(spectral lines from magnetic and spin interactions between electrons and nucleus)
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.