©The term black hole is of very recent origin.It was coined in 1969 by the American scientist John Wheeler as a graphic description of an idea that goes back at least two hundred years,to a time when there were two theories about light:one,which newton favoured,was that it was composed of particles;the other was that it was made of waves.We know that really both theories are correct,by the wave/particle duality of quantum machines,light can be regurded as both a wave and a particle.Under the theory that light is made up of waves,It was not clear how it would respond to gravity.But if light is composed of particles,one may accept them to be affected by the gravity in the same way that canon balls,rockets and planets are.At first,people thought that particles of light travelled infinitely fast,so gravity would not have been able to slow them down,but the discovery by Roemer that light travels at a fine speed meant that gravity might have an important effect.
On this assumption,a Cambridge don,John michell,wrote a paper in 1783 in the philosphical transactions of the royal society of london in which he pointed out that a star that was sufficiently massive and compact would have such a gravitational field that light could not escape:any light emitted from the surface of the star would be dragged back by the star’s gravitational attraction before it could get very far.Michell suggested that there must be a large no. of stars like this. Although we would not be able to see them because the light from them would not reach us,we would still feel their gravitational attraction.Such objects are what we now call black holes ,because that is what they are:Black voids in space.A similar suggestion was made a few years later by the french scientists Marquis de Laplace,apparently independently of Michell.Interestingly enough ,Laplaceincluded it in only the first and second editions of his book The system of the World, and left it out of later editions;perhaps he decided that it was a crazy idea.
To understand how a black hole might be formed,we must need an understanding of the life cycle of a star.A star is formed when a large amount of gas ,especially Hydrogen starts to collapse in on itself due to its gravitational attraction.As it contracts the atoms of the gas collide with each other more and more frequently and at greater and greater speed-the gas heats up.Eventually,the gas will be so hot that when the hydrogen atoms collide they no longer bounce off each other,but instead coalesce to form helium.The heat released in this reaction,which is like controlled hydrogen bomb explosion,is what makes the star shine.This additional heat also increases the pressure of the gas until it is sufficient to balance the gravitational attraction,and the gas stops contracting.Here we can take example of a balloon.
In reality,with more fuel a star starts,sooner it will ends up and turning into a black hole.Luckily,our sun has got enough fuel that it will lights up for atleast five billon years[5000000000].
In 1928, an Indian graduate student ,Subrahmanyan Chandrashekhar set sail for England to study at Cambridge with the british astronomers Sir.Arthur Eddington,an expert on generel relativity.During his voyage ,Chandrashekhar worked out how big a star could be and still support itself against its own gravity after it had used up all its fuel.The idea was was this:when the star becomes small,the matter particles get very near each other,and so according to Pauli Exclusion principle,they must have very different velocities.This makes them move away from each other and so tends to make the star expand.A star can therefore maintain itself at a constant radius by a balance between attraction of gravity and the repullsion that arises from the exclusion principle ,just as earlier in its life gravity was balanced by the heat.
Chandrashekhar realised that however ,that there is a limit to the repullsion that the exclusion princple can provide.The velocitiedmof the matter particles in the star to the speed of light.This means that when the star got sufficiently dense,the repulsion caused by the exclusion principle would be less than the attraction of gravity.Chandrashekhar calculated that a cold star of more than about one and half times the mass of the sun would not be able to support itself against its owb gravity.This mass is known as Chandrashekhar Limit. A similar discovery was made by a Russian scientist in same time named Lev Davidovich Landau.
Stars with masses above the Chandrashekhar limit on the other hand have a big problem when they come to the end of their fuel.In some cases they may explode or manage to throw off enough matter to reduce their mass below the limit and so avoid catastrophic gravitational collapse but it is difficult to believe that this always happens, no matter how big the star.How would it know that it had to lose weight?And even if every star manage to lose enough mass to avoid collapse,what would happen if you added more mass to the white dwarf or neutron star to take it over the limit?Would it collapse to infinite density?Eddington was shocked by that implication and he refused to believe Chandrashekhar’s result,Eddington thought,it was simply not possible that a star would collapse to a point.This was a view of most of the scientists:Einstein himself wrote a paper in which he claimed that star would not shrink to zero size.
Chandrashekhar had shown thatthe exclusion principle could not halt the collapseof a star more massive than the Chandrashekhar limit,but the problem of understanding what would happen to such a star, according to generel relativity,was first solved by a young American, Robert Oppenheimer,in 1939.His result suggested that there would be no observational consequences that could be detected by the telescopes of the day.No doubt,James Web Telescope is the most adcanced telescope in this world of Homo sapiens.
Mr Universe 