M. They developed the technique of plotting absolute magnitude for a star versus its spectral type to look for families of stellar type. These diagrams, called the Hertzsprung-Russell or HR diagrams, plot luminosity in solar units on the Y axis and stellar temperature on the X axis, as shown below. Notice that the scales are not linear.
Hot stars inhabit the left hand side of the diagram, cool stars the right hand side.
Bright stars at the top, faint stars at the bottom. Our Sun is a fairly average star and sits near the middle. We will see in a later lecture that stars begin their life on the main sequence then evolve to different parts of the HR diagram. Several regions of the HR diagram have been given names, although stars can occupy any portion. The brightest stars are called supergiants.