This element was discovered in 1828 by J. Berzelius in Stockholm. The name comes from Thor, the Scandinavian god of war.

Ionization energies
ThI 6.1 eV, ThII 11.5 eV, ThIII 20.5 eV.

Absorption lines of ThII
The equivalent width of ThII 4019(3) in the sun is 0.009Å. ThII (4019) was observed in several G-type dwarfs and giants (W appeq 0.010) by Butcher (1988).

Behavior in non-normal stars
ThII was identified by Jaschek and Brandi (1972) in one Ap star of the Cr-Eu-Sr subgroup and by Cowley et al. (1976) in other objects. Cowley et al. (1977) published a detailed study of Th and U and its isotopes in Ap stars.

In the solar system Th is more abundant than U, but in stars one generally finds the opposite.

Th occurs in the form of 12 isotopes and isomers. The longest lived isotope is Th232 with a half life of 1.4 × 1010 years. All Th in the solar system occurs in this form. Of the other isotopes, Th230 has a half life of 8 × 104 years and the remainder are shorter lived.

Th232 can be used as a radio-chronometer, if one compares its abundance with that of a stable r-process element like Eu (Francois 1992).

Th can only be produced by the actinide-producing r process.

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