The hyperfine structure of hydrogen is specifically exemplified by the so called 21 cm line. This 1420MHz emission of hydrogen was predicted in 1944 by the Dutch astronomer Dr. Hendrik van de Hulst. It was first observed in inter-stellar hydrogen by H.Ewen and E.M. Purcell at Harvard in 1951, and shortly afterwards by observers elsewhere. Observations of this spectral line of hydrogen were subsequently instrumental in revealing the spiral nature of the Milky Way galaxy.
The 21 cm line in inter-stellar hydrogen results from an electron transition between a hyperfine orbital and the ground state orbital in the first orbit shell.

The background for the development of the hyperfine structure of hydrogen in the resurrected theory, is presented in the preliminary discussion in the next Section. The remainder of the Section then provides a detailed derivation of it, in which all hyperfine orbitals and the transitions between them are identified, and the mathematical representation given.
Because the 21cm line is an emission resulting from inter-stellar hydrogen, the environment within which this development is carried out is an inter-stellar one.

In Appendices A and B, the resulting spectra are presented in both tabular, and spreadsheet form for download, as is the resulting amendments to the fine structure spreadsheet initially provided in [2]. In Appendix C, the transition type table, initially presented in [9], is extended to incorporate the hyperfine orbitals, and to show how they thereby extend the inter-orbit transition Selection Rules.

A parameter will only be defined in this paper if it has not previously been so in [1], [2] or [3], with which familiarity is assumed.