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The emission nebulosities in Messier 33 are similar in spectral characteristics to those of our own Gal- axy and to the low-excitation planetary IC 418. The ratio of A 5007/X 3727 is found to be the most re- liable excitation criterion for these nebulosities. The available evidence suggests that in most of these nebulosities the oxygen is predominantly in the singly ionized condition
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L. H. Aller (1942) studied this question.
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