Emission, Absorption & Spectral Lines: Physical Mechanisms

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.

Excitation and ionization determine which transitions can occur. A hot gas may contain an element but show weak neutral lines because most atoms are ionized; a cooler gas may lack enough particles in an excited state to produce a particular line. Collisions, radiation fields, and density determine level populations. This is why the strongest hydrogen Balmer lines occur over a particular stellar-temperature range rather than simply increasing with hydrogen abundance.