plural of galactic year

(humorous) A notional space monster invoked to explain the high failure rate of probes sent to Mars.

plural of active galactic nucleus

(astronomy) A compact region at the center of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum.

plural of galactic habitable zone

(astrobiology, astronomy) the zone in a disk galaxy lying within the disk which is neither too far from the core so as to be lacking in heavier elements, nor too close to the core as to experience excessive exposures to supernovae, where intelligent life may develop