These color-magnitude plots are known as Hertzsprung – Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell.

In Potsdam in 1906, the Danish astronomer Ejnar Hertzsprung noticed that the reddest stars — classified as K and M in the Harvard scheme — could be divided into two distinct groups.

The first color-magnitude diagrams of open clusters were published by Ejnar Hertzsprung in 1911, giving the plot for the Pleiades and Hyades star clusters.

He befriended fellow students Bart Bok and Pieter Oosterhoff and was taught by Ejnar Hertzsprung, Antonie Pannekoek, Willem de Sitter, Jan Woltjer, Jan Oort and the physicist Paul Ehrenfest.

* 1910 — Ejnar Hertzsprung and Henry Norris Russell study the relation between magnitudes and spectral types of stars

Ejnar Hertzsprung (, 8 October 1873 – 21 October 1967 ) was a Danish chemist and astronomer.

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