Scientists believe such concentrations of matter can occur under certain conditions, as when a massive star (one with a mass three or more times that of the sun) runs out of fuel for thermonuclear reactions and collapses in one itself. In the constellation Cygnus, a star has been discovered that appears be in a binary (two-star) system with a small, invisible object that may be a black hole formed from a star (Abell, 34-36). The area around the object is a strong source of X-rays, possibly produced by gases heated to very high temperatures as they are drawn into the black hole.
Very massive black holes may form at the centre of a galaxy, where there is a high concentration of stars and other matter. Astronomers have found evidence for the existence of massive black holes at the centre of several galaxies, including the Milky Way. Black holes ranging down to microscopic size may have formed when the universe was very dense, shortly after its creation. According to a theory by the English physicist Stephen Hawking, black holes of very small size lose a significant amount of mass through subatomic processes at their boundaries.
According to this theory, once a black hole become extremely small, it emits all its remaining mass in an explosion of high-energy particles. However, evidence for such explosions has not been found. Moreover, the term “black hole” was coined to describe such an object more than 50 years ago, long before there was any evidence that such object existed. Today, there is ample evidence that black hole exist (Snow, 111). If the core contains more than 3 solar masses, its collapse leads to the formation of a black hole. In this case, the degenerate neutron gas pressure cannot halt the collapse.
There may or may not be a supernova explosion, depending on whether a neutron star forms temporarily (causing a rebound of the infalling outer layers of the star) before collapsing further. A black hole never stops collapsing; mathematically, it can be described as a single point containing all the mass of the collapsed stellar core, but physically it is difficult to describe. In other words, if a star more than 2 or 3 solar masses in its core collapses, it will exceed the mass limit for formation of a neutron star.
When a star collapses beyond the point where neutron gas pressure degenerate can support it, the collapse never stops as mentioned earlier. Thus, a black hole is not in hydrostatic equilibrium because there is no known force that can counteract the inward force of gravity and it is said that the mass of the star forms a singularity, described mathematically as a single point having infinite density. As the collapse proceeds, the surface gravity of the star becomes stronger (Chaisson, 16-17).
The gravitational force of the star remains the same at distance outside of the original surface of the star; the immense increase in gravity occurs only at closer distances. As the surface gravity increases, it has an increasingly significant effect on photons of light. Eventually a point is reached where the surface gravity is so great that light cannot escape. At this point, the star is said to have passed through the event horizon because it is impossible to observe anything that happens to it after this.
The radius of the star at this point is called the Schwarzschild radius and the Schwarzschild radius is proportional to the mass of the star; for a star of 1 solar mass, it is 3 km. A black hole cannot be directly observed, but its presence may be detected through its gravitational effects. If a binary system is found to have an unseen member whose mass is too great to be neutron star, then it must be a black hole. Such binary systems are most easily recognized if mass transfer takes place from the companion star to the black hole. In this case, the matter that is transferred forms an accretion disk so hot that it emits X-rays (Abell, 34-36).
Thus, X-ray binaries are likely places to look for black holes. Several X-ray binaries have been observed in which the analysis of the orbit of the visible star indicates that the unseen companion has too much mass to be a neutron star and must therefore be a black hole. Reference: 1. Abell, G. D. Exploration of the Universe (96h edition), pp. 34-36. Philadelphia: W. B. Suanders Co. , 2001. 2. Chaisson, E. Astronomy Today. Pp. 16-17. Englewood Cliffs, N. J. : Prentice Hall, 2002. 3. Snow, T. P. The Dynamic Universe: An Introduction to Astronomy (6th edition), p. 111. St. Paul: West Publishing Co. , 2001.