What’s up with Black Holes?

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                On April 10 2019 first image of the Black Hole was released by the team of Event Horizon Telescope Collaboration and it broke the internet. Everyone around the world was discussing about it on the social media, Television, etc. but does everyone know what is a Black Hole? When this word strikes to our ears we suddenly think of some sci-fi movies demonstrating time travel or some science show that has shown that black holes are massive objects in the Universe with a very high force of gravitation or Einstein or Stephen Hawking….. There are many myths and misconceptions about black holes. Black Holes are one of the biggest unsolved mysteries of 20th & 21st centuries!
 Credits: Event Horizon Telescope Collaboration

The term ‘Black Hole’ was first coined by Albert Einstein in the year 1916 and theoretically proving its existence by general relativity. According to him black hole was a singularity with very strong gravitation that even electromagnetic waves cannot escape. Its mass continuously increases as it keeps on absorbing matter but it shrinks due to its own gravity, hence its density is infinite. Even the laws of physics break down on the surface of a black hole!

Source: Google Images (An artist's illustration of a Black Hole)
Black Hole is formed from a star quite bigger than our sun. Such stars fall under the category of Massive Stars. For a star to shine continuously, it should burn its fuel. When a certain critical amount of fuel is burnt, dark patches start to appear on the star. At this time sun like stars become Red Giants whereas Massive stars become Red Super-giant. Burning of fuel also keeps the star in its shape. But when its fuel is completely combusted, that is the next step after Red Giant/Super-giant, a star cannot withhold its gravity, so it starts evaporating. A normal star reaches the stage of Nebula which is a dense cloud of gases formed from exhaust gases of the star, and then rebirth of a star from Nebula takes place. But for a Super-giant, it’s a different case.
Credits: NASA and the Night Sky Network
After complete combustion of the fuel in a Super-giant, star does not evaporate slowly rather a big explosion takes place called Supernova. Nebula and Supernova are the two most beautiful things to observe in the sky! After a Supernova explosion if the Massive star is big enough to retrieve a large portion of itself then it forms a Black hole! After a Black hole is formed, as it has very high gravitational attraction, it starts pulling nearby celestial bodies; but also cannot resist itself from shrinking.
The largest Black Hole in our galaxy (Milky Way): Sagittarius A* is assumed to be located in the center. Its estimated mass is approx 4.6 million times that of our sun. Our galaxy as we see today is due to the presence of this black hole. Milky Way was born due to the birth of Sagittarius nearly 13.7 billion years ago. And our solar system is believed to be 4.6 billion years old. The Sun is at the outermost edge of the Milky Way, and about 27,000 light years away from Sagittarius! All the stars in the galaxy revolve around the galactic centre; and since the birth of the Sun, it hasn’t even completed half of the revolution! Temperature on the surface of Sagittarius A* is approximately calculated to be 1.4×10-14Kelvin which can be considered absolute zero. Hence from the Second law of Thermodynamics, entropy on the surface of the Black hole is infinite. Infinite density, infinite entropy, ridiculous gravitational pull, absolute zero temperature, escape velocity greater than velocity of light, etc. ….seems like a very different type of Physics!
Credits: NASA/JPL-Caltech
Well, the Black Holes aren’t that scary. Movies, Novels, TV shows might have horrified the scene of a black hole; but the truth is a black hole is no “vacuum cleaner”, it is just like a normal giant star with its own gravitational force. It doesn’t just go on swallowing celestial objects randomly. The stars, nearby the black hole, that lose energy due to the gravitational influence, tend to cross the event horizon and are eventually pulled inside. Event Horizon is a boundary around a black hole, which, if any object crosses, is pulled into the Black Hole. But the rest of the matter outside the event horizon just keeps revolving around a black hole. Take Earth for instance; the satellites in the Low Earth Orbit, if deviates from their path or lose its energy, it will fall into the Earth. But the Geostationary satellites in higher orbits keep on revolving around the Earth. Our solar system is just like the geostationary satellite to the Black Hole in the centre of the Milky Way and hence it will continue to revolve around Sagittarius A* forever. Even if Black Holes were “vacuum cleaners” per se, it would take around 1030 (a nonillion) years until that tragic day comes for our solar system! So until then you have nothing to worry about; because “Black Holes ain’t so Black!”