Class 11 nebulaedit this page

From Memory Alpha, the free Star Trek reference

A class 11 nebula is a type of nebula, consisting of primarily oxygen and argon with trace amounts of theta-xenon, fluorine, and sirillium. Class 17 nebulae can be superficially similar to type 11 nebulae.

The Azure Nebula was a class 11 nebula. (VOY: "Flashback")