Closely spaced spectral lines from a given element. Fine structure has two causes. One is the interaction of electrons in an atom with the magnetic field of the atom itself, and the second is relativistic effects on the electrons’ kinetic energies due to their orbital motion. These combined effects create a fine splitting of the various principal energy levels and produce several closely spaced lines instead of one. For hydrogen the fine structure is so fine (0.006 nm) that it is unobservable in stars. However, more complex atoms such as sodium can have more discernible fine structure because of the presence of many other electrons in the atom. The sodium D lines are an easily observable example of fine structure, with a separation of 0.6 nm. Hyperfine structure is on a still smaller scale.