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So what’s a black hole? A celestial body with such an intense pull that nothing, not light, not electromagnetic radiation – nothing can escape its pull; hence the name black hole. It possesses an infinite density and is a one-way pathway because things can go in but cannot escape out. Its core is termed as singularity and the outer boundary is the event horizon, the end point beyond which anything and everything is sucked into the cosmic whirlpool of infinite density.
But how do blackholes form?
There are many theories to that question.
Most common theory is where a colossal star with a mass of more than 3 times the Sun’s reaches the end of its life, gets crushed under its own gravity, leaving behind a compact blackhole.
Let’s see how that intriguing process takes place.
When a gigantic star reaches the final stage of its life and is about to go supernova (which normally takes billions of years), it spends all the nuclear fuel by then. So it stops burning and heating up and cannot create the nuclear energy required to feed the star and let it make a pivotal balance to support its own gravitational draw against the intense pressures brewing inside.
Therefore its stability cracks under its own gravity.
The radius of the star shrinks to a critical size, called the Schwarzschild radius and it starts to devour anything and everything that comes a bit too close, including light. Gravity does its job and the core of the star caves in and implodes.
The outer shells of the star explode into the space. They may even fall into the already dense black hole making it even heavier and denser. And that’s how you get a stellar mass black hole.
Now let’s explore the various reactions going on inside the star that result in a stellar mass black hole.
Basically what happens is that nuclear fusion reactions take place in the core of the star which causes an acute outward pressure but that pressure is optimally balanced by the intense inward pull of gravity by the star’s mass. But when a star is in its death throes, the fusion reactions combining hydrogen into helium (like in the Sun) stop and a new kind of nuclear reaction take place that convert helium into carbon.
This is followed by carbon turning into oxygen and oxygen to silicon and then to iron.
That’s the point where nuclear fusion stops and the outer layers of all the elements produced (hydrogen, helium, carbon & silicon) keep burning around the central core of iron.
The mammoth iron core builds up and finally explodes which is called a supernova explosion.
Well after that there can be a few outcomes of the fate of the now-blown-up star –
1. A star with a mass 1.4 times more than that of our sun will after a supernova explosion simply compresses further into a mass of dense neutrons (they are so dense that 100 million tons of them would be equal to just one teaspoon!) and become a massive neutron star held up by neutron degeneracy.
2. But if the neutrons degenerating are not able to prevent the star’s collapse due to the gravitational forces lurking inside, its shrinks and compresses into an infinite void of blackness or in other words – a stellar mass black hole.
There’s another category of black holes known as the ‘supermassive’ ones.
Supermassive black holes are present in the centers of most galaxies, including our very own the Milky Way. Until now whatever black holes that scientists and astronomers have identified have been either supermassive ones or the size of normal stars.
If you put billions of suns together, you will know the mass of one supermassive black hole. And that’s why the name as they are super-colossal in size.
Astronomers aren’t very sure as to how black holes are formed of the supermassive variety. Maybe a couple of smaller black holes coagulate together, or humongous gas clouds cave in to form them.
The British Scientist Stephen Hawking proposed that trillions of nonstellar black holes or mini or primordial black holes were created along with the universe in accordance with the ‘big bang’ theory.
But that’s just another ‘unproven’ theory along with one that suggests high energy collisions produce the required dense matter that can create black holes.
Some intermediate black holes are supposed to be formed by the amalgamation of many smaller and compact cosmic bodies.