As the two conversed, the third amongst them spread his awareness out... seeking the subtle and not so subtle signs of radiation in the city.  He could sense radiation from many of the ships in orbit... their drives, no doubt.  The city itself however held a distinct lack of anything of the sort aside from personal vehicles and ships.  As a matter of fact, as he spread his senses to the edges of the city where the massive alien generators sat, he could detect nothing... no radiation of any kind that he could quantify.  Whatever the city ran on, it did not seem to emit radiation.  There was something there that he could -almost- detect, but it was like seeing something on one's peripheral.  Barely there.  If Black Starfire chose to scan the area with his own sensors, he would detect that the strongest energy source in the city aside from the generators themselves seemed to be in that massive dome in the city's center.  In fact, the majority of the city's populace seemed to be flocked around the dome... various alien beings chattering in every conceivable language as they appeared to be waiting in line in the distance.