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).
See also: → second; → collapse.
