2007ApJ...662L..75S

We consider the formation of OB associations from two perspectives: (1) the fractional gas consumption in star formation, ε, per dynamical timescale t_dyn in a galaxy, and (2) the origin of the so-called Kennicutt-Schmidt law, that the rate of star formation per unit area is proportional to a power, α, of the surface density in H I and H₂ gas when certain thresholds are crossed. The empirical findings that ε~10^–2 and α~1.4 or 1.5 have simple explanations if the rate of star formation is magnetically regulated. An empirical test of the ideas resides in an analysis of why giant OB associations are ``strung out like pearls along the arms'' of spiral galaxies.