So anyway, we went to dinner with phyrra and nihilus last night at the local Cheesecake Factory, which is (for some reason) decorated in a style that can accurately be described as "trendy chain restaurant meets shrine to the Great God Cthulhu." The dining room is flanked by tall pillars with glowing orange eyes of the Greater Old Ones ato them, for reasons not entirely clear to your humble scribe.
So we ate beneath the scowling glower of the Great Old One, and nihilus and I started talking about Dyson spheres, and specifically what a Dyson sphere would look like from the outside.
The idea behind a Dyson sphere is pretty straightforward: you are a member of a super-advanced alien race, you have the technology to construct objects on the scale of solar systems, and you need energy. Lots and lots of it. So what do you do? Why, you take apart your solar system and build a Dyson sphere--a shell, about 90,000,000 miles or so in diameter, that completely encloses your sun. You live on the inside surface of the sphere, which has the land area of hundreds of millions of earth-sized planets, and you capture all the available energy from your star.
Now, the laws of quantum dynamics and the Second Law of Thermodynamics impose limitations on how efficiently it is possible to capture and use energy to perform useful work. The law of entropy always wins out in the end; no matter how advanced you are, there are certain limits imposed by the laws of physics themselves on how efficiently you can capture the energy from your sun. Some of it is going to leak out in the form of waste heat. A Dyson sphere cannot be a perfectly dark body; there must be some radiated waste, there's no way around it.
I believe that a Dyson sphere, when viewed from the outside, would appear as a dark stellar-mass object radiating strongly in deep infrared. nihilus believes the limitations imposed by the laws of physics are much smaller, and that a Dyson sphere created by an advanced enough civilization can capture and use the star's energy with such efficiency that it would appear as a dark stellar-mass body radiating only a few microdegrees Kelvin above the universal microwave background radiation. Unfortunately, I don't have the background to be able to calculate which is more likely--anyone?
During the course of this conversation, nihilus and I apparently missed Shelly doing something lascivious with a cherry, which is a pity, but at least phyrra got to see it...