The Hertzsprung-Russell iagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude diagram, or CMD) shows the relationship between absolute magnitude, luminosity, classification, and effective temperature of stars. The diagram was created circa 1910 by Ejnar Hertzsprung and Henry Norris Russell, and represented a huge leap forward in understanding stellar evolution, or the 'lives of stars'.