Fr.: deuxième effondrement
An early evolutionary period in the process of star formation which succeeds the → first collapse. When the mass of the → first core has increased by about a factor 2 and the radius has decreased by a similar factor, the central temperature of the core reaches about 2000 K. At this point the → molecular hydrogen begins to dissociate into atoms. This reduces the → adiabatic index (γ) below the critical value 4/3, with the result that the material at the center of the core becomes unstable and begins to collapse. Most of the gravitational energy generated by this collapse goes into the → dissociation of H2 molecules, so that the temperature rises only slowly with increasing density. In this second collapse phase, as in the first, the density distribution in the collapsing region becomes more and more sharply peaked at center, and the time scale becomes shorter and shorter with increasing central density. The central collapse of the core continues until the hydrogen molecules are nearly all dissociated and γ again rises above 4/3. The central pressure then rises rapidly and once again becomes sufficient to decelerate and stop the collapse at the center. A small core in the → hydrostatic equilibrium then arises, bounded by a shock front in which the surrounding infalling material is suddenly stopped. The initial mass and radius of the second core are about 3 x 1030 g (1.5 x 10-3Msun) and 9 x 1010 cm (1.3 Rsun) respectively, and the central density and temperature are about 2 x 10-2 g cm-3 and 2 x 104 K, respectively. The second core will evolve into a → young stellar object (R. B. Larson, 1969, MNRAS 145, 271).