Born Halle, Germany, 6 July 1883
Died Bonn, (Germany), 28 May 1969
Arnold Kohlschütter and Walter Adams found subtle criteria that could distinguish ordinary giants from dwarf stars.
Kohlschütter was educated at Göttingen University, a student of Karl Schwarzschild . He spent 3 years at Mount Wilson Observatory, California, from 1911 to 1914. There he cooperated with Adams in the work that led to a new method for determining the distances to stars.
Kohlschütter and Adams examined the spectra of stars with both large and small parallaxes, but similar apparent magnitudes. The stars with smaller parallaxes were necessarily more luminous. The two Mount Wilson spectroscopists found differences in the absorption-line strength ratios between the two sets of stars – even within the same spectral class. Once calibrated using stars of known distance, these differences could be observed in stars without measured parallax in order to determine their distances. The method refined the technique known as spectroscopic parallax. When coupled with the apparent magnitudes of the stars involved, this method allowed the determination of stellar distances greater than the limit measurable by trigonometric parallax, surpassing the quality of traditional parallax measurements at 25 pc.
In 1897, Antonia Maury had identified a few peculiar spectrograms characterized by some of the absorption lines being unusually sharp and others unusually strong for the stellar colors. Ejnar Hertzprung recognized in 1905 that these stars were supergiants, the brightest sort known. Kohlschütter and Adams found that differences in line properties arise from the lower gas densities in giant atmospheres, which sharpen those lines whose width is due mostly to Stark effect broadening and strengthen lines produced by ionized atoms, because recombination proceeds more slowly at low density.
In a second Mount Wilson collaboration, Kohlschütter and Harlow Shapley concluded that even strong absorption lines typically have a residual flux at their centers that is 20–30°% of the continuum, meaning that the region of a stellar atmosphere responsible for the absorption features must be located near an optical depth of 0.2.
In 1918 Kohlschütter was appointed to the staff of Potsdam Observatory, and in 1925 became professor of astronomy at Bonn and director of the observatory there. He undertook the Bonn portion of the Zweiter Katalog der Astronomischen Gesellschaft, completed in 1958.