Worked example - the Schwarzschild radius of the Sun and Earth. Rs = 2GM/c^2. For the Sun: M = 2 x 10^30 kg. Rs = 2 x (6.67 x 10^-11) x (2 x 10^30) / (3 x 10^8)^2 = 2.67 x 10^20 / 9 x 10^16 = 2,960 m ~ 3 km. So the entire Sun squeezed into a 6-km-wide ball would be a black hole. For Earth: M = 6 x 10^24 kg, giving Rs ~ 8.9 mm - about the size of a marble. This shows why only collapsed stellar cores (and giant galactic centres) actually form black holes: ordinary matter is nowhere near dense enough.