Galaxies are some of the largest structures in the universe, consisting of billions of stars which rotate around their common centre of gravity in a similar way that the planets rotate around the Sun. Our local star, the Sun, along with another ten billion stars, is also part of a galaxy, revolving around the galactic centre once every 250 hundred million years. It’s is estimated that the observable universe contains billions of galaxies and observations reveal that they come in different sizes, shapes and with different properties.
One of the most recently discovered type of galaxies is the Active Galaxies. The reason for their late discovery is that Active Galaxies are of the most distant ever observed and therefore appear very dim. However, after taking in account the large distances involved, astronomers discovered that these galaxies were in fact extremely bright, a hundred times brighter than the nearby galaxies previously observed. Not only were these galaxies unusually bright, but observations revealed that most of the brightness was generated in a small (in astronomical scales) region at their centres. This discovery captured the interest of many astronomers at the time, since according to current theories, it was not possible to explain the brightness as the result of ordinary stars; a different, yet unknown mechanism must have been at work.
After years of observational evidence and theoretical work, it is now generally accepted that the mechanism responsible for the brightness of Active Galaxies are super-massive black holes (with mass of tens of billion times the mass of the Sun) that are present in the galaxies’ centres. Although it is currently believed that all galaxies contain super-massive black holes (including our own galaxy), active galaxies are different because their black holes are surrounded by clouds of gas and dust. The intense gravitational attraction of the black hole accelerates these clouds to velocities close to that of light, and in the process, heats them up (through friction) to very high temperatures. In a similar manner where metal produces light when heated, the intense temperatures experienced by the gas and dust clouds causes them to radiate the huge amount of light that we observe.
Since their initial discovery of Active Galaxies, many more have been observed and it has been found that they, too, come in different varieties. Although in the years of study there has been significant progress in understanding Active Galaxies, there are still many unanswered questions about their nature and for this reason they still remain an interesting subject of research for many astronomers around the world.