Strömgren, Bengt Georg Daniel (1908–1987)

Swedish-born Danish astronomer whose 1940 hypothesis about the so-called Strömgren spheres – zones of ionized hydrogen gas surrounding hot stars embedded in gas clouds – proved fundamental to our understanding of the structure of interstellar material.

Strömgren was born in Göteborg. He studied in Denmark at Copenhagen, where he became professor in 1938; in 1940 he succeeded his father Elis Strömgren as director of the observatory there. From 1946 he held mainly US academic posts; he was professor at the University of Chicago 1951–57 and director of both the Yerkes and McDonald observatories, and was a member of the Institute for Advanced Study at Princeton 1957–67.

Some gaseous nebulae that can be observed within our Galaxy are luminous. Strömgren proposed that this light was caused by hot stars within obscuring layers of gas in the nebulae. He suggested that these stars ionize hydrogen gas and that the dimensions of the ionized zone (the Strömgren sphere) depend on both the density of the surrounding gas and the temperature of the star. His calculations of the sizes of these zones have been confirmed by observations.

Strömgren's other work included an analysis of the spectral classification of stars by means of photoelectric photometry, and research into the internal composition of stars.