The name Black Holes (BH) was given to dying stars at the end of their life or to the residual matter after the collision of two stars, by an American scientist, John Wheeler, at New York in 1967. Interestingly, there is nothing black about them and, in fact, BH are invisible. These represent un-imaginably huge vacant spaces (but no surface) inside all galaxies. However, they do not have vacuum inside them. Stars, planets or any other matter entering these spaces, after crossing a certain region around them, get sucked in. This region is known as the Event Horizon and may extend to millions of miles from the centre of the BH. These are invisible (and are called black) because even rays of light crossing the Event Horizon cannot come out of the BH. Stephen Hawking, the British scientist who passed away recently, however, showed that BH leak some energy, generated by their heating and spinning. This seepage is named Hawking Radiation in his honor.
The very existence of BH had been doubted initially and even Einstein considered it preposterous. Recent research (described in his last interview by Hawking, in October 2017) has shown that the very strong gravitational waves emanating after the collision of two decaying neutron stars has led to their collapse. This is the first direct proof of the birth of a hyper-massive BH, creating an infinitely dense point called Singularity. Advanced X-ray and radio telescopes have now established the presence of millions of BH in all galaxies, including our own.
The black hole in our Milky Way has mass about 4.3 million times that of our Sun and is about 26,000 light years away from us. This BH’s location has been plotted from its effects on the surrounding stars. Normally the repulsive nuclear forces inside a star are countered by the attractive gravitational forces and a dynamic equilibrium exists. As the nuclear fuel is consumed with the passage of time, the enormous gravitational forces overpower and ultimately the aging star implodes into a very heavy dense mass. For such strong gravitational forces to exist it is necessary that mass of the star should be more than at least 20 suns. Stars lighter than 20 suns degenerate into neutron stars that can become BH only after collisions. Stars having masses less than 10 suns become white dwarfs. These generally do not implode under gravity.
The density of matter inside a BH can be judged from the fact that if earth were to become as condensed and compact, it would have a diameter of about 2.5 cm only! To visualize how a BH is formed, consider a tight trampoline held on all sides and a heavy metallic sphere rolled onto it. It will go to the centre where the net will dip deep downwards making a curved cone. If another lighter sphere is now dropped on any one side it will roll over immediately into the curved dip already made in the centre of the trampoline and will not be able to come out. Very strong gravitational pull of a gigantic star likewise bends the space into a curved cone-like shape with a wide top (similar to the Event Horizon) and a deep bottom narrowing downwards. Matter of any size or mass that falls into this conical trap has no chance of ever coming out. The adjective black is used for these galactic traps perhaps to indicate that just as black objects have very high power to absorb (and do not reflect) incident radiations, the black holes also absorb everything that gets into them. ……………………………………………………………………………………………………………