Hyperfine Structure
Extremely small energy level splittings in atoms caused by interaction between nuclear magnetic moments and electron magnetic fields.
Governing formula
ΔE_HFS ∝ I_nucleus · J_electron
SI unit
eV or MHz (Small Energy Splitting)
In depth
Hyperfine splitting is roughly 1000 times smaller than fine structure because nuclear magnetic moments are much smaller than electron moments (due to heavier nucleon mass).
Examples in the real world
The 21-centimeter (1420 MHz) neutral hydrogen radio wave line mapping spiral arms of the Milky Way, and cesium atomic clocks defining the SI second.