A Hertzsprung-Russell diagram refers to an illustration in which the absolute magnitude or essential brightness of stars are plotted vertically against the surface temperatures or corresponding spectral types. There seems to be a strong connection between brightness and surface temperatures, with types of blue stars having higher luminosity and types of red stars having lower brilliance. The more-or-less diagonal line which moves from the upper left of the diagram to the lower right depicts this association and is termed as principle series.
From the chart shown below, the following facts could be derived.
The earliest is a galaxy of stars with fairly high brightness and comparatively colder shadowy category (to the right) of the principal progression. Supergiants are the name given to this constellation. The second is a galaxy of stars at high luminosities spread across the top of the HR diagram, representing the full range of spectral types. Another name for this constellation is giants. Midway between the two is the bright giants.
It is seen that a few stars also are clustered at the lower end of the HR diagram, at comparatively high surface temperatures but low brightness. These stars have been called white dwarfs, and the demarcation of their visual aspects from major succession stars shows that they must be a very diverse type of stars from within.