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Black Holes

Author: Raja H R Bobbili, MIT

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I would not know if a normal high school curriculum goes into astronomy, but I can assure you that almost every high school student has learnt something about black holes. It is a topic which has attracted great interest around the world. It is said that there are super black holes trying to suck every material object of the universe into them. This should contract the universe, shouldn t it? But astronomical observations suggest that the universe is expanding. This is a major contradiction. There are many such interesting circumstances associated with the presence of black holes. The study of black holes is the key to understanding our universe. I covered this topic in passing in The Evolution of the Sun , one of my previous articles, but it was never covered in great depths.

Any giant body of mass, such as a star, has gravitational forces. There is always a gravitational pull from the centre of the star. Therefore, we may assume that this pull from the centre causes the star to contract. However, it is known to us that energy is produced in the star by thermonuclear fusion the process by which two hydrogen atoms combine at high temperatures to produce one helium atom. During this process, mass is lost and is converted into energy. This energy, which is radiated away from the centre, produces an outward pressure, opposing the gravitational pull. Therefore, we have two forces acting on the same star: a force toward the centre, trying to contract the star and a force away from the centre, trying to expand the star. These two forces cancel out and the star neither expands nor contracts. Its radius and size is now constant. Even as you read this article, a mass of hydrogen equivalent to 1 million cars is being converted to helium every second. There is an enormous amount of energy produced due to thermonuclear fusion. What happens when all the hydrogen finishes? When hydrogen finishes, thermonuclear fusion stops. When thermonuclear fusion stops, the balance between the gravitational pull and the outward pressure is lost, causing the star to contract.

What will happen to this contracting star? This star will keep on contracting until thermonuclear fusion restarts. If for some reason thermonuclear fusion does not restart, then the star will contract to a point mass it will have a zero volume and an infinite density. This, dear readers, is what scientists call a Black Hole. It is a hole in the spacetime axis. The point mass is called a singularity. This black hole has an accretion disk, which sucks in every object near it. This accretion disk has a pathway called the event horizon . No object which enters the event horizon can escape. Not even light can escape the strong gravitational forces. According to Einstein, light is the fastest moving object and hence, if light can not escape, nothing can. The radius of the event horizon is called the Schwarzchild radius. Even if black holes are too dark to be seen, their mass, rate of rotation and their charge can be measured.

There are two types of black holes: a Schwarzchild black hole and a Kerr black hole. A Schwarzchild black hole is a simple black hole, which does not rotate. It has an event horizon and a singularity. A Kerr black hole was formed from a rotating star. Because it is formed from a rotating start, the black hole itself is rotating. This black hole has a singularity, an event horizon, an ergosphere which is the distorted space around the even horizon and the static limit, which is the space which divides the ergosphere and normal interstellar space. It is important to note that when an object enters the ergosphere, it can still escape if it acquires the required speed. However, no object can ever escape out of the event horizon.

You would be asking yourself How do we detect black holes? Most black holes are in a binary system. That is, they are present with a companion star. By studying the movements of the companion star, we can detect the mass of the black hole. For example, there is a galaxy called NGC 4261, in which a disk is rotating. The disk is as big as our solar system but has a mass of more than 1 billion times that of the Sun. This shows that there is a black hole present. And we can estimate the mass of the black hole. We also use gravitational lens to measure some of the properties of a black hole. It was proved theoretically by Einstein s theory of General Relativity that space bends when gravity acts on it. This was later proved experimentally when a Solar Eclipse occurred. The Star s position was measured before the eclipse, during the eclipse, and after the eclipse. It was seen that the star s position shifted, showing that light bent due to the gravity of the star. Similarly, when light bends due to an invisible object, it can be concluded that there is a black hole. If there is a companion star to a black hole, the black hole will try and suck chunks of the companion star. The companion star heats up as its mass is pulled towards the even horizon. It gets to such high temperatures that it will emit X-rays. The intensity and the rate of emission of these X-rays can be used to find the mass of the black hole.

I hope that this fairly in-depth analysis of black holes has helped you. This is a major topic in International Baccalaureate and Advanced Level astrophysics syllabus. It is therefore important to understand the nature of black holes and the techniques used to detect them.

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