Alderson disc

An Alderson disc (named after Dan Alderson) is a megastructure similar to a Dyson sphere.

It would weigh about 6 x 1033 kg, or about 3000 solar masses. The innermost radius is about 50 million km, just inside the orbit of Mercury; the outermost radius is set at 600 million km from the sun, about midway between the asteroid belt and Jupiter. The disc is 5000 km thick, so the surface gravity is 0.14 gees (like the moon). The hole would be surrounded by a 1000km high wall to prevent the atmosphere from drifting into the Sun

The energy required to assemble the Alderson Disk is in the region of 5 x 1044 J. The mechanical stresses within the disc would be so great that only something with the tensile strength of magmatter would be used in its construction.

Because the Sun remains stationary, there is no day/night cycle, only a perpetual twilight. This could be solved by forcing the Sun to bob up and down within the disk, lighting first one side then the other. Computer-controlled Shklovsky Grasers mounted on the inside edge could induce vertical motion, and would also serve to nudge the star back to center if it begins to stray towards the inside annular edge of the disc.

Life could exist on either side of the disk, though close to the Sun the heat would make life impossible without protection. Conversely, farther away from the Sun living beings would freeze. Therefore, for the entirety of such a structure to be made habitable, it would have to include a vast number of life support systems. Even without such systems the habitable surface area could be up to 100s of millions of Earths for a late Type II civilization.

To scale the useful living surface to more than 4 billion Earths would require a cooperative venture of a group of Type II cultures (perhaps 10-100 of them) or a single Type III galactic civilization.