|Died||21 October 1967 (aged 94)|
|Alma mater||Copenhagen Polytechnic (DTU)|
|Known for||Hertzsprung gap|
|Spouse||Henriette Mariette Augustine Albertine Kapteijn|
|Awards||Bruce Medal (1937)|
Gold Medal of RAS (1929)
Hertzsprung was born in Frederiksberg, Denmark, the son of Severin and Henriette. He studied chemical engineering at Copenhagen Polytechnic Institute, graduating in 1898. After spending two years working as a chemist in St. Petersburg, in 1901 he studied photochemistry at Leipzig University for a year. His father was an amateur astronomy, which led to Ejnar's interest in the subject. He began making astronomical observations in Fredericksberg in 1902, and within a few years had noticed that stars with similar spectral type could have widely different absolute magnitudes. In 1909, he took a position at the Göttingen Observatory under director Karl Schwarzschild.
In 1913 Hertzsprung determined the distances to several Cepheid variable stars by 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. Among his graduate students at Leiden was Gerard Kuiper.
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. He used the earlier classification system developed by Antonia Maury in his work. The so-called "Hertzsprung–Russell Diagram" has been used ever since as a classification system to explain stellar types and stellar evolution. He also discovered two asteroids, one of which is 1627 Ivar, an Amor asteroid.
- Named after him
- Sky & Telescope, January, 1968, Sky Publishing Corporation, Cambridge
- Van Berkel, Klaas; Van Helden, Albert; Palm, L. C. (1999). The History of Science in the Netherlands: Survey, Themes and Reference. BRILL. p. 460. ISBN 9789004100060.
- Hellyer, B. (October 1973). "Ejnar Hertzsprung, 1873–1967". Journal of the British Astronomical Association. 83: 460–461. Bibcode:1973JBAA...83..460H.
- Hertzsprung, E. (1913). "Über die räumliche Verteilung der Veränderlichen vom δ Cephei-Typus" [On the spatial distribution of variable [stars] of the δ Cephei type]. Astronomische Nachrichten (in German). 196 (4692): 201–208. Bibcode:1913AN....196..201H.
- Hoffleit, D. "Reminiscences on Antonia Maury and the c-Characteristic." The MK Process at 50 Years: A Powerful Tool for Astrophysical Insight. Vol. 60. 1994.
- Schmadel, Lutz D. (2003). "(1627) Ivar". Dictionary of Minor Planet Names – (1627) Ivar. Springer Berlin Heidelberg. p. 129. doi:10.1007/978-3-540-29925-7_1628. ISBN 978-3-540-29925-7.
- Schmadel, Lutz D. (2003). "(1693) Hertzsprung". Dictionary of Minor Planet Names – (1693) Hertzsprung. Springer Berlin Heidelberg. p. 135. doi:10.1007/978-3-540-29925-7_1694. ISBN 978-3-540-29925-7.