Black holes are known to exist at the centres of most galaxies, including our own Milky Way. The masses of those black holes are correlated to many of the properties of their host galaxies, which strongly suggests that galaxies and black holes evolve together. Measuring their masses and comparing them in a variety of different galaxies is crucial to the understanding of the interactions between the two components.
Light. Most of us take it for granted during the day. And at night, we have learned to domesticate it. Light, the natural agent that stimulates our sense of sight and makes things around us visible.
Early astronomers already make the distinction between stars and planets, as the former remain relatively fixed for centuries, while the latter wander an appreciable amount in a comparatively short time. But that’s not all!