Original sentence: A core region forms with gas falling toward the center.The thermal energy dissociates the H2 molecules from the cloud, then ionizes the hydrogen and helium atoms
Edited sentence: A core region forms where the collapse is essentially halted.After the density of infalling material has dropped below about 10-8 g cm-3, the material becomes sufficiently transparent to allow radiated energy to escape.The combination of convection within the protostar and radiation from the exterior allow the star to contract in radius.The gas falling toward this opaque region creates shock waves that further heat the core.This is followed by the ionization the hydrogen and helium atoms.This occurs when the density of about.When the core temperature reaches about , the thermal energy dissociates the H2 molecules