Stars are ejected from galaxies all the time. Usually this causes no problems to experiment designs which assume that each galaxy acts as an isolated Petri dish

Stars are ejected from galaxies all the time. Usually this causes no problems to experiment designs which assume that each galaxy acts as an isolated Petri dish, but sometimes a star is ejected with a planetary system capable of supporting life, and on a trajectory where it may interact with another galaxy during the stellar life time. In that case, of necessity, when the star enters the sterilization zone between galaxies it must be sterilized to preserve the validity of the Experiments.