- The two most powerful phenomena in galaxies are active galactic nuclei (AGNs) and starbursts. The former are intense, concentrated sources of light¿probably matter falling into a supermassive black hole. (Quasars are the best-known example.) Starbursts are galactic fireworks shows during which stars form at a frenetic pace.
This article was originally published with the title The Galactic Odd Couple.