and naked singularities.

A black hole has been appropriately described by Chandrashekhar as the most beautiful macroscopic object known to man. Only a few parameters suffice to describe the most general black hole solution, and these objects have remarkable thermodynamic properties. Further, excellent observational evidence for their existence has developed over the years [@rees]. Thus, there can be no doubt about the reality of black holes, and the gravitational collapse of very many sufficiently massive stars must end in the formation of a black hole.

However, the following question is still very much open. If the collapsing core is heavy enough to not end as a neutron star, does this guarantee that a black hole will necessarily form? The answer to this question has to come from the general theory of relativity, and unfortunately this remains an unsolved problem.

What we do know from general relativity about gravitational collapse is broadly contained in the celebrated singularity theorems of Geroch, Hawking and Penrose. It has been shown that under fairly general conditions, a sufficiently massive collapsing object will undergo continual gravitational collapse, resulting in the formation of a gravitational singularity. The energy density of the collapsing matter, as well as the 