In 2373, despite supernovae generally occurring in the Milky Way Galaxy only once in a century, a number of stars went supernova. These included at least four Delta Quadrant stars in close proximity, three of them known to be in the same sector.
The first star was observed from a safe distance by the USS Voyager. Less then three days later, two other supernovae happened near-instantaneously in the same cluster, close enough that the three shockwaves combined.
It was learned that the supernovae were caused by the Q Civil War. Some time thereafter, the ship managed to find a fourth star about to go supernova with the help of a Q. (VOY: "The Q and the Grey")