Ejnar Hertzsprung

Lore & Background

Ejnar Hertzsprung was a Danish chemist and astronomer, born in Frederiksberg to Severin and Henriette. His father’s amateur astronomical work sparked his own interest in the subject. After studying chemical engineering at Copenhagen Polytechnic Institute and working as a chemist in St. Petersburg, he spent a year studying photochemistry at Leipzig University. He began making astronomical observations in Frederiksberg in 1902, and within a few years noticed that stars of similar spectral type could have widely different absolute magnitudes. In 1909, he took a position at the Göttingen Observatory under Karl Schwarzschild. Hertzsprung is best remembered for developing the Hertzsprung–Russell diagram, a classification system for stars based on spectral type, stage of development, and luminosity, which he created in 1911, independently of Henry Norris Russell in 1913. He used Antonia Maury’s earlier classification system in his work. In 1913, he determined distances to several Cepheid variable stars by parallax, calibrating the period-luminosity relationship discovered by Henrietta Leavitt, though a mistake made the stars appear ten times too close. He used this to estimate the distance to the Small Magellanic Cloud. From 1919 to 1946, he worked at Leiden Observatory, serving as director from 1937. Among his graduate students was Gerard Kuiper. He also discovered two asteroids, including 1627 Ivar. He married Henrietta, daughter of astronomer Jacobus Kapteyn, and died in Roskilde in 1967.

Reader's Guide

Ejnar Hertzsprung’s most significant scientific achievement was the creation of a stellar classification scheme that sorted stars according to their spectral type, evolutionary stage, and luminosity. This system built upon the earlier classification work of Antonia Maury. The resulting diagram, now known as the Hertzsprung–Russell diagram, became a cornerstone of astrophysics, used continuously to interpret stellar types and the processes of stellar evolution. Hertzsprung’s path to this discovery began with his early astronomical observations in Frederiksberg, where he noticed that stars of the same spectral type could possess vastly different absolute magnitudes. This insight led him to develop the diagram in 1911, a concept independently arrived at by Henry Norris Russell in 1913. Beyond this diagram, Hertzsprung made other notable contributions. In 1913, he used parallax to measure distances to several Cepheid variable stars, calibrating the period-luminosity relationship discovered by Henrietta Leavitt, though a calculation error placed the stars ten times too close. He then applied this relationship to estimate the distance to the Small Magellanic Cloud. During his long tenure at Leiden Observatory, where he served as director from 1937, he mentored students including Gerard Kuiper. He also discovered two asteroids, among them the Amor asteroid 1627 Ivar. His work fundamentally reshaped how astronomers classify and understand stars, linking their spectral properties directly to their intrinsic brightness and life cycles.

Did You Know?

• By 1908, Danish astronomer Ejnar Hertzsprung had realized the value of Antonia Maury's classifications and used them in his system of identifying very bright red giant stars from faint dwarf stars.
• Hertzsprung wrote to Pickering: 'In my opinion the separation by Antonia Maury of the c-and ac stars is the most important advancement in stellar classification since the trials by Vogel and Secchi.'
• In 1922, the International Astronomical Union modified its classification system based on work done by Maury and Hertzsprung, a formulation known as the Hertzsprung–Russell diagram.