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Analysis and Interpretation of Astronomical Spectra 41

9.3 Spectral Lines of Other Atoms

For all other atoms, i.e. with more than one electron, the description of line formation can
be very complex. All atoms have different energy levels and are therefore distinguished by
different wavelengths of the spectral lines. Another factor is the number of valence elec-
trons on the outer shell, or how many of the inner levels are already fully occupied. Further
the main levels are subdivided in a large number of so-called Sublevels and Sub-Sublevels,
each of them with completely other implications in respect of quantum mechanics. The en-
ergy differences between such sublevels must logically be very low. This explains why
metals often appear in dense groups, a few with distances even <1Å! Typical examples are
the sodium lines at 5896 Å and 5890 Å, and the famous Magnesium Triplet at 5184, 5173,
5169Å in the solar spectrum. By the hydrogen atoms these sublevels play no practical role,
particularly for amateurs, because they are degenerated here [5]. The stay of the electrons
within this complex level system is also subject to a set of rules. The best known is proba-
bly the so called Pauli Exclusion Principle which demands that the various sublevels may be
occupied by only one electron at the same time.

For most spectroscopic amateur activities, detailed knowledge of this complex matter is not
really necessary. For those, deeper interested in quantum mechanics, [5] is recommended
as a “first reading”, which provides a good overview on this matter and is relatively easy
and understandable to read.