Astrobiologists have long known that these events are capable of causing mass extinctions by stripping a planet of its ozone layer and exposing the surface to lethal levels of radiation. The likelihood of being hit depends on the density of stars, which is why the center of galaxies are thought to be inhospitable to life.
Like many here, I'm sure, I first considered the possibility that the galactic core was inhospitable to life when I read Larry Niven's 1968 short story "At the Core" (collected with his other "Beowulf Shaeffer" stories in Crashlander ). In his science-fiction tale, Niven had an astronaut visiting the core and witnessing the wash of radiation from so many supernovas placed so close together.
Niven's story, however, ended with the astronaut coming back and warning that this massive wave of radiation would be moving towards Earth at the speed of light. If that were true, and even the edges of galaxies were not safe in the end, then every galaxy would be ultimately hostile to life, not just in their cores. Is this the case, or did Niven get it wrong?