Last year, a star (KIC 8462852 aka Tabby’s Star) was discovered with a flickering that couldn’t be explained by yet unknown natural phenomena. So of course, most people jump to the alien megastructure conclusion. Stuff like Dyson’s Sphere, or Larry Niven’s Ringworld make for great sci-fi, but are very impractical in the real world.
The megastructures in question are ones big enough to surround a star or in one case an entire star system. The amount of resources and man power (or robot power) required would be astronomical, and not as pragmatic as building ships, even if they are generational, to travel to other stars.
Some Types of Megastructures
Dyson Sphere: This is probably the simplest but still very massive. First postulated by Freeman Dyson. It can either consist of a huge sphere the collects massive amounts of energy, or a more massive one that supports an entire civilization (like the one seen in Star Trek: The Next Generation episode “Relics”). Its interior would have a radius within Earth’s and Venus’ orbits (otherwise know as the Goldilock’s Zone), if it’s a star of similar size to the Sun.
Alderson Disk: A large disc with a star in center (essentially a giant CD) , and an outer edge approximate to Mar’s orbit. A thickness of several thousand miles. Its mass would be greater than the Sun’s.
Ringworld: Popularized in Larry Niven’s sci-fi series, it’s mass is the sum of all the planets in the Solar System with a radius approximate to Earth’s orbit.
Matrioshka Brain: A Dyson Spheres or series of Dyson Spheres largely made of computers and powered by a star.
Stellar Engine: May consist of a Dyson Sphere. Is a giant engine that moves the star it’s powered by and it’s entire system.
Now to show an example of Dyson Sphere based on our Solar System (The math might give you a headache. The size of numbers is astronomical.) The first few calculations are how I determined the actual volume of the Dyson Sphere minus that empty space inside the sphere using the formula V = (4/3) × pi × r3.
Radius of Exterior (approximate to Earth’s orbit) = 150,000,000 km
Radius of Interior (with thickness of sphere being 1.6 km) = 149,999,998.4 km
Vol of Exterior sphere = 1.413716694115407 x 10^25 cubic km (141,371,669,411,540,700,000,000,000 cubic km)
Vol of Interior sphere = 1.4137166489764734 x 10^25 cubic km (141,371,664,897,647,340,000,000,000 cubic km)
Vol of Dyson Sphere (Exterior – Interior Vol) = 4.241150024 x 10^17 (424,115,002,434,126,000 cubic km)
Compare this to our largest planet Jupiter, who’s mass is 1.43 x 10^15 (1,430,000,000,000,000 cubic km) that makes the Dyson Sphere’s mass that of 297 Jupiter’s. Which means it require almost everything in our Solar System just for the materials alone. That doesn’t take in account the material you would need to build millions, if not billions of robots to construct it. And even then it will take hundreds, if not thousands of years of completion.
Unless the Universe is near its death and there are only a handful of stars left (and even then all races in the Universe may have evolved beyond their corporeal form or found a way to travel to other universes), then it doesn’t make practical sense to construct megastructures of this magnitude. A civilization’s time and resources are better spent building ships to travel to other worlds, whether they are sleeper, generational, or hopefully one day capable of FTL (faster than light) speed.
More info on these megastructures (1 through 5) http://listverse.com/2010/03/07/top-10-theoretical-megastructures/