I read many times that black holes is created when center of a very massive star collapses in upon itself but what does it mean ? Does weight of center of massive star increase so much that itself can't bear weight or I can say space itself cant bear weight of such center of massive star that it collapse. I tried to read answer on google but want to know in layman language as I am not astro physics researcher or student. Thanks
This Wikipedia page on Stellar Evolution is a good starting point with lots of links to more info.
A star is a huge sphere of mostly hydrogen.
Like any large mass it has a gravitational field and that field acts on star itself.
So it would always want to collapse to as dense a sphere as possible under it's own gravity.
Now as the density increases, particularly at the center, the pressure also increases and so does the temperature. When these are high enough the core of the star will be able to fuse hydrogen (and later other materials) into heavier elements. Doing so releases huge amounts of energy and this is what creates all that light we see.
But the energy released also prevents the star from collapsing more.
And it can stay that way (more or less) for billions of years. It is this energy produced by fusion at the core that balances the gravitational force (weight) of the rest of the star trying to crush the core. So fusion that generates more energy than it requires to happen is essential to keep a star "alive".
After a long time, the hydrogen at the core runs out and the star starts to burn heavier elements at the core (helium, carbon, etc. ). It does that until those run out and (roughly) we reach elements like Iron remaining. At that point fusing those elements no longer generates enough energy to stop the star collapsing and it starts to collapse again, but very, very rapidly.
Exactly what happens next depends mostly on the mass of the star. It can end up as a white dwarf, neutron star or black hole.
But the final collapse is a very rapid process and quite violent.
Once the useful fuel at the core runs out, the stars dies a spectacular death one way or another.
What is a black hole (in simple terms) ?
Gravity is a result of the way mass (or even energy) can distort space.
Beyond a certain point this distortion becomes so great that everything on one side becomes trapped inside a "surface" called the event horizon. An event horizon is what a black hole actually is - no event horizon, no black hole.
Within the black hole, space is distorted in a way that's basically impossible to describe without some mathematics. The effect is that there is no direction you can move in and no speed you can travel at which will not lead you to the center of the black hole - the singularity. Don't even try and visualize it, it just isn't a common sense idea.
This even works with light.
Now an event horizon is not a surface you can feel. You can actually travel across an event horizon (but only inward, not outward !). So this change in the way space-time is distorted is not something that is sudden or obvious to the person doing the falling.
Back to our star collapsing.
As it collapses (without energy from fusion to stop that), it becomes denser and denser.
Now if it is large enough it will collapse to a black hole, because the core will become so dense from the collapse that it will be enough to form an event horizon.
Our Sun is not large enough to do that.
A star is always a balance of gravity trying to collapse it inward and nuclear fusion trying to blow it up. Usually, the forces are in balance, as the gravity causes more nuclear fusion from hydrogen all the way to iron. However, iron takes an exponentially higher amount of energy to fuse, so gravity wins and crunches the star inward. This weight forms the black hole and also generates a final burst of nuclear fusion, causing the burst of light that people see, and also forming the elements beyond iron. Hope that this answer helps!