What external event can serve as a trigger for the initial gravitational collapse leading to star formation?

Answer

The shockwave generated by an exploding supernova

Star formation in cold molecular clouds can be initiated by external disturbances that overcome the internal pressure and magnetic fields. These triggers include shockwaves from ancient, exploding stars (supernovae) or the collision of two interstellar clouds.

What external event can serve as a trigger for the initial gravitational collapse leading to star formation?
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