Black Holes Explained: What Is a Black Hole? How They Form?

Black holes are one of the few natural phenomena that were mathematically predicted first, and then observed later. They were first predicted in the 1780s by natural philosopher John Mitchell, who suggested that the gravity of a sufficiently dense star could be so great that even light emitted by it would be unable to escape its pull. The discovery in Cygnus relieved astronomers, because otherwise they would have been labeled crazy to believe in an object so massive that it's invisible. Because Mitchell's star doesn't emit any light, it wouldn't look like this, but perhaps something like this. This is why they are called black holes, because that's what they are, a black void in space. To understand why a star would grow so withdrawn, we must take a look at its rough childhood. A star is born when gravity forces a huge volume of mostly hydrogen gas to collapse in on itself. This compression increases the gas's temperature, causing its atoms to violently collide with each other. These collisions further heat the gas until the hydrogen atoms don't collide and ricochet, but coalesce to form helium atoms. The mass of a helium atom is less than the combined mass of two hydrogen atoms. The remaining mass is released as the energy, whose magnitude is given by Einstein's famous E equals mc squared. The energy released might be small for two coalescing atoms, but for billions and billions of atoms, the cumulative release is tremendous. This same principle that makes a star shine is replicated inside a devastating hydrogen bomb, albeit in a controlled manner. To survive, the star's expansion, driven by the explosion, must neutralize the compression driven by its gravity, but eventually the star runs out of fuel. All the hydrogen has fused to form helium, all of which has then fused to form carbon, and so on, until finally iron is synthesized. Iron refuses to fuse any further, so the star is now jam-packed with heavy elements. With no more fuel to burn, the star begins to cool, and with no heat to combat the compression, it begins to contract. In 1928, during his voyage from India to England, Chandra Shekhar realized that a star could survive if the gravity's contraction is counteracted by the We now call such stars white dwarfs. These are hundreds of tons per cubic inch dense, as all the mass is packed in a sphere of just 1,000 miles in diameter. However, if the star is any denser, its gravity would overcome even those repulsive forces between the electrodes. He calculated that a cold star 1.5 times the mass of the sun would undergo further contraction, yet it could hold on to life by neutralizing gravity's pressure with the repulsive forces between its neutrons and protons. Such a star is thus known as a neutron star. These are millions of tons per cubic inch dense, as all the mass is packed in a sphere nearly 20 miles in diameter. But what would happen if a star were even denser? After arriving in England, when Chandra Shekhar showed his results to Arthur Eddington, the astronomer couldn't believe that a star could become infinitely dense. He refused to believe that a star such as the sun could collapse to a point. In the 60s, Stephen Hawking and Roger Penrose predicted that an even denser star would contract into a point of infinite density and hence spacetime curvature, a point where all known laws of physics would break down. This point is called a singularity. The star becomes a black hole because the singularity distorts the spacetime around it so severely that any light which falls into the pit is held captive forever. Nothing escapes. The boundary where the pit begins is known as the black hole's event horizon. It is imperative to know that black holes don't suck up everything like a galactic vacuum cleaner. If a massive or supermassive black hole, the mass of the sun was to replace it, we'd still be revolving undisturbed. Survival, however, would not be guaranteed. So we developed a mathematically rigorous model of a black hole, but how was one supposed to find evidence for its existence? How was one supposed to find, as Hawking asks, a black cat in a dark room? Then we pointed our telescope towards the constellation Cygnus. Cygnus X-1 is one of the strongest sources of x-rays that is visible from Earth. Astronomers realized that matter from the rotating star was blown off into orbit around its invisible companion. The rotation caused it to heat so greatly that it gave off x-rays. The unseen thing, however, didn't necessarily have to be a black hole. It was equally likely to be a massive star just too faint to be visible. But with the knowledge of the dynamics of the star's orbit, astronomers determined the object's mass to be six times that of the sun. The object was too massive to be a white dwarf or a neutron star. While we lack any explicit evidence due to its withdrawn nature, we have a multitude of indirect evidence that suggests that Cygnus X-1 is definitely a black hole. Hawking believed that the universe is replete with such black holes. He audaciously speculated that the number of black holes is greater than the number of visible stars. Surely, millions of stars have exhausted their fuel during the billions of years this universe has existed. Supermassive black holes, billions of times the mass of the sun, are believed to exist at the center of our own and probably every galaxy there is. What's more astonishing is that Hawking, who went on to occupy the prestigious Lucasian chair at Cambridge, once held by Newton, showed that black holes aren't so black after all. He found that they emit very tiny amounts of radiation, which we now call Hawking radiation. A black hole radiates particles and eventually vanishes, but it takes one billions and billions of years to completely evaporate. Lastly, a singularity is the most notorious phenomenon in the universe. No one knows what mystery lies at the bottom of the pit, but it seems to combine the two final pieces of physics. Its large-scale properties concern classic general relativity, while its point size concerns the microscopic quantum mechanics. Together, they combine to form the theory of everything. For over 60 years, no one, including Einstein, has been able to fully get a hold of this theory. It would combine every little discovery that man has made throughout his endless quest for certitude. The theory of everything will surely be the greatest triumph of human reason.