Definitions for "Supergiant"
A star of at least eight solar masses that has evolved off the main sequence and depleted a significant portion of its hydrogen fuel supply. Helium becomes the new fuel, and "burns" at a much higher temperature in the core while hydrogen continues to "burn" in a surrounding shell. This shell expands the outer atmosphere of the star to over 10 up to 1000 solar radii. The entire life of the star from main sequence to supergiant is only a few million years.
A post-main-sequence phase of evolution of stars of more than about 4 solar masses. Supergiants fall in the extreme upper right of the H-R diagram.
a representative of the largest possible stars.