Ejnar Hertzsprung
Erling Poulsen  

Amateur astronomers.
Ejnar Hertzsprung at workMany of the famous astronomers of the past started as amateurs, even Tycho Brahe and Copernicus were not "professionals" in the modern sense of the word. Olbers was a physician, Bessel was an apprentice to a commercial firm and Herschel was a musician. One of the last to become professor in astronomy and an observatory director without having attended a lecture in astronomy, was Ejnar Hertzsprung.

His life.
Hertzsprung was born October 8, 1873 in Copenhagen, his father had studied astronomy but could not get a job in that field, so he became a director of an insurance company. After he left school, Hertzsprung studied chemistry at Copenhagen Polytechnics. From 1898 to 1901 he worked in St. Petersburg with the technology of acetylene-lighting. In the summer of 1902, he began his studies of chemistry at W. Ostwald in Leipzig, but after his brother died he returned to Copenhagen to live with his sister and mother. Here he started to live as a private scientist (the family was not poor), and his first publications were on stereo-photography and spectrophotometry. These publications do not mention astronomy. From 1902 he began visiting the University Observatory and the privately owned Urania Observatory in Copenhagen.

Then, in 1905, he published "Zur Strahlung der Sterne" in "Zeitschrift für Wissenschaftliche Photographie". Here he published the following results:
Stars in the late spectral-classes (G, K and M) are divided into two series with different luminosity.
The luminous red stars must be very big.
The little number of red giants (he does not use that term) shows that these stars are in a stage of fast evolution.
He supposes that there must be a connection between the spectrum and the luminosity of stars.

In 1907, he published "Zur Bestimmung der photographischen Sterngrössen", in which he combines his experience from his hobby, photography, with the important astrophysical questions. He sent a preprint to Schwarzchild, the director of the Göttinger Observatory, and after they met in 1908, he proposed Hertzsprung as an extraordinary professor. In 1909 they both moved to Potsdam Observatory.
During a travel to the USA in 1910, Schwarzchild met Russell, who had come to the same results as Hertzsprung. The work of Hertzsprung and Russell was first published in graphical form in 1911: the Hertzsprung-Russell Diagram.

The first HRD The diagram from 1911. For the Hyades above and the Pleiades below. The ordinate shows the effective wavelength of the light in Angstrom, the abscissa shows (from above) the visual magnitude, absolute magnitude and visual magnitude. From "Publikationen des Astrophysikalischen Observatorium zu Potsdam", 1911, Nr. 63.

In 1913 Hertzsprung found the distance to the Small Magellan Cloud, the first distance to an object outside the Milky Way, by using the delta-Cephei type of variable stars.
He worked at Leiden University Observatory from 1919 to 1945, becoming director there in 1935.
Later in his life he made discoveries in the evolution of open star-clusters and variable stars.
Hertzsprung remained an active researcher until he was over 90 years old.
He died in 1967 in Roskilde, Denmark, and his papers (12,153 pages with original measurements) were given to the U. S. Naval Observatory Library where his former student K. Aa. Strand was the director.