Supergiant

A supergiant is a star with exceptionally high luminosity and a large radius, classified above giant stars on the stellar luminosity scale.

Hertzsprung–Russell diagram: A plot that relates stars’ luminosities to their surface temperatures or spectral types. It places supergiants in the luminous regions above the main sequence and giant branch.
Stellar evolution: The sequence of physical changes a star undergoes as it forms, burns fuel, and eventually ends its life. Supergiant phases arise during particular stages of massive stars’ evolution.
Betelgeuse: A nearby red supergiant in the constellation Orion, variable in brightness and strongly extended in the infrared. Its changing light and apparent size make it a prominent case for studying red supergiants.
Giant star: An evolved star larger and more luminous than a main-sequence star of similar temperature, but below supergiants in luminosity classification. It is the neighboring luminosity category that supergiants outrank.
Stellar luminosity class: A spectral classification system that groups stars by properties associated with their luminosity and atmospheric pressure. Supergiants occupy luminosity classes I and, in some schemes, intermediate classes.
Hydrostatic equilibrium: The balance between inward gravity and outward pressure in a stable star. This balance governs the expanded structures of supergiants.
Rigel: A blue supergiant system in Orion and one of the brightest stars visible from Earth. Its high temperature contrasts with the cool red-supergiant examples.
Main sequence: The evolutionary phase in which a star primarily fuses hydrogen into helium in its core. Supergiants are far more luminous and generally more evolved than main-sequence stars.
Stellar classification: The system of assigning stars spectral types and luminosity classes from their observed properties. Supergiant status combines spectral type with a luminosity class.
Stellar nucleosynthesis: The creation of chemical elements through nuclear reactions inside stars and during stellar explosions. Advanced fusion stages in massive supergiants build elements beyond those made by ordinary hydrogen burning.

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