stellar-spectroscopy

# Stellar Spectroscopy A stellar spectrum is the single most information-dense observation in astronomy. From a dispersed beam of starlight you can read temperature, luminosity, surface gravity, chemical composition, radial velocity, rotation speed, magnetic field strength, and binary companionship. This skill covers the core techniques for turning a spectrum into astrophysics: how the continuum forms, how absorption lines are produced, how to classify a star on the OBAFGKM sequence, how to identify and measure lines, how to extract Doppler velocity, and how to run a curve-of-growth abundance analysis. Cecilia Payne-Gaposchkin's 1925 dissertation — the first analysis to show that stars are mostly hydrogen — is the worked example at the end. **Agent affinity:** payne-gaposchkin (composition analysis), burbidge (nucleosynthesis signatures) **Concept IDs:** astro-hr-diagram, astro-stellar-classification, astro-nuclear-fusion ## Where a Spectrum Comes From A star's spectrum has three components, each produced by a different physical process: 1. **Continuous spectrum (continuum)** — thermal emission from the hot, dense gas below the photosphere, approximately a blackbody with small deviations. 2. **Absorption lines** — cooler gas in the photosphere absorbs specific wavelengths as electrons jump between bound energy levels. 3. **Emission lines** (less common in stars) — hot tenuous gas re-emits at specific wavelengths, most prominent in Be stars, Wolf-Rayets, and the solar ch...