(b. Schweidnitz, Germany [now Świdnica, Poland], 7 May 1851; d. Potsdam, Germany, 7 July 1925)
Müller’s father, a merchant, died when Gustav was only six years old. Müller was educated at a private school and then at the Gymnasium of his native city. After passing the final secondary school examination in 1870, he began the study of mathematics and natural science at Leipzig. From 1872 he continued his studies in Berlin, where the lectures of Wilhelm Foerster induced him to give up his original plans of becoming a teacher and to dedicate himself to astronomy. His other professors included Helmholtz, Weierstrass, and Ernst Kummer. Even before completing his studies Müller took part in the calculations made for the Berlin Astronomisches Jahrbuch and in Auwers’ new reduction of Bradley’s observations. He also assisted Hermann Vogel in his spectroscopic work and in 1877 followed him to the newly created astrophysical observatory at Potsdam. The work done there in spectrophotometry provided the decisive impetus for Müller’s enduring interest in photometry. In 1877 he earned his doctorate with Untersuchungen über Mikrometerschrauben. He became an observer in 1882, chief observer in 1888, and professor in 1891.
Müller’s contributions to the development of astrophysics, especially to the gathering of primary data, were distinguished less by bold innovations than by a clear grasp of the needs of an organically growing science and by the persevering and precise execution of the vast programs of research required by such growth. His photometric studies began in 1877 with investigations of the luminosities of the planets and of the absorption of starlight in the earth’s atmosphere. He published extensive results in 1883 and 1893. His absorption tables for Potsdam were used for decades.
Müller’s most important photometric project was the Photometrische Durchmusterung des nördlichen Himmels—the Potsdamer Durchmusterung—which he planned and, for the most part, carried out himself. Observations were begun in 1886, in collaboration with Paul Kempf. Utilizing an astrometer that he had constructed in accordance with the principle described in 1861 by Zöllner, he ascertained the luminosities of more than 14,000 stars in the Northern sky listed in the Bonner Durchmusterung (BD) to a magnitude of 7.5. The work devoted to this undertaking lasted for several decades. Partial results appeared in 1894, 1899, 1903, and 1906; the general catalog appeared in 1907. The Photometry Durchmusterung furnished, with the Harvard Photometry of Pickering and his coworkers, the most exact photometric information on stars then available; and it is still an indispensable standard work. Moreover, through its consistent use of Pogson’s scale it played a decisive role in the general adoption of this scale.
In 1909 Müller began with E. Kron a further series of zonal observations of the luminosities and colors of the BD stars from magnitude 7.6 to 9.5. However, World War I hindered the progress of this undertaking, and Müller was unable to publish a compendium of the results.
Müller’s proposal at the 1900 meeting of the Astronomische Gesellschaft that a complete catalog of the variable stars be produced led to the Geschichte und Literatur des Lichtwechsels der bis Ende 1915 als sicher veräderlich anerkannten Sterne (GuL). This three-volume work (1918–1922), which exercised an extremely positive influence on variable-star research, was continued by R. Prager, who published two volumes of a new edition in 1934 and 1936; the third volume, prepared by H. Schneller, appeared in 1952.
Müller’s work in the field of spectroscopy includes his contribution to Vogel’s Spektroskopische Beobachtungen der Sterne (1883) and his determination with Kempf (1886) of the absolute wavelengths of 300 lines of the solar spectrum.
Müller also produced a series of works that contributed to increasing the precision of observations and to improving reduction elements. This series included an investigation of the influence of temperature on the refraction of light in various types of glass (1885).
Müller participated in many of the great scientific expeditions sponsored by the Potsdam observatory. He led the 1882 expedition to the United States to observe the transit of Venus and participated in the expeditions to Russia to observe the total solar eclipses of 1887 and 1900. He also assisted in absorption studies conducted in Tenerife.
Müller exercised important functions in the organization of scientific research; from 1896 to 1924 he was secretary of the Astronomische Gesellschaft. Following the death of the second director of the Potsdam observatory, Karl Schwarzschild, he directed the institution from 1917 to 1921. During this time the Einstein Foundation was created and the Einstein Tower for solar physics was constructed. Admitted to the Prussian Academy of Sciences in 1918, Müller became chairman of the commission on the Geschichte des Fixsternhimmels (“history of the fixed stars”). Although not active in popularizing astronomy, he wrote the well-known monograph Die Photometrie der Gestirne (1897).
A complete bibliography of Müller’s 107 published works, compiled by his son Rolf, is in Vierteljahrsschrift der Astronomischen Gesellschaft, 60 (1925), 174–177. See also H. Ludendorff’s obituary, ibid., 158–174; and his notice on Müller, in Astronomische Nachrichten, 225 (1925), cols. 199–200.
Dieter B. Herrmann