A nebula near Klingon space that the USS Excelsior entered in 2293 during Captain Sulu's unauthorised attempt to rescue Kirk and McCoy, as relived by Tuvok in Flashback.
The Azure Nebula is a class 11 nebula lying on the border between the Federation and the Klingon Empire. Its gases include oxygen and argon along with traces of the volatile compound sirillium.
It is seen in the Voyager episode Flashback, in which Tuvok relives his service as a young ensign aboard the USS Excelsior in 2293. Captain Hikaru Sulu takes the ship into the nebula hoping it will conceal an approach to Qo'noS, where Kirk and McCoy are being held after the events of Star Trek VI: The Undiscovered Country.
Inside, the Excelsior is intercepted by a Klingon battle cruiser under Kang, who insists on escorting it out. Sulu disables Kang's ship by igniting the sirillium, but further Klingon vessels force the Excelsior to abandon the attempt.