The most talked about and the most interesting celestial object that really demands extraordinary imagination of mankind is nothing but an unseen star that has exhausted all its nuclear fuel at its center and are known as called black holes. Even primary standard students are heard talking about it. Black holes are the most ingenious kind of system that a universe can have. Black holes even today hold many secrets to be revealed by scientists of today and tomorrow. Let us try to understand such far-fetched celestial object in the simplest terms without going in to the hassles of mathematics.
A basic fact that every body should know is that a black hole is nothing but a star that ended its life in a very special manner and got converted from a star in to a black hole. A black hole can form only when a specific type of star dies. But not all stars can become black holes after their death. This we shall see a little later.
So, what a black hole is? Black hole is a region of space that contains enormous amount of mass at very high density. How high this density should be? To understand this let us take an example of our Sun. The mass of sun is trillions and trillions of kg, and it covers an area of diameter 14 lakh kilometers. If all the mass of such a star is squeezed in to an area of roughly 3 kilometers then the density inside such a ball of 3 kilometers will be unimaginably high. One spoonful of matter from such a high-density area will weigh billions of tons and a spoonful of sand on Earth will hardly weigh just a few grams!
Due to mass concentration in a small area, gravitational force gets very strong, so strong that not even a beam of light can escape from the its surface, and so is the name black hole and they are also invisible.
Let us say that you throw a stone up in the sky, after going up a for a few meters, the stone will stop its upward flight and will start falling in downward direction. This is because, the gravitational pull of the Earth gets back the stone on to the surface. But if more force is applied then the stone will raise a few more meters than it had gone the last time. But even than it comes back to the surface. What if you want to throw the stone such that it should not come back to the ground? There is a term called Escape Velocity. Earth's escape velocity is 11.2 km/s. If you can throw something with a speed of 11.2 km/s, then it will never come back to earth, it will fly away in to the space overcoming the gravitational pull of the Earth. Moon's escape velocity is 2.4 km/s. This indicates that a lighter body will have smaller escape velocity and a larger body will have greater escape velocity. Escape velocity also depends on how far you are from the planet's center: the closer you are the higher the escape velocity.
To send space shuttles in space, they are accelerated up to the speed of 11.2 km /sec, than only they are able to escape free from the earth's gravitational pull and if a rocket is on the Moon than only a speed of 2.4 km/s is required for the rocket to leave the surface of the Moon.
The escape velocity on the surface is 617.7 km/s, the reason for such high escape velocity is the Sun's mass. There are stars in space that have escape velocities as high as 10,000 km/s and more. In the case of a black hole, an enormous concentration of mass in a very small radius makes the escape velocity as high as the velocity of light. Then, since nothing can go faster than light, nothing can escape the gravitational field. Even a beam of light would be pulled back and would be unable to escape from it.
This gives us a rough idea of what a black hole is and why light cannot escape from its surface. Now we will understand about how does a black hole form. As already mentioned earlier, a black hole forms out of a star's death, but not all stars at the end of their life can result into black holes. Because, for a star to result in to a black hole it should have mass roughly equal to 3 times the mass of our Sun. In other words our sun can never became a black hole.
In the interior of every star nuclear reactions are going on. Under high pressure and temperature four hydrogen atoms combine to become a helium atom, this is called fusion reaction. In this process large amount of energy is liberated, because the mass of a helium atom is little less than the mass of four hydrogen atoms, so the difference of mass is converted in to energy as per the famous equation of Albert Einstein "E=mc2". Every moment the energy equivalent to the explosion of millions of atom bombs is released from the core of the star where nuclear reactions are going on. This makes the star shine.
The outward radiation pressure generated due to the nuclear reactions balances the inward gravitational force and this is how a star remains stable. Usually a star is comprised of 90 to 95 per cent of hydrogen and remaining are the other elements. Abundance of hydrogen fuels the star's nuclear reactions for millions and even billions of years. But a time comes when all hydrogen gets exhausted to sustain the nuclear reactions within the core of the star. Nuclear reaction dwindles and there is no force left at that time to stop the contraction of the star due to the inward pull of gravitational force.
This is where the mass of the star plays an important role. If a collapsing star is of one solar mass (i.e. it has the mass equal to the mass of Sun) than the star will collapse until all the electrons of the matter inside are thrown out around the core giving an outward pressure. This pressure will halt the collapse and the star will be called a white dwarf star. This is the fate of our Sun after 5 billion years.
But if the mass of a collapsing star is greater than 1 solar mass than even the shell of electrons around the core will not be able to stop the inward contraction. In such a case, electrons will collide with the protons inside the core to form neutrons. Here again the collapse halts. The inward gravitational pull is not enough to further contract the star. Such a star is called a Neutron star. A neutron star may even rotate very fast on its axis emitting light from a fix angle due to accelerated electrons in the strong magnetic field. We receive light pulses from various such stars when the its beam of light passes over the earth. Such stars are called Pulsars (pulsating stars).
Collapse of a star cannot be halted if the mass is equal to three times the mass of the Sun. In such cases the star collapses to infinite density and small size (1 to 3 kilometers only) and becomes a black hole. A neutron star is around 10 to 12 kilometers in radius.
There are two important terms connected with a black hole that we need to understand, Singularity and Event Horizon. An event horizon is a spherical surface that marks the boundary of a black hole. At the event horizon the escape velocity equals to the velocity of light. One can pass through event horizon, but can never come out of it. Once an object approaches event horizon, it experiences incredible pressure and temperature and gets accelerated to the speed of light. The entering object gets transformed in to elementary particles and falls towards the center called Singularity. Physicists say that the singularity is infinitely small, dense and hot. The laws of physics do not seem to work inside the event horizon. We can only imagine and make speculate.
The next thing that we need to understand is that how are black holes detected even though they are invisible. Since there is no direct evidence of black holes, we have to rely on indirect evidences for detecting black holes. Scientists have spotted several binary star systems, in which a star is seen to revolve another star. In such systems, the center body is known to be of very high mass and compact and is invisible. The center body is also seen to be pulling matter from its revolving star. While the matter goes spirals towards the center it emits x-ray radiations, this form of radiation can be detected. Such binary systems are speculated to have a black hole. There are also speculations that huge black hole exists at the center of a galaxy.
American scientist John Wheelar coined the name black hole in 1969, but in fact, the theory has been around for much longer. As far as back in 1783, English astronomer John Michell suggested that if a star were massive enough, it would have such a strong gravitational field that no light can escape from it. Later in 1959 with the help of Einstein's General theory of Relativity, Physicists like Robert Oppenheimen, Volcoff and Snyder came to a conclusion that it would be possible for a star to collapse to the point where it would become a black hole, though the name black hole was given in 1969.
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