F3 - 21-cm line

the ground state energy level of H can split into two levels due to the parallel (higher E), or anti-parallel (lower E) alignments of the proton and electron spins
this is classified as a hyperfine transition
the difference in the energies, $Δ E = 6 \times 10^{- 6} e V$

Δ E = E_{γ} = \frac{h c}{λ} ⟹ λ = 21 cm

this gives the 21-cm line
the transition rate is very low, on the order of $10^{- 14} s^{- 1} {atom}^{- 1}$
but due to the high mass fraction of H, the 21-cm line is the most prominent transition coming from the ISM