dimensionless quantity $\xi=|\rho_{3}|/\rho_{0}^{5/2}$. The singularity is naked if $\xi\geq 25.48$ and covered if $\xi$ is less than this number. If $\rho_{1}$, $\rho_{2}$ and $\rho_{3}$ are all zero, the singularity is covered, the Oppenheimer-Snyder collapse being a special case of this.

It is known that the collapse of null dust (directed radiation), described by the Vaidya spacetime, also gives rise to both black hole and naked singularity solutions, depending on the rate of infall [@vaidya].

Spherical Collapse of Fluids with Pressure
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Exact solutions of Einstein equations describing collapse of fluids are rare. Hence little is known about the end state of collapse in these systems. To some degree, numerical methods have been used to integrate Einstein equations and study light propagation. In comoving coordinates, the energy-momentum tensor is diagonal and its components are the energy density, the radial pressure and the tangential pressure. For a perfect fluid, the two pressures are identical.

A significant development was the work of Ori and Piran [@op], who investigated the self-similar gravitational collapse of a perfect fluid with an equation of state $p=k\rho$. It is readily shown that the collapse leads to the formation of a curvature singularity. The assumption of self-similarity reduces Einstein 