The Azure Nebula was a Class 11 nebula located on a border of the Federation and the Klingon Empire. The nebular gases it was composed of included oxygen, argon, theta-xenon, fluorine, and sirillium. (VOY episode & novelization: Flashback) 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 & novelization: Flashback)