He had no formal education in astronomy but studied in technological colleges in Denmark and became a chemical engineer. Keenly interested in the chemistry of photography, he turned to astronomy in 1902, working in small Danish observatories, where he applied photography to the measurement of starlight. In two papers, published in 1905 and 1907, he showed that a relationship exists between the colours of the stars and their true brightness and that giant and dwarf stars must exist. The correlation of colour with true brightness became the basis of a widely used method of deducing the so-called spectroscopic parallaxes of stars (estimates of their distances from the Earth). Because the spectrum of a star is a reliable index of its absolute magnitude, its distance can be inferred from the known relationship between the apparent and absolute magnitudes. This work so impressed Karl Schwarzschild, director of the observatory at Potsdam, that he found a place for Hertzsprung on the staff at Göttingen Observatory (1909) and later the same year as a senior astronomer at Potsdam. Hertzsprung was appointed assistant director of the university observatory at Leiden, The Netherlands, in 1919 and became director in 1935. He retired in 1945 and returned to Denmark.