Question: Although there are just a few types of galaxies (spiral, elliptic, irregular) why are there so many different shapes of galaxies? One would think that since the forces of nature are the same everywhere, the shapes should be very similar, the only difference should be the size. Is it because of collisions? — Matt
Answer: Galaxy collisions is one reason why there are a wide variety of shapes of galaxies. Another reason, which is the most important driver for galaxy shape variety, is that galaxies evolve as gravity turns gas and dust into stars. In a way, what you are seeing in the diversity of galaxy shapes is the range of evolutionary states in galaxies. Galaxies within a given type with different shapes tend to be at different stages of evolution, largely driven by the star formation process in these galaxies.