely in individual size and design, and possibly include space habitats for biological creatures to live in, but as a group they would collectively intercept almost all of the star's total light output. A number of different orbital patterns for the collectors have been proposed, each with different benefits and drawbacks; whatever pattern is chosen, some solar collectors will spend part of their orbits in the shadows of other solar collectors, reducing the sphere's efficiency somewhat. Since the collectors operate largely independent of each other, a Dyson swarm can be constructed incrementally over a long period of time and provide useful output throughout.
A second type of Dyson sphere is a uniform solid shell around the star, sometimes called a "Dyson shell", often with a layer of atmosphere and soil on the inner surface to provide an astronomically large living space for organic life forms. This form of Dyson sphere is much more popular in science fiction, but is not physically feasible for a variety of reasons. One is the immense strength that would be required for such an enormous structure, and another is the fact that the net gravitational force exerted by a uniform hollow sphere on anything inside is zero; there would be nothing holding the atmosphere to the sphere's surface, and it would fall into the sun. The problem of gravity cannot be solved even using hypothetical gravity generators, since it is a characteristic of the fundamental inverse square falloff nature of gravitational force, but the sphere could be rotated to produce centrifugal pseudogravity around its equator. However, the structural strength requirements for the shell material becomes even larger in this case.
A third type of Dyson sphere called a "Dyson bubble" is occasionally considered, composed of statites that hover motionless relative to the englobed sun using light pressure; this form of Dyson sphere has such low mass requirements that it could potentially be built from the material contained in a single small moon or large asteroid. However, a Dyson bubble has few practical applications (harvesting energy would be difficult due to its low mass and dependence on high reflectivity) and so it is not often discussed.
Another possibility is the "Dyson net", a web of cables strung about the star which could have power or heat collection units strung between the cables. The Dyson net reduces to a special case of Dyson shell or bubble, however, depending on how the cables are supported against the sun's gravity.
A Dyson Sphere appeared in the Star Trek: The Next Generation episode "Relics."
See also: Ringworld