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by kuldeep singh and vishwdeep singh



Introduction of Black Holes

Black holes as the name suggest is not any hole, black in color, but it is the region in space-time where such a tremendous gravitational force is activated, so that not even light can escape through.It was way back in 1783 when John Michell used the concept of Newtons Corpuscular Theory of Light to develop the idea of black holes.He argued that if light consisted of a stream of particles, it should be then influenced by gravity.Actually Michell developed this idea from a well established formula ,according to which the escape velocity of any object on Earth depends upon two quantities, mass of the body and its radius.If the mass of the body is very high and the radius is very small ,then the escape velocity becomes quite high.From this observation,Michell pointed out that a star that was sufficiently massive and compact would have such an enormous gravitaional field that light would not be able to espace from it.

The Einstein View

supernovae It was in 1915 when Einstein proposed The General theory of Relativity,that scientists actually understood how gravity affects light.His theory explained how space and time are destorted due to gravity of a massive body.Let us visualize how Einstein's model works. Consider the space-time to be a gaint trampoline,stretched out tight .When a stone is placed on the surface of the trampoline ,then the weight of the stone causes the stretchy material to drop downwards .Though the trampoline is still mainly flat ,it now has a slight dent in the middle.Now if we place several stones on the trampoline ,it would get more curved. In this manner,massive bodies in the universe cause the cosmic fabric to become distorted.Just as several stones bend a trampoline's surface much more than one does, an object of much greater mass than the sun would create greater distortion in space.If a tennis ball is rolled across the top of a fully stretched trampoline it follows a straight line. On the other hand,if a tennis ball is rolled across the top pf a curved trampoline,then the ball would follow a curved path.In the same manner, objects travelling through the flat regions of space-time continue along straight-line paths while those those travelling through curved regions move in curved trajectories.

Now imagine placing an extremely massive boulder on a trampoline. Naturally this would affet considerably.If a tennis ball is now rolled over the surface of this trampoline, it would fall through the whole created by the boulder.This massive boulder can be compared with a black hole. Actually the existence of a black hole in a region of the cosmos would serve to rip its very fabric apart.This tear in the structure of space-time is called a Singularity.An object moving near a black hole simply gets captured by its steep gravitational well.

Formation of black holes

life cycle of starTo understand how a black hole might be formed we first need to understand the life cycle of a star.Actually a star is nothing but a huge mass of burning gasses .In the evolution of star, the gas and dust particle start falling toward a common center due to which the star's density slowly increases.As more and more gas and the dust come together, the temperature inside also rises steadily.In course of time, may be a few hundred thousand years, as it contracts, the atoms of the gas collide with each other more and more frequently,at a greater speed.Due to generation of heat, the temperature inside the star rises to a few thousand degrees.The pressure also increases tremendously until it become high enough to balance the pull of gravity and halts the fall of overlying material.At this point the collapse stops and a protostar is formed.

A protostar is not yet a full-fledged star.Its nuclear furnace is yet to be lit, and it does not give off any visible light.Although the collapse stops after a protostar is formed, gravitational contraction continues for several million years with generation of energy.With further heating of the central core due to contraction, a time ultimatelycomes when the temperature as high as 10 million degrees or more, which is enough to trigger nuclear fusion of hydrogen into helium.At this point, the protostar becomes a full-fledged star.

The nuclear reactions not only infuse life into a star,they also make stable.However, this stability ends when a star has used up12% of its total hydrogen.At this stage, all that left in the core of the star is helium,which cannot sustain nuclear reaction.As a consequence, with to push against gravity, the core starts contracting, generating heat which in tern heats the layers just outside the core, that still contain unburnt hydrogen.Soon conversion of hydrogen into helium starts again outside the core.

milky way galaxyWhen hydrogen burning is over,the outer layer of the star expands and cools off, while the core shrinks.As a result of this expansion, not only does the star becomes enormously large, often swelling to a thousand times its original volume, and at the same time, it also cools to a temperature of a mere 3000 degrees or so. The star then presents a dull, red apperance and is clled a Red gaint . On the other hand, the core becomes sufficiently hot due to contraction.For continues contraction of the core, its density becomes almost a 1000 times the density of the core in a normal star. What happens next depends mainly on the total mass of the star.If it is a Sun-sized star nothing much happens. Unable to counter the intense pressure generated in the core by helium fusion, the outer envelop of the red giant is literally blown away.

All that remains of the star is the naked core about the size of the Earth. As it is extremely hot, yet small in size, this remnant of a star is called a White dwarf.The volume of a white dwarf is inversely proportional to its mass. So more massive white dwarf are actually smaller. This mass-volume relation is a result of the star deriving its support from electron Degeneracy pressure.The electron must be more closely confined to generate the larger degeneracy pressure required to support a more massive star.

chandra ray observatory

In 1931, an Indian physicist ,Subrahmanyan Chandrashekhar showed that when a star ran out of its nuclear fuel it collapses into an entirely new dense matter-so dense that a matchbox- full of it would weigh 12 tons!.Chandrashekhar also proved therotically that a white cannot be stable if its mass is more than 1.44 the mass of the Sun- critical mass that is called the Chandrashekhar Limit. It is natural to wonder what would happen if a star with a mass greater than 1.44 M. (Solar Mass) were to run out of fuel and start shrinking. Chandrashekhar theory is of course silent about this.Electrons cannot resist the greater gravitational collapse of such stars and are pushed into atonic nuclei, where they combine with protons to form uncharged tightly packed neutrons. At very high pressure electrons and protons combine to produce neutron. A neutron star is much more complex than a ball of neutrons because the pressure varies from a low value on the outside to a very high pressure inside and hence the state of matter varies. Neutron stars are very few kilometer in diameter. They weight about one million tons per cubic centimeter.

What happens to a dying star that is more than twice as large as the sun? Even strong force cannot halt its falling momentum, and it collapses completely, beyond the neutron star stage, to an even smaller, denser object, the Black hole.Complete collapse does not mean that black hole vanishes from the universe. The structure of space-time, as described by Einstein, precludes an infinite collapse and produces instead an immaterial, invisible but real curvature of space. A black hole can be compared to a hefty invisible man who sits on a couch. He cannot be seen, but his weight creates a depression in the seat. According to Dr.John Wheeler and Dr.Remo Ruffini of Princeton University, unlike every other physical object,black holes have neither shape nor size in the conventional sense.But they function with in the diameter of about 15 kilometers;they have masses ranging from that of Sun to a hundred million times as much;and they act like vortices.any stray matter or energy that passes too close to a black hole- within a critical distance called its horizon-is irresistibly drawn into the vortex of black hole.

Existence of black holes

Hanging space and time By definition, one cannot see a black hole with the eye. So there is no point in looking for it with an ordinary, or even a radio telescope. Radiowaves, infrared radiation etc., are nothing but electromagnetic radiation and the black hole sucks them all. So how can the scientists confirm the black hole? At least half of all star in the sky are actually mutiple system, consisting of two or more stars moving in an orbit about their common centre of mass. Now consider binary stars of which one is black hole.Due to strong gravitational pull, the black hole can suck matter from its companion stars. In course of time, this matter accumulates around the black hole and forms, what is known as the Accretion disc. Strong beams of x-rays are produced when matter from the companion star come close to the black hole and is sucked into the disc.

In 1970, an artificial satellite of United States, named Uhuruwas launched to detect stellar sources of x-rays. Uhuru has found over 100 stars that give off x-ray pulses. One of them is Cygnus X-1 in the constellation Cygnus. But unlike conventional pulses, Cygnus X-1 emits no radio waves. It has been found to be present very close to a massive, old supergiant star. This suggests that Cygnus X-1 is a member of binary star system. In 1974, it was designated as a black hole.

If a black hole gets crushed to a point, the wrapping of space-time would result in a sharp pin like feature called as Singularity. But physicists do not like singularities, and one way of avoiding them is to have a Worm hole, which is a hypothetical tunnel between two points in space-time separated by an arbitrarly great distance.

Whether this wormhole connects two different universes or parts of our own universe, the theoretical possibilities alone are, indeed, fascinating.

Fascinating Facts About Black holes

  • The black hole concept was developed by the German astronomer Karl Schwarzschild in 1916.
  • Theoretically if a black hole is been formed by a planet like saturn then its schwarzschild radius would be of 1 meter!
  • There are evidences that black hole converts 30% of all matter falling over it into energy.
  • Surrounding the black hole, where the escape velocity from the whole just equals the speed of light, is the Event Horizon, beyond which we cannot see any events.
  • Just as a black hole can only suck things in, a White Hole can only spit things out.
  • The combination of black and white holes are called Worm Hole.