Ejnar Hertzsprung

Alex Kiefer

Mr. Percival

Astronomy

25 February 2011

Ejnar Hertzsprung

Ejnar Hertzsprung was born October 8^th 1873 in Copenhagen. His father studied astronomy but was unable to get a job, opting instead to be director of an insurance company. After high school, Ejnar studied chemistry at Copenhagen Polytechnics. From 1898 to 1901, Hertzsprung worked on acetylene-lighting in St. Petersburg. He began studying chemistry at W. Ostwald in Leipzig in 1902. Unfortunately, the death of his brother caused him to return to Copenhagen to live with his mother and sister. He worked as a private scientist, publishing his first results on stereo-photography and spectrophotometry, surprisingly having nothing to do with astronomy. After 1902, he regularly visited the University Observatory and the privately owned Urania Observatory in Copenhagen.

Karl Schwarzschild invited Hertzsprung to work in Guttingen in 1902. In 1909, he stayed at the Potsdam Astrophysical Observatory until moving to Leiden to join De-Sitter. He worked at the Leiden Observatory from 1919-1944, directing it after 1935. He retired in Denmark, although he kept working into his 90's.

Hertzsprung published “Zur Strahlung der Sterne” in “Zeitschrift für Wissenschaftliche Photographi.” in 1905. He obtained the following results: Stars in the late spectral-classes are divided into two series with different luminosity; luminous red stars must be very large; the scarcity of “red giants” shows that these stars are in a stage of fast evolution; there must be a connection between the spectrum and luminosity of stars. In 1907, he published “Zur Bestimmung der photographischen Sterngrössen,” combining photography with astronomy. He sent a preprint to Swarzschild, prompting him to propose Hertzsprung as an excellent professor.
While traveling to the US in 1910, Swarszchild met Henry Norris Russell, who had independently reached the same results as Hertzsprung. It was published as the Hertzsprung-Russell diagram in 1911. The Hertzsprung-Russell diagram plots absolute magnitude versus spectral type, and naturally reveals the main-sequence of stars, red giants and white dwarfs. In modern Hertzsprung-Russell diagrams, the rather vague measurement of spectral type has been replaced with the B-V color index, resulting in the alternate name color-magnitude diagram. Sometimes versions of the CMD with apparent magnitude on the y-axis are used in clusters where all the stars are similar distances. Another version of the H-R diagram plots the luminosity versus the effective surface temperature, useful for describing the evolution of stars.

From 1913 to 1917, Hertzsprung claimed that the color of the star is directly related to its temperature. He thought blue stars were the hottest and largest while red dwarfs were the smallest and coolest. He also theorized that stars begin as hot and gradually cool to a red dwarf, which has since been disproven.

Hertzsprung determined the distance to the Small Magellan Cloud, the first extra-galactic distance ever determined, in 1913 using the delta-Cephei type of variable stars.

Most of Hertzsprung's work was not done in the field, but desk work analyzing data from other scientists. He was the first astronomer to advocate the use of absolute magnitude. At Potsdam, he developed a technique for observing double stars, using a great refractor which eliminated errors, and improved results to ten times the accuracy of ordinary refractors. He also took a large number of exposures on one plate to achieve greater accuracy.