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.
Star clusters are rich in stars just off the main sequence called red giants.
Main sequence stars are called dwarfs. And the faint, hot stars are called white dwarfs. On a log-log plot, the R squared term in the above equations is a straight line on an HR diagram. This means that on a HR diagram, a star's size is easy to read off once its luminosity and color are known. The HR diagram is a key tool in tracing the evolution of stars.