Noun
Schwarzschild radius (plural Schwarzschild radii)
(astronomy) The radius of an object such that, if all its mass were compressed within that radius, the escape velocity would equal the speed of light.
Because of this property, the collapsed stars were called "frozen stars", citation because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius. Source: Internet
For instance, the gravitational wave signal suggests that the separation of the two object prior to merger was just 350 km (or roughly 4 times the Schwarzschild radius corresponding to the inferred masses). Source: Internet
For non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. Source: Internet
The redshift is not defined for photons emitted inside the Schwarzschild radius, the distance from the body where the escape velocity is greater than the speed of light. Source: Internet
When the photon is emitted at a distance equal to the Schwarzschild radius, the redshift will be infinitely large and it can't escape to any finite distance from this Schwarzschild sphere! Source: Internet