A Federation starbase to which the Enterprise is heading for resupply when a black star hurls it back to twentieth-century Earth in Tomorrow Is Yesterday.
Starbase 9 is a Starfleet facility mentioned in the Original Series episode Tomorrow Is Yesterday. Kirk records that the Enterprise was on course for the base to take on supplies when it passed too close to a black star of enormous gravitational pull.
The ship had to use full warp power to tear itself free, and the recoil threw it through space and back in time to Earth in the late 1960s. There it was photographed by a United States Air Force interceptor, whose pilot, Captain John Christopher, had to be beamed aboard.
The starbase is never seen, and the episode ends with the ship returning to its own century and calling Starfleet Control. The slingshot manoeuvre found on the way home became a standard method of time travel in later stories.