  The abundant forms in which the major elements in the universe exist have
been determined from numerous astronomical observations and meteoritic
analyses. Iron (Fe) is an exception, in that only depletion of gaseous Fe has
been detected in the interstellar medium, suggesting that Fe is condensed into
a solid, possibly the astronomically invisible metal. To determine the primary
form of Fe, we replicated the formation of Fe grains in gaseous ejecta of
evolved stars by means of microgravity experiments. We found that the sticking
probability for formation of Fe grains is extremely small; only several atoms
will stick per hundred thousand collisions, so that homogeneous nucleation of
metallic Fe grains is highly ineffective, even in the Fe-rich ejecta of Type Ia
supernovae. This implies that most Fe is locked up as grains of Fe compounds or
as impurities accreted onto other grains in the interstellar medium.
