These are megastructures that require the presence of a star for construction or stability.

Ringworld

After the Death Star, the Ringworld is the most famous example of a megastructure.

Take a band of unbelievably strong material a million miles wide and stretch it around a stable star at a distance comparable to Earth's distance from the sun. Spin it to simulate gravity on the inner surface, Put a wall a thousand miles high on the inner edges to keep in the atmosphere. Now sculpt the interior surface to resemble an Earth-like world, complete with soil, water, oceans, forests, and so on. Further in, orbiting nearer the sun, would be thirteen enormous squares, casting shadows on the inner surface and simulating the day-night cycles of natural worlds. What you end up with is a vast world with a habitable surface area equal to approximately 3 million Earths. Totally selfsustaining, it could easily support tens of trillions of people.

The Ringworld described above was first proposed by Larry Niven in his novel, Ringworld. As large as the preceding megastructures were, the scale of the Ringworld and the following artifacts truly staggers the imagination. For example, On Niven's Ringworld, two at Oceans exist on either side of the Ring, each reaching from one rim wall to the next, making them early a million miles wide. That's large enough to swallow the planet Saturn if the oceans were made deep enough. Entire civilizations can rise and fall on artifact over the millennia, exploring many planlengths in all directions, but never knowing more than a tiny portion of their world.

Ringworlds would require the disassembly of an the solar system for building materials, as well as fantastically advanced technology, such as large-scale matter conversion and the production of near-indestructible materials. The Ringworld can also serve the secondary functions of power production, drawing in energy from its central sun, and defense.

In Niven's version' superconductor grids within the Ringworld created powerful magnetic fields that could draw and focus gigantic flares from the central star, create a laser-like beam that could incinerate an Earthed world within seconds. Besides Niven's novels, tails on the construction and inhabitants of a Ringworld can he found in two sources: from Chaosium's old Ringworld game, now a rare collector's item and from the more recent The Guide to Larry Niven's Ringworld, by Kevin Stein. Both are well-detailed and generously illustrated, and can provide a wonderful starting point for GMs for a Ringworlded campaign.

A gossamer ring is a variation of the Ringworld. Its in purpose is pure energy gathering. Instead of a band of solid material, only a sparse frame would be needed. In between the struts would be stretched furls very light composite material, like those proposed for use in solar sails. This material is similar to the coating of solar cells, only much thinner (on the order several microns -- thousandths of a centimeter thick).

As the ring is not meant to support life, it can be built with a smaller radius and width so as to a greater percentage of the star's energy. A gossomer ring would take up far less material and be easier to build than a life-supporting Ringworld.

Dyson Sphere

First proposed in 1960 by famed physicist Freemanson, this is a physical shell completely encasing a star. Such a sphere constructed with a radius equal to Earth's distance from the sun would have a habitable area of about one billion Earth-like planets. If entire surface were made life-bearing, it could hold over one hundred trillion inhabitants.

However, like a Ringworld, a habitable dyson sphere presents a great deal of logistic problems. It would have to be made out of unbelievably strong material far more resilient than anything now known, order to withstand the tidal stresses it would have to endure.

Even if it is set spinning, some means of artificial gravity generators will be needed to keep the objects not on the interior equator from flying off ward the sun from centrifugal force. And there is the question of whether there would be enough silicates in an entire star system to form soil for the entire surface, or enough nitrogen and oxygen to provide enough breathable atmosphere. Chances are exotic technologies such as matter conversion would have to be employed or material would have to be ported from dozens of other star systems.

Like a Ringworld, a Dyson Sphere has such a mindboggling amount of habitable room that it could support millions of diverse ecologies and thousands of independent civilizations with no strain whatsoever. If used for defense, the Sphere would have the energy output of an entire sun to draw upon to power its weapons.

Because powerful gravitic technology is required for a Dyson Sphere, it should be possible to very slowly move the sphere using the combined gravity generators. (One portion of the sphere would increase its gravity slightly, tugging the sun ever so gently in its direction a few inches at a time.

The sun's gravity would pull the rest of the sphere along with it. Since it is already assumed that the sphere is made of nearindestructible material, it should handle the stres- ) One can imagine a very advanced civilization roaming through cosmos in such sphere for billions of years, exploring, trading with, and/or conquering whatever other races they encounter.

Dyson Spheres can be found in a number of sources, including the novels Wall Around a Star, by Jack Williamson and Frederick Pohl, and The Ring of Charon and The Shattered Sphere, both by Roger MacBride Allen. The latter details how a Sphere could possibly be constructed. A Dyson Sphere can also be seen in the Star Trek: The Next Generation episode "Relics."

A variation on the Dyson Sphere is the gossamer sphere. Like the gossamer ring, it would be composed mostly of framework and solar,cell material. Its main purpose would be energy gathering, and can he constructed with much smaller dimensions than an inhabited sphere.

A compromise can be built between the solid and gossamer sphere, and avoid some of the logistic problems of the former. The equator of the sphere would be a habitable ringworld, as described above. The rest of sphere, however, can be made from gossamer material for energy gathering, and supply power to the trillions of inhabitants in the ring.

Another variation on the Dyson Sphere is the solid Sphere. It has the same dimensions as a standard sphere, but is solid all the way through nearly to the surface of the central star. Even with matter conversion, it would take material from dozens, if not hundreds, of star systems to construct such a mammoth artifact. If self-supporting, it could hold over a quadrillion sentient beings, and have millions of continent-sized habitats spread throughout its volume.

The purpose of a Solid Sphere would be as inscrutable the ultra-advanced civilization capable of constructing it. However, if made mobile, it could become a true planet-eater, roaming through the galaxy, constantly consuming the vast raw materials needed to support itself and the cultures within.

Godwheel

The largest megastructure so far proposed, this disc-like world has not one star at its center, but two. The central stars orbit each other about a common center of gravity, giving them both highly elliptical orbits. A disk built around them, perpendicular to their orbit, would take advantage of their mutual circling to simulate a day-night cycle over its alternate sides.

The suns would rise and set successively over each surface during their circular, short orbital period. If the disk is built large enough, its habitable surface can cover the entire habitable zone around a star, theoretically from about the orbit of Venus to the orbit of Mars. Its actual surface would have to extend quite a ways further inward, in order to block one of the suns as the star dipped in its orbit to illuminate the other side of the wheel. Gravity would have to be created by purely artificial means.

Put both sides of the disc together, you get a habitable surface area equal to trillions of Earth-like planets. Like a Solid Sphere, the Godwheel could conceivably hold a quadrillion inhabitants, but spread out over its two surfaces. And it would also require the gutting of many star systems to construct. The Godwheel was created by Larry Niven for Malibu Comics' Ultraverse books, and was a central feature in that brief-lived but intriguing super-hero universe.

MegaStructures Sci-fi Settings That Challenge the Scale of the Universe

Introduction
Free-Floating Structures
Planet-Centric Structures
Star-Centric Structures
Other Genres