- Graph showing the relationship between the surface temperature and brightness of stars, first drawn in 1910 independently by Ejnar Hertzsprung and Henry Norris Russell. The H-R Diagram is a fundamental underpinning of modern astronomy and astrophysics. Far from showing a uniform or random distribution of stars, the diagram shows that most stars lie on a single band called the main sequence, with white dwarfs below and giant and supergiant stars above. It is also possible to produce more detailed H-R diagrams for particular groups of stars, such as all those in a specific cluster – this is particularly simple because they can be assumed to be at essentially the same distance from the observer. This yields information about the distinguishing characteristics of particular clusters. In addition, the information on the H-R diagram is now so well established that it can with confidence be used ‘backwards,’ to yield the distance of a star of known type from its apparent magnitude, since the relationship between brightness (absolute magnitude) and spectral type is fixed on the H-R diagram.
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