ARGON Ar      Z = 18

This element was discovered in 1894 by D. Rayleigh and W. Ramsay in Great Britain. The name comes from the Greek argos (inactive). The name of the element has also been abbreviated as A instead of Ar (for instance by Moore (1945)).

Ionization energies

ArI 15.8 eV, ArII 27.6 eV, ArIII 40.7 eV, ArIV 59.8 eV.

Absorption lines of ArII

Table 1: Equivalent widths of ArII

  4371(1) 4431(1)   4590(21)  

Group V V Ia IV III

B 0.5   0.004      
B 1       0.0025  
B 2       0.0065  
B 3 0.006 0.008 0.052    
B 6         0.004

Source. Data are from Keenan et al. (1990).

ArII (for instance the line at 4590) is represented by weak lines in early type B-stars. It has a maximum around B2. The line at 4431 (see table) shows a positive luminosity effect.

Emission lines of ArII

The lines at 920 and 932 (UV M.1) are seen in emission in the ultraviolet solar spectrum (Feldman and Doschek 1991). Supernova 1987A also showed ArII lines in emission (Arnett et al. 1989).

Emission lines of ArIII

The 7135 line of [ARIII] (M.1F) has been observed in at least one nova (Thackeray 1953).

Figure 2

Behavior in non-normal stars

ArII lines are strong in the spectrum of the extreme He star upsilon Sgr (Morgan 1935).

Forbidden lines of ArIII, ArIV and ArV are often visible in symbiotic objects (see for instance Freitas Pacheco and Costa (1992)). In at least one recurrent nova Joy and Springs (1945) found forbidden lines of ArV, ArX and ArX1.


There exist eight isotopes. The three stable isotopes are Ar 36, 38 and 40. In the solar system 99.6% of Ar is in the form of Ar4O.


Ar36 produced by explosive nucleosynthesis, Ar38 by the same process or by oxygen burning and Ar40 by the s process or neon burning.

Published in "The Behavior of Chemical Elements in Stars", Carlos Jaschek and Mercedes Jaschek, 1995, Cambridge University Press.