Ejnar Hertzsprung

Ejnar Hertzsprung , 1873-1967, Danish astronomer. Although trained as a chemical engineer, Hertzsprung made his career in astronomy, specializing in exacting photographic observations of stars. In 1905 he discovered high-luminosity, or giant, stars. In 1913 he calculated the distance to the Small Magellanic Cloud by a method still used for measuring galactic and intergalactic distances. His 1922 catalog of star colors and luminosities disclosed the absence of bright stars of intermediate color, called the Hertzsprung gap. Working independently, both Hertzsprung and the American astronomer H. N. Russell developed a graph in which the luminosity of a star is plotted against its surface temperature. Such a graph is now called a Hertzsprung-Russell diagram and is the fundamental piece of observational evidence that the theory of stellar evolution must explain.

Hertzsprung, Ejnar (1873–1967)Danish astronomer. 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.