Q:   What is the typical time between head-on collisions for a
           star in the disk?


                        mean free path
              time  =  --------------------
                         relative speed


                         6.4 x 10^(31) m
                    =  ------------------
                         25,000 m/s


                    =   2.5 x 10^(27) s  

                    
                    =   8 x 10^(19)   years