Hertzsprung and Russell independently plot luminosity against temperature

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Hertzsprung, working at Leiden, plotted luminosity against colour around 1911 and distinguished giants from dwarfs; Russell, unaware of that work, plotted absolute magnitude against spectral class in 1913. Both found that stars fall into distinct groups rather than scattering. Because Russell was the more prominent figure the plot was first called the Russell diagram, then the Russell-Hertzsprung diagram, and only later given the order that matches the chronology.

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