The Hertzsprung-Russell diagram (or HR diagram) is constructed in logarithmic coordinates. In this graphic construction, each star is a point marked on the ordinate by its brightness (luminosity or absolute magnitude) and on the abscissa by its spectral type (or surface temperature). By making such a graph for a cluster of stars, H.N. Russell publishes an article in Nature in 1914 where he notes that stars do not place themselves randomly in his graph.
He thus identifies three settlement areas:
Such a diagram is a powerful analytical tool because it allows one to draw conclusions about the mass, size, chemical composition, age and evolutionary stage of a star.
Luminosity is expressed in solar light (L⊙). The radius of the stars is expressed in comparison with the radius of the Sun (R⊙).
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