Abstract
THE relationship which exists between the dark galactic clouds, the luminous diffuse nebulosities and the involved or neighbouring stars is now generally recognised, but it has been only during the last twenty years or so that the real connexion between them has become evident, owing principally to the work of Slipher and Hubble. The luminous nebulosities, such as that surrounding 0 Orionis and p Ophiuchi, are regions on the near side of obscuring clouds, illuminated by stars within effective range. The medium composing the clouds is lit up in two ways, depending on the temperature of the stars involved. Stars of Bo type and higher temperatures excite and ionise the atoms of the cloud by intense ultraviolet radiation, the resulting characteristic radiations being principally hydrogen, oxygen (singly and doubly ionised in the metastable state), and helium. With stars of lower temperature than J52, the illuminated cloud gives a continuous spectrum of the same type as the stars, which we may reasonably interpret as reflected light, while nebulosities surrounding intermediate stars show a combination of both types of spectrum.
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Reynolds, J. The Galactic Nebulæ. Nature 137, 601–604 (1936). https://doi.org/10.1038/137601a0
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DOI: https://doi.org/10.1038/137601a0