What characteristic directly determines a star's spectral type letter?
Answer
Surface temperature
The spectral type, represented by the letters O, B, A, F, G, K, M, is primarily determined by the star's surface temperature. This temperature dictates the specific pattern of absorption lines observed in the star's spectrum. Different temperatures cause various elements in the star's outer atmosphere to absorb photons at unique wavelengths, creating a signature pattern highly sensitive to thermal conditions. For example, very hot O-stars show ionized helium lines, while cooler M-stars show molecular bands like titanium oxide.
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