This element was discovered in 1839 by C. Losander in Stockholm, Sweden. The name comes from the Greek lanthanein (to lie hidden).

Ionization energies
LaI 5.6 eV, LaII 11.1 eV, LaIII 19.2 eV.

Absorption lines of LaI

Table 1: Equivalent widths of LaI 6395(7)

Group III

K5 0.022
M0 0.028

Source: Data are from Gopka et al. (1990).

Absorption lines of LaII

Table 2: Equivalent widths of LaII

  4333(24)     6262(33)    

Group V III Ib V III Ib

F0     0.049      
F5 0.024   0.105      
F8     0.290      
G0 0.070         0.083
G1           0.109
S 0.035     0.003    
G2 0.040         0.103
G5 0.090         0.085
G6           0.168
G8 0.024 0.050   0.011 0.040 0.173
K0         0.020  
K2         0.173 0.223
K3         0.162  
K5 0.100       0.141 0.177

Table 3: Equivalent widths of LaII 6320(19)

Group III Ib

G0   0.060
G2   0.083
G5   0.072
K0 0.026-0.031  
K2 0.032 0.158
K3   0.148
K5   0.129
M2   0.126

La II lines (see for instance those at 4333 and 6262) appear in F-type stars and increase toward a maximum for K-type. A positive luminosity effect exists.

Emission lines of LaII
Some LaII lines like 6130(47) and 6627(61) have been observed in emission in one Mira-type variable (Grudzinska 1984). They are excited by fluoresence from H beta.

Behavior in non-normal stars
LaII lines are normal or strong in the spectrum of Ap stars of the Cr-Eu-Sr subgroup (Adelman 1973b). Typically W(4942) = 0.035 (Sadakane 1976). Cowley (1984) observed that LaIII is weak or absent in Ap stars. LaII is observed exceptionally in a Bp star of the Si subgroup (Cowley 1984).

LaII is strong in Am stars. Typically W(4087) = 0.150 for late Am stars (Smith 1973, 1974).

La behaves in a manner parallel to that of Fe in metal-weak G and K dwarfs (Magain 1989), though Gilroy et al. (1988) find that it is overabundant with respect to iron (see also Part Two, section 2.2).

In globular cluster stars it behaves in a manner parallel to that of Fe (Wheeler et al. 1989).

As with other rare earths, La II is enhanced in Ba stars (Lambert 1985), which leads to large overabundances. Typically W(4322) = 0.19 for a K O Ba star (Danziger 1965).

LaII lines are enhanced in S-type stars (Bidelman 1953) by factors of the order of three; and to a lesser degree in MS stars (Smith et al. 1987).

LaI lines are highly enhanced in the spectra of many SC stars (Wallerstein 1989, Kipper and Wallerstein 1990).

La lines are strong in the spectra of C stars (Keenan 1957), which was confirmed later by detailed analysis (Kilston 1975) for stars later than C 3 (Dominy 1984, Utsumi 1984).

Figure 28

La occurs in the form of two stable isotopes, La 138 and La139. In the solar system they contribute respectively O.1% and 99.9% of all La. There exist also 17 short-lived isotopes and isomers. The longest lived (La137) has a half life of 6.7 × 104 years. La138 has a half life of 1.1 × 1011 years and can be used for radioactive dating.

La138 can be produced only by the p process and La139 by either the s process or the r process.

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