  The spherical gravitational collapse of a compact packet consisting of
perfect fluid is studied. The spacetime outside the fluid packet is described
by the out-going Vaidya radiation fluid. It is found that when the collapse has
continuous self-similarity the formation of black holes always starts with zero
mass, and when the collapse has no self-similarity, the formation of black
holes always starts with a finite non-zero mass. The packet is usually
accompanied by a thin matter shell. The effects of the shell on the collapse
are also studied.
