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Analysis and Interpretation of Astronomical Spectra 45

11 Ionisation Stage and Degree of Ionisation

11.1 The Lyman Limit of Hydrogen

In the second diagram of sect. 10.3 the energy for the lowest excitation level of the Lyman
. Converted with formula {10}, this gives the well-
Series results to

known Lyman limit or Lyman edge in the UV range with wavelength λ = 912 Å. It is func-

tionally equivalent to the “Balmer edge” of the Balmer Series (sect. 10.4). This value is very

important for astrophysics, because it defines the minimum required energy to ionise the H-

atom from its ground state . This level is only achievable by very hot stars of the O-

and early B-Class. [3]. The very high UV radiation of such stars ionises first the hydrogen
clouds which are shining due to emitted photons by the subsequent recombination (H II re-
gions, eg M42, Orion Nebula, sect. 22).

11.2 Ionisation Stage versus Degree of Ionisation

The term “Ionisation stage” refers here to the number of electrons, which an ionised atom
has lost to the space (Si IV, Fe II, H II, etc.). This must not be confused with the term “De-
gree of ionisation” in plasma physics. It defines for a gas mixture the ratio of atoms (of a
certain element) that are ionised into charged particles, regarding the temperature, density
and the required ionisation energy of the according element. This “Degree” is determined in
astrophysics with the famous Saha equation.

11.3 Astrophysical Form of Notation for the Ionisation Stage

Unfortunately, in astrophysics the chemical form of notation is not in use but instead of it
another somewhat misleading version. The neutral hydrogen is denoted by chemists with
H, and the ionised with H+, which is clear and unambiguous. On the other hand astrophysi-
cists, denote already the neutral hydrogen with an additional Roman numeral as H I and the
ionised with H II. The doubly ionised calcium is referred by chemists with Ca++, for astro-
physicists this corresponds to Ca III. Si IV is for example triply ionised silicon Si+++. This sys-
tem therefore works according to the “(n–1) principle”, ie in astrophysics the ionisation
stage of an atom is always by 1 lower than the Roman numeral. A high ionisation stage of
atoms always means that very high temperatures must be involved in the process.