Hertzsprung-Russell Diagram — In stellar astronomy, the Hertzsprung-Russell diagram (H-R diagram) shows the relation between the absolute magnitude and the spectral types of stars. It was invented around 1910 by Ejnar Hertzsprung and Henry Norris Russell.
There are two equivalent forms. One is the observer’s form which plots the color of the star on one axis and the absolute magnitude on the other axis.
The theoretician’s form plots the temperature of the star on one axis and the luminosity of the star on the other. The transformation from one to the other is not trivial, and depends on the model being used and their parameters (like age and composition).
The H-R diagram is used to define different types of stars, and to match theoretical predictions of stellar evolution using computer models with observations of actual stars.
An examination of the diagram shows that stars tend to fall only into certain regions on the diagram. The most predominant is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence, which is where the majority of stars reside.
This line is so pronounced because both the spectral type and the luminosity depend on a star’s mass only to zeroth order as long as it is fusing hydrogen — and that is what almost all stars spend most of their “active” life doing.
At closer inspection, one notices that the main sequence is not exactly a line but instead somewhat fuzzy. There are many reasons for this fuzziness, the most important one still being observational uncertainties which mainly affect the distance of the star in question but range all the way to unresolved binary stars.
But even perfect observations would lead to a fuzzy main sequence, because mass is not the only parameter a star has after all. Chemical compositon and — related — its evolutionary status also move a star slightly on the main sequence, as do close companions, rotation, or magnetic fields, to name just a few.
Actually, there are very metal-poor stars (subdwarfs) that lie just below the main sequence although they are fusing hydrogen, thus marking the lower edge of the main sequence’s fuzzyness due to chemical compositon.
Stars usually enter and leave the main sequence from above when they are born or they are starting to die, respectively.
In the lower-left is where white dwarfs are found, and above the main sequence are the red giants and supergiants.