The Critical Importance of Russell's Diagram
  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.
