What observational evidence confirms to astronomers that a star has ceased core hydrogen burning and become a red giant?
Answer
Absorption lines in the spectrum show chemical enrichment of elements like carbon and nitrogen on the surface
Astronomers use spectroscopy to probe the chemical composition of a star's outer layers, even if they cannot directly view the core. When a star transitions to the red giant phase, internal convection mechanisms dredge up material from deeper layers that have been processed by prior fusion stages. This material, enriched with products of earlier core burning like carbon and nitrogen, appears in the star's light spectrum as altered absorption lines. Observing this surface enrichment acts as a definitive spectral signature indicating that core hydrogen exhaustion and subsequent shell burning are underway.
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