Johann Georg Hagen, (born March 6, 1847, Bregenz, Austria—died Sept. 5, 1930, Rome, Italy), Jesuit priest and astronomer who is noted for his discovery and study of dark clouds of tenuous, interstellar matter sometimes known as Hagen’s clouds.
Hagen served as director of the Georgetown College Observatory, Washington, D.C., from 1888 to 1906, when Pope Pius X appointed him director of the Vatican Observatory. In 1893, while compiling a general catalog of bright and dark nebulae, Hagen began an intensive study of dark nebulae. Over a period of several decades, he became convinced that he sometimes observed dark interstellar matter through the telescope, although he failed to detect it photographically, presumably because of its faint luminosity. Modern astronomers tend to discount the existence of Hagen’s clouds and attribute his observations to other causes, such as inconsistencies in the reaction of the human eye.
Hagen is also noted for his work on variable stars (stars whose intensity changes cyclically). His studies on variable stars appear in Atlas Stellarum Variabilium (1890–1908; “Atlas of Variable Stars”), and his observations of Hagen’s clouds are summarized in a paper that appeared in Specola Astronomica Vaticana 14 (1931; “Vatican Astronomical Observatory”).