Two papers, one plane of lights

Ejnar Hertzsprung, in Europe, plots star brightness against color and sees that red stars come in two brightnesses. Henry Norris Russell, in Princeton, plots absolute magnitude against Cannon’s spectral types and finds a sequence and a cloud of luminous red stars. Neither is copying the other’s figure in the moment of first publication. The sky, plotted, has families.

A giant is not a metaphor. It is a star that is bright for its color. A dwarf is faint for its color. Sirius B will later sit as a white dwarf, off the sequence Clark could not yet draw.

The Hertzsprung–Russell diagram remains the most compact picture of what kinds of star there are. Evolution along it is a later reading. In 1911 and 1913 it is a sorting, hard won from parallaxes, photometry, and spectra.