This element was discovered in 1735 by G. Brandt in Stockholm. The name comes from the German Kobold (goblin).
Ionization energies
CoI 7.9 eV, CoII 17.1 eV, CoIII 33.5 eV.
Absorption lines of CoI
| 4121(28) | 5352(172) | ||||
| Group | V | Ib | V | III | Ib |
| A2 | 0.003 | ||||
| F0 | 0.050(II) | ||||
| F4 | 0.012 | ||||
| F5 | 0.135 | 0.120 | 0.007 | ||
| F6 | 0.021 | ||||
| F8 | 0.160 | 0.012 | |||
| G0 | 0.120 | ||||
| G1 | 0.024,0.022 | 0.057 | |||
| S | 0.125 | 0.021 | |||
| G2 | 0.130 | 0.070 | |||
| G5 | 0.160 | 0.016 | 0.074 | ||
| G6 | 0.123 | ||||
| G8 | 0.037(IV) | 0.044 | 0.139 | ||
| KO | 0.02 | 0.065 | |||
| K2 | 0.02 | 0.066 | 0.152 | ||
| K3 | 0.114 | 0.135 | |||
| K5 | 0.240 | 0.162 | |||
| M0 | 0.083 | ||||
| M2 | 0.153 | ||||
Co is represented by numerous faint lines in late type spectra. As far as the number of lines are concerned, CoI ranks after FeI, CrI and TiI.
CoI (see the lines at 4121 and 5352) appears in A-type stars and increases toward later types. A positive luminosity effect is present. In the infrared one important CoI line is that at 8093(189). In the sun, W(8093) = 0.026.
Emission lines of CoI
In long-period variables of type S, some CoI lines (those at 3894,3997,
4118 and 4121) appear in emission around maximum light (Merrill 1952).
Absorption lines of CoII
The equivalent width of CoII 3388(2) in the sun is 0.031.
CoII has been identified in the ultraviolet spectrum of an A 0V
star (Rogerson 1989).
Emission lines of CoII
CoII 2202(M.1) is often seen in emission in T Tau stars
(Appenzeller et al. 1980).
The supernova 1987A presents some strong CoII emissions and it
is possible that these are due to radioactive Co56 decaying from
Ni56 (Arnett et al. 1989).
Varani et al. (1990)
identified
an emission at 1.547µm with [CoII] and have proved that it
is due to Co56.
Behavior in non-normal stars
CoI is enhanced in some Ap stars of the Cr-Eu-Sr subgroup
(Adelman 1973b). Typically
W(3873) 
0.070. CoI is also enhanced in many stars of the Hg-Mn subgroup
(Takada-Hidai 1991).
Cowley (1979) called attention to a few Ap stars in which the CoII lines are very strong. Sadakane (1992) studied one of these stars in detail and found very strong CoII lines, as well as other anomalies. The Co overabundance is of the order of 3.6 dex.
Co is about normal in Am stars (Smith 1973, 1974).
Co behaves in a manner parallel to that of Fe in globular cluster stars (Wheeler et al. 1989). For field stars the evidence is somewhat contradictory, but on the whole it seems that Co behaves like Fe (Luck and Bond 1985, Gilroy et al. 1988).
Strong absorption lines of CoII appear in the spectra of supernovae of types Ia and II. [CoII] and [CoIII] appear in the nebular stages of supernovae of types II and Ia respectively (Branch 1990).
Isotopes
Co has 14 isotopes and isomers - 13 short-lived ones and one stable one,
Co59. In the solar system all the Co is in this form.
Origin
Co59 is produced either by the nuclear statistical
equilibrium process or by carbon burning.
Published in "The Behavior of Chemical Elements in Stars", Carlos Jaschek and Mercedes Jaschek, 1995, Cambridge University Press.