Which two astronomers independently developed the H-R diagram concept around the early 1900s?

Ejnar Hertzsprung and Henry Norris Russell

The fundamental framework of the Hertzsprung-Russell diagram was conceptualized through the simultaneous, independent work of two key figures in astronomy during the early years of the 1900s. Ejnar Hertzsprung, a Danish chemist and astronomer, and Henry Norris Russell, an American astronomer, each recognized the non-random clustering of stars when plotting their properties. Their combined efforts established a tool that revealed deep connections between stellar characteristics. The diagram is named in honor of both contributors, recognizing the parallel nature of their discoveries regarding the fundamental organization of stellar properties on such a graph.