What does the $ ext{I}$ luminosity class signify within the Morgan-Keenan ($ ext{MK}$) system?
Answer
Supergiants
The luminosity class component of the $ ext{MK}$ system uses Roman numerals to categorize a star's size and evolutionary state. Class $ ext{I}$ specifically denotes Supergiants, indicating they are very large stars that are significantly more luminous than standard main-sequence stars ($ ext{V}$). This system differentiates stars based on size even if they share the same spectral type (temperature); for instance, a star classified as $ ext{III}$ (Giant) and one classified as $ ext{I}$ (Supergiant) of the same temperature will differ drastically in total luminosity due to size.
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