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Hertzsprung, Ejnar

stars stellar results colour

[hert sprung] (1873–1967) Danish astronomer: discovered stellar spectral type/luminosity relationship.

Trained as a chemical engineer, Hertzsprung did research in photochemistry before appointment as an astronomer at the Potsdam Observatory in 1909. His work on photography led to his success in classifying stars. Hertzsprung was the first to realize that there was a relationship between the spectral colour of stars (as defined and classified by ) and their luminosity; for most stars the more blue the colour, the brighter the star. He also found that a small proportion of stars did not fit this pattern, being far brighter than might be expected for their colour. These two groups are now called main sequence stars (the numerous faint dwarfs) and red giants (fewer, more luminous) respectively. His results were published in 1905 and 1907 in obscure journals. Independently, in 1913, came to the same conclusions, the usual representation of their results being known as the Hertzsprung–Russell . This discovery had a great effect on ideas about stellar evolution.

His second important achievement was to utilize the period–luminosity relationship of Cepheid variable stars, discovered by in 1912, as a means of calculating stellar distances. In 1913 Hertzsprung was able to determine the distance of some nearby Cepheids from their proper motion (the only method available for measuring stellar distance up to that time), and using Leavitt’s results he began to calibrate the Cepheid variable technique. These first measurements of distances outside our Galaxy were developed by . Hertzsprung was an active researcher until he was over 90.

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