Henry Norris Russell

Today is the birthday of astronomer Henry Norris Russell, who was born in Oyster Bay, New York, in 1877. He received his PhD from Princeton University in 1900. He became a professor at Princeton in 1911 and took over as director of the university’s observatory in 1912. Russell used parallax measurements to determine both the distance to and the intrinsic brightness of various stars. He then compared the stars’ brightness with their color, as measured by spectroscopy. He plotted this data on a graph that became known as the Hertzsprung–Russell (H–R) diagram. (Danish astronomer Ejnar Hertzsprung had made similar observations a few years prior.) The diagram reveals a narrow strip of main sequence stars that includes hot, bright blue stars and cool, dim red ones. Russell’s data analysis fueled the study of stellar evolution and the rise of astrophysics. Beginning in the 1920s Russell analyzed the spectra of stars to determine physical characteristics such as temperature and chemical compositions. Today astronomers can make basic observations of a star, quickly place the star on the H–R diagram, and immediately know what stage the star is in its life. (Diagram credit: ESO, CC BY 4.0)

Date in History: 25 October 1877