  A star collapsing gravitationally into a black hole emits a flux of
radiation, known as Hawking radiation. When the initial state of a quantum
field on the background of the star, is placed in the Unruh vacuum in the far
past, then in the exterior Hawking radiation corresponds to a flux of positive
energy radiation travelling outwards from near the surface to future infinity.
Based on pair creation, the evaporation of the collapsing star can be
equivalently described by the absorption of an ingoing negative energy flux of
radiation travelling towards the center of the star. Here, we are interested in
the evolution of the star during its collapse. Thus we include the backreaction
of the negative energy Hawking flux in the interior geometry of the collapsing
star when writing the full 4-dimensional Einstein and hydrodynamic equations.
Hawking radiation emitted before the star passes through its Schwarzschild
radius slows down and reverses the collapse of the star. The star evaporates
without forming an horizon or a singularity. This study provides a more
realistic investigation than the one first presented in [1], since the
backreaction of Hawking radiation flux on the collapsing star is studied in the
case when the initial state of the field is in Unruh's vacuum.
