a hypothetical chemical element that was assumed to exist in gaseous nebulae. The hypothesis of the existence of nebulium arose in the second half of the 19th century as an attempt to explain the many emission lines in the spectra of gaseous nebulae that could not be identified with the spectral lines of chemical substances known on the earth. However, in 1927 the American astrophysicist I. Bowen showed that these lines are actually forbidden lines of a number of known elements. It is possible for gaseous nebulae to emit the forbidden lines because of the high degree of rarefaction of the gas and because of the incident stellar radiation. The term “nebular lines” is used as a convenient designation for these lines in the spectra of nebulae.