This page contains a list of the Megastructures used by the Nivenian Empire.
The building block of the Nivenian Empire is the Niven ring, or ringworld, a band encircling either the white dwarf or the red dwarf. Typically 25 meters of dirt and stone are on the surface of a ringworld, with 200 meters of structural material below that. For a white dwarf-based ringworld, almost seven kilometers of more structural material sit below the ringworld proper, suspended from each other by neodymium magnets. The ringworld proper spins at 593 km/s to generate 8.00 m/s^2 of gravity, while the bulk structural material is non-rotating so that the stellar gravity can be used to hold the ringworld proper in place (if this was not there, the ringworld proper would be torn to shreds by its own centrifugal tension). The material itself is composed of 90% poly(p-phenylene-2,6-benzobisoxazole) and 10% 6061-T6 Al-Mg-Si alloy. Carbon dioxide lasers protect the rings from meteorites and muonic fusion thrusters provide stability assist systems. Ringworlds based around the red dwarf have less bulk material as the gravity is stronger at the habitable zone. Note: the white dwarf has an accretion disk and is fusing the material that lands on its surface due to the extreme gravity, leading to a luminosity only an order of magnitude below sol.
The ringworlds orbiting the M9.9V-type secondary were the first ones built, 930,000 kilometers in radius and 12,700 kilometers wide. They were built with agriculture in mind, are 75% land and 25% water, and have the land capacity for 6.7 trillion Nivenians to farm conventionally on each one. Recently, though, a decision to cover these partly in greenhouses has resulted in a tenfold population capacity increase. These ringworlds now have a population capacity of 67 trillion each, with a combined population of 6,459 trillion. The ringworlds around the secondary are actually one political unit, as they each have a (relatively, 64 trillion is no laughing matter) small population on their own, but have a large population and strong economy when operating together.
The ringworlds orbiting the DB3.3-type primary orbit between Nivenia Prime and Nivenia Secondus, at 42,000,000 kilometers in radius and 720,000 kilometers wide. These were also built with agriculture in mind (conventional agriculture is less expensive than hydroponics), and also have 75% land and 25% water. These contain the vast majority of the population of the Nivenian Empire, with each having the land capacity for 17,129 trillion to farm conventionally on each one. 35 of these have been built so far, but they are partially unoccupied, having a total population of 333,234 trillion.
The maintenance of a ringworld, while still small compared to the ringworld's construction cost, is still gargantuan. The ringworlds are composed of 5 km x 5 km 90% poly(p-phenylene-2,6-benzobisoxazole)-10% 6061-T6 Al-Mg-Si plates, held in a 6061-T6 Al-Mg-Si frame. The plates can be removed if punctured, and duct tape can be used as a temporary patch. The broken plates are taken to huge stations near the ringworld's sun, and recycled using the abundant solar power into new plates, which are then used to fill the hole covered in duct tape. While each plate has a half-life of 400,000 years, this still means that, for a ringworld around the primary, 12,119 plates fail and must be replaced every day. The station orbiting the primary is thus over six thousand kilometers in diameter (The station is a ring and is spinning very slowly so that it does not crumple into a sphere.).
Nivenia Prime is an example of a planet with everything completely maxed out. The normal difficulty for Ecumenopolises (planet-wide cities) is the removal of waste heat. The Nivenian Empire has found a way around this by just attaching giant radiators to 5% of the planet's surface, and using air conditioning (a reasonably effective method of using energy to put heat somewhere else) to heat these panels to 2800 K, effectively supporting the infrastructure to hold 15,394 trillion inhabitants. 20% of these Nivenians work simply maintaining the huge air conditioners, fusion reactors, mass transport systems, and Carbon Dioxide reprocessors needed on a planet with 15,394 trillion inhabitants. The planet itself is covered by five massive sheets of metal, each 2.5 kilometers apart, with skyscrapers in between. Air is constantly pumped upward to equalize pressures, and each cityscape has walls twice as thick as the one above it. The top layer is also covered in a cityscape, and orbital rings are accessible from here.
Other examples of Ecumenopolises show two key differences from Nivenia Prime: First, they typically have lower radiator temperatures and higher percentages of area covered (e.g. 1200 K and 75%, instead of 2800 K and 5%), to maximize the percentage of energy that can be utilized. Second, they are typically much more efficiently planned. This second difference is largely due to the fact that while Nivenia Prime's population and waste heat dispersal mechanisms grew continuously, creating a patchwork of different styles of buildings and roads, and a highly amorphous sewer and chemical reclamation system, newer Ecumenopolises are typically built from a DIY kit almost all at once. This means that the entire planet typically becomes extremely modular, covered in self-sufficient "Ecumenopolis hexagons" a few hundred kilometers wide, which prevents the entire atmosphere from being ruined in something on one of them breaks. Two ecumenopolises in this style exist in the Nivenian Empire, the ecumenopolis Nivenia A12.1, which is under construction, but will cover the entire planet when complete, and the ecumenopolis Caeruleo 4, which is only a partial ecumenopolis, with a carrying capacity in the hundreds of trillions instead of quadrillions.
The fourth planet from the close binary, Nivenia Secondus / RSC 5649-2399-4-2271-56 A4, has much the same gravity as Nivenia Prime, making it a suitable target for colonization (never mind the fact that it is a gas giant, we have a workaround). To colonize this planet, aerostat habitats were built in the 1.5 MPa region of the atmosphere by creating giant rigid aluminum spheres, expelling all the air, leaving a perfect vacuum inside, and attaching these to the colonies in question for buoyancy. These colonies currently hold 1,937 trillion, making them a common sight in the lower atmosphere. Carbon, nitrogen, oxygen, and hydrogen can be extracted from the atmosphere, turned into poly(p-phenylene-2,6-benzobisoxazole), and shipped to construction sites.
Another 1,045 trillion make their residence in the colder gas giant B3, in similar aerostat colonies, although 1,181 trillion lived there before a devastating attack that killed hundreds of trillions (The population was 587 trillion for a short period of time before a burst of immigration). B3's industry fills much the same purpose as A4's. Interestingly enough, over twice the mass of earth has already been extracted from these gas giants to build ringworlds with.
A system-wide empire needs a system-wide transport system. When the system-wide empire has hundreds of quadrillions of inhabitants, eighty-one cubic kilometers of compressed soda can pull tabs must be transported to recycling centers every day. When hundreds of interplanetary freighters are needed just to transport pull tabs to recycling centers, the infrastructure needed for a homogeneous free-trade economy is staggering. 57 trillion workers are needed to pilot transport ships and another 89 trillion must work as clerks, accountants, supervisors, janitors, and all of the other necessary support professions needed. Trying to manage and house this system became insane hundreds of years ago. This is why large-scale orbital habitats were created. These habitats are connected to stations in "wings." Large stations may have dozens of wings, smaller ones may just have two. The stations around Nivenia Prime and Nivenia Secondus both have over two hundred, and dozens exist with more than twenty wings. Each wing is a hexagonal aluminum rod, five kilometers wide and 800 kilometers long. Six 80 kilometer-long spokes extend every 100 kilometers from the center in a hexagonal pattern, attached to 200 kilometer-long, 60 kilometer-wide O'Neill cylinders made out of the same 90% poly(p-phenylene-2,6-benzobisoxazole) and 10% 6061-T6 Al-Mg-Si alloy structural material as the ringworlds. These are small enough not to rip themselves apart, and usually have a density between a dense suburb and a full-blown city (although limited in the latter case to 10-floor buildings due to the structural integrity of the floor). On larger stations, those with more than six wings, the wings are attached to a larger hexagon (800 kilometers on a side), usually with a spaceport on the inner side of the hexagon opposite each wing on the outside. These hexagons are stacked, with the station proper suspended in the middle. The following image illustrates this:
These habitats account for 287 trillion Nivenians.
Two other habitat-based provinces exist, one around the planet A6, and one just outside of the orbit of A6. These provinces have more independence than other provinces in the Nivenian Empire due to their status as swarms of detached pairs of O'Neill cylinders. The swarm around A6 is focused on utilizing the material inside the smaller moons of A6, and the other is a very thin Dyson swarm focused on scientific and legal pursuits. Both of these provinces have noticibly different cultures than the Nivenian Empire Proper, but still pay Nivenian taxes and obey Nivenian laws. The swarm around A6 has a population of 1,349 trillion and the partial Dyson swarm has a population of 8,722 trillion.
The Nivenian Empire has colonized many of the planets, moons, and large asteroids in their system for strip-mining. The materials from these planets are vitally needed to continue construction of more ringworlds and habitats. 112 trillion Nivenians live and work on conventionally inhabited planetary colonies, and 291 trillion live and work in other colonies and mining outposts (and there is B5.1, which is an exception as it holds 508 trillion). Notable mining colonies include A11, A12, A12.1, and B4, with smaller bases on dozens of small planets, moons, and large asteroids. Nivenia Prime, A11, A12, A12.1, and B4 are collectively called "the habitable five," as they have the correct gravity for Nivenians.