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| Black hole - from an artist's point of view |
Stan Lee created a fictional character for Marvel comics which was debuted in the series Fantastic Four in 1996. The character was 'Galactus'. He was like a demi god with god like powers. He consumed planets, rich with both thermal and organic energy. That's why he was called 'the devour of planets'. But unlike Galactus who feeds to survive, black holes feed because it's hunger is infinite. No matter how much it eats, it will keeping eating and eating forever.
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| Galactus |
I would like you to get clear about how gravity works before reading any further.The life a star depends of the dynamic equilibrium between nuclear force and gravitational force. When nuclear fusion stops and nuclear energy runs out, gravity crushes the star until it's destruction.
But When stars die, they leave behind a corpse. The nature of the corpse depends on the mass of the star. when a mid sized star like our sun dies, it leaves a white dwarf behind. When a massive star goes supernova, it creates different kinds of neutron stars. But when a super massive star dies in a hypernova, it gives rise to a baby 'black hole'.
The theory of general relativity predicts that a sufficiently compact mass will deform space-time to form a black hole. So anything with a sufficiently compact mass could create a black hole. If our earth is compressed enough to the size of a ball with a radius of 9 millimeters then it would produce such strong gravity that the escape velocity at it's surface will be more than the speed of light. so light will be trapped itself if it gets too close.
This critical radius for any object to become a blackhole is called 'Schwarzschild radius'. The Schwarzschild radius for the sun is 3 kilometers. So if the sun is compressed to that radius then it will be a black hole but, the solar system will remain unchanged because the gravity of the sun will stay the same. Because we know gravity depends on the mass and the mass of the sun is still the same as it was but, only the density is greatly increased even more than a neutron star. That's why only at the surface of the compact sun (now it's a black hole) the gravity will be so strong that the escape velocity will be greater than the speed of light.
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| Schwarzschild radius |
When a super massive star runs out of nuclear fuel, the gravity crushes the star's core to zero volume and infinite density. It is called a 'singularity'. The laws of physics breaks down at this point. There is a region around the singularity, where the gravity is so strong that even light itself gets sucked in. It is called the 'event horizon'. As light can't reflect back from the event horizon, it appears black. The singularity creates a hole in the space-time fabric. Hence the name black hole.
But why can't a supernova create a black hole?
Because to create a black hole gravity must be strong enough to crush the core of a star to such extent, that space-time continuum disrupts itself. From Newton's law of universal gravitation, we know that objects with more mass creates more attraction or gravity and the more the mass, the bigger the size. So the equation to have sufficient mass to produce enough gravity for creation of a black hole could only fit inside a super-massive star. That's why only the super-massive stars will have sufficient gravity to form black holes.
What does it exactly mean that the black holes deform space-time continuum ?
To understand what it means let's see a video clip about a simple example. 'the dead and alive paradox' from Leonard Susskind. He is the man who proved that black holes obey the law of 'conservation of information'. That is whatever a black hole consumes, the information about it is stored in it's core and it's 2D surface area or in the even horizon.
Please watch the video clip before going further.
Well after seeing the video you must be wondering how is it possible ? This is ridiculous. Well the math of this phenomenon is woven into the very fabric of a black hole.
Our milky way galaxy contains may be a thousands of stellar mass black holes( formed by gravitational collapse of a super massive star). We have detected only a dozen. So we have no idea where the rest of the black holes are, they are wandering through out the galaxy.
As black hole consumes matter or other black holes, it grows in size.
What is the significance of the black holes in our universe? Are they here only to eat and destroy?
To know more about the black holes we have to go 14 billions years back to the time of the 'big bang'. The initial stars were formed from the clouds of the dust and gas which were almost everywhere in our universe. In regions where this cloud was the thickest, eventually the dust and gas get collected with the help of gravity forming an accretion disc( An accretion disc is a structure formed by diffuse material in orbital motion around a central body, typically a star or a black hole ). As the gases begin to gather and rotate with high speeds, the tempreture rose and when the tempreture was high enough to sustain nuclear fusion, then a new star was born.
But the initial stars were super massive. They burnt through their fuel very quickly and exploded to give rise to black holes. so in the early phase of the universe, there were a lot of black holes moving around. eventually over hundreds to millions of years they attracted each other and merged. So some of the black holes became super massive like millions or billions times more than the mass of the sun. They were called 'supermassive black holes'.
As black holes grew bigger and bigger, they produced stronger gravity and pulled in more and more gas clouds. As the gas cloud became thick as even gravity drew them together, new stars formed and eventually over millions of years, galaxies shaped from it having the super massive black hole at their center. Most galaxies are formed like this. But super-massive black holes and gravity are not completely responsible for creating galaxies, they merely play a small part. We will discuss about that later at the end of this article.
The super-massive black hole eats anything it can quickly. When the galaxy was young and new stars were forming, there was a lot of gas all around the super massive black hole. So as the black hole attracts more and more gas it also feeds on it. But the rate of consumption of gas was so high that it was way over the consuming capacity of the black hole itself so it spits most of it back as 2 beams of light in the opposite directions emerging from the black hole.
The matter coming close to the event horizon forms an accretion disc around the it as it gets drawn towards it. The spinning disc has huge velocities and high tempretues. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms, and when the atoms reach a few million Kelvin, they emit X-rays. The X-rays are sent off into space.
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| These are the first, focused high-energy X-ray views of the area surrounding the supermassive black hole at the center of our Milky Way galaxy, called Sagittarius A*. The three images on the right side show Sagittarius A* before, during and after an X-ray flare that was spotted in July. (pictures taken from NASA's NuSTAR telescope) |
Gradually as the galaxy starts to grow in size, the gas accumulation slows down. As the black hole eats and spits gases at such a high rate that the gas cloud will eventually disappear and star formation will stop. The galaxy will stop growing and the quasar jets will fade away. So scientists speculate that the size of a galaxy truly depends upon the size of the super-massive black hole at it's center.
So now it's clear that almost every large galaxy has a super-massive black hole at it's center and small galaxies have dense star clusters in their centers. Our own galaxy 'Milky way' is home to a super-massive black hole which has a mass of 4 million suns is situated at 28,000 light years from earth. Our neighbor Andromeda galaxy has a super-massive black hole which has 140 millions times more mass than the sun.
Some galaxies even contain dual nucleus, as a supermassive black hole and a dense star cluster.
The M-s relation was discovered in the year 2000. It is called the "Faber-Jackson law for black holes". From the relation we can found out that the mean ratio of black hole mass to bulge mass ( mass of the galaxy) is now believed to be approximately 0.1%, that means a galaxy of one billion solar masses contains a super-massive black hole of approximately one million solar masses. It's been used ever since.
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| Hobby-Eberly Telescope Facility |
Types of black holes
Previously scientists thought that , a black hole is either small like few miles across ( around 2,3 times the mass of sun ) or supper massive ( millions or billions times more heavy than our sun). But recently scientists have found out that mid sized black holes do exist.
Well this is not all. In the early moments of the big bang, the tempreture and densities were huge so it is possible that micro black holes or quantum primordial black holes or tiny black holes were created in the high-density environment of the early Universe (or big bang), or possibly through subsequent phase transitions.
These mini black holes can be produced inside the large hydron collider providing the 1st experimental evidence of Hawking's radiations. These mini black holes would be so tiny that they would evaporate almost instantly.
To know more, please visit - arxiv.org
Blackholes as wormholes!
The best description of a spinning black hole was given in 1963 by the New-Zealand mathematician Roy Kerr, using Einstein’s equations of gravity. He predicted that if one fell into a black hole, one might be sucked down a tunnel (called the “Einstein-Rosen bridge”) and shot out a “white hole”( the opposite side of a black hole) in a parallel universe! Kerr showed that a spinning black hole would not collapse into a point (singularity), but to a “ring of fire.” Because the ring was spinning so rapidly that the centrifugal forces would keep it from collapsing. This “wormhole” may connect two parallel universes, or even distant parts of the same universe.
So if the above statement is true then it could be possible that our black hole is only the flip side of a 'white hole' so whatever our black hole consumes it spits it back through the white hole on the other side. Scientists have calculated that when you put the parameters (Like mass, size) of our universe into the equation of a black hole, then it fits perfectly So our universe solves the equation for a black hole. Well if it's true then we are inside a black hole ourselves and there could be billions of other universes out there.
Black hole is still an unsolved mystery. So may be some of the things we have discussed till now could be meaningless in the future.
So it's clear that the black holes are the biggest mystery out there. But there are things even more mysterious than black holes.
What are the supreme forces of the universe and how galaxies are built?
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