STEPHAN, ÉDOUARD JEAN MARIE

(b. Ste.-Pezenne, Duex Sèvres, France, 31 August 1837; d. Marseilles, France, 31 December 1923)

astronomy.

Stephan was admitted first in his class to the École Normale Supérieure in 1859. Upon graduating in 1862 he was invited by Le Verrier to the Paris observatory, where he learned observational techniques. At the same time he worked on his doctoral thesis, on second-order partial differential equations, which he defended in 1865.

Around this time Le Verrier had founded a branch of the Paris observatory at Marseilles. The city was selected as the appropriate site for the eighty-centimeter reflecting telescope, for which Léon Foucault had just constructed the mirror. In 1866 Stephan was assigned to equip and direct the observatory, and when it became independent of the Paris observatory in 1873, he was named director. In 1879 he became professor of astronomy at the University of Marseilles, holding both posts until his retirement in 1907. He became a corresponding member of the Bureau des Longitudes in 1875 and a corresponding member of the Académie des Sciences in 1879.

The scientists at the Marseilles observatory devoted their efforts primarily to the exploration of the sky. Stephan’s collaborators, Alphonse Borrelly and J. E. Coggia, discovered a great number of asteroids and comets. Stephan directed his attention mainly to the search for nebulae and to the determination of their positions. He discovered approximately 350 of them, including a compact group known as “Stephan’s quintet,” which consists of five galaxies, one of which has a radial velocity very different from that of the others. The existence of this group poses two problems that are still unsolved: that of the instability of clusters of galaxies and that of abnormal red shifts. For his work as a whole, Stephan was awarded a prize by the Académie des Sciences in 1884.

The first to study stellar diameters, Stephan used a procedure suggested by Fizeau in 1868, in which the surface of the mirror of a telescope is masked by a screen, except for two separated areas that play the role of the slits in Young’s experiment. The image of a point source is formed by the superposition of two groups of diffraction fringes; if the source has a perceptible apparent diameter, the fringes disappear when the distance between the areas attains a certain value, which is inversely proportional to this diameter. Stephan employed the procedure with the eighty-centimeter reflector in 1873 and observed the principal bright stars. The fringes did not disappear, and he concluded that the stellar diameters were less than 0.16″. The experiment was repeated in 1920 by Michelson, whose determinations of diameters confirmed the upper limit obtained by Stephan.

Among his other accomplishments, Stephan contributed to the geodetic connection of Africa and Europe. In collaboration with Maurice Loewy and François Perrier, he determined the differences of longitude between Algiers, Marseilles, and Paris (1874–1876). In this undertaking the astronomical measurements were associated with telegraphic transmissions of time signals.

BIBLIOGRAPHY

I. Original Works. For Stephan’s observations of asteroids and comets, see “Discovery of a New Planet,” in Monthly Notices of the Royal Astronomical Society, 27 (1867), 15; and “Comet I 1867, Discovered at Marseilles, 27 January 1867,” ibid., 255; as well as thirty–five notes in the Comptes rendus hebdomadaires des séances de I’ Acadeémie des sciences, 63–128 (1866–1899). On the observations of eclipses, see “Voyage de la Commission francaise ...éclipse totale de soleil du 19–8–1868,” in Annales scientifiques de I’ Ecole normale supémie des sciences, 99 (1884), 106 (1888), 130 (1900), and 136 (1903). On the positions and discoveries of nebulae, see “Nebulae Discovered at Marseilles,” in Monthly Notices of the Royal Astronomical Society. 32–34 and 37 (1872–1877); the last article reports on “Stephan’s quintent,” which is catalogued under the numbers 19–22,corresponding to the objects NGC 7317, 7318 A and B, 7319, and 7320. See also “Note sur les Nébuleuses découvertes á I’Observatoire de Marseille,” in Bulletin astronomique, 1 (1884), 286–290; “nébuleuses découvertes et observées á I’ Observatorie de Marseille,” in Astronomische Nachrichten, 105 (1883), 81–90, and 111 (1885), 321–330; and eleven notes in Comptes rendus hebdomadaires des séances de I’ Académie des sciences, 74–100 (1872–1885). For his work on stellar diameters, see “Sur les franges d’interférence ...,” ibid., 76 (1873), 1008–1010; and “Extréme petitesse du diamétre apparent ...,” ibid., 78 (1874), 1008–1012.

Three other publications should be noted: “Équations aux dérivées partielles du second ordre,” in Annales scientifiques de I’École normale supérieure, 3 (1866), 7–53, his dissertation; “Détermination de la différence des longitudes entre Paris et Marseille et Alger et Marseille,” in Travaux de I’ Observatoire de Marseille, 1 (1878), 1–214, written with M. Loewy; and “Notice sur I’Observatoire de Marseille,” in Bulletin astronomique, 1 (1884), 122–132.

II.Secondary Literature. See J.Bosler, “Édouard Stephan,” in Bulletin astronomique, 4 (1924), 5–8, with portrait; G. Bigourdan, “Annonce de la mort de Stephan,” in Comptes rendus hebdomadaires des séances de I’ Académie des sciences, 178 (1924), 21–24; and M. Hamy, “La détermination interférentielle des diamétres des astres,” in Annuaire publié par le Bureau des Longitudes (1919), B9–B15.

Jacques R. LÉvy