An early phase in the process of star formation which begins when the mass of
a → molecular cloud → clump
exceeds the → Jeans mass.
The collapse is initially → optically thin to the
thermal emission from → dust grains, and the
compressional heating rate is much smaller than the cooling rate by the
→ thermal radiation. The collapse proceeds
→ isothermally.
The isothermal condition is broken when the central density reaches about
10-13 g cm-3 and a small region at the center of
the cloud starts to become → opaque.
The heat generated by the collapse in this
region is no longer freely radiated away, and the compression becomes approximately
→ adiabatic.
The central temperature and pressure then begin to rise rapidly, soon
becoming sufficient to decelerate and stop the collapse at the center.
There then arises a small central core, called the → first core,
in which the material
has stopped collapsing and is approaching → hydrostatic equilibrium.
Outside this core, the material is still nearly isothermal and continues to fall
inward almost in → free fall.
Consequently a shock front arises at the boundary
of the core, where the infalling material is suddenly stopped. The initial mass and radius
of the core are about 1031 g and 6 x 1013 cm, respectively,
and the central density and temperature are about 2 x 10-10
g cm-3 and 170 K, respectively. As the collapse proceeds, the core grows
in mass due to the infall of the surrounding material; at the same time, however, the core
radius decreases because of radiative energy losses from the outer layers of the core.
The process leads to the → second collapse
(R. B. Larson, 1969, MNRAS 145, 271).
See also: → first; → collapse.
