Short answers, pulled from the story.
Modern astronomy places supergiants in the top region of the Hertzsprung Russell diagram with absolute visual magnitudes between about negative 3 and negative 8. Their temperatures range from approximately 3,400 K to over 20,000 K.
Antonia C. Maury divided stars based on spectral line widths in 1897 identifying class c as having the narrowest lines. Morgan and Keenan formalized the spectral luminosity classes in 1943 assigning class I to supergiant stars.
Supergiants are destined to end their lives violently leaving behind neutron star or black hole remnants usually after a core collapse supernova explosion. Stars with initial masses above eight solar masses quickly initiate helium core fusion until developing an iron core that collapses into a Type II supernova.
Rigel stands as the brightest star in the constellation Orion representing a typical blue white supergiant. The three stars forming Orion's Belt are all blue supergiants including Deneb which is the brightest star in Cygnus.
Red supergiants can be distinguished from luminous but less massive AGB stars by unusual chemicals including enhancement of carbon-13 lithium and s process elements. Cool supergiants show enhanced helium and nitrogen caused by convection of these fusion products during the main sequence of very massive stars.