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Published online by Cambridge University Press: 03 August 2017
A common form of planetary nebula consists of a high temperature nuclear star surrounded by an ovoid or ellipsoidal shell of gas which has originated in some manner in the nucleus. The radiation from the nebula consists of the ordinary spectral lines and continua of hydrogen and helium plus the so-called forbidden lines of atoms and ions of oxygen, nitrogen, neon, argon, etc. It has become clear that, as in other cosmic bodies, the bulk of the nebula consists of hydrogen and helium, and the remainder of the chemical elements occur only as a slight admixture of impurities. For instance, recent data by Aller and Minkowski lead to relative numbers of atoms of various chemical species in a particular nebula (NGC 7027) as follows (oxygen arbitrarily taken as 104):
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