HertzsprungRusselldiagrammet

The Hertzsprung-Russell diagram, often abbreviated as the HR diagram, is a two-dimensional plot that shows the relationship between the luminosities of stars and their effective temperatures (or spectral types). On the traditional diagram, luminosity or absolute magnitude runs vertically, with brighter stars toward the top, while temperature or color runs horizontally, typically with hotter stars to the left and cooler stars to the right. The diagram was developed independently in the early 20th century by Ejnar Hertzsprung and Henry Norris Russell, using observed properties of stars and clusters.

The HR diagram reveals distinct stellar populations. Most stars form a diagonal band known as the main sequence, where hot, luminous stars sit at the upper left and cool, dim stars occupy the lower right. Above and to the right lie the red giants and supergiants, which are cool but highly luminous. In the lower left are the white dwarfs, compact and hot but faint. The diagram also serves as a framework for understanding stellar evolution: stars spend most of their lives on the main sequence, then move off it as they exhaust hydrogen in their cores, ascending to giant or supergiant phases, and, for lower- and intermediate-mass stars, ending as white dwarfs.

Applications of the HR diagram include estimating the ages of star clusters through main-sequence turnoff points, constraining stellar masses via the mass-luminosity relation, and testing theoretical models of stellar structure and evolution. Limitations arise from uncertainties in temperature and luminosity determinations, interstellar extinction, distance measurements, and variations in chemical composition, which can shift stars’ positions on the diagram.