In 1905 he showed how a star’s luminosity was related to the width of lines in its spectrum, thus establishing spectroscopic parallax as a means of distance-finding. He inferred the distinction between red giant and dwarf stars from the fact that, although they were of the same spectral type, their spectra had different linewidths. Hertzsprung also proposed the modern definition of absolute magnitude. He went on to plot the first Hertzsprung–Russell diagram, for the stars of the Pleiades, in 1906. His work remained largely unknown, and in 1910 H. N. Russell independently developed the diagram in a slightly different form. In 1911 Hertzsprung discovered that Polaris is a Cepheid variable, and in 1913 estimated the distance to the Small Magellanic Cloud from the brightness of Cepheids within it.