where "G" is the gravitational constant, "M" is the mass of the central body, and "c" is the speed of light in a vacuum. In cases where the radius of the central body is less than the Schwarzschild radius, formula_16 represents the radius within which all massive bodies, and even photons, must inevitably fall into the central body (ignoring quantum tunnelling effects near the boundary). When the mass density of this central body exceeds a particular limit, it triggers a gravitational collapse which, if it occurs with spherical symmetry, produces what is known as a Schwarzschild black hole. This occurs, for example, when the mass of a neutron star exceeds the Tolman-Oppenheimer-Volkoff limit (about three solar masses).