This element was discovered by C. Mossander in Stockholm in 1842. The name comes from the city of Ytterby in Sweden. This city is also referred to in the names of the elements yttrium and terbium.

lonization energies

ErI 6.1 eV, ErII 11.9 eV, ErIII 22.7 eV, ErIV 42.6 eV.

Absorption lines of ErII and ErIII

ErII 3616 is seen in one FOIb star (W=0.015) according to Reynolds et al (1988).

Behavior in non-normal stars

ErII lines are strengthened in the spectra of some Ap stars of the Cr-Eu-Sr subgroup. Aikman et al. (1979) also observed ErIII in the spectra of stars with strong ErII lines, and this was confirmed by Cowley and Greenberg (1987). The ErIII line at 4000 has W= 0.040. ErII and ErIII lines were also detected in the spectrum of one Bp star of the Si subgroup (Cowley and Crosswhite 1978).

ErII lines are also seen in at least one Am star (van t'Veer-Menneret etal 1988) with W[4009)=0052.

ErII lines are enhanced in at least one Ba star (Lambert 1985) and in at least one S-type star (Bidelman 1953).

Isotopes

Er has six stable isotopes and ten unstable isotopes and isomers. The stable ones are Er 162, 164, 166, 167, 168 and 170. In the solar system Er166 represents 33% and Er 167, 168 and 170 respectively 23%, 27% and 14%.

Origin

Er is made by several processes, Er162 by the p process, Er167 and Er170 by the r process and the others can be made by two processes, namely Er164 by the p or the s and Er166 and Er168 by either the r or the s process.