Subject: biography, astronomy
Danish astronomer and physicist who, having proposed the concept of the absolute magnitude of a star, went on to describe for the first time the relationship between the absolute magnitude and the temperature of a star, formulating his results in the form of a graphic diagram that has since become a standard reference.
Hertzsprung was born in Frederiksberg on 8 October 1873. His father was interested in astronomy and stimulated a similar fascination for the subject in his son, but the poor financial prospects of an aspiring astronomer led Hertzsprung initially to choose chemical engineering as his career. He graduated from the Frederiksberg Polytechnic in 1898 and then went to St Petersburg, Russia, where he worked as a chemical engineer until 1901. Returning to Copenhagen via Leipzig, he studied photochemistry under Wilhelm Ostwald and began to work as a private astronomer at the observatory of the University of Copenhagen and the Urania Observatory in Frederiksberg. Under the generous tutelage of H Lau, Hertzsprung rapidly acquired the skills of contemporary astronomy.
Following a correspondence with Karl Schwarzschild at the University of Göttingen, Hertzsprung was invited to take up the post of assistant professor of astronomy at the Göttingen Observatory in 1909. When Schwarzschild moved on to the Potsdam Astrophysical Observatory later that year, Hertzsprung went with him. Willem de Sitter was appointed director of the Leiden Observatory in the Netherlands in 1919, and he appointed Hertzsprung head of its department of astrophysics. Within a year, Leiden University appointed Hertzsprung a professor, and on the death of de Sitter he also became director of the Leiden Observatory. He retired in 1945 and returned to Denmark, but he did not cease his astronomical research until well into the 1960s.
Hertzsprung's outstanding contributions to astrophysics were recognized by his election to many prestigious scientific academies and societies and with the award of a number of honours. He died in Roskilde, Denmark, on 21 October 1967.
It was quite early in his work at the observatory in Copenhagen that Hertzsprung realized the importance of photographic techniques in astronomy. He was extremely well qualified to apply these methods and did so with great precision and energy. In 1905 he published the first of two papers (the second appearing in 1907) in a German photographic journal on the subject of stellar radiation. He proposed a standard of stellar magnitude (brightness) for scientific measurement, and defined this ‘absolute magnitude’ as the brightness of a star at the distance of ten parsecs (32.6 light years). As a further innovation, he described the relationship between the absolute magnitude and the colour - that is, the spectral class or temperature - of a star. During the following year (1906), Hertzsprung plotted a graph of this relationship in respect of the stars of the Pleiades. Later, he noticed that there were some stars of the same spectral class that were much brighter, and some that were much dimmer, than the Sun. He named these the red giants and the red dwarfs respectively.
Publication of his papers in a photographic journal, and refusal altogether to publish his diagrammatic material (because of diffidence in the quality of his own observations), meant that his discoveries were simply not known by Hertzsprung's fellow astronomers. And in 1913 Henry Russell, a US astronomer, presented to the Royal Astronomical Society a diagram depicting the relationship that Hertzsprung had previously and independently discovered, between the temperature and absolute magnitude of stars. Credit was eventually accorded to both astronomers equally and the diagram named after both of them.
The Hertzsprung-Russell diagram, one of the most important tools of modern astronomy, consists of a log-log plot of temperature versus absolute magnitude. As plotted, the stars range themselves largely along a curve running from the upper left (the blue giant stars) to the lower right (the red dwarf stars) of the graph. This apparent arrangement is simply a reflection of the mass of each star, which is responsible for its temperature and luminosity. Approximately 90% of stars belong to this ‘main sequence’; most of the rest are red giants, blue dwarfs, Cepheid variables, or novae. The blue giant stars are giant hot stars, the red dwarfs are compact cooler stars. Our Sun lies near the middle of the main sequence, and is classed as a yellow dwarf.
One of the earliest uses of the Hertzsprung-Russell diagram was devised in 1913, when Hertzsprung developed the method of ‘spectroscopic parallax’ (as distinct from ‘trigonometric parallax’) for the determination of the distances of stars from the Earth. His method relied on data for the proper motions of the nearest (galactic) Cepheids and on Henrietta Leavitt's data for the periods of the Cepheids (which are variable stars) in the Small Magellanic Cloud (which is, it turned out, extragalactic). He deduced the distances of the nearest Cepheids from their proper motions and correlated them with their absolute magnitude. He then used Henrietta Leavitt's data on the length of their periods to determine their absolute magnitude and hence their distance. He found the Small Magellanic Cloud Cepheids to be at an incredible distance of 10,000 parsecs. His method was excellent, but there was a serious source of error, which led to an overestimation of the distance: he had not accounted for the effect of galactic absorption of stellar light. Nevertheless, this work earned Hertzsprung the Gold Medal of the Royal Astronomical Society.
The Hertzsprung-Russell diagram has also been essential to the development of modern theories of stellar evolution. As stars age and deplete their store of nuclear fuel, they are believed to leave the main sequence and become red giants. Eventually they radiate so much energy that they then cross the main sequence and collapse into blue dwarfs. Larger stars may follow a different pattern and explode into novae, or collapse to form black holes, at the end of their lifespans.
In 1922 Hertzsprung published a catalogue on the mean colour equivalents of nearly 750 stars of magnitude greater than 5.5. This catalogue was notable for the particularly elegant manner in which Hertzsprung managed to analyse the data to uncover a linear relationship.
Most of Hertzsprung's later work was devoted to the study of variable and of double stars. He worked on variable stars (especially Polaris) at Potsdam in 1909, and later in Johannesburg (from 1924 to 1925) and at the Harvard College Observatory (1926) and at Leiden.
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