The presence and behaviour of the emission lines in the spectra of the long-period variable stars pose one of the most interesting problems yet unsolved in astronomical spectroscopy.

There is no time in a ten-minute paper to go into detail. Let me say, though, that the primary question is ‘Where and how, in the atmospheres of the cool late-type giants of spectral types M, N and S which make up the long-period variables, can there exist the source of energy for production of the observed emission lines?’