Sirillium is a volatile element found within nebulas. One use for sirillium was as a warp plasma catalyst. As a gas it could also be used to boost the efficiency of deflector shields. In 2293, the USS Excelsior, under the command of Captain Hikaru Sulu, ignited the sirillium gas within the Azure Nebula in order to escape from a Klingon battle cruiser commanded by Kang. The plan was effective, as it disabled the Klingon vessel. In 2373, the USS Voyager encountered a class 17 nebula composed of 7,000 parts per million of sirillium. (VOY: "Flashback")