How Does A Black Hole Form ?
And What Happens When It Forms ?

 A black hole forms when an object reaches a certain density and its gravity cause it to collapse.  It sucks and pulls in anything that is near it.

Explanation:

Black Holes form from the Gravitational Collapse of large stars. This occurs usually after the star is unable to keep up with its current shape as a giant star, and starts to contract under its own gravity. A giant star uses its 'fuel' inefficiently, burning lots of it in a short time. After the process of stellar nucleosynthesis is unable to provide the star enough energy to maintain its state, the star starts contracting under its own gravity.

This may have several consequences, and the result depends upon the mass of the remaining matter in the star. This is because the outer layers of the star could be exploded away due to a supernova explosion, or from a pulsating star (pulsar). This remaining mass can be substantially smaller than the original star.

If the mass of the remaining matter of a star exceeds 3-4 times that of the sun, which can happen due to the large mass of the original star or the remnant creating accretion to engulf matter, then the contraction in the remaining matter under gravity will be unstoppable by almost any kind of internal pressure inside the matter. In this case the remaining matter will go on collapsing to form a black hole.

As the collapse occurs, the gravity of the contracting mass becomes increasingly intense in the nearby region, and the Escape Velocity from its surface goes on increasing. Eventually, the escape velocity reaches the speed of light, and after that point nothing can escape from the surface when it is at a particular distance from the centre of the black hole. This region is known as the event horizon of a black hole.

At the centre of most galaxies, super-massive black holes are believed to exist. The rest of the matter in the galaxy revolves around it.

Black Holes exist for a very long time. No black hole in the universe has evaporated completely yet, since the time of Big Bang.

Explanation:

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