Galaxies are huge collections of stars which are held together by gravity. Often, large galaxies have centers (also called nuclei) which contain supermassive black holes. Some galaxies appear “active” when large amounts of stars, gas and other material get fed into the black hole and this process also releases matter and light away from the center. Depending on which orientation this process is viewed from, Active Galactic Nuclei can be observed as huge amounts of light of different energies which changes over time (but doesn’t usually repeat periodically like other variable objects). At first, these objects were given different names because astronomers didn’t know they were usually AGN viewed from different angles. Some of the most powerful objects in the Universe, studying AGN can help us understand how physics works at the extreme limits where huge amounts of matter and energy come together in small spaces. NASA’s Fermi mission has measured thousands of AGN and helped link these objects to the production of strange particles called neutrinos which are incredibly difficult to detect on Earth.