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E. Hertzsprung was born near Copenhagen (Denmark) in 1873. He studied in technical colleges and became a chemical engineer, working for a while in St. Petersburg (Russia) and then Berlin (Germany). After a year and a half, he quit his job to study physical chemistry in Leipzig (Germany). Following the death of his mother in 1902, Hertzsprung went back to Copenhagen, living off a private income for a while and getting involved in amateur astronomy. Not long afterwards, he began working at both the Urania and University observatories and quickly made a name for himself. In 1909, the German astronomer K. Schwarzschild invited Hertzsprung to Göttingen (Germany) to work “as extraordinary professor of astronomy with research and teaching duties in astrophysics.” When Schwarzschild became the director of the Astrophysical Observatory in Potsdam (Germany) later that year, Hertzsprung went with him. He is best known for the Hertzsprung-Russell diagram, which he developed in 1911. This is a plot of the luminosities and colors of stars, which became an important tool in studying stellar evolution. In 1913, he developed the Cepheid variable star luminosity scale, which became an important tool in determining the distances of galaxies. He worked at Leiden Observatory (Netherlands) from 1919 to 1944, serving as the director during the last 9 years, but also worked at other observatories in the United States and South Africa during that time [1].