Nymyn was a star, a neutron star located in the space of the galaxy's Alpha or Beta Quadrants.
History and specifics[]
In the year 2267, the Federation starship USS Enterprise entered Nymyn's orbit with the engines shut down to effect repairs to the ship's pressure gauge. However, it was then that the star underwent gravitational collapse, turning into a black hole which drew the Enterprise into superspace. (TOS comic: "Siege in Superspace")