What causes the densest parts of the nebula to collapse?

The densest parts of a nebula collapse primarily due to gravitational forces. As regions within the nebula become denser, their gravitational pull increases, attracting surrounding gas and dust. When the pressure and density reach a critical threshold, the intense gravitational forces overpower the internal thermal pressure, leading to the collapse of these regions. This process can initiate star formation as the collapsing material forms a protostar.