Wikipedia(https://en.m.wikipedia.org/wiki/Photon_sphere): 'This equation entails that photon spheres can only exist in the space surrounding an extremely compact object (a black hole or possibly a neutron star)' But how can a neutron star? I have a doubt because 1. When а mass of а star increases, radius decreases. 2. The radius of the most massive NS exceeds the schwarzschild radius. 3. We know Oppenheimer-Volkoff limit roughly. It is not known exactly. I suppose that the existence of such neutron stars is questionable. Is it correct?