CESIUM Cs      Z = 55

The element was discovered by R. Bunsen and G. R. Kirchhoff in 1806 in Heidelberg. Its name comes from the Latin cacsius (sky blue).

Ionization energies

Cs 13.9 eV, CsII 25.1 eV.

Behavior in stars

Cs has been observed in the spectrum of sunspots. The resonance line is 8521. CsII was identified as marginally present by Bidelman in one Ap star of the Sr-Cr-Eu subgroup according to Ch. Cowley (private communication).


Cs has one stable isotope, Cs132, and 20 short-lived isotopes and isomers. The longest lived one is Cs135 with a half life of 3 × 106 years.


Cs132 is produced by both the r and the s process.

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