Pleiades star cluster, one of the best known and studied, is so young that many of its massive, blue, short-lived stars are still shining. The cluster was the subject of the first published "periodic table" of astronomy, which became known as the Hertzsprung-Russell diagram. Image: Rogelio Bernal Andreo deepskycolors.com
- Astronomy is remarkable for how it gleans so much knowledge from such meager slivers of light. A graph known as the Hertzsprung-Russell diagram, which marks its centennial this year, is part of the secret.
- The diagram plots the luminosity and temperature of stars. On it, striking patterns emerge, classifying stars by their stage of life and revealing that most of their properties are ultimately determined by mass.
- These patterns helped astronomers deduce that most stars, including the sun, shine by nuclear fusion reactions. They still guide research on stars today.
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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.
This article was originally published with the title The Periodic Table of the Cosmos.