Hertzsprung–Russell diagram

An H–R diagram is a graph, not a map of positions in the sky. Its vertical axis represents intrinsic brightness—often luminosity relative to the Sun or absolute magnitude—and its horizontal axis represents temperature, colour or spectral class. In the common temperature convention, hotter stars appear on the left and cooler stars on the right, so temperature decreases from left to right. Absolute magnitude uses an inverse scale: more luminous stars have numerically lower, often more negative, magnitudes. Some versions use different axes or scales, so the labels must be checked when reading a particular diagram.

Most stars lie along the main sequence, a diagonal band running from hot, luminous stars to cool, faint stars. These stars generate energy mainly by fusing hydrogen into helium in their cores. Giants and supergiants occupy regions above the main sequence: at a given surface temperature, their high luminosity generally indicates a much larger emitting surface area. White dwarfs lie at the hot but faint end of the diagram; their low luminosity despite high temperature reflects their small size. These groupings make it possible to compare stars with different temperatures and luminosities in a single framework.

A star’s position provides evidence about its physical state, but it is not by itself a complete measurement of its age or history. Astronomers interpret the diagram using stellar physics, spectra, distance measurements and models. As stars change over time, their temperature and luminosity change, and their paths across the diagram depend strongly on properties such as mass and composition. A cluster is especially useful: its member stars are at nearly the same distance and formed in a related setting, so a colour–magnitude diagram can reveal a population pattern such as a main-sequence turnoff. The H–R diagram is therefore both a classification tool and a way of testing ideas about how stars change.

The diagram emerged from independent work by Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell. The Australia Telescope National Facility dates Hertzsprung’s plot of absolute magnitude against colour to 1911 and Russell’s work using spectral class and absolute magnitude to 1913. Their comparisons revealed that stars occupy structured groups in brightness–temperature space, helping establish a powerful way to organize observations and investigate stellar physics.