What distinguishes a normal galaxy from one that contains an AGN? It is a principle of science, often attributed to Einstein, that one should make things as simple as possible, but not simpler. Explain how this principle is at work in the distinction between normal galaxies and AGNs.

A normal galaxy is a collection of stars, gas, and dust bound together by gravity, but it does not have an Active Galactic Nucleus (AGN). Typical activities in these galaxies include star formation, stellar evolution, and the regular emission of light. These galaxies exhibit the standard processes governing the life cycle of stars within them.
Normal galaxies emit light due to these stellar and interstellar processes. The light comes from stars in various stages of their lifecycles, and from nebulae that are either forming new stars or are remnants of dead ones.
They glow because stars within them burn hydrogen and other elements, creating energy in the form of electromagnetic radiation. This radiation spans the electromagnetic spectrum but is relatively uniform. These processes are what most people picture when they imagine a galaxy.

A supermassive black hole is an incredibly massive object with a gravitational pull so strong that nothing, not even light, can escape its grasp. These black holes typically reside at the centers of galaxies and are millions to billions of times more massive than our Sun.
In galaxies with an Active Galactic Nucleus (AGN), the supermassive black hole plays a crucial role. Its immense gravitational force attracts vast amounts of matter, forming an accretion disk around it. As this matter spirals inwards, it heats up and emits tremendous amounts of energy, often outshining the rest of the galaxy.
The energy released by this process spans the entire electromagnetic spectrum, from radio waves to gamma rays. This makes AGNs some of the most brilliant and energetic objects observed in the universe.

The emission spectrum of an object is a unique pattern of wavelengths (colors) of light that it emits. It's like a fingerprint that can tell us about the object's temperature, composition, and physical processes occurring within it.
In a normal galaxy, the emission spectrum is dominated by the combined light from its stars, giving it a relatively uniform appearance. Elements within stars emit light at specific wavelengths, creating bright lines in the spectrum.
However, galaxies with AGNs have very different emission spectra. The intense energy from the AGN's supermassive black hole causes the surrounding matter to emit across a much wider range of wavelengths and at much higher intensities. This leads to emission lines that are broader and often more intense than those found in normal galaxies.
This distinct emission pattern is one of the simplest ways to distinguish between a normal galaxy and one containing an AGN, supporting the scientific principle of keeping explanations as simple as necessary.