Arhennius was a star in or near Federation space, and the center of the Arhennius system. The Arhennius star was a quarter more massive than Sol, with twice the energy output. The star system here had no habitable planets.
The Arhennius system had been scanned by Federation sensors nearly a parsec away, and by passing ships, looking for lifesigns and habitable worlds, but there were neither; the system contained two gas giant planets roughly the size of Saturn and two small rocky planets slightly smaller than Venus without atmosphere. There was no record of any Federation, Romulan, or Klingon vessel ever entering the system. No large explosions had been recorded, but there was evidence of a half-dozen low-yield photon torpedoes detonating near the end of the 22nd century.
The innermost planet contained amounts of dilithium ore in quantities too small or deep to be detected at long range.
In 2370, retired Starfleet Captain Montgomery Scott obtained a Klingon bird-of-prey and planned to travel back in time and rescue James T. Kirk from the USS Enterprise-B and save him from apparent death, and used the Arhennius star to slingshot back in time, with the USS Enterprise-D following.
In an alternate timeline where Scott succeeded in rescuing Kirk before he could be absorbed into the Nexus, the USS Enterprise-D found evidence of a Klingon mining operation that had been abandoned at roughly the same time as the recorded torpedo detonation. The Klingons also mined the system with interphase cloaked torpedoes, one of which caught the Enterprise-D off guard. (ST novel: Engines of Destiny)