Hertzsprung–Russell diagram

The single plot that organises everything about stars.

Brightness up the side, temperature along the bottom — backwards, hot on the left, for historical reasons nobody has fixed. Almost every star falls into one of a few regions: the main sequence running diagonally, red giants above and to the right, white dwarfs below and to the left. A star traces a path across it as it ages, so the diagram is simultaneously a census and a life story. Plot a whole cluster and the point where its stars peel off the main sequence gives you the cluster's age. It also made stellar astrophysics possible as a science. Hertzsprung and Russell were plotting what they had, independently and without a theory of how stars work, and the fact that the points fell into bands rather than scattering was itself the discovery: it meant something simple was governing the whole population, and that something turned out to be mass.