An Active Galactic Nucleus (AGN) is a compact, extraordinarily luminous central region of a galaxy powered by matter spiraling into a supermassive black hole containing millions to billions of solar masses. As gas and dust form an accretion disk around the black hole, gravitational energy converts to thermal energy, heating material to millions of degrees and producing radiation across the entire electromagnetic spectrum—from radio waves to gamma rays.</p><p>AGNs can outshine their entire host galaxies by factors of 100 or more, with luminosities reaching 10^12 times that of our Sun. This phenomenon manifests in various forms: quasars (quasi-stellar objects) appear as brilliant point sources visible across cosmic distances; Seyfert galaxies show broad emission lines from rapidly moving gas; and blazars beam intense radiation directly toward Earth via relativistic jets traveling at nearly light speed.</p><p>First discovered in 1943 by astronomer Carl Seyfert, AGNs revolutionized our understanding of galaxy evolution and black hole physics. The landmark 1963 identification of quasar 3C 273—located 2.4 billion light-years away yet visible through small telescopes—demonstrated that these cosmic beacons serve as lighthouses illuminating the early universe. Today, AGNs help astronomers study galaxy formation, test Einstein's relativity, and trace cosmic history back to when the universe was less than one billion years old.