An Etymological Dictionary of Astronomy and Astrophysics
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Closely spaced components seen at high resolution in a → spectral line. The phenomenon is explained by the fact that instead of a single → energy level corresponding to a given value of the → quantum number  n, there are actually a number of energy levels lying close to one another. → fine-structure constant, → fine-structure line.

→ fine; → structure.

In spectroscopy, the → splitting of a spectral line into a number of very thin components. It results from a small perturbation in the energy levels of atoms or molecules due to the magnetic dipole-dipole interaction arising from the interaction of the nuclear → magnetic moment with the → spin of the electron. It can be observed only at high spectral dispersion. → fine structure.

→ hyperfine; → structure.