After the collapse of a nebular cloud, atoms begin gravitating together to form a condensed center. what happens next in the star formation process?

The condensed center will act as a central mass that will supply sufficient gravitational pull to continue to gather atoms around it. As the mass becomes larger, the atoms at the core (likely hydrogen as this is nebula) will be subject to greater and greater pressure. What happens next depends on the amount of mass that has been gathered.