Galactic whirlpool

The Galactic whirlpool was a spacial anomaly. It was a maelstrom caused by Polo's Bolos, two orbiting black holes located 15 light years from Ellison's star.

Overview
Orbital patterns of the two singularities produced an enormously destructive effect through space like a propeller through water. Over time the gravimetric disruptions had churned thousands of stellar objects, including a trio of neutron stars in the year 2267. Its core area was four light-years across. Hard radiation was lethal six light-years away. The fabric of spacetime was dramatically curved even from a distance of ten light-years, with time dilation and other effects reaching as far away as 17 light years.

The whirlpool’s path was on a course to destroy Earth and its solar system three million years in the future.

History
In 2267, the Wanderer generation spaceship was 17 light-years away from Polo's Bolos, and its fusion engines needed to be fired immediately for a course-correction to avert disaster. The USS Enterprise (NCC-1701) mounted a rescue mission.