Introduction To Black Hole

DEFINITION AND FORMATION OF BLACK HOLE

In this section the possible definitions of black holes will be discussed. Most scientists agree on one fundamental definition, there is believe that, however there is no complete understanding of the black holes. A Black hole can be defined as a region formed from the high gravitational collapse of massive stars. That is when massive stars undergo a very intensive gravitational collapse which leads to the creation of bodie s of small volume and great mass stars called black holes (Hawking 1994, Begelman & Rees 2010).

The Idea of the formation of black holes from gravitational collapse was first introduced by the physicists Robert Oppenheimer and Hartland Snyder 1939. However, the whole notion of the existence of black holes in modern physics came in to consideration after Einstein’s General Theory of Relativity in 1915. Hawking says that the first person who talked about these stars was John Michell in 1783 at Cambridge and the first time that the term black hole was used was in 1967 by physicist John Wheeler (Hawking 1994).

In 1915 Einstein introduced the concept of the velocity of light in his theory of general relativity. He stated that light travel faster than everything and there is nothing that can exceed that velocity. He proposed that even light can be influenced by gravity and he produced unsolved equations dealing with that situation (Hawking 1994). The solution of those equations resulted in introducing the study of black holes. Karl Schwarzschild was a person who found a solution and he introduced a Schwarzschild radius which is the radius of a body in which the escape velocity is equal to the velocity of light (Begelman and Rees 2010).

In the range of the Schwarzschild radius nothing can escape the gravitational attraction
force, even light (Hawking 1994). Black holes are stars with attraction force and with Schwarzschild radius. The space behind this radius is called the event horizon beyond which there is no return once anything enters (Hawking 1994, Begelman and Rees 2010).

According to the main sequence diagram, which explains the formation of all types of star from the beginning of its formation to its death passes through different stages. In each state it have different characteristics for example different mass, volume and luminosity. Some stars will die out to become a supergiant and others to white dwarfs. Black holes are one of the stars dying stages in which due to the gravitational collapse the volume will be reduced to a very small scale this leads to the great mass to small volume ratio (Hawking 1994, Begelman and Rees 2010)

THE DENSITY OF BLACK HOLE

As it has been mentioned before, black holes are formed after a strong gravitational collapse of stars. This resulted in reducing the volume of the star with remaining its own mass that produce a very intensive density, since density is equal to mass of the object per its volume (Preskill 1994, Frolov and Zelnikov 2011).

Some discoveries by the method of gravitational lensing found a black hole, Cygnus X-1,
in 1965. The calculations showed that this black hole has a mass of 6 to 20 times greater
than mass of sun. This enormous mass produces a very dense star since their volume is
small, so that black holes considered being the densest object in the space. In addition,
by another method, X-ray detection, the X-ray observatory satellite, Chandra, observed galaxies with high luminosity at their centre. Observers conclude that this very bright luminosity that comes from a small centre of a galaxy should imply to the existence of a black hole with giant density (Raine and Thomas 2010).