6.1 Introduction

It is appropriate to recall, in the context of this volume, that just over a century ago the first color-magnitude diagram (CMD) was published. The author of this landmark paper was not Ejnar Hertzsprung nor Henry N. Russell, but Hans O. Rosenberg, a colleague of Karl Schwarzschild at Göttingen. Rosenberg had been working since 1907 on getting spectral properties of stars by measuring plates obtained with the Zeiss objective prism camera (Hermann, 1994). To maximize the number of spectra per plate, he observed the Pleiades cluster and obtained spectra for about 60 of them, over 1907–1909, noting that their inferred effective temperatures correlated with their apparent magnitudes in the first ever published CMD (Rosenberg, 1910). His goal was to “make the most accurate determination of the spectral types of stars in the Pleiades” by using a “physiological blend” of the depth and width of the Ca II K line (393.37 nm) with the Balmer Hδ and Hζ lines. He excluded the Ca II H line at 396.9 nm as it was blended with H∈ in the very low dispersion spectra he used (1.9 mm from Hγ to Hζ). With an exposure time of 90 minutes he could measure spectra down to the 10th photographic magnitude, finding that for the actual members of the Pleiades “there is a strict relation between the brightness and the spectral type, with no exception in the interval from the 3rd to the 9th magnitude.”