Short answers, pulled from the story.
A supergiant is one of the most massive and most luminous types of star, occupying the top region of the Hertzsprung-Russell diagram. Supergiants have absolute visual magnitudes between about minus 3 and minus 8 and surface temperatures from roughly 3,400 K to over 20,000 K. They carry masses from 8 to 12 times the Sun and upward and luminosities from about 1,000 to over a million times the Sun.
Blue supergiants are the hottest, with spectral classes O to A or temperatures above 7500 K. Yellow supergiants fall in classes F and G, or between 4800 and 7500 K. Red supergiants are the coolest, in classes K to M or below 4800 K, and for the same luminosity they are physically larger than blue supergiants.
Astronomers use the MK spectral luminosity system formalized by Morgan and Keenan in 1943, in which class I denotes supergiants. The system assigns Ib to ordinary supergiants, Ia to luminous supergiants, and 0 or Ia+ to hypergiants, with intermediate labels such as Iab. Classification relies on spectral lines that reveal a star's low surface gravity and high luminosity.
Rigel, the brightest star in Orion, is a blue-white supergiant, and the three stars of Orion's Belt are all blue supergiants. Deneb is the brightest star in Cygnus, while Delta Cephei and Polaris are Cepheid variables and yellow supergiants. Antares and Betelgeuse are red supergiants, with Betelgeuse the second-brightest star in Orion.
Supergiants are destined to end violently, usually in a core-collapse supernova. Massive supergiants fuse elements all the way to iron, after which the core collapses and the star explodes, leaving behind a neutron star or a black hole. Most Type II supernova progenitors are thought to be red supergiants, while Type Ib/c supernovae come from hotter Wolf-Rayet stars.
Supergiants are short-lived because their extreme masses cause them to burn through their fuel extremely fast, with lifespans between 30 million years and a few hundred thousand years. They are found mainly in young structures such as open clusters, spiral arms, and irregular galaxies, and are rare in elliptical galaxies and globular clusters made of old stars.