black colored solid sphere of radius "R" and mass "M" is inside a cavity with vacuum inside.The walls of the cavity are maintained at temperature `T_(0)` .The initial temperature of the sphere is `3T_(0)` .If the specific heat of the material of the sphere varies as `alpha T^(3)` per unit mass with the temperature of the sphere,where `alpha` is a constant,then the time taken for the sphere to cool down to temperature `2T_(0)` will be ( `sigma` Stefan Boltzmann constant)