Emission, Absorption & Spectral Lines: Observation, Measurement & Evidence

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.

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.

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.

Line identification uses calibrated wavelength scales, known laboratory transitions, multiple lines from the same species, and physically consistent velocity shifts. Line ratios can measure electron temperature or density, while profile widths reveal thermal motion, turbulence, rotation, or pressure broadening. Equivalent width helps compare line strength when continuum brightness changes. Reliable abundance analysis fits many transitions rather than inferring composition from one isolated feature.