Black Holes

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.

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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.

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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:

                                                            F=G(m1m2)/r2

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.

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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.

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Einstein’s General Theory of Relativity

One of the most important theories in the history of Astrophysics which has changed our views to see the universe entirely. One of the most brilliant minds the world has ever seen, Albert Einstein has proposed this theory of general relativity. The General theory of relativity and Time Dilation go hand in hand. This theory further expands the views of time dilation and combines it with space expansion. Let’s get into it. Again, this might shatter some of our preoccupied notions and might be thought as sci-fi but, it has been accepted by many scientists and its time we do too.

Let’s again start with a hypothetical experiment. We are sitting in the car which is at rest and the headlights are blinked. The speed of light emitted from headlights is measured and is recorded accurately as speed of light. Now, if the car is moved at a speed of say 80 and the headlights are again blinked, and the speed is recorded, the speed remains constant. Yes, as the speed of car is negligible as compared to that of light but still there is not even negligible increase in the speed of light when measured accurately enough. The reason for this as we know now is Time Dilation. The time moves slowly so as to keep the speed of light constant. But, Einstein says that it’s not only time that moves slowly but, the space (or in case of car ‘Distance’) expands. So, basically, we know Speed= distance(Space)/Time. Einstein says not only time slows down but also the space expands and both of them thus keep resultant speed of light constant. This led to a new revolutionary theory that space and time are not different entities at all. They are related and linked to each other. This new entity, Einstein called it as Spacetime.

Continue reading “Einstein’s General Theory of Relativity”

The Stars

Let’s start the discussion with something that we know and see daily. We have studied from childhood that stars are the massive bodies in the universe that shine by themselves. We also know the closest star to us, The Sun. But, what is it that makes The Sun and other stars like it shine so brightly for almost forever?

Firstly, let’s address the fact that there are many types of stars. The Sun, is a Main sequence Star. Some stars are bigger than sun viz, Giants and Super Giants. Some stars are smaller than The Sun viz, White dwarfs. Basically, at the time of birth stars are a huge mass of gas. In general, all the stars are made up of two thirds of Hydrogen and one thirds of Helium. These gases form a cloud which collapses when the pressure and temperature are high enough to start a Nuclear Fusion reaction. Thus a Star is born.

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The Nuclear Fusion fuses two Hydrogen atoms to a Helium atom generating a massive amount of energy at very high temperatures almost to millions of degrees. This process is continued for the most of the lifespan of the star and these stars are called the Main sequence stars. You might be thinking that fusing two Hydrogen to helium is the cause of the light and the heat of The Sun but, the Hydrogen deposits will be extinct some time or later. Yes. You are right. The Fusion reactions on The sun Gallop Hydrogen at the rate of almost 4 Million Kg per second. But, the mass of The Sun is so huge that even at this rate it will still take Billions of years for the Hydrogen deposits to become extinct. Continue reading “The Stars”

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