The Enterprise approaches a pulsar in the Lonka Cluster
The Lonka Cluster was a star cluster which contained the nearest pulsar to Cor Caroli V and Browder IV. The pulsar was a rotating neutron star of approximately 4.356 solar masses. It was 31 hours away from Cor Caroli V at warp 2 and 34 minutes away at warp 7.
In reality, neutron stars and pulsars are believed to have a mass limit of about 2.5 solar masses; any more massive and they will collapse into a black hole.
In 2366, an alien posing as Jean-Luc Picard ordered the USS Enterprise-D to divert to the Lonka Cluster, and delayed their mission to rendezvous with the USS Hood. (TNG: "Allegiance")