Birkhoff's Theorem

Birkhoff's Theorem states that the gravitational field outside a spherically symmetric object, like a star or planet, is the same as if all its mass were concentrated at its center. This means that, regardless of how the mass is distributed inside the sphere, the space around it behaves as if all that mass were compressed into a single point. The theorem helps simplify calculations in astrophysics, showing that the gravitational effects outside such an object do not depend on its internal structure, only on its total mass and size.