An Alderson Disk is an enormous interstellar megastructure. It is shaped like a giant CD, with an entire Star inside the middle hole. The radius of the disk would be about the same as the orbit of Jupiter. The disk would be so massive, it would have greater gravity than the Sun it was built around. The gravity would pull perpendicular to the plane of the disk, not toward the sun.
A society could exist on both surfaces of the disk, though close to the sun you'd bake and far from the sun you'd freeze. See Temperature of Space. Environmental controls, life support, and habitat engineering could cover this, but man, what a project. This is definitely the territory of Sufficiently Advanced Technology. Just making such a disk might take more matter than is in our solar system, and the gravitational forces involved would demand you made it out of something extremely resilient.
The Alderson Disk has no day or night, just a constant twilight. However, since you're obviously scoring very high on the Kardashev Scale, there is the option of manipulating gravity to cause the Sun at the center to bob, creating artificial day and night cycles for the two surfaces of the disk.
Game and Story Use
- Sufficiently Advanced Aliens, especially ones who can survive temperature extremes, might engineer (and live on) such a disk. Over the millenia, the environmental conditions at various parts of the disk might cause them to evolve into two or more subspecies - one adapted to heat and light, the other adapted to gloom and cold.
- We need huge amounts of matter to build this thing, and we'd rather not have to move it astronomical distances. Perhaps it's built from a large supply of Dark Matter that was previously hidden (Dark Matter is invisible) just outside the solar system in question. If that's the case, large sections of the disk's superstructure may be translucent.
- So, from a distance, it may look more like a cloud of debris or an asteroid belt, unless the surface has been adorned with normal matter.
- A big question might be why such a structure would be built, given its huge cost and limited utility - the same resources could yield (probably at least one) fully habitable Niven Ring, any of the Dyson structures, or a family of smaller solar orbital ringworlds.