When the ISM clouds get dense enough or there are perturbations within the cloud (shock waves and such) regions within can begin to collapse under their own self gravitation. As material is falling in on a region the pressure in the region begins to rise which causes the temperature to rise as well. Gravitational energy is being converted into heat. A protostar forms at the center of the collapsing region. It is very luminous and is radiating by gravitational heating only. The star is working toward achieving hydrostatic equilibrium. This collapse may take only a few million years.
