BornRingarum, Östergötland, Sweden, 2 November 1906
DiedLund, Sweden, 10 February 1993
Swedish spectroscopist Bengt Edlén solved a 70-year-old puzzle by identifying emission lines in the solar corona (discovered in 1869 by Thomas Young) with transitions in very highly ionized atoms, thereby demonstrating that the corona is much hotter than the visible surface of the Sun. He received his secondary education in Norrköping, Sweden, and entered the University of Uppsala in 1926, earning a series of degrees ending with a doctorate in 1934.
By 1925, optical spectroscopy had reached a shortest wavelength of 155 Å, while X-ray spectroscopy had reached a longest wavelength of 17 Å. Karl M. G. (“Manne”) Siegbahn of Uppsala, who had received the 1924 Nobel Prize in Physics for his work on X-ray spectroscopy, suggested that Edlén should try to fill in the gap. This led to a doctoral thesis on the ultraviolet spectra of light elements from lithium to oxygen, with wavelength measurements and identifications of energy levels extending up to carbon and nitrogen with four electrons removed and oxygen with five electrons missing. This early work led to the 1932 identification of emission lines of ionized carbon, nitrogen, and oxygen in Wolf-Rayet stars, whose spectra had been something of a mystery since their discovery in the nineteenth century.
After obtaining his degree, Edlén remained at Uppsala as a docent, finally being appointed to the professorship of physics at the University of Lund in 1944, a chair previously held by spectroscopists Anders Ångström and Johannes Rydberg . Edlén continued to work on atomic spectra, focusing on the similarities of atoms that have the same numbers of electrons, following ionization. (For instance, singly ionized magnesium is like sodium, and singly ionized argon like chlorine.) He took a suggestion from Walter Grotrian to follow such sequences right on up to very highly ionized atoms of argon, calcium, iron, and nickel, allowing him to predict the wavelengths that these atoms should emit or absorb. A very important result was that a line at 5,303 Å would be produced by iron deprived of 13 electrons. This wavelength corresponded to a green emission feature seen in the spectrum of the solar corona during eclipses since 1869 and sometimes attributed to a nonexistent new element called “coronium.” In 1942, Edlén identified this and a number of other coronal lines. Because of wartime barriers to transatlantic communication, the news first reached the United States the following year in a paper written by Belgian astronomer Polydore Swings .
At Lund, Edlén established a large group of spectroscopists to work on other elements in other ionization states. The lines they predicted very often turned out to occur in the spectra of stars, gaseous nebulae, and even quasars, and the identifications made it possible to use these lines to determine the compositions and temperatures of the astronomical objects. Other features, like a pair of lines due to carbon missing three electrons, proved to be signatures of hot gas flowing away from stars in massive winds. The beginning of ultraviolet astronomy from satellites in the 1970s revealed many more of Edlén’s lines, just as he was reaching emeritus status in 1973. Nonetheless, he continued to be an active member of the community for a number of years beyond retirement.
Among the honors Edlén received for his work were medals and prizes from the Royal Astronomical Society, the Optical Society of America, and the United States National Academy of Sciences.