AFAIK, star destroying takes place as soon as the order is executed. So if you order a ship to destroy the star, that ship is a goner. If you have a separate fleet on the warp point and your star destroyer hasn't gotten to the star yet, though, the fleet will make it through. (Each turn is split into 30 "days" in which orders are executed, so if your star destroyer isn't actually on the star when you end your turn, and you order the fleet to warp, the fleet will warp as soon as it can, and if it's fast enough (speed 10 is pretty fast
) it will make it out of the system before the star destroyer reaches the star and executes its destruction order.)