Stellar Spectral Types: Foundations

Each spectral class is divided into numerical subclasses, such as G0 through G9. Spectra contain temperature-sensitive atomic, ionic, and molecular features: hot stars show strong ionized species, intermediate-temperature stars can show strong hydrogen Balmer lines, and cool stars show neutral metals and molecules.

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.

The OBAFGKM spectral sequence is fundamentally a temperature sequence encoded in absorption-line patterns. It is not an abundance ranking and it is not alphabetical because the historical classes were reordered once the underlying ionization physics became clear.