Coronium And Nebulium

This is not the end of the story of the coronal spectrum. As seen in Figure 5-7, there are myriad spectral lines, each of which may have its wavelength measured, and its elemental parentage perhaps allotted. Inspired by the discovery of Janssen and Lockyer in the previous year, astronomers flocked to observe the 1869 total eclipse in North America using their spectrometers, and the new methods of photography to record the spectra for later analysis.

When the dust settled and all the "easy" spectral lines had been accounted for, still there were many that could not be ascribed to any known element. A novel species was invented to explain these, and it was called "coronium" because it was found only in the solar corona. Astronomers also turned their telescopes towards the distant nebulas of the cosmos, and found evidence, they thought, for yet another element. This was christened "nebulium."

Coronium and nebulium were both, in the event, figments of the astronomers' imaginations. The lines they detected were real, but their interpretation was wrong. It is possible to get known elements to produce those spectral lines if their atoms are subjected to extreme physical conditions, such as the huge temperatures of the solar corona. Physicists could not produce a temperature of a million degrees in their laboratories, and so these lines had not been seen previously.

When one supplies an atom with some energy, by heating it or by illuminating it with light of a wavelength below some threshold, an electron (a negatively charged particle) can be ejected, leaving an ion: a positively charged atom. It is possible to strip off another electron, making the ion doubly charged, and maybe an other, but it gets progressively more difficult to remove extra electrons. That limits what can be done on Earth (at least in a controlled way: a nuclear explosion is another matter).

In the solar corona, however, the phenomenal temperatures mean that the ions are multiply charged. As each successive electron is removed, the resulting ion produces a new, distinctive set of spectral lines. For example, greatly ionized iron atoms may have lost ten electrons and emit a series of wavelengths that one could not hope to duplicate in a laboratory. No wonder the astronomers were confused.

How then can we identify the atom responsible? The answer comes from theoretical calculations, although again there is a twist to the tale. There are simple selection rules that usually work in spectroscopy, corresponding to the known properties of atoms. According to these rules, many of the lines detected appeared to correspond to "prohibited" transitions. Such "forbidden lines" were not fully understood until after the developments in quantum theory that took place in the 1930s. From the correct identification of the "coronium lines" it was eventually inferred that the corona is exceedingly hot: over a million degrees, as we saw earlier.

Before leaving coronal spectroscopy, consider an interesting coincidence. In Figure 5-7 the numbers give the wavelengths of various lines in angstroms (one angstrom, which is given the symbol A, equals one ten-billionth, or 10-10, of a meter). The spectrum we see with our eyes extends from about 4,000 A (the violet/blue end) through to 7,000 A (the red end). (Some physicists like to use angstroms for wavelengths, while others use the strict metric system, so you will also find wavelengths given in nanometers. One nanometer [1 nm] is a billionth, or 10-9, of a meter, and so equals ten angstroms.) The angstrom unit gets its name from Anders Angstrom (pronounced ong-struh-m), a Swedish astronomer who lived from 1814 to 1874. The coincidence is that it was he who first identified the hydrogen lines in the solar spectrum, showing us why the chromosphere is red.

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