What external event is mentioned as capable of triggering the gravitational collapse of a molecular cloud segment?

Answer

A passing shockwave from a nearby supernova explosion

Star formation is often initiated externally, such as by a shockwave from a supernova explosion or the collision between two molecular clouds, which compresses the gas sufficiently to start the collapse.

What external event is mentioned as capable of triggering the gravitational collapse of a molecular cloud segment?
evolutionenergystarprotostarcloud