  We provide a detailed analysis of quantum field theory around a collapsing
shell and discuss several conceptual issues related to the emission of
radiation flux and formation of black holes. Explicit calculations are
performed using a model for a collapsing shell which turns out to be
analytically solvable. We use the insights gained in this model to draw
reliable conclusions regarding more realistic models. We first show that any
shell of mass $M$ which collapses to a radius close to $r=2M$ will emit
approximately thermal radiation for a period of time. In particular, a shell
which collapses from some initial radius to a final radius
$2M(1-\epsilon^2)^{-1}$ (where $\epsilon \ll 1$) without forming a black hole,
will emit thermal radiation during the period $M\lesssim t \lesssim M\ln
(1/\epsilon^2)$. Later on ($t\gg M \ln(1/\epsilon^2)$), the flux from such a
shell will decay to zero exponentially. We next study the effect of
backreaction computed using the vacuum expectation value of the stress tensor
on the collapse. We find that, in any realistic collapse scenario, the
backreaction effects do \emph{not} prevent the formation of the event horizon.
The time at which the event horizon is formed is, of course, delayed due to the
radiated flux -- which decreases the mass of the shell -- but this effect is
not sufficient to prevent horizon formation. We also clarify several conceptual
issues and provide pedagogical details of the calculations in the Appendices to
the paper.
