The Marrat Nebula was a nebula in a strategically important area of space to a dozen cultures including Arcturans, Khodini, Klingons, Orions and the Federation. Starfleet personnel have described this area as a "treacherous expanse of charged-gas clouds and ion storms".
Several major routes ran through the nebula, however its properties often made sensor and communications technologies unreliable and the region was subsequently also an area popular with those fond of illegal activity. In response, Starfleet built Starbase 13, in the Marrat system, to police the region, however the eventual result was for the starbase to become a target itself, provoking the nickname of "Fort Apache".
Some time later Starfleet began a more ambitious scheme to bring order to the region in Project Pharos, in which a huge lighthouse was built on a planet in the nebula to broadcast navigational data to all vessels in the area and chart and log illegal traffic. The Pharos project itself became a target when the Starfleet engineers discovered huge quantities of extremely pure dilithium on the planet, which the Klingon commander Kaaj also became aware of, prompting him to encourage the Nebula's criminal elements to form an alliance to launch more effective attacks on Starbase 13. With the starbase's resources stretched in self-defence Kaaj was able to launch his own offensive to take control of Pharos, however the USS Enterprise had been called in to assist Starbase 13, and after driving off the most recent brigand attack proceeded tot he Pharos siteworld to investigate the loss in communications.
Shortly after the Enterprise arrived Kaaj attacked, striking a crippling blow to the starship. With the situation threatening to escalate to a larger conflict between the Federation and the Klingons Captain Pike saw only one way to prevent a more extended conflict; he fired on Pharos. The pure dilithium ignited and the planet became a huge fireball which burned for decades providing the navigational beacon the region required. (EV comic: "The Fires of Pharos")