Pair-instability supernovae can only happen in stars with a mass range from around 130 to 250 solar masses () (and low to moderate metallicity (low abundance of elements other than hydrogen and helium – a situation common in Population III stars)). However, this mass gap is expected to be extended down to about 45 solar masses by the process of pair-instability pulsational mass loss, before the occurrence of a "normal" supernova explosion and core collapse. In nonrotating stars the lower bound of the upper mass gap may be as high as 60 . The possibility of direct collapse into black holes of stars with core mass > 133 , requiring total stellar mass of > 260 has been considered, but there may be little chance of observing such a high-mass supernova remnant; i.e., the lower bound of the upper mass gap may represent a mass cutoff.