It could be a cylindrical or circular core, producing light or heat, usually generating some kind of sound - regardless of whether or not humans can hear it. Usually in containment of some kind, in a chamber or room with a cleared space around it before another room buffering it where the interaction controls are.

[ Jim thinks that's the best ballpark description he's going to be able to give without starting to draw diagrams; the basic parameters cover warp cores to old Romulan hyperdrives and 20th century space station engines. It's a solid pattern. But-- ]

If it's magic, I have no clue. Probably glowing, that shit loves to glow.

[ Thanks, Jim. ]