A spectral line is a dark or bright line in a spectrum, representing the absorption or emission of light at specific wavelengths by atoms or molecules. Spectral lines occur when electrons in an atom or molecule transition between energy levels; they either absorb or emit photons with wavelengths that correspond to the energy difference between those levels.
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A narrow spectral feature records enhanced emission or absorption near a transition frequency. In a stellar atmosphere, its strength depends on atomic populations, opacity, velocity fields and the source function. Lines therefore diagnose composition as well as the physical atmosphere in which they form.
A single line in the emission spectrum.
Has been the leading motivation of the development of quantum mechanics, all the way from the:
- Schrödinger equation: major lines predicted, including Zeeman effect, but not finer line splits like fine structure
- Dirac equation: explains fine structure 2p spin split due to electron spin/orbit interactions, but not Lamb shift
- quantum electrodynamics: explains Lamb shift
- hyperfine structure: due to electron/nucleus spin interactions, offers a window into nuclear spin