In astronomy the Hertzprung-Russell diagram is a distribution chart showing the relationship between the absolute magnitude or luminosity versus spectral type or stellar classification and effective temperature. The Hertzsprung-Russell diagrams are not pictures or maps showing the location of stars. Instead, they put each star a graph showing the magnitude (or brightness) against its temperature and color. Hertzsprung-Russell Diagram

HR Diagram adapted from Powell. Shown are 22,000 stars of the catalog of Hipparchus and 1,000 of the Gliese Catalogue of nearby stars.

Most stars occupies the area of the diagram along line called the main sequence. During this stage stars are fusing hydrogen nuclei. The concentration of stars is in the next horizontal branch (helium fusion and burning hydrogen in the layer covering the core). Hertzsprung-Russell Diagram

HR Diagram of two open clusters M6 and NGC188, showing off the main sequence at different ages.

Another important area is the failure of Hertzsprung located in the area between A5 and G0 spectral types and between +1 and -3 absolute magnitudes (i.e., between the top of the main sequence and giant the horizontal branch), where the density of stars is smaller. The RR Lyrae variable stars are found to the left of this failure. The variables of Cepheids located in the upper section of the instability strip (one almost vertical region of the diagram occupied by pulsating variable stars). Hertzsprung-Russell Diagram

The sun is on the main sequence, the magnitude 1. The HR diagram can also be used by scientists to measure approximately the distance between a star cluster and earth. This can be done by comparing the apparent magnitudes of the stars of the cluster with the absolute magnitudes of stars with known distances (stars or model). The group observed is then moved in vertical direction until the two major sequences coincide. The difference in magnitude that was covered to match the two groups is called the distance modulus and is a direct measure for the distance. This technique is known as sequence matching primary or spectroscopic parallax.

Hertzsprung-Russell Diagram