Hydrostatic equilibrium is a fundamental physics and astrophysics principle describing the balance between inward gravitational force and outward pressure gradient force, and no citation is needed for this basic definition.
The claim states a foundational definition in physics and astrophysics regarding hydrostatic balance, which is trivially true by definition and everyday observation in stellar and fluid mechanics. Following the instructions, common knowledge claims require no citation and return COMMON_KNOWLEDGE with empty index arrays.