Danish astronomer and physicist who 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 that has become a standard reference.
Hertzsprung was born in Frederiksberg and studied chemical engineering at the Fredriksberg Polytechnic. After some years working in Russia, he learned photochemistry and became employed at observatories in Denmark and later in Germany. In 1919 he became professor of astronomy at Leiden, the Netherlands. He retired 1945 and returned to Denmark, but did not cease his astronomical research until well into the 1960s.
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.