The solution of the Einstein equations they represent a static gravitational field, with spherical symmetry and in the absence of matter, implies the existence of an ideal boundary, called the "horizon of events", characterized in that no matter what goes beyond it will no longer be able to go back, because, attracted by the gravitational field. For even light can not cross the horizon of events from the inside out, the inner region on the horizon it behaves in all respects like a black hole.
In the 1930s, as astronomers began to ponder the future of a star whose nuclear fuel had run out, they ran into difficulty describing its fate. When energy generation within a star falters, its own gravity begins to take over, compressing the star’s mass. Our own sun will end up as a white dwarf, whose core is a dense mix of atomic nuclei in a sea of electrons. But the fates of larger stars were less clear.
In 1939 Robert Oppenheimer published a fundamental work, in whose abstract he writes:
Nel 1939 Robert Oppenheimer pubblica un lavoro fondamentale, nel cui abstract scrive: “When all thermonuclear sources of energy are exhausted a sufficiently heavy star will collapse. Unless fission due to rotation, the radiation of mass, or the blowing off of mass by radiation, reduce the star's mass to the order of that of the sun, this contraction will continue indefinitely. In the present paper we study the solutions of the gravitational field equations which describe this process. In I, general and qualitative arguments are given on the behavior of the metrical tensor as the contraction progresses: the radius of the star approaches asymptotically its gravitational radius; light from the surface of the star is progressively reddened, and can escape over a progressively narrower range of angles. In II, an analytic solution of the field equations confirming these general arguments is obtained for the case that the pressure within the star can be neglected. The total time of collapse for an observer comoving with the stellar matter is finite, and for this idealized case and typical stellar masses, of the order of a day; an external observer sees the star asymptotically shrinking to its gravitational radius.” 10 July 1939. [2]
MAQ/March 2018/40