Question

What are hyperfine levels of ground state?

Answer 1

The interaction between the magnetic moment, due to the spin of the nucleus, and the larger magnetic moment, due to the electron's spin, results in energy shifts which are much smaller than those of the fine structure. The hyperfine corrections may be difficult to measure in transitions between states of different n, however, they are quite measurable and important because they split the ground state. The different hyperfine levels of the ground state are populated thermally. Hyperfine transitions, which emit radio frequency waves, can be used to detect interstellar gas.