What causes clouds of dust and gas to form a protostar?

Mainly the gravitational instability in the clouds of dust (molecular clouds)

Explanation: Stars are born within molecular clouds, which are mostly made of hydrogen and lesser amounts of helium and other heavier elements. The formation of a protostar begins when gravitational instability is present in the molecular cloud, caused by regions of higher density. Eventually, the clouds of dust start to collapse to due gravity, the protostar forms. The protostar, is still an underdeveloped star, meaning it does not perform nuclear fusion in its core, but it still produces its own heat from the extreme pressure.

a- all cells come from pre-existing cells

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