Not every huge, brilliant star counts as a true supergiant
Some stars shine thousands of times brighter than the Sun yet are not supergiants at all. Astronomers define the label two ways, by the look of a star's spectrum and by the stage of life it has reached, and the two definitions do not always agree.
Supergiants sit at the top of the Hertzsprung-Russell diagram, among the heaviest and brightest stars, with surface temperatures from about 3,400 kelvin to more than 20,000. Colour labels follow spectral type or temperature: below roughly 4,800 kelvin they are red, between 4,800 and 7,500 yellow, and hotter still blue. The name grew out of Ejnar Hertzsprung's giants, the big, bright stars of types A to M. Some of those showed no measurable parallax, meaning they were farther and thus far more luminous than the rest, and super-giant soon became supergiant.
Spectra give them away, because a huge, bloated star has weak surface gravity and narrow lines. In 1897 Antonia Maury flagged stars with the narrowest lines as class c, not knowing they were the brightest of all. Morgan and Keenan formalised luminosity classes in 1943, with class I for supergiants, a scheme still used with finer grades: Ib, Ia for luminous supergiants and 0 or Ia+ for hypergiants. Real stars blur these bands, so intermediate labels like Iab also appear.
In life-story terms, a supergiant is a star born with more than about 8 to 10 solar masses. After burning its hydrogen it smoothly moves on to helium and then heavier elements until an iron core forms and collapses in a Type II supernova. Its atmosphere inflates as soon as it leaves the main sequence. Lighter stars never build iron cores; they climb the asymptotic giant branch, can reach thousands of times the Sun's brightness, then shed their outer layers and leave white dwarfs.
Several lookalikes complicate the picture. Classical Cepheids such as Delta Cephei, about 2,000 times the Sun's luminosity at 4.5 solar masses, often carry supergiant classes. Wolf-Rayet stars are hotter and smaller but frequently more luminous, and hypergiants are essentially the most extreme, unstable supergiants of all.
Source: Supergiant
