This element was discovered by R. Bunsen and G. Kirchhoff in Heidelberg, Germany in 1861. The name comes from the Latin rubidium (deep red).

Ionization energies
Rb I 4.3 eV, Rb II 27.3 eV.

Absorption lines of RbI

Table 1. Equivalent widths of RbI 7800(1)

Group V III

G2 0.005  
S 0.005  
K2   0.039

This element is prominent in sunspot spectra.

The resonance lines of Rb I are 7800 and 7948 from M.1. These lines become intense in M 9 and M 10 dwarfs (Solf 1978).

Emission lines of RbI
RbI 7800 was detected in emission in one S-type star (Wallerstein 1992).

Behavior in non-normal stars
Rb is overabundant with respect to iron in Ba stars (Lambert 1985).*

Rb has two stable isotopes, Rb85 and Rb87, which occur in the solar system with frequencies 72% and 28% respectively. There exist also 18 short-lived isotopes and isomers.

Lambert and Luck (1976) analyzed the resonance lines of RbI 7800(1) and 7947 (1) in one K-type giant and found the isotope ratio to have the solar value. The same result was found by Malaney and Lambert (1988) from an analysis of the 7800 line in two Ba stars. From a preliminary analysis, Lambert (1991) suggested that the same is true for a sample of M-, MS- and S-type stars.

Rb87, with a half life of 4.9 × 109 years, can be used for radioactive dating.

Rb87 is a pure s process product, whereas Rb85 is produced by both the r process and the s process.

* Note added in proof. According to Gratton and Sneden (1994) AA 287, 927, Rb is proportional to Fe in metal-weak stars.

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