Stellar Spectral Types: Structures & Vocabulary

Stellar spectral types classify photospheres primarily by temperature through the pattern and strength of absorption lines. The modern sequence O, B, A, F, G, K, M runs from the hottest common stars to the coolest, with additional classes used for still cooler objects and chemically unusual spectra.

Line strength depends not just on elemental abundance but on how temperature controls excitation and ionization. Hydrogen is abundant in most stars, yet its visible Balmer lines are strongest only in a limited temperature range where enough atoms occupy the relevant excited state without being mostly ionized.

High-resolution stellar spectra show repeatable line patterns correlated with color and temperature. Laboratory spectroscopy identifies the transitions, while parallax and luminosity measurements show that spectral class alone does not specify size or evolutionary state.

Spectral types are subdivided numerically and supplemented by luminosity classes. Hot O stars show highly ionized species, A stars display strong Balmer absorption, solar-type G spectra contain many metal lines, and cool K/M stars develop strong neutral and molecular features.