Danish astronomer (1873–1967)
Hertzsprung was born in Frederiksberg, Denmark, the son of a senior civil servant who had a deep interest in mathematics and astronomy but who was anxious to see that his son received a more practical education. Consequently Hertzsprung was trained as a chemical engineer at the Copenhagen Polytechnic, graduating in 1898. He worked as a chemist in St. Petersburg and then studied photochemistry under Wilhelm Ostwald in Leipzig before returning to Denmark in 1902. His first professional appointment as an astronomer was in 1909 at the Potsdam Observatory. The bulk of his career, from 1919 to 1944, was spent at the University of Leiden where from 1935 he served as director of the observatory. After his retirement in 1944 he returned to Denmark where he continued his studies for a further 20 years.
Hertzsprung's name is linked with that of Henry Russell as independent innovators of the Hertzsprung–Russell (H–R) diagram. In the late 19th and early 20th centuries, techniques used in photographic spectroscopy were being greatly improved. With his background in photochemistry, Hertzsprung was able to devise methods by which he could determine the intrinsic brightness, i.e., luminosity, of stars. He showed that the luminosity of most of the stars he studied decreased as their color changed from white through yellow to red, i.e., as their temperature decreased. He also found that a few stars were very much brighter than those of the same color. Hertzsprung thus discovered the two main groupings of stars: the highly luminous giant and supergiant stars and the more numerous but fainter dwarf or main-sequence stars. Hertzsprung published his results, although not in diagrammatic form, in 1905 and 1907 in an obscure photographic journal. His work therefore did not become generally known and credit initially went to Russell who published the eponymous diagram in 1913. It would be difficult to exaggerate the importance or usefulness of the H–R diagram, which has been the starting point for discussions of stellar evolution ever since.
Much of Hertzsprung's work concerned open clusters of stars. In 1911 he published the first color-magnitude diagrams of the Pleiades and Hyades clusters, showing how the color of member stars varied with observed brightness. He also measured the proper motions of stars, i.e., their angular motions in a direction perpendicular to the observer's line of sight, and used the results to establish membership of clusters.
One other major achievement of Hertzsprung was the development of a method for the determination of stellar and galactic distances. In the 19th century Friedrich Bessel and Friedrich Georg Struve had been the first to use measurements of annual parallax to calculate stellar distances but this was only accurate up to distances of about a hundred light-years. In 1913, when Hertzsprung announced his results, astronomers had made little progress in measuring distances. The work of Henrietta Leavitt in 1912 had shown that the period of light variation of a group of stars known as Cepheid variables was related to their observed mean brightness. These Cepheids lay in the Magellanic Clouds. Hertzsprung assumed that at the great distance of the Clouds all member stars could be considered to have approximately the same distance. Since observed and intrinsic brightness of a star are directly linked by its distance, the periods of light variation of Cepheids in the Clouds were thus also related to their intrinsic brightness. By extrapolation, Cepheids could thus be an invaluable means of measuring the distance of any group of stars containing a Cepheid by observing the period and apparent brightness of the Cepheid.
The work of establishing the period-luminosity relation on a numerical basis was begun by Hertzsprung and continued by Harlow Shapley. Hertzsprung determined the distances of several nearby Cepheids from measurements of their proper motions. Using his results and Leavitt's values for the periods and apparent brightness of Cepheids in the Small Magellanic Cloud (SMC) he was then able to calculate the distance to the SMC. Although smaller than today's value this was the first measurement of an extragalactic distance.
