  The far-infrared [C II] 158 micrometer fine structure transition is
considered to be a dominant coolant in the interstellar medium. For this
reason, under the assumption of a thermal steady state, it may be used to infer
the heating rate and, in turn, the star formation rate in local, as well as in
high redshift systems. In this work, radio and ultraviolet observations of the
Galactic interstellar medium are used to understand whether C II is indeed a
good tracer of the star formation rate. For a sample of high Galactic latitude
sightlines, direct measurements of the temperature indicate the presence of C
II in both the cold and the warm phases of the diffuse interstellar gas. The
cold gas fraction (~ 10 - 50% of the total neutral gas column density) is not
negligible even at high Galactic latitude. It is shown that, to correctly
estimate the star formation rate, C II cooling in both the phases should hence
be considered. The simple assumption, that the [C II] line originates only from
either the cold or the warm phase, significantly underpredicts or overpredicts
the star formation rate, respectively. These results are particularly important
in the context of the Damped Lyman-alpha systems for which a similar method is
often used to estimate the star formation rate. The derived star formation
rates in such cases may not be reliable if the temperature of the gas under
consideration is not constrained independently.
