A galaxy is a system of stars, gas, and dust bound together by their mutual gravity. A typical galaxy has billions of stars, and some have trillions. Although they come in many different shapes, the basic structure is the same: a dense core of stars called a ‘nucleus’ surrounded by stars and gas. In Normal galaxies the core is small, relatively faint, and composed of older, redder stars. However, in some galaxies the core is intensely bright, shining with power equivalent to trillions of suns, easily outshining the rest of the light of the galaxy combined. A galaxy that emits such tremendous amounts of energy is called an active galaxy, or AG for short. Active galaxies are actually rare, but so bright they can be seen clear across the visible universe.
At the center of these Galaxies it is believed to contain a suppermassive black hole, millions to billions of times the mass of the Sun.