Reading: Chapters 5 and 19

Here, we consider an empirically deduced relationship between two fundamental properties of stars. This relationship is best shown using what is referred to as the Hertzsprung-Russell diagram after its discoverers. Before I forget, however, let me mention that under certain simplifying assumptions (which are in fact satisfied by most stars), the properties of a star are determined by only its mass. This statement is so powerful that it has been codified as the Russell-Vogt theorem, although it is not really a theorem in the mathematical sense. We will explain why the Russell-Vogt theorem is true when we talk about stellar structure. For now, let us discuss the Hertzsprung-Russell diagram, arguably the most important astronomical discovery of this century and certainly one of the top half-dozen or so discoveries.

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