The HR Diagram

 

Few diagrams are more fundamental to astronomy than the Hertzsprung-Russell diagram.  Basically one plots the brightness of stars as a function of their surface temperature.  The brightness is expressed in magnitudes (-2.5 log luminosity) or the logarithm of the luminosity and the surface temperature is logarithmic and increases toward the left.  Alternates on the temperature scale are photometric color indices or spectral types.  Details can be found in elementary astronomy textbooks.  For our purposes here, there are some demarcations or boundaries of note.  They are shown in the figure:

A star is represented as a single point on such a graph.  Most stellar points lie along the thick line, called the ‘main sequence,’ labeled luminosity class V, called dwarfs, although they are the run of the mill normal stars.  The thin blue lines give means for other luminosity classes: IV for subgiants, III for giants, II for bright giants, & Ib for supergiants.  Stars spend most of their time on the main sequence, happily burning hydrogen into helium.  When that process is expended, i.e., the hydrogen in the stellar core has all been processes, the star grows in size and its point in the HR diagram move upward and to the right into the higher luminosity classes (lower Roman numerals).

Now for the more technical material: there are three special boundaries shown, 1) the Granulation Boundary - bisectors of spectral lines have C shapes for stars on the cool (right) side and a reversed-C shape on the hot side; 2) the Rotation Boundary - stars rotate slowly (≲ 6.5 km/s) on the cool side, while on the hot side stars rotate much faster with some individuals reaching ~ 450 km/s; and 3) the Coronal Boundary - stars on the hot side (left) have hot outer portions of their atmospheres called corona, while those on the cool side do not.