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.
The Hertzsprung-Russell (H-R) diagram is simple. It plots two basic properties of stars: their luminosity (intrinsic brightness) and their surface temperature (as revealed by their color). In doing so, it anchors stellar astronomy just as the periodic table anchors chemistry. Whereas the periodic table groups together similar chemical elements—for example, placing all noble gases, such as helium, neon and argon, into one column—the H-R diagram groups together stars passing through similar stages of life. When astronomers invented the diagram, no one knew why the sun and other stars shine. No one knew how stars are born or how they die. No one could even assure the public that the sun would never explode. Nor did anyone know that the stars had forged most of the elements that make up Earth and our bodies.