grant hutchison: But the question seems to relate to a star collapsing to become a black hole, not a star approaching a black hole.

dcl: Thank you, grant hutchison, for waking me up. You are right. Sorry, I led myself astray from the question at hand. Following, hopefully, is the answer to the question that was asked:

The scenario most frequently cited as leading to production of a black hole is collapse of the core in a star with mass beyond the Chandrasekhar limit when it exhausts its supply of hydrogen. As the pressure from the core falls below the amount necessary to support the weight of the overlayers, the latter begin to fall into the core, and the process cascades until the core density becomes so great that the core can no longer support that weight and collapses into free fall along with the rest of the star. As the density increases, an event horizon suddenly comes into existence at a point at the very center of the core and rapidly expands. Matter overtaken by the event horizon falls into it and the added mass inside the expansion of the event horizon to accelerate, taking in more and more mass. The process is explosive, sweeping over the rest of the star in an instant. In the meantime, a shock wave from the initial explosion sweeps outward through the rest of the star, resulting in what rest of the world sees as a nova or supernova explosion, depending on the original mass of the star. The conversion from a star to a black hole is thus not instantaneous but is nevertheless extremely fast since it takes only a very brief instant for the shock wave to reach the surface of the star. The star does not disappear at the instant that the event horizon comes into existence. In fact, the star does not disappear even at that instant, for the shock wave throws extremely hot gases into the space surrounding the star, and it is only when those gases cool sufficiently that what's left of the star disappears. No radiation escapes when mass falls through the event horizon, but the escaping gases continue to radiate heat radiation after they cease to be visible to the human eye and to astronomical instruments.