Elis Strömgren, father of Bengt, was an astronomer of distinction who served as director of the Copenhagen Observatory. His son was born at Gothenburg in Sweden and studied at the University of Copenhagen. After obtaining his PhD there in 1929 he joined the staff and was appointed professor of astronomy in 1938. He succeeded his father as director in 1940. He later moved to America, serving from 1951 to 1957 as professor at the University of Chicago and director of the Yerkes and McDonald observatories. He was a member of the Institute for Advanced Study, Princeton, from 1957 until 1967, when he returned to Copenhagen as professor of astrophysics.

In the 1930s and 1940s Strömgren engaged in pioneering work on emission nebulae – huge clouds of interstellar gas and dust shining by their own light. He showed that they consist largely of ionized hydrogen, H II to the spectroscopist. If hot young stars were embedded in uniformly but thinly distributed neutral hydrogen, then the emission by them of ultraviolet radiation would virtually ionize the gas completely. To meet this condition the stars would need a surface temperature of some 25,000 kelvin. At a certain distance from the star, the Strömgren radius, the emitted photons of radiation would no longer possess sufficient energy to ionize the hydrogen, leading to a sharp boundary between ionized and cooler nonionized regions. Strömgren showed that this distance would depend on the density of the hydrogen and the stellar temperature.

A typical example of the process described by Strömgren is to be found in the Orion nebula. Later work has however shown that there are three types of emission nebulae, two of which are produced by different mechanisms.