Starlight is a barcode. Every element absorbs light only at specific wavelengths. In spectra, these appear as dark lines (Fraunhofer lines). Relativistic Doppler: λ_obs = λ₀·√((1+β)/(1−β)). The simulator shows 10 main Fraunhofer lines with interactive zoom. Spectral classes O-M span 3200–40000 K. Galactic rotation curves (all stars rotate too fast) reveal dark matter. Stellar radial velocity precision: ~1 m/s with modern spectrographs (ESPRESSO).
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