This element was discovered by W. Ramsay and M. W. Travers in 1898 in London. Its name comes from the Greek kryptos (hidden).

Ionization energies
KrI 14.0 eV, KrII 24.4 eV, KrIII 36.9 eV.

Absorption lines of KrI
No Kr line is seen in the solar spectrum.

Absorption lines of KrII
Following Bidelman's (1960) discovery of KrII in 3 Cen A, this element was observed in some other hot Bp stars. W(4355) = 0.025 according to Hardorp (1966).

Emission lines of KrIII
Lines of [KrIII] are seen in the spectrum of at least one recurrent nova Joy and Swings 1945).

Kr occurs in the form of six stable isotopes and 17 short-lived isotopes and isomers. The stable isotopes are Kr 78, 80, 82, 83, 84 and 86. In the solar system they represent 0.3%, 2%, 12%, 11%, 57% and 17% abundances respectively. Among the unstable isotopes, Kr81 has a half life of 2 × 105 years.

Kr 83, 84 and 86 can be produced by both the r and the s process, Kr82 is a pure s process product, Kr78 a pure p product and Kr80 can be produced by either the s or the p process.

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