What is an active galactic nucleus (AGN)?

An active galactic nucleus (AGN) refers to the highly luminous central region of certain galaxies where significant amounts of energy are produced, often outshining the entire galaxy. This luminosity is typically associated with the presence of a supermassive black hole at the galaxy's core, which actively accretes gas and dust. As material spirals into the black hole, it heats up and emits substantial electromagnetic radiation, which can include visible light, X-rays, and radio waves.

The defining characteristic of an AGN is its activity, which results in the high luminosity that differentiates it from the more quiescent galactic cores that contain older stars without the same energetic processes. Therefore, stating that an AGN is the core of an active galaxy with high luminosity accurately captures its essence, emphasizing the dynamic and powerful nature of these regions compared to other quiet, star-rich areas typically found in less active galaxies.