We consider the formation of OB associations from two perspectives : ( a ) the fractional gas consumption in star formation , \epsilon , per dynamical time scale t _ { dyn } in a galaxy , and ( b ) the origin of the so-called Kennicutt-Schmidt law that the rate of star formation per unit area is proportional to a power , \alpha , of the surface density in H I and H _ { 2 } gas when certain thresholds are crossed .
The empirical findings that \epsilon \approx 10 ^ { -2 } and \alpha \approx 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 .