Sirillium is a volatile element found within nebulas.

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")