Problem $12($Halliday Problem $13\mathrm{.}41)$

Two neutron stars are separated by a distance of $1\mathrm{.}0{10}^{10}m.$ They each have a mass of $1\mathrm{.}0{10}^{30}kg$ and a radius of $1\mathrm{.}0{10}^{5}m.$ They are initially at rest with respect to each other. As measured from that rest frame, how fast or they moving when $($a$)$ their separation has decreased to one$-$half its initial value and $($b$)$ they are about to collide?