"A region of space-time in which there is such an immense concentration of material within a small volume that space-time curves over on itself and matter and energy cannot escape".

The variable possible evolutionary paths that stars follow in their life span are still under theoretical investigations. Some calculations show that under certain conditions, stars with mass, greater that specific value, in their last stages of evolution, collapse very rapidly. Their densities increase to even greater than those ascribed to be Neutron Stars. As the density of such stars increase, so does their gravity. A stage comes when the gravitational pull of such a star would permit neither matter nor light to escape from the star and it would become a Black Hole - a little universe in itself. Such Black Holes are now the subject of much research.
If light cannot escape, then how can astronomers ever hope to detect a Black Hole? Increased use of satellites have permitted astronomers to study objects in the Universe that produce x-rays. X-rays are very energetic forms of radiant energy that are blocked by out atmosphere. One of the source of x-rays is special type of binary system of super giant star and Black Hole. Cases from the super giant star spill over and fall into the companion Black Hole. These gases are heated to tremendous temperatures during their fall towards the Black Hole and before reaching the point of no return, x-rays are emitted into space. Therefore the existence of Black Holes can only be determined by observations of its influence on matter and space around it.

CONTACT: Ray Villard, STScI Wednesday, May 25, 1994
(410) 338-4514
PRESS RELEASE NO.: STScI-PR94-23

Dr. Holland Ford, STScI/JHU
(410) 338-4803
(410) 516-8653

HUBBLE CONFIRMS EXISTENCE OF MASSIVE BLACK HOLE AT HEART OF
ACTIVE GALAXY

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Black Holes till today have not yet been fully investigated and hence what happens to the matter which enters a Black Hole is not completely known. It is said that in the constellation Cygnus, there is a Black Hole we call it Cygnus X-1. It is also predicted that at the center of our Milky Way Galaxy there lies a super massive Black Hole. The White Dwarf , the Neutron Star and the Black Hole are now considered the ends of three main paths of evolution that stars of various masses can take once they have passed the Helium burning stage of their lives. What conditions cause a star to evolve one way or the other at this point is not fully understood. But surely it all depends on the initial mass and the composition of the star's evolutionary progress upto this point, together with the specific environment including the existence of any binary companion, etc.
If theoretically, a Black Hole is the penultimate residue of a massive star, then what happens to an average star such as the Sun, once it has exhausted all its nuclear fuel?
It will finally become a White Dwarf. But there are atleast another four billion years for the Sun to reach such a stage and four billion is next to eternity for us humans on Earth!! Black Holes are formed when matter reaches sufficiently high density. Light from a star of low density, such as a Red Giant, is, scarcely deflected by the gravitational field. A White Dwarf, which has a density of one tonne per cubic cm, also does not trap light, although the light rays are appreciably curved.