The photoionisation of the interstellar gas

By 1939, the existence of various forms of interstellar matter had been established. From the study of interstellar absorption lines and the variation of interstellar extinction with distance, it was known that diffuse gas and dust are present in the interstellar medium (Plaskett and Pearce, 1933; Joy, 1939). Gaseous nebulae had been known to be constituents of the Galaxy since the time of Huggins’ pioneering observations in the 1860s. During the first two decades of the twentieth century, Edward Barnard (1857–1923) made extensive studies of the forms of the dark clouds apparent in photographs of the MilkyWay (Barnard, 1919). The nature of these clouds was studied by MaxWolf (1863–1932), who determined the amount of extinction they cause by making star counts in their vicinity (Wolf, 1923). He correctly attributed the extinction to dust grains rather than gas because in the latter case the strong dependence of Rayleigh scattering upon wavelength would have resulted in much greater reddening of background stars than was observed.

On the theoretical side, it was recognised in the early 1920s that both the central stars of planetary nebulae and the O stars are very hot and so radiate a great deal of energy in the ultraviolet waveband. Russell suggested that the excitation of the emission lines seen in gaseous nebulae and planetary nebulae were due to photoexcitation (Russell, 1921), and Eddington showed that, as a result, the gas would attain a temperature of about 10 000K (Eddington, 1926b).