This element was discovered by A.von Welsbach in Vienna in 1885. The name comes from the Greek prasios didymos (the green twin).
Ionization energies
PrI 5.4 eV, PrII 10.6 eV, PrIII 21.6 eV, PrIV 39.0 eV.
Absorption lines of PrI
No PrI lines are visible in the sun.
Absorption lines of PrII
| 5219(35) | 5259(35) | ||||
| Group | V | Ib | V | III | Ib |
| F5 | 0.012 | ||||
| G0 | 0.058 | ||||
| G1 | 0.031 | ||||
| S | 0.002 | 0.003 | |||
| G2 | 0.035 | 0.058 | |||
| G5 | 0.031 | 0.069 | |||
| G8 | 0.035 | ||||
| K0 | 0.062 | ||||
| K2 | 0.062 | 0.105 | |||
| K3 | 0.123 | ||||
| K5 | 0.059 | 0.105 | |||
PrII (for example 5259) is present in F-type supergiants and grows toward later types. A positive luminosity effect is present.
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Behavior in non-normal stars
PrII lines are strong in the spectra of Ap stars of the Cr-Eu-Sr
subgroup (Adelman 1973b). A typical value of W(4222) is 0.035
(Sadakane 1976). In normal stars of the same temperature, the 4222(4)
line is invisible (Smith 1972, 1973). The presence of PrIII was signaled
by Bidelman (1966) in
one Ap star. Later Aikman et al. (1979) detected PrIII in some of the Ap
stars with strong PrII lines. A detailed study by Mathys and Cowley
(1992) in
the red spectral region showed that Pr III is very strong in many Ap stars,
without a clear relation to the peculiarity subgroups
(W(6160) = 0.1 Å).
PrII is also strengthened in Am stars. For instance,
W(4222) 
0.050
for late Am stars.
In Ba stars PrI and PrII lines are enhanced (as are those of other rare earth elements), which leads to large overabundances (Lambert 1985). Typical values of W are twice as large as in normal giants of the same temperature (Danziger 1965). Because of the similar pattern of rare earths in Ba and in Ap stars, it is worth recalling the observation of Cowley and Dempsey (1984) that, whereas Pr is generally strong (or very strong) in Ba stars, there do exist some Ap stars in which Pr is weak.
Pr II lines are fairly strong in the spectrum of one S-type star (Bidelman 1953).
Pr II lines have been observed in metal-weak G- and K-type dwarfs by Gilroy et al. (1988). These authors find Pr to be overabundant with respect to iron (see also the discussion on rare earths).
Isotopes
Pr has one stable isotope, Prl41, and 14 short-lived ones.
Origin
Pr can be produced by both the r process and the s process.
Published in "The Behavior of Chemical Elements in Stars", Carlos Jaschek and Mercedes Jaschek, 1995, Cambridge University Press.