  The gravitational collapse of a star is a warmly discussed but still puzzling
problem, which not only involves the dynamics of the gases, but also the subtle
coordinate transformation. In this letter, we give some more detailed
investigation on this problem, and reach the results: (I). The comoving
coordinate system for the stellar system is only compatible with the
zero-pressure free falling particles. (II). For the free falling dust, there
are three kind of solutions respectively corresponding to the oscillating, the
critical and the open trajectories. The solution of Oppenheimer and Snyder is
the critical case. (III). All solutions are exactly derived. There is a new
kind singularity in the solution, but its origin is unclear.
