Supervisor:  Maximilian Stritzinger 

In this colloquium, I would like to give you a very brief introduction to Active Galactic Nuclei.  

  We will barely scratch the surface but it will be insightful. Even more than the object in itself the interesting things will be the thinking processes that are behind some of the discoveries made regarding this particular class of objects. Most of them will not be rigorous as I will use the classical point of view, but the results will still be valid, and their consequences easy to understand.

  I will start by giving a brief historical background and then proceed with the first important aspects of AGNs, their luminosities and their sizes. Another important aspect I will go through is the reason why a more efficient process than the Proton-Proton chain, the most efficient process for the production of energy in the stars, is needed for these objects, and in this regard, a rough estimate for the efficiency of gravitational accretion will be obtained. I will also present the unified model and then end by showing observations that brought to the discovery of the supermassive black hole at the center of our galaxy.