O9

O9 is a designation that can refer to several unrelated concepts depending on the context.

In military nomenclature, O-9 designates a senior commissioned officer pay grade across the U.S. uniformed services and many NATO allies. In the United States, O-9 is the rank of Lieutenant General in the Army, Air Force, and Marine Corps, and Vice Admiral in the Navy and Coast Guard. The grade sits above O-8 and below O-10; officers at this level typically hold commands of large formations or senior staff positions, or serve as high-level planners and policymakers. Pay and benefits are tied to the grade and the service branch.

In astrophysics, O9 denotes a spectral subclass within the O-type class of stars. O-type stars are among the hottest and most massive in the galaxy. An O9 star is cooler than earlier O subclasses and near the boundary with B-type stars, yet still extremely hot and luminous. These stars drive strong ultraviolet radiation fields and often reside in or near regions of active star formation. They have short lifespans in astronomical terms and end their lives in energetic supernovae or direct collapse.

Outside these domains, O9 may appear as a model or catalog designation in various fields; its meaning is context-dependent and requires clarification.