Free Astronomy Magazine September-October 2024

7 SEPTEMBER-OCTOBER 2024 ASTRO PUBLISHING R endering of a nearly completed Dyson shell sphere. For all the variants of these hypothetical megastructures, it is not clear how the stellar energy collected by the inner surface of the sphere could be transferred to the home planet of the builder civilization or to other destinations. [Neil Blevins] T he electromagnetic spectrum, with the subregion that includes near, medium, and far infrared radiation highlighted. [Technology Networks] Dyson suggested looking for at wave- lengths between 8 and 12 microns, one of the best windows (the so- called N-Band) accessible by ground- based telescopes (the first infrared space telescope, IRAS, was launched over twenty years later and ob- served more than 250,000 sources at 12, 25, 60, and 100 microns). According to Dyson’s initial vision, a civilization technologically much more advanced than ours would have the means to directly take from the planets of its system all the ma- terial needed to build a sphere. As- signing to the latter a radius of two astronomical units and a thickness of a few meters, Dyson calculated that the megastructure could be (theo- retically) built by reconverting the entire mass of a planet as large as Jupiter. It is easy to imagine that Dyson spheres remained for a long time nothing more than a bizarre idea, with almost no astronomer even attempting research into them. Nonetheless, in recent decades, they have occasionally returned to the fore, thanks to the multiplication of infrared surveys carried out both from the ground (for example, 2MASS) and from space (for exam- ple, WISE). Although the purposes of these surveys had nothing to do with the search for alien megastruc- tures, the wealthy databases they helped create could contain traces of suspicious sources, that is, stars with an infrared emission that is too