Color and stellar luminosity

Hertzsprung devised an absolute measure of star brightness, measuring its luminosity from a standard distance of 10 parsecs (about 32 light years) away. An interesting finding was that there were no red stars of intermediate luminosity. This helped compare the real luminosities of different stars.
 Ejnar Hertzsprung Denmark 1905 Color and stellar luminosity

Hertzsprung devised an absolute measure of star brightness, measuring its luminosity from a standard distance of 10 parsecs (about 32 light years) away. An interesting finding was that there were no red stars of intermediate luminosity. This helped compare the real luminosities of different stars.