The Megasphere was gigantic artificial construct which was an exact model of the Milky Way housed at the Prometheus Science Center in 2268. Distances between the stars, nebulae, black holes, neutron stars, and other objects were to scale – though the stars and other intragalactic objects were not. All galactic coordinates for starship navigational computers and all scientific referents to galactic structure were drawn from the Megasphere. Only two originals existed – one on Earth, the other on Vulcan.
New stellar discoveries were being added by specially designed computer controlled microhandlers. It took weeks of preparation merely to design new microbandlers to perform the insertion of new objects. Banks of special lights flooded the Megasphere from every angle. The light was picked up, amplified, and thrown back into near darkness by special photosensitive material. Bulbs or any kind of powered light source were too clumsy and too difficult to use. The photosensitive material was designed to never wear out, never need replacement.
Half-a-dozen special gravity field generators held the stellar model intact, and any one of which could maintain the Megasphere. If all six failed simultaneously, the work of years would fall to the chamber floor in a shower of tiny glowing pebbles. (TOS - Log Three novelization: The Magicks of Megas-Tu)