Hertzsprung, Ejnar (1873–1967)

Danish astronomer and physicist. He introduced the concept of the absolute magnitude (brightness) of a star, and described the relationship between the absolute magnitude and the temperature of a star, formulating his results in the form of a diagram, known as the Hertzsprung-Russell diagram, that has become a standard reference.

His astronomical interests were very wide, but his observations were mainly of variable stars, double stars, and clusters.

Hertzsprung was born in Frederiksberg and studied chemical engineering at the Fredriksberg Polytechnic, becoming interested in astronomy 1902. After some years working in Russia, he learned photochemistry and became employed at observatories, at first in Copenhagen, Denmark, and later in Göttingen and Potsdam, Germany. In 1919 he became professor of astronomy at Leiden, the Netherlands, and associate director of Leiden Observatory, becoming director 1935. He retired 1945 and returned to Denmark, but did not cease his astronomical research until 1966.

In 1905, Hertzsprung proposed a standard of stellar magnitude for scientific measurement, and defined this ‘absolute magnitude’ as the brightness of a star at the distance of 10 parsecs (32.6 light years). He also described the relationship between the absolute magnitude and the colour – the spectral class or temperature – of a star, plotting it on a graph in 1906. He did not publish the diagram, which was independently arrived at by US astronomer Henry Russell in 1913.

In 1922, Hertzsprung published a catalogue on the mean colour equivalents of nearly 750 stars of magnitude greater than 5.5.