The Schwarzschild radius of a black hole is that radial distance from the center of a sphere within which not even light can escape. It was first discovered mathematically by Schwarzschild in 1916 after Einstein published his general relativity theory. It can be calculated from a star's mass M as: R = 2GM/c2. Take the radius of both stars A and B as the Schwarzschild radius. If the mass of star A is twice as much as the mass of star B, the average density of star A, compared to star B will be

Einstein's principle of equivalence:  No experiment can be performed that could distinguish between a uniform gravitational field and an equivalent uniform acceleration. or Gravitational mass is equivalent to inertial mass.