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

corona around sun, with stars marked

The corona seen during the 1919 solar eclipse. This photograph, incidentally, was one of the first confirmations of Einstein's theory of general relativity. It revealed that the sun's mass bends starlight passing by.

ESO/Landessternwarte Heidelberg-Königstuhl/F.W. Dyson, A.S. Eddington & C. Davidson.

In 1666, Issac Newton realized that sunlight is not simple, but consists of many different “rays” which are bent to different extents in passing through a glass prism.Thus a prism spreads sunlight into the familiar rainbow band of colors, a spectrum.¹ But the glass of Newton's time had imperfections that smeared the spectrum and hid fine details.

In late 18th century Bavaria, patrons recognized the ability of a teenage apprentice glassmaker, Joseph von Fraunhofer, and supported his education and experiments. By 1814 Fraunhofer had created vastly improved glass, from which he made prisms that he used to invent the modern spectroscope. Turned on sunlight, his spectroscope's improved resolution revealed hundreds of dark lines in the solar spectrum. He found other lines, bright and dark in light from bright stars. Angelo Secchi used these spectral lines to classify 600, then 4000 stars, into four groups.

What causes the spectral lines?

An astronomical spectroscope circa 1869. Notice the two prisms. Ganging up prisms is a way of spreading (dispersing) the spectrum over a wider area, making it easier to observe spectral lines. Eventually, prisms were superceded by diffraction gratings¹.

From Roscoe, On Spectrum Analysis (1869)


At the University of Heidelburg Bunsen and Kirchhoff, professors of chemistry and physics, respectively  One of the techniques they used was to a sample of the elelment they were investigating into the flame of a Bunsen burner, observed by a spectrograph.

For example, sprinkle a pinch of table salt (sodium chloride) into a flame on a gas kitchen range. The bright yellow light that appears is characteristic of the element sodium. The yellow is the same yellow in the light of old-fashioned sodium vapor streetlights. It appears as a bright yellow line in a spectrum (with other bright lines characteristic of sodium).

Kirchhoff and Bunsen demonstrated that an element could be identified by the unique set of wavelengths of light it emitted, Using a spectroscope, Bunsen discovered two new elements in mineral water.

Light from an incandescent lamp is continuous; it doesn't contain bright lines. passes through a gaseous cloud of sodium atoms. will appear as a dark line against and it will appear at exactly the (and to Bohr's atom).

So even if that light had travelled millions of light-years.   The path was opened to studying the chemistry of stars. See also nebulium.

proves the existence of sodium in that light source.

From Roscoe, On Spectrum Analysis (1869)

Astronomers studied the solar corona with a spectrograph for the first time during the eclipse of August 1868 (before the invention of the coronagraph, the corona was only visible during eclipses). Unfamiliar lines in its spectrum suggested the existence of a new element, dubbed helium.² In 1895, helium was found on Earth, but even before that helium's example raised the exciting possibility that one might discover new elements by studying other unidentified spectral lines. Light from non-terrestrial sources seemed a promising place to look for undiscovered elements.

During an 1869 solar eclipse, two observers spotted a bright green spectral line in light from the corona. No element tested in the lab had emitted light at that wavelength.

Now move to the periodic table devised by Mendeleev in atomic weight. weights by compaing one elemnt to another

Recall that, in 1868, the very existence of atoms was controvrsial

The discovery of the neutron, and the sorting of the periodic table by atomic number instead of atomic weight, closed the gap that coronium and nebulium had been thought to fill. Hydrogen and helium were no longer 1 and 4, but 1 and 2. Atoms with atomic weights of (roughly) 2 and 3 were not new elements, but isotopes of hydrogen. They have, uniquely, been given their own names, deuterium and tritium (2ium and 3ium).

Finally, in 1941 Bengt Edlén³ showed that the spectral lines associated with coronium are emitted by very highly ionized iron atoms. Kirchhoff and Ångstrom's hunch was borne out, but in solving one mystery Edlén created another.

The iron atoms in the corona are ionized (stripped of electrons) to an extent indicating very, very high temperatures. More than two million degrees, in the case of iron atoms missing 12 electrons, the atoms that emit the green line discovered in 1869.³ The sun we see in the sky, the photosphere, lies below the corona, but the photosphere is only around 6000 K. How the sun's outer atmosphere can be hotter than the “surface” below is still being investigated, but it seems to be done with magnets.

the sun photographed in light emitted by iron-14 atoms


1. See source 1, below.

2. Not that the manufacture of high-quality prisms has ceased.

2. “Frankland and Lockyer find the yellow prominences to give a very decided bright line not far from D, but hitherto not identified with any terrestrial flame. It seems to indicate a new substance, which they propose to call Helium.”

William Thomson [future Lord Kelvin].
Inaugural address, The British Association Meeting at Edinburgh.
Nature, vol 4, footnote page 268 (Aug 3 1871).
doi: 10.1038/004261a0

3. Walter Grotian identified two of the lines. The remaining publications are by Edlén; the cascade illustrates the difficulty of disseminating scientific advances during a World War.

Walter Grotrian.
Zur Frage der Deutung der Linien im Spektrum der Sonnenkorona.
Naturwissenschaften, vol 27 page 214 (1939).

An Attempt to Identify the Emission Lines in the Spectrum of the Solar Corona.
Arkiv för Matematik, Astronomi och Fysik 28B, no. 1, pages 1–4 (1941)

Die Deutung der Emissionslinien im Spektrum der Sonnenkorona.
Zeitschrift für Astrophysik, vol. 22, pages 30–64 (1942).

The Identification of the Coronal Lines.
Monthly Notices of the Royal Astronomical Society, vol 105, pages 323–333 (1945).

3. A set of images illustrating the relationship between temperature, degrees of ionization of iron, and the resulting emission. Or see NASA



[Page 3075] To perform my late promise to you, I shall without further ceremony acquaint you, that in the beginning of the Year 1666 (at which time I applyed my self to the grinding of Optick glasses of other figures than Spherical,) I procured me a Triangular glass-Prisme, to try therewith the celebrated Phænomena of Colours.

[Page 3709] And so the true cause of the length of that Image [of the spectrum] was detected to be no other, then that Light consists of Rays differently refrangible, which, without any respect to a difference in their incidence, were, according to their degrees of refrangibility, transmitted towards divers parts of the wall.

When I understood this, I left off my aforesaid Glass works; for I saw that the perfection of Telescopes was hitherto limited, not so much for want of glasses truly figured according to the prescriptions of Optick Authors, (which, all men have hitherto imagined,) as because that Light it self is a Heterogeneous mixture of differently refrangible Rays.

Isaac Newton.
A Letter of Mr. Isaac Newton, Professor of the Mathematics in the University of Cambridge ; containing his New Theory about Light and Colors : sent by the Author to the Publisher from Cambridge. Febr. 6, 16 71/72 in order to be communicated to the R. Society.
Philosophical Transactions: giving some accompt of the present Undertakings, Studies, and Labours, of the Ingenious in many Considerable Parts of the World, , vol. 6, no. 80, February 19, 16 71/72


[Page 88:] At least, two additional elements, as yet unidentified with any terrestrial substances, are recognized in the chromosphere by bright lines—one of them the unknown substance, which is most conspicuous in the corona, the other the hypothetical helium, as Frankland named it; there may, probably enough, be others also.

[Page 193:] 3. 5874·9, D₃. Unknown element :-Frankland's "helium."
                        4. 5315·9. The corona-line, element unknown.

[Page 224:] The question was first settled in 1869 by the observations of Professor Harkness and the writer, who, independently, found the spectrum of the corona to be characterized by a bright line in the green—the “1,474 line”—so called because on Kirchhoff's map of the solar spectrum, then generally used for reference, the line in question falls at this point of the scale.

[Page 230:] This bright line, as has been said before, was first recognized as coronal at the eclipse of 1869. It had been seen reversed in the spectrum of the chromosphere a few weeks previously, both by Mr. Lockyer, and, independently, by the writer, who, however, did not know of the earlier observation until some time after the eclipse. In the ordinary solar spectrum it appears as a fine, dark line at 1,474 of Kirchhoff's scale, or 5,315·9 of Ångstrom's—a line in no way conspicuous as compared with hundreds of others, and barely visible with a single-prism spectroscope. With a spectroscope of high dispersion it was found, in 1876, to be closely double, the upper (more refrangible) component being slightly hazy, while the other is sharp and well-defined. The upper component is the true coronal line, and is always seen without much difficulty, reversed in the spectrum of the chromosphere. Both Kirchhoff and Ångstrom give the line as belonging to the spectrum of iron a fact which was for a time very perp1exing, since it is hardly possible that the vapor of this metal could really be the prevailing constituent of the corona, surmounting even hydrogen itself. This difficulty, however, no longer exists, for it is now clear that the iron-line is the lower component of the double, its close proximity to the other being only accidental.

C[harles] A[ugustus] Young.
The Sun.
New York: D. Appleton and Co., 1881.


A gas having the properties above represented seems to be the hitherto unknown corona substance, which emits the only at present known ray 1474 of Kirchoff's scale, or λ = 5315·9 of Angström's, and which I may be here permitted to name briefly “coronium.” We may therefore possibly assume that the primary constituent, a, of hydrogen is identical with this coronium, which is in any case a distinct substance from helium, since the corona line 5315·9 often remains unchanged, whilst at the same time the helium line, D3, is either expanded or distorted and displaced, displaying Lockyer's forms of motion.

A. Gruenwald.
The spectrum of watery vapour and the line spectra of hydrogen and oxygen, and on the chemical structure of the latter two bodies and their dissociation in the atmosphere of the sun.
Chemical News, vol 56, no. 1462, page 232 (2 Dec 1887).


It has long been known that the distribution of coronium throughout the corona was not concurrent with the streamers themselves, the '1474 K' line often being as strong in rifts as elsewhere.

W. Shackleton.
The Use of a Coloured Screen in Photographing the Corona during an Eclipse.
Monthly Notices of the Royal Astronomical Society, vol 60, page 433 (1900)


Later, in the Bakerian Lecture for 1901, I was led to infer that another member of the helium group might exist having the atomic weight about 2, and this would give us a gas still more volatile, with which the absolute zero might be still more nearly approached. It is to be hoped that some such element or elements may yet be isolated and identified as coronium or nebulium.

James Dewar.
Liquefaction of gases and low temperatures.
Popular Astronomy, vol 11, page 16 (1903).


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.


There is a famous bright coronal line in the green at wave length 5303. This line was discovered by Young, in 1870, and it has been seen with more or less distinctness at many subsequent eclipses. It does not correspond in wave length to a line of any known substance, or to a photospheric line, so that it is ascribed to a hypothetical element "coronium." As the element helium was found in the earth after its spectrum had long been known in the sun and stars, so it may happen with “coronium.”

Charles G. Abbot.
The Sun.
New York: D. Appleton and Company, 1911.
Page 134.


The first of these primary substances is an atom containing a single ring of two electrons rotating round a positive nucleus. When this paper was communicated to the British Association, it was suggested that this element would be coronium, the unknown substance (or one of them) present in the solar corona. Subsequent work on its spectrum, which at that time was incomplete, has justified this supposition.

Helium can therefore be expressed as a simple type of compound atom. With the symbols Cn for coronium, Nu for nebulium, and Pf for protofluorine, we may write

He = Nu + Pf,

where the equation is not to be interpreted in the chemical sense.

J. W. Nicholson.
Philosophical Magazine, series 6, vol. 22, no. 132, page 867 (Dec. 1911).


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).


Should coronium, nebulium, &c. exist, then from this we must conclude that they must be isotopic with hydrogen or helium, but not that the periodic system must be changed, or that atomic numbers are incorrect.

A. van den Broek.
Ordinals or atomic numbers?
Philosophical Magazine, vol 28, page 232 (1914)


Many scientists have despaired at the solution of the problem presented by coronium. It is very true that the past 68 years have yielded little positive information but we must remember that the actual time for eclipse observation which has been placed at the disposal of the astronomer during this period is only a little greater than one hour. The work of the experimental physicist has been that of attempting to match lines. This is certainly not a very direct and forward attack on the problem, especially under conditions which do not begin to approximate those under which we find the lines in question originating. Perhaps due to the rapid changes in our ideas regarding the structure of the atoms and lack of accurate data, the theoretical physicist has only occasionally given attention to the problem. No one knows how long we will have to wait for the key to the riddle of coronium. Its solution may come from a new interpretation of work already done or from a source entirely unsuspected at the present time.

George C. Claridge.
Journal of the Royal Astronomical Society of Canada, vol. 31, no. 8, page 346 (October 1937).
NASA/ADS Bibcode: 1937JRASC..31..337C

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 Edlén.
Kungl. Fysiografiska sällskapets handlingar, n. f., bd. 65, nr. 21.
Lund: C.W.K.Gleerup, [1955].

Pol Swings.
Edlén’s Identification of the Coronal Lines with Forbidden Lines of Fe X, XI, XIII, XIV, XV; Ni XII, XIII, XV, XVI; Ca XII, XIII, XV; A X, XIV.
Astrophysical Journal, vol 98, pages 116–128 (1943).

Myles W. Jackson.
Spectrum of Belief: Joseph Von Fraunhofer and the Craft of Precision Optics.
MIT Press. (2000)

Info Trek

1. What is a “forbidden line” and what does that have to do with coronium?

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