A study of the sodium D1 interstellar line in the spectrum of a Cygni with a PEPSIoS spectrometer at a resolving power of about 5 X 100 indicates that, of the previously found five components, each of the two extreme components is doubled, with a separation that could be taken to be the 0.057 hyperfine structure separation; pending further study, the structure is tentatively attributed to hyperfine structure, which implies an upper limit of 0.64 km/sec to the turbulent velocity spread in the component cloud.
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L. M. Hobbs (1965) studied this question.