What commonly triggers the gravitational collapse within a diffuse molecular cloud?
Answer
Shockwave from a supernova explosion
While molecular clouds possess immense mass, the material is spread so thinly that gravity often struggles against the internal pressure resisting contraction. Therefore, the collapse leading to star formation rarely begins spontaneously; it usually requires an external catalyst or trigger to sufficiently compress a region, raising its density past a critical threshold. The most cited examples of such triggers include the powerful shockwaves generated by the violent death of a massive star in a supernova explosion, or the gravitational instability induced when two large molecular clouds pass near or collide with one another.
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