Why do stellar spectra show different chemical elements at different temperatures?

The variation in chemical elements observed in stellar spectra is primarily a result of thermal ionization rather than differences in the actual chemical composition of the stars. Most stars share a similar chemical makeup, but their spectra appear different because temperature dictates the ionization state of the atoms in their atmospheres.1

This phenomenon is explained by the Saha equation, developed by the astrophysicist Meghnad N. Saha. The process works as follows:

• Low Temperatures: Atoms retain their electrons, producing a spectrum characteristic of neutral atoms. For example, neutral calcium is prominent in cool M-type stars.1
• High Temperatures: Collisions between atoms and electrons, along with the absorption of radiation, detach electrons from atoms. This creates ionized atoms, which produce different spectral lines. In hotter G-type stars, for instance, ionized calcium lines become the dominant feature.1
• Extreme Temperatures: In very hot stars, such as O-type stars, atoms may become multiply ionized, requiring significantly more energy to produce the observed spectral features.1

Thus, the appearance of specific elements in a spectrum is a diagnostic tool used to determine a star's temperature and atmospheric conditions.23