By the turn of the 20th century, astrophysicists had already gathered extensive data. For example, they had determined the masses of many stars; they knew their apparent and absolute (actual, independent of distance) brightnesses, their colors and surface temperatures, and their spectral types. In 1913, American Henry N. Russell (1877 to 1957) had the idea to check whether the different characteristics were in some way connected. Some years earlier, the Dane Ejnar Hertzsprung (1873 to 1967) had been wondering much the same thing. Hertzsprung had examined many stars closely on paper and compared their luminosity and spectral types.
Russell drew a simple system of coordinates with x- and y-axes, familiar to schoolchildren from geometry lessons. On this diagram, he entered the stars according to their spectral type or temperature (x-axis) and absolute brightness or luminosity (y-axis). Contrary to what might have been expected, the resulting distribution was far from random. Instead, the majority of stars were located on or close to a line running diagonally from the top left corner to the bottom right corner: the main sequence.
Some stars, however, were located at the bottom left of the diagram. They are extremely hot and belong to the spectral types O, B, A and F. At the same time, their absolute brightness is low – that is, they shine weakly. Something that radiates hot but appears only as a weak light must have a relatively small surface area. These miniature balls of gas are thus located on the dwarf sequence. Finally, there is a second large group outside the main sequence. Stars in this group are all located in the upper area of the Hertzsprung- Russell diagram with very high absolute brightness values. Some of them have relatively cool surfaces. Nevertheless, they are brighter than the stars in the main sequence in the spectral classes K or M. They must therefore have very large radii. They are located in the giant sequence.
The color-magnitude diagram (CMD) is equivalent in practice to the Hertzsprung-Russell diagram (HRD). The spectral type is replaced by the “color index,” which describes the brightness of a star in two different spectral ranges and is a measure for the color temperature of the star’s surface. The rule is that the higher the color index, the redder the star’s appearance. Since the members of a cluster are all located the same distance from the Earth, their apparent brightness in the sky reflects the actual differences in their luminosity. The apparent brightness is therefore often substituted for the absolute brightness in color-magnitude diagrams.