Spectroscopy is the technique of splitting light into its component wavelengths to read what a source is made of. Atoms and molecules absorb and emit at characteristic wavelengths, leaving a pattern of lines that works as a chemical fingerprint. The same spectra also reveal temperature, density, magnetic fields, and motion, since a source moving toward or away from us shifts its lines toward blue or red. Nearly everything known about the composition of stars, galaxies, and exoplanet atmospheres was learned this way.