October 8, 1873 - October 21, 1967
Ejnar Hertzsprung was a Danish chemist and astronomer.
In the period 1911-1913, together with Henry Norris Russell, he developed the Hertzsprung-Russell diagram.
In 1913 he determined the distances to several Cepheid variable stars by statistical parallax, and was thus able to calibrate the relationship discovered by Henrietta Leavitt between Cepheid period and luminosity. In this determination he made a mistake, possibly a slip of the pen, putting the stars 10 times too close. He used this relationship to estimate the distance to the Small Magellanic Cloud.
From 1919 to 1946 Hertzsprung worked at Leiden Observatory in The Netherlands, from 1937 as director. Perhaps his greatest contribution to astronomy was the development of a classification system for stars to divide them by spectral type, stage in their development, and luminosity. The "Hertzsprung-Russell Diagram" was used for many years as a classification system to explain stellar types and evolution.
Hertzsprung received the Gold Medal of the Royal Astronomical Society in 1929 and the Bruce Medal in 1937.