  In the outskirts of nearby spiral galaxies, star formation is observed in
extremely low gas surface densities. Star formation in these regions, where the
interstellar medium is dominated by diffuse atomic hydrogen, is difficult to
explain with classic star formation theories. In this letter, we introduce
runaway stars as an explanation for this observation. Runaway stars, produced
by collisional dynamics in young stellar clusters, can travel kiloparsecs
during their main-sequence lifetime. Using galactic-scale hydrodynamic
simulations including a treatment of individual stars, we demonstrate that this
mechanism enables the ejection of young massive stars into environments where
the gas is not dense enough to trigger star formation. This results in the
appearance of star formation in regions where it ought to be impossible. We
conclude that runaway stars are a contributing, if not dominant, factor to the
observations of star formation in the outskirts of spiral galaxies.
