This element was discovered by M. H. Klaproth in Berlin in 1789. Its name alludes to the planet Uranus, discovered by Herschel in 1781, which in turn alludes to the Greek god Urania.

Ionization energies
UI 6.1 eV, UII 14.7 eV.

U is the heaviest stable element. It has not been observed in the sun.

Behavior in non-normal stars
UII was discovered independently by Guthrie (1969) and by Jaschek and Malaroda (1970) through the presence of 3859 in Ap stars of the Sr-Cr-Eu subgroup. The two groups each found this line in a different Ap star. It was also observed by Cowley et al. (1974) in other stars of this subgroup. Typically W(3859) = 0.030.

A detailed study of U and its isotopes was made by Cowley et al. (1977).

The longest lived isotope of U is U238, with a half life of 4.5 × 109 years. It is followed by U235 with a half life of 7.0 × 108 years and by 13 isotopes and isomers with shorter half lives. In the solar system U238 makes up 99.3% of all U. The isotopes of U can be used for radioactive dating.

The U isotopes are 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.