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How is Rigel classified as a blue supergiant?

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Open cluster NGC 290 — Star Stars in the open cluster NGC 290, as seen by the Hubble Space Telescope.

Rigel is classified as a blue supergiant because of its high intrinsic luminosity and its specific position and characteristics within the Hertzsprung-Russell diagram.12

In the context of stellar classification, supergiants are rare, short-lived, and highly luminous objects that can be observed at great distances due to their tremendous energy output. While supergiants exist across all spectral types, Rigel’s classification as "blue" corresponds to its high surface temperature and spectral characteristics, which place it among the hot, luminous stars in the upper regions of the Hertzsprung-Russell diagram.213

The term "supergiant" refers to the star's evolutionary stage; as stars evolve away from the main sequence, their physical properties—such as radius and luminosity—change based on their mass, chemical composition, and evolutionary history. Rigel, located in the constellation Orion, is approximately 47,000 times as luminous as the Sun, a level of brightness consistent with the supergiant category.41