  There are models of gravitational collapse in classical general relativity
which admit the formation of naked singularities as well as black holes. These
include fluid models as well as models with scalar fields as matter. Even if
fluid models were to be regarded as unphysical in their matter content, the
remaining class of models (based on scalar fields) generically admit the
formation of visible regions of finite but arbitrarily high curvature. Hence it
is of interest to ask, from the point of view of astrophysics, as to what a
stellar collapse leading to a naked singularity (or to a visible region of very
high curvature) will look like, to a far away observer. The emission of energy
during such a process may be divided into three phases - (i) the classical
phase, during which matter and gravity can both be treated according to the
laws of classical physics, (ii) the semiclassical phase, when gravity is
treated classically but matter behaves as a quantum field, and (iii) the
quantum gravitational phase. In this review, we first give a summary of the
status of naked singularities in classical relativity, and then report some
recent results comparing the semiclassical phase of black holes with the
semiclassical phase of spherical collapse leading to a naked singularity. In
particular, we ask how the quantum particle creation during the collapse
leading to a naked singularity compares with the Hawking radiation from a star
collapsing to form a black hole. It turns out that there is a fundamental
difference between the two cases. A spherical naked star emits only about one
Planck energy during its semiclassical phase, and the further evolution can
only be determined by the laws of quantum gravity. This contrasts with the
semiclassical evaporation of a black hole.
