active galactic nucleus
/ AY-jee-en (AGN) /
Most galaxies, like our own Milky Way, are mostly starlight: add up all the stars and that is roughly how bright the galaxy is. But in a small fraction of galaxies, the very center outshines everything else — a single point at the core can blaze brighter than all the hundreds of billions of stars around it combined, and it flickers in ways no star ever could. That dazzling central engine is an active galactic nucleus.
An AGN is powered not by nuclear fusion (the way stars are) but by gravity feeding a supermassive black hole. Gas spirals inward, forms a hot, swirling accretion disk, and as it falls it gives up enormous gravitational energy as light, from radio waves to X-rays and gamma rays. The energy comes from a tiny region — sometimes smaller than our Solar System — yet can reach 10^38 to 10^41 watts, trillions of times the Sun's output. We know the source is tiny because the brightness can change noticeably in days or even hours, and nothing can vary faster than light can cross it.
AGN matter because they are the most luminous persistent objects in the universe, visible across nearly the whole cosmos, and they shaped the galaxies that host them. The same engine appears under many disguises — quasars, Seyfert galaxies, radio galaxies, blazars — and a big idea called AGN unification argues these are mostly the same beast seen from different angles. Note that 'active' does not mean a galaxy is alive or special forever; activity switches on and off as the black hole's gas supply changes, and our own galaxy's center is quiet today but may have flickered in the past.
The galaxy NGC 4151 looks like an ordinary spiral in a wide image, but its core varies in brightness from one week to the next. That flicker betrays a supermassive black hole, roughly 10 to 40 million times the Sun's mass, feeding at the center — a textbook active galactic nucleus close enough to study in detail.
A point of light at a galaxy's heart that outshines all its stars and flickers in days.
An AGN is the powered central engine, not the whole galaxy; the surrounding stars carry on more or less normally. And 'active' is a phase, not a permanent species — most big galaxies, including ours, host a dormant supermassive black hole that is simply not feeding right now.