FORMATION OF OB ASSOCIATIONS IN GALAXIES

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.