The Azure Nebula was a class 11 nebula located on the border of the Federation and the Klingon Empire. It was composed of oxygen, argon, theta-xenon, fluorine, and sirillium. (VOY episode: "Flashback")
The nebula was created by the supernova explosion of the star which was charted as FGC-SR37-758, around which formerly orbited the planet Erigol, the home world of the Caeliar. That explosion also created a series of subspace tunnels leading to numerous points across and beyond the Milky Way galaxy. (Star Trek: Destiny novel: Gods of Night)
In 2293, the USS Excelsior encountered a Klingon battle cruiser commanded by Captain Kang during an attempt to rescue Captain James T. Kirk from Klingon custody. The Excelsior escaped when Captain Hikaru Sulu ignited the nebula's sirillium gas. (VOY episode: "Flashback")
In 2381, the Borg began using the Azure Nebula's subspace tunnel network to invade Federation space. The Starfleet vessels USS Enterprise and USS Aventine eventually discovered this network, and called for an allied armada to gather in the Azure Nebula against an anticipated Borg invasion. The USS Titan also independently located the other terminus of subspace tunnels by unintentionally uncovering the hidden planet New Erigol by detecting the high-energy byproducts of the tunnels. By extrapolating the energy pulses, Chief Tactical Officer Tuvok determined that one path intercepted the nebula.
While the Enterprise and Aventine were investigating the network, an armada of over 7000 Borg vessels entered the Alpha Quadrant through the Azure Nebula, destroying the assembled fleet of more than 300 vessels including the USS Voyager which was the only ship to survive the conflict. (ST - Destiny novels: Gods of Night, Mere Mortals)
- The Star Charts places the Azure Nebula near Cardassian space, but well within Federation territory. However, Flashback and the Destiny series state the nebula is at the triborder between the Federation, Klingon Empire, and Romulan Star Empire.
External links
Azure Nebula article at Memory Alpha, the wiki for canon Star Trek.