Ira Sprague Bowen
American astronomer (1898–1973)
Bowen, who was born in Seneca Falls, New York State, graduated from Oberlin College, Chicago in 1919 and gained his PhD from the California Institute of Technology, Pasadena in 1926. He taught physics at Cal Tech from 1921 to 1945, serving from 1931 as professor. In 1946 he was made director of the Mount Wilson Observatory and in 1948 of the newly opened Palomar Observatory, posts he continued to hold until his retirement in 1964.
In 1928 Bowen tackled the problem of the strange lines first observed by William Huggins in the 1860s in the spectrum of planetary nebulae and the Orion nebula. The difficulty was, according to Bowen, that the strong lines had not been reproduced in the laboratory. Spectrographic evidence showed that such lines must be emitted by an element of low atomic weight. Talk of a new element, known as ‘nebulium’, that could produce the observed spectral lines was however dismissed by Bowen as nonsense.
Bowen was able to show that the lines were in fact due to radiation emitted from ionized atoms of oxygen and nitrogen as they decayed into more stable lower-energy levels. Specifically he was able to show that triply and doubly ionized oxygen as well as doubly ionized nitrogen would radiate at the wavelengths attributed to ‘nebulium’ but only in the highly rarefied conditions of nebulae where collisions between atoms are very infrequent. It is this radiation that contributes to the green and red colors observed in emission and planetary nebulae.
