This element was discovered by J. A. Marinsky, L. E. Glendenin and C. D. Corycll at Oak Ridge, USA. Its name comes from the hero Prometheus, who stole fire from the Gods in the Greek legend.

Ionization energies
PmI 5.5 eV, PmII 10.9 eV.

Pm in stars
No Pm lines are observed in the sun. Pm was discovered in the Ap star HR 465 by Aller and Cowley (1970). The presence of this element was not confirmed by Wolff and Morrison (1972), but Poli etal. (1987) found it to be present in another Ap star.

Aslanov et al. (1975) showed that the element is present in at least one Ap star, but its intensity varies over the spectroscopic variability cycle of the star. The strongest line of PmII, that at 3877, has W leq 0.016 Å.

Pm has 14 unstable isotopes and isomers. The longest lived, Pml45, has a half life of about 18 years, implying essentially that, if it is found in cosmic sources, then it has been made in situ.

The element is produced by the r process.

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