
A galaxy with a compact, highly luminous central region powered by accretion onto a supermassive black hole.An active galaxy is a galaxy whose center, or nucleus, shines far more brightly than the combined light of its billions of stars. This extraordinary luminosity is not produced by stellar fusion but by matter spiraling into a supermassive black hole, which can have a mass millions to billions of times that of the Sun. As gas and dust fall toward the black hole, they form a hot, rapidly spinning accretion disk, where friction and magnetic forces heat the material to millions of degrees, releasing immense energy across the electromagnetic spectrum, from radio waves to X-rays and gamma rays.
The concept emerged in the mid-20th century with the discovery of quasars in 1963 by Maarten Schmidt, who identified their redshift as cosmological. The connection to supermassive black holes was established in the 1970s, notably by Donald Lynden-Bell and Martin Rees.
The engine of an active galaxy is the accretion disk around a supermassive black hole. Material drawn in by gravity loses angular momentum and spirals inward, heating up as it goes. The intense radiation and relativistic jets—collimated streams of plasma ejected along the black hole's spin axis—can outshine the entire host galaxy. The activity is regulated by the supply of fuel and by magnetic fields, which can either channel matter inward or launch powerful winds that push gas away, sometimes shutting off the accretion and making the galaxy 'sleep' until new material arrives.
Active galaxies serve as natural laboratories for studying extreme gravity, jet physics, and the co-evolution of black holes and their host galaxies, and they are used as beacons to probe the distant universe.
The Milky Way's own supermassive black hole, Sagittarius A*, is currently quiet, but it shows signs of having been active in the recent past, and astronomers have witnessed it 'waking up' with increased flaring.




















