Modern astronomy paints a vivid picture of the universe having been born in a cataclysmic bang and filled with exotic stars ranging from gargantuan red supergiants that span the size of a modest solar system to hyperdense white dwarf stars and black holes that are smaller than Earth. These discoveries are all the more remarkable because astronomers infer them from the faintest glimmers of light, sometimes just a handful of photons. A key to this success is a graph that two astronomers introduced 100 years ago.