Ejnar Hertzsprung (8 October 1873 – 21 October 1967) was a Danish chemist and astronomer born in Copenhagen. 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[clarification needed] 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. 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.
His wife Henrietta (1881–1956) was a daughter of the Dutch astronomer Jacobus Kapteyn. Hertzsprung died in Roskilde in 1967. The asteroid 1693 Hertzsprung was named in his honour.
- 1627 Ivar (25 September 1929)
- 1702 Kalahari (7 July 1924)
- Gold Medal of the Royal Astronomical Society in 1929
- Bruce Medal in 1937
- Named after him
- lunar impact crater Hertzsprung
- main-belt asteroid 1693 Hertzsprung
- Sky & Telescope, January, 1968, Sky Publishing Corporation, Cambridge
- Hertzsprung, E., "Über die räumliche Verteilung der Veränderlichen vom δ Cephei-Typus." Astronomischen Nachrichten, 196 p. 201–210 (1913)
- Schmadel, Lutz D. (2003). Dictionary of Minor Planet Names – (1627) Ivar. Springer Berlin Heidelberg. p. 129. ISBN 978-3-540-29925-7. Retrieved 1 November 2015. Search this book on
- Schmadel, Lutz D. (2003). Dictionary of Minor Planet Names – (1693) Hertzsprung. Springer Berlin Heidelberg. p. 135. ISBN 978-3-540-29925-7. Retrieved 1 November 2015. Search this book on
- Bruce Medal page
- Awarding of Bruce Medal: PASP 49 (1937) 65
- Awarding of RAS gold medal: MNRAS 89 (1929) 404
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