Black Hole is a black coloured mass or a place in spacetime which have a massive gravitational force so that even not a light particle can escape. It have a massive mass squeezed in a small volume. The black hole which is present at the center of the Milkyway galaxy is known as Sagittarius which is a supermassive black hole, it have mass of 4 million Sun and have volume of few million Earth, and it contains about 4.3 million solar masses. You can imagine- 'How much mass it have?' The colour of black hole is black because it does not reflect light particles in spite of this, it trap light particles. A black hole in spacetime is completely invisible for human eyes, even if we are in outerspace. So scientists use cameras and telescope with special tools to find a black hole.
But actually scientists can't see black at first, they observe any star which is orbiting the black hole or near the black hole. If any star is present close to black hole, then its light will bend towards the black hole. In this way scientists find black holes.
Most of the scientists believed / generally consensus that most of the galaxies which is present in this universe have a centrally located supermassive black hole. And they also contribute that the supermassive black hole which is present at the center of the Milkyway galaxy was formed when this universe came in existence. It is described by the space scientists that the size of a black hole, may be equal to the size of an Atom and it may be equal to the size of the millions of the Sun. But a black hole can expand till to the end of the universe by absorbing other stars and by merging with other black holes which are present in this universe. Here a question arise -' Can black hole pull the earth towards it?'
~ It may be possible, but their is not any black hole which is present close to the earth, and black holes does not move in space.
How Black Holes form?
It is expected that, when very massive stars collapse at the end of their life cycle, then a black hole is form. The smallest black holes formed when the universe began. Stellar black holes are made when the center of a very big star falls in up on itself, or collapse, when this happens, it causes a supernova.
John Michell was the first (in the year 1784), who describe the idea of a body whose gravitational force is so massive that even electromagnetic waves like light particles could not escape. He was astronomical pioneer and english clergyman. He simply assumed that such a body might have some density as the Sun, and concluded that such a body would form when a star's diameter exceeds the Sun's by factor of 500, and the surface escape velocity exceeds the usual speed of light.
Black Hole Radiation
It is also known as Hawking radiation or Hawking-Bekenstein radiation on the name of physicist Stephen Hawking, who provided a theoretical argument on the existence of radiation in black holes. And Jacob Bekenstein, who predicted that black holes should have a finite entropy.
Hawking radiation is blackbody radiation that is predicted to be released by black holes, it is due to quantum effects near the event horizon. According to the quantum mechanical uncertainty principal- a rotating black should create and emit particles.
Black Hole evaporation- Hawking radiation reduces the mass and energy of black holes. Because of this, a black hole that do not gain mass through other means, were expected to shrink with time. In the year 1975, when Stephen Hawking published his theory of black hole radiation, everyone who takes quantum theory on account were shocked. According to Hawking radiation theory, black holes are not quite black, but it radiate or glow slightly with consisting photons, neutrinos and to a lesser extent all sorts of massive particle. This contradiction by Hawking, has never been observed by any scientist. Since the only thing we know about it was- Black holes always growing by trapping light, gases, or other solar particles, before Hawking give his theory. Stephen Hawking contradict that, if mass of a black hole is 'M' then it glows like a black body of temperature,
6×10^-8/M kelvins
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