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Hyperfine structure of atomic energy levels and spectral lines are based upon very small energy-level differences due to spin and/or to magnetic interactions between electrons and the nucleus (hyperfine levels). They are orders-of-magnitude smaller than fine structure differences, which, in turn is orders of magnitude smaller than gross structure differences/transitions from electron movement between larger/smaller electron shells (all the potential electron orbits that have a certain orbital radius), that form lines such as hydrogen's Balmer series. One hyperfine structure line widely used in astrophysics is hydrogen's 21-cm line, due to its one electron changing its spin.