But what happens for stars heavier than about 3-4 solar masses? In this case the pressure from the squashed nuclei cannot stop the gravitational attraction and collapse continues. In fact no known effect can stop the collapse and it will go on and on until the star collapses to a point. This how a black hole is created (see Sect. 7.5).
For this object the gravitational force is so big that even light cannot leave its vicinity: as mentioned in section 7.5, if a light beam comes too close to the center of such an object, the bending effect is so severe that it spirals inwards. Light emitted from up to a certain distance will be bent back into the star. This distance defines a horizon: nothing inside the horizon can ever come out, nothing that crosses the horizon ever leaves the black hole. The more massive the black hole, the larger the horizon.
For a very massive black hole an astronaut may cross the horizon without feeling any personal discomfort, only later he realizes that he is inside a cosmic Venus fly-trap (or roach motel ) out of which there is no escape.
General relativity together with our knowledge of subatomic physics guarantees that a sufficiently large star will eventually collapse to the point where a horizon appears. The manner in which such a star evolves thereafter is impossible to know since no information from within the horizon can be sent to the outside universe. There might be some new kind of effects which will stop the collapse of even the most massive stars, but even then the horizon will remain. The point is that our present knowledge of physics predicts the existence of black holes, even if we do not know all physical effects in Nature. The fact that we have several excellent black-hole candidates supports (albeit indirectly) our understanding of gravitation and physics in general.
The detection of black holes is difficult: one looks not for the object itself but for certain characteristics of the radiation emitted by matter falling into the black hole; see Fig. 7.11. Anything coming near the black hole will be strongly attracted to it, it will swirl into the black hole, and in the process it will heat up through friction, this very hot matter emits electromagnetic radiation in a very characteristic way and it is this patter what the astronomers look for (see Sect. 7.5).
The best candidate for a black hole was, for a long time an object in the constellation Cygnus and is called Cygnus X1. Very recently (May 1995) an object with the name GRO J1655-40 in the constellation of Sgittarius became an excellent black-hole candidate. In this object a star is accompanied by an object that emits no light, there is material falling into the companion and the X-rays from this material are unique to black-holes. Moreover, the mass of the companion can be determined to be heavier than 3.35 solar masses. The companion has then all the properties of a black hole.
Black holes are also supposed to be the engines at the center of active galactic nuclei and quasars (see Fig. 9.11). These are very distant objects which, by the mere fact of being detectable on Earth, must be immensely luminous. So much so that nuclear energy cannot be the source of that much radiation (you'd need more nuclear fuel than the amount of matter in the system). On the other hand, a black hole of several million and up to a billion solar masses can, by gulping down enough stellar material (a few suns a year) generate in the process enough energy.