The Hertzsprung-Russell diagram is a graph in which the absolute magnitudes (intrinsic brightness) of stars are plotted against their spectral types. Of great importance to theories of stellar evolution, it evolved from charts begun in 1911 by the Danish astronomer Ejnar Hertzsprung and independently by the U.S. astronomer Henry Norris Russell.
On the diagram stars are ranked from bottom to top in order of decreasing magnitude (increasing brightness) and from right to left by increasing temperature (spectral class). Stars of the galactic arm in which the Sun is located tend to fall into distinct regions on the diagram. The group called the main sequence extends in a rough diagonal from the upper left of the diagram (hot, bright stars) to the lower right (dim and cool). The giant sequence of large, bright, though cool, stars appears in the upper right, and the white dwarfs, dim, small, and hot, lie in the lower left. The Sun lies near the middle of the main sequence.
Diagrams drawn for the same stars with colour instead of temperature plotted against magnitude give similar results and are called colour-magnitude diagrams. Colour-magnitude diagrams drawn for stars from other parts of the Milky Way Galaxy--e.g., globular clusters--show different patterns from that for the local stars.
Excerpt from the Encyclopedia Britannica without permission.