Emission, Absorption & Spectral Lines: Structures & Vocabulary

Spectral lines are narrow wavelength features produced when atoms, ions, or molecules exchange energy in quantized transitions. Emission lines add light at characteristic wavelengths, while absorption lines remove light from a background continuum at those wavelengths.

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.

A useful structural distinction is continuum versus line, neutral versus ionized species, and permitted versus forbidden transitions. Each spectral feature has a rest wavelength, profile, width, depth or intensity, and sometimes fine or hyperfine structure. Molecules add rotational and vibrational bands. Astronomers organize these features into line lists so observed spectra can be matched to laboratory physics with known uncertainties.