Curiosity builds up as this topic emerges. One of the most intriguing and exciting mysteries in the Universe. An Astrophysical body that practically gallops everything in its vicinity. Einstein’s General theory of relativity theoretically proved the existence of black holes. The Phenomenon was so strange that even Einstein couldn’t believe that such a body could practically exist in the Universe despite his theory proving their existence.
A Black hole is nothing but, an astrophysical body having enough mass so as to warp the space-time continuum so much that nothing in the vicinity, not even light can escape its gravitational pull. What causes them? When a Massive star, having mass 100s of times mass of the sun starts dying, the fusion reactions start the formation of heavier elements like silicon, Copper and finally Iron. Once heavy metals like iron are formed from the fusion reaction, no more energy can be radiated, only the mass of the core is increased. When the core is filled with enormous amounts of iron, the core collapses under its own gravitation pull unable to sustain its volume. This will cause the core to shrink in size increasing the density of the core enormously. This will then cause a supernova explosion disbursing the gas clouds around the core leaving away a Black Hole. The core in the center is so dense that it causes a space-time warp so deep that nothing in its vicinity can escape its pull. This dense core is called Singularity.
The Black Holes get their name because of the fact that even light is absorbed by them. So, what does this exactly mean? If you consider a light approaching black hole from a distant galaxy, the light very close to black hole will get absorbed, light with some distance from black hole will change its path when near black hole circling around it. Sounds weird? We know that gravitation can bend the light in its direction but, the gravitation field of black hole is so strong that instead of bending light a little bit and curving its path, its gravity causes light to move in circles around it. Literally, light orbits around black hole. The circling around of light causes a ring type structure around the black hole. This is called an Event Horizon. So, if you are standing at the edge of an event horizon and you look at your right, you can see yourself standing right besides you because light reflected from your face is circled back to you. Basically Event Horizon is a point near black hole from where there is no return. Once a body is moved past the event horizon, it will need to move at a speed more than speed of light to stop falling into the singularity which means nothing can escape not even light once you pass the event horizon of a black hole. If you pass the event horizon of black hole, a distant observer will never actually see you falling inside the singularity, they will just see you become dimmer and dimmer nearing the edge of event horizon eventually vanishing.
The black holes which are formed by collapsing of a huge star are termed as stellar black holes. Scientists believe that there are another kinds of black holes called super massive black holes. These black holes have masses billions of times that of sun and volume up to millions of times that of sun. They are believed to be formed during galaxy formation. Scientists believe that every galaxy has a super massive black hole in its center.
What makes the study of black holes a tough task is that they are invisible. They don’t reflect light, they absorb it all so they are invisible. The Event horizon can be seen but, it requires specialized devices mounted to detect the gas clouds surrounding the event horizon. The scientists try to study the black holes by observing the effects they have on neighboring bodies like planets or stars. No one has seen a black hole. So, everything is based on theories. But, why should we believe those theories? What is the proof that actually something like a black hole can exist? Actually, there is. It all is in Newton’s law of gravitation and its formula. Newton’s formula for gravitation is:
It can be inferred from that, more the mass of the body and lesser its radius more is the gravitational force. So, if we decrease the radius of earth keeping the mass constant, it will increase its gravitational force. In other words, the escape velocity will increase. Now, if we could compress the volume of earth up to radius say, of Mumbai, it will have the escape velocity close to speed of light. Thus, we have a black hole. Any object if compressed of certain radius can give us a black hole. That certain radius is called Schwarzschild radius. Example, if Mt.Everest is compressed to a radius of basket ball, we have a black hole. If a basket ball is compressed to radius of one atom, we have a black hole. But, of course it’s not possible to achieve that type of enormous compression as of now.
Where do we stand in all this? We Earthlings need not worry about black holes galloping earth because even the closest black hole from earth is millions of light years away. And Sun will never turn into a black hole as it does not have sufficient mass. And even if it somehow does turn into a black hole, earth will still continue to revolve around it, apart from the fact that everything will be dead frozen.