How do numerical subdivisions (3 through 9) within the 'O' spectral class relate to temperature?

Answer

Lower numbers indicate greater heat

Within the O group, numerical subdivisions run from 3 (hottest) down to 9 (coolest), meaning lower numerical designations correspond to higher surface temperatures.

How do numerical subdivisions (3 through 9) within the 'O' spectral class relate to temperature?

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Why Are O-Type Stars the Hottest and Bluest? - YouTube

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