Jeff M. Hopper said,
Why not build the Dyson Sphere around a Brown Dwarf?
The same question could be asked of a Gas Giant. Why not build a Dyson Sphere around a Gas Giant?
The answer to both questions is that neigther brown dwarfs of gas giants have sufficient mass for their cores to undergo nuclear fusion. Without nuclear fusion all a gas giant and brown dwarf does is radiate its internal hear as it cools. As the brown dwarf or gas giant cools they will radiate less heat and less energy will be intercepted by the Dyson Sphere. If at first the Dyson Sphere is temperate, it will grow colder and colder as it does not shrink with the reduced radiation from the Gas Giant or Brown Dwarf. A type M V class star does undergo fusion at its core and at the rate that it fuses hydrogen, it can last 100 billion years, and as it runs out of fuel it will grow hotter, not cooler.
By the way ever read Stephen Baxter's Book
Time Ships? It is a sequel to H.G. Wells famous book,
The Time Machine, it follows the adventures of the character known simple as the Time Traveler after his first trip into the future. In his second trip into the future, our intrepid inventor finds a different future from the one he previously visited, in this future at about 600,000 AD or so, the Morlocks instead of being the canibalistic underground dwellers of his previous visit, have built a Dyson Sphere around the Sun at the distance of Venus's orbit. The Dyson sphere was constructed out of material mined from the Sun through magnetic manipulation over a period of 10,000 years. This Dyson sphere has a number of layers, the outermost one being transparent and the one that the Morlocks live on. The sphere rotates to produce gravity, which is strongest at the equator and diminishes as you move towards the poles, it is not explained how the atmosphere doesn't all rush to the equator, but their is some mention of vast chambers. Basically, I believe this would mean that the inside of the sphere would have a transparent "roof" to hold in atmosphere.
As one stands on the inner surface at the equator, one would see flat land merging with flat sky at a seemingly infinite distance rather than their being a horizon. The landscape would simple fade into a haze as you had to look though more at more atmosphere at the area where the horizon of a planet would normally be. The sky looks blue but mottled with slightly different shades of blue representing the varied terrain above, and their would be clouds and the sun at a permanent "high noon" position. As one moved away from the equator this would change a little bit, with the atmosphere growing thinner, but at no time becomeing too thin to breath and then you would come across a transparent wall. The traveller would then either have to proceed through an airlock or through an opening with air gushing out, above these openings their would be other openings blowing air back in. The traveller would proceed through the opening and find the air pressure back at sea level values, he would continue away from the equator and find more and more of these transparent walls as he proceeded toward the equator, the ground would become more and more sloped as he continued away from the equator, to accomodate inner surface dwellers, the land would also become more rugged and mountainous so portions at this higher latitude would still be level with relation to gravity. As the traveler went through more walls he would also get lighter until he would only feel the sun's gravity pulling him away from the inner surface at the poles. Another feature of the sphere is that it incorporates nanotechnology and can construct just about any object whose pattern that it has in memory. Typically the Morlocks lived only on the outer layer where their was no sunlight while the Eloi lived on the inner surface and frequently fought wars with each other.
