|The Critical Importance of Russell's Diagram
|471, Origins of the Expanding Universe: 1912-1932
|The idea of dwarf and giants stars, but not the nomenclature, was first
established by Eijnar Hertzsprung in 1905; his first diagrams in support appeared in 1911.
In 1913 Henry Norris Russell could demonstrate the effect far more strikingly because he
measured the parallaxes of many stars at Cambridge, and could plot absolute magnitude
against spectral type for many points. The general concept of dwarf and giant stars was
essential in the galactic structure work of Harlow Shapley, Russell's first graduate
student. In order to calibrate the period-luminosity relation of Cepheid variables, he was
obliged to fall back on statistical parallax using only 11 Cepheids, a very sparse sample.
Here the insight provided by the Russell diagram became critical. The presence of
yellow K giant stars in globular clusters credentialed his calibration of the
period-luminosity relation by showing that the calibrated luminosity of the Cepheids was
comparable to the luminosity of the K giants. It is well known that in 1920 Shapley did
not believe in the cosmological distances of Heber Curtis' spiral nebulae. It is not so
well known that in 1920 Curtis' plot of the period-luminosity relation suggests that he
didn't believe it was a physical relation and also he failed to appreciate the significance
of the Russell diagram for understanding the large size of the Milky Way.