Black holes are neither holes nor it is black as it is painted. A Blackhole is a region of spacetime where gravity is so strong that nothing, no particles or even electromagnetic radiation such as light can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. It is said that facts are sometimes stranger than fiction. Well, that's absolutely true in case of black holes. The scientists believed that the massive stars at its end-stage could collapse in on themselves, under their own gravity and wondered how did they behave. Albert Einstein even wrote a paper in 1939 claiming that stars could not collapse under gravity because the matter could not be compressed beyond a certain point. Many scientists shared Einstein's gut feeling.
Black hole in Spacetime curvature
The principal exception was the American scientist John Wheeler, who in many ways is the hero of Blackhole story. In his work in the 1950s and 1960s, he emphasized that many stars would eventually collapse, and pointed out the problems that possibility posed for theoretical physics. He also foresaw many of the properties of the objects which collapsed stars become that is, black holes.
The phrase 'Blackhole' is simple enough, but it's hard to one out there in space. Think of a giant drain with water spiraling down into it. The analogy is similar to that of a Blackhole. The edge is known as the Event horizon (there is no way back). Because black holes are so powerful, even light gets sucked in, so we can't actually see them. But scientists know they exist because they rip apart stars that get too close to them because they can send tremors through space. It was a collision between two black holes more than a billion years ago that triggered what is called Gravitational waves, the recent detection of which was a hugely significant scientific achievement.
In a couple of papers in 1939, Robert Oppenheimer along with George Volkoff and Hartland Snyder showed that stars with greater mass than a white dwarf or neutron star when exhausted their nuclear fuel, could not be supported by the outward pressure; and that, if you take the pressure out of the calculation, a uniform spherically systematic symmetric star would contract to a single point of infinite density. Such a point is called a singularity. It refers not only to the end of a star but also to a far more fundamental idea about the starting point for the formation of the entire universe.
Einstein's Theory of General Relativity says that objects distort the spacetime around them. Imagine a bowling ball lying on a trampoline, changing the shape of the material and causing smaller objects to slide towards it. This is how the effect of gravity is explained. But if the curves in spacetime become deeper and deeper, and eventually infinite, the usual rules of space and time cease to apply.
Distortion of Spacetime continuum due to Black hole
From the outside, one can't tell what is inside a black hole. You can throw anything into a black hole, and all the black hole will remember is the total mass, the state of rotation and the electric charge. A black hole has a boundary, called the event horizon. This is where gravity is just strong enough to drag light back and prevent it's escaping. As nothing travel faster than light, everything else will get dragged back as well. If you fall towards a black hole first, gravity will pull harder on your feet than your head, because they are nearer the black hole. The result is you will be stretched out longways, and squashed in sideways. If the black hole has a mass of a few times the sun's you will be torn apart and made into spaghetti before you reach the horizon. However, if you fall towards a much larger black hole, with a mass of a million times the sun's, you'll reach the horizon without much difficulty. So, if you wanna explore the inside of a black hole, make sure you choose a big one. There is a black hole with a mass of about four million times of the sun at the centre of our Milky Way galaxy. It is named Sagittarius A*
Basically, we have got two types of black holes :
Stellar-mass Black holes
Supermassive Black holes
A Stellar black hole (or Stellar-mass black hole) is a black hole formed by the gravitational collapse of a star. They have masses ranging from about 5 to several tens of solar masses. The process is observed as a hypernova explosion or as a gamma-ray burst. These black holes are also referred to as collapsars.
A Supermassive black hole is the largest type of black hole, containing a mass of the order of hundreds of thousands to billions of times the mass of the Sun. Black holes are a class of astronomical object that has undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light. Observational evidence indicates that nearly all large galaxies contain a supermassive black hole, located at the galaxy's centre.
Supermassive Black hole
Messier 87* black hole image by ETH
Recently, A black hole and its shadow have been captured in an image for the first time, a historic feat by an international network of radio telescopes called the Event Horizon Telescope (EHT). The stunning new image shows the shadow of the supermassive black hole in the center of Messier 87 (M87), an elliptical galaxy some 55 million light-years from Earth. This black hole is 6.5 billion times the mass of the Sun. Catching its shadow involved eight ground-based radio telescopes around the globe, operating together as if they were one telescope the size of our entire planet.
One interesting fact about the black holes is that it slows down the time near its spacetime continuum. To oversimplify the explanation, you have to understand the curvature of space-time around a black hole. The basic principle is that because of the curvature of spacetime around a black hole, the amount of "distance" a beam of light has to cover is greater near a black hole. However, to an observer in that gravitational field, light must appear to always be 300,000 km/sec, the time has to slow down for that individual as compared to someone outside that gravitational field as related by the time/distance relationship of speed.