What astrophysical phenomenon is cited as delivering a sudden, external push to initiate gravitational collapse in a stable cloud?
Answer
A Supernova shock wave compressing local density.
One of the most powerful mechanisms for triggering the final collapse into a protostar is the external shock wave generated by the violent explosion of a massive star, known as a supernova. When this expanding shock wave slams into a nearby molecular cloud, it instantaneously compresses the gas and dust. If this external compression elevates the local density sufficiently high, it tips the balance in favor of gravity, overcoming the cloud's initial thermal resistance and causing the runaway gravitational contraction necessary for star birth. This can lead to a chain reaction called propagating star formation.
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