Emission, Absorption & Spectral Lines: Foundations

Each species has allowed energy states and therefore a characteristic pattern of transitions. A spectrum can contain a continuum plus emission or absorption features whose positions, strengths, widths, and shapes encode physical conditions.

An electron or molecule absorbs a photon when the photon energy matches an allowed upward transition. Excited matter can later emit photons during downward transitions. Collisions, ionization, density, temperature, and radiation fields control which states are populated.

Laboratory spectra identify the rest wavelengths of elements and molecules. Matching those patterns in stellar and nebular spectra reveals composition, while line ratios and widths constrain temperature, density, pressure, magnetic fields, and motion.

Spectral lines appear because atoms and molecules possess quantized energy states. An emission line occurs when a transition releases a photon, while an absorption line occurs when incoming radiation is removed at a transition energy. The pattern of line wavelengths identifies species, but line strength also depends on temperature, density, ionization, abundance, and radiative-transfer geometry. A line is therefore both a chemical fingerprint and a physical-condition diagnostic.