Americanastrophysicist and solar astronomer. In the 1920s, with H. N. Russell, he established the field of theoretical astrophysics. In the 1930s he used quantum physics to interpret the Sun's flash spectrum, ascertaining the composition of the chromosphere and demonstrating the preponderance of hydrogen. Also in the 1930s he used similar techniques to study nebulae, discovering much about planetary nebulae and the later stages of stellar evolution. After World War II, as an astronomy administrator, he was responsible for founding many institutions and observatories, including the Harvard–Smithsonian Center for Astrophysics and the Kitt Peak National Observatory.
Subjects: Astronomy and Astrophysics.