An Expedition into the Singularity: A brief study of Black holes

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In this vast observational universe, there are billions of celestial bodies existing, among which humans have explored only a few of them in real life. Beyond these, there lie several mysterious bodies in the infinite space that always intrigued the minds of astronomers. Indeed, black holes were the predominant among them. This article is an expedition to the world of black holes.

On April 10, 2019, the world witnessed a historic moment, when scientists captured the first-ever black-hole picture of M87 in Messier 87 which is a super-giant elliptical galaxy in the Virgo constellation, it became a big turning point in the whole course of study about these dark giants. Researchers are still after the quest to unravel the mysteries of this void which is a great concern to the whole scientific world.

What is a black hole? 

In 1783, a famous scientist John Michell, in his paper introduced the idea of black holes as a massive, compact star with a strong gravitational field. He also stated that we didn’t see these stars because the light cannot escape from them to reach us. Black holes were believed to be the black voids in the universe as their name suggests. Later from the detailed studies, scientists realized that black giants are formed in the last stage of the life of a star, whose mass is very much greater than the Chandrashekar limit (1.4Mʘ ). Chandrashekar limit is the maximum mass of a star to become the white dwarf. Beyond this limit, a star can be either a neutron star or black hole.

Everything in the universe is conceptualized to have a start and an end. This idea compelled us to think that stars also have a beginning and an end.

Birth of a star

A star is born from the region called ‘Nebula’ which is a combination of dust and gas. These mixtures will come together and reach a stage where the gravity pulls everything into a point. This is called the gravitational collapse. This leads to the increase of pressure inside the nebula and which in turn increases the temperature inside this system. The temperature will become so high that the gas inside the nebula initiates nuclear fusion. Gradually, nuclear fusion results in an outward pressure that will balance the gravitational collapse (hydrostatic equilibrium). This balance eventually comes to a stage, where the star becomes stable. This stage is called the main sequence in which nuclear fusion of hydrogen and helium occurs inside the core of the star.

 

Death of a star

Gradually, inside the core, heavier elements start undergoing fusion, and finally carbon or iron remains. At this stage, the stars run out of fuel and reach a point where fusion no longer occurs. The last stage of the star is dependent upon its mass. The higher the mass, the hotter it will be because of the need for high temperature to maintain hydrostatic equilibrium, then the faster it will burn up the fuel and eventually run out of fuel. This leads to the death of a star. There can be a few crucial cases to be considered.

The crucial stages: White Dwarf, Neutron star, … Black Hole

Firstly, if the mass of the star is less than the Chandrashekar limit, they will explode (planetary nebula) and the remaining core exists as a small hot white ball called a white dwarf.

Secondly, If the star has a greater mass than the Chandrasekar limit, it will undergo a supernova explosion in which the outer shell of the star will burst out in space. If the star has a mass in between 1.45 to 3 solar masses then the core will shrink into a size where the star becomes a ball of neutrons and neutrinos. Neutrinos are so light that they will fly out into space and the core will remain as a ball of neutrons called a neutron star. Neutron star will have a magnetic field around it that sweeps the star surface and produces a pulsating light. Thus, a neutron star is also called a pulsar. When the star has a mass four times greater than the Chandrashekar limit, then after the supernova explosion the core will undergo a constant gravitational collapse and will eventually become a black hole.

Black hole structure

Black holes are only a few miles in size but very much denser than we can expect. Most people think that a black hole is like a vacuum cleaner which will suck everything surrounding them.

But in reality, black holes are like a deep well in the dark. If you step inside the boundary, you will enter a region where nothing can escape, not even the light! This boundary is called the event horizon. The radius of the event horizon is termed as the Schwarzschild radius. What is inside this black hole is still a mystery. Scientists say that there is a point of Singularity which is the center point of a black hole where the gravity is infinite and the laws of physics no longer applicable.

Outside the event horizon, different stellar materials that are close to the black hole are spinning towards the center due to gravity. This region is called an accretion disk. In rotating black holes the region around it will also rotate due to a phenomenon called space dragging. This area is called the ergosphere.

Types of black holes

For a long time, black holes were just a few mathematical solutions for scientists. But years later, they started to observe these hidden mysteries by the effect of them in their surroundings. There are two classes of the black hole – those with masses about five to several tens of times that of the sun, are called stellar-mass black holes, and those with masses millions to billions of times that of the sun, are called supermassive black holes.

Stellar-mass black holes are formed when a massive star dies and collapses. The evidence of these types of black holes can be seen in a binary star system.

Mostly in a binary star system, one among the two can be a white dwarf and if the companion is not visible then it’s for sure that it is a black hole. For astronomers, it is a clue for the existence of the black hole.

The other class of black holes is Supermassive black holes found at the center of nearly every large galaxy. The origin of this kind of black hole is still under study. Our galaxy; the Milkyway has a supermassive black hole at the center called Sagittarius A*.

Einstein’s General theory of relativity and black holes

Albert Einstein’s General theory of relativity is a geometric theory of gravity in which he explains gravity as a geometric property of space and time and he called it ‘space-time’. Gravity depends upon the mass of the body. If the body is massive, it can bend space-time. He represented these theories in set partial differential equations known as field equations. Until then scientists believed that light travels only in a straight line. But the General relativity explained that light does not always travel in a straight line. Instead, it will bend near massive objects because of the bending of space-time fabric. This was proved in 1919 on a solar eclipse day when the scientists confirmed the bending of light at an angle of 1.75 arc seconds (0.0005°). This was a revolutionary theory and got a lot of attention.

Inspired by Einstein’s general relativity, a German physicist Karl Schwarzschild in 1915 tried to find the solution for the Einstein field equation. He surprisingly came across the idea of a black hole. He also found the radius at which a star becomes a black hole. It is called the Schwarzschild radius also called a black-hole radius. At this radius, the escape velocity will become equal to the speed of light. So that even light cannot escape from it.

He surprisingly came across the idea of a black hole, and he solved the radius at which a star becomes a black hole,

\( r=\frac{2GM}{c^2} \)

where G is the gravitational constant, M is the object mass, and c is the speed of light.

Gravitational waves

Another interesting consequence of the general theory of relativity is gravitational waves. Gravitational waves are the ripples in spacetime. Billion years ago, two black holes or neutron stars collided with each other and made a huge impact in space-time. This impact travelled across space-time as the ripples in a pond.

Albert Einstein from his general theory of relativity explained these strange waves which can affect everything in the universe by stretching and compressing. But they are coming from a long distance thus will be very light. He also included that no instruments can detect these waves. But that statement was proved wrong in 2015 where the scientist detected the gravitational waves for the first time in LIGO Observatory. This experiment also won the Nobel prize for physics in 2017.

The existence of a black hole has been proved by scientists, and further studies are still going on. Recently in the year 2019, Scientists took the real image of the black hole, which was very close to what scientists had predicted earlier. These large mysterious celestial bodies keep us thinking that we have a lot to discover about this universe.  

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