A postulated chemical element of low atomic weight, in the late 19ᵗʰ and early 20ᵗʰ centuries thought to exist in nebula, on the basis of spectroscopic evidence. It does not exist.

The Cat's Eye Nebula photographed by the Hubble Space Telescope

The Cat's Eye Nebula (NGC 6543)

NASA, ESA, HEIC, and The Hubble Heritage Team (STSci/AURA)

For background, please read the discussion of coronium.

before Hubble made it clear that some of the nebula were galaxies.

A chemical element which the Swedish physicist J. R. Rydberg conjectured existed between hydrogen and helium in the periodic table, based on spectral lines which I. S. Bowen later showed to be forbidden lines in ionized nitrogen and oxygen.

two bright green lines

1. I. S. Bowen.
The Origin of the Nebulium Spectrum.
Nature, vol 120, issue 3022, page 473 (1 October 1927).



Students of the solar corona and of the gaseous nebulae are discussing the properties of the hypothetical elements coronium and nebulium almost as familiarly as if they had actually handled them. Out of some 20,000 absorption lines mapped by Rowland, more than half are awaiting laboratory identification.

W. W. Campbell.
An address on astrophysics.
The Popular Science Monthly, page 305 (February 1905).

Campbell was Director of the Lick Observatory. His address was delivered at the St. Louis International Congress of Arts and Science.


Some strong independent evidence in favour of this view is found in a recent discussion by J. R. Rydberg on the system of the elements.† He identifies each element by the integral number giving its place in the table, regarding this as the “independent variable” functions of which must give all the properties of the atom. For a reason given below this number is two units above that chosen by Moseley for the “atomic number.”

On the other hand, there should be a series corresponding to p = 1 of 4 elements. These are H (1), two unknowns, and He (4), and at the beginning stands the rare gas, Electron. And he suggests Coronium and Nebulium for the unknowns.

† R. Rydberg (Lunds Universitets Arsskrift, N. F. Afd. 2, Bd. ix. Nr. 18, or Kongl. Fysiografiska Sällskapets Handlingar, N. F. Bd. xxiv. Nr. 18.

W. M. Hicks.
High frequency spectra and the periodic table.
Philosophical Magazine, 6th series, vol. 28, page 144 (1914).


In his letter to Nature of Oct. 1, p. 473. Mr. I. S. Bowen has made the important suggestion that several of the chief lines in the spectra of gaseous nebulæ may be due to what spectroscopists have called “forbidden” combinations of terms in the spectra of ionised nitrogen (N II), ionised oxygen (O II), and doubly-ionised oxygen (O III). It is, of course, no longer permissible to suppose the existence of hypothetical elements to account for the long-standing mysteries of nebular spectra, and we must accordingly regard the nebular lines as being produced by known elements under conditions of excitation which have not yet been imitated in the laboratory.

A. Fowler.
The Origin of the Nebulium Spectrum.
Nature, vol 120, issue 3025, pages 582-583 (October 1927).

further reading

Wolfgang Pauli.
Rydberg and the periodic system of the elements.
Proceedings of the Rydberg Centennial Conference on Atomic Spectroscopy.
Papers and abstracts, edited by Bengt Edlèn.
Kungl. Fysiografiska sällskapets handlingar, n. f., bd. 65, nr. 21.
Lund: C.W.K.Gleerup, [1955].

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