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One of astronomy's greatest conceptual revolutions began 100 years ago, when scientists demonstrated that stars follow specific patterns of brightness and color. Not only did these patterns lead to the discovery that stars generate energy by nuclear fusion, they also allowed astronomers to sort stars into types both mundane and exotic. The key development was a sort of periodic table of stars, the Hertzsprung–Russell diagram, named for two of its inventors. The diagram continues to be an essential tool for astronomers trying to make sense of the strange things they find in the sky, as this article in the July issue of Scientific American explains.