How stars form within a nebula?

Interstellar gas is inelastic. To compress it some event typically must occur. A supernova, for example, produces a shockwave that collapses interstellar gas, precipitating nearby star formation. Another force is gravity--we have seen stars form in the swirling wake of passing black holes.

When two nebula collide, we might infer star formation across the "impact zone," much as water droplets precipitate when a moist warm air mass collides with a cooler, denser mass of air.