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Not a physicist just a simple guy, don't know much about physics as well. Just like to read things. I was reading about black holes and a question popped in my mind.

If I understand it correctly then the black hole is created by collapse of a super massive star at the end of its life. This star will have a finite mass. Then what leads to the black hole to have such massive gravity so that nothing can escape it (because if it was a star before with a finite mass then it would also have some finite gravity, right)?

And then they start absorbing more and more matter. So they grow bigger and bigger. Do black holes die?

Do all galaxies have a black hole at their center?

Thanks & Regards

Update-1

Found answer to the question, do black holes die?

Update-2

Found answer to the question, Do all galaxies have black hole in the center?

Posting the answer link here so anyone having this questions can find all answers in one place.

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  • $\begingroup$ I'm not sure I follow your question. The star that collapses into a black hole has less mass as a black hole, but much smaller size. It's the mass to size ratio that makes the black hole possible. More specifically, any mass has a corresponding Schwarzschild radius. As for your 2nd question, yes, all galaxies are believed to have a very large black hole at their center. $\endgroup$
    – userLTK
    Commented Sep 27, 2019 at 6:35
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    $\begingroup$ These are three different questions. In order for the Q&A format of stack exchange to work properly, it is better if you ask them as separate questions. $\endgroup$
    – TimRias
    Commented Sep 27, 2019 at 6:39
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    $\begingroup$ From en.wikipedia.org/wiki/Supermassive_black_hole Unambiguous dynamical evidence for supermassive black holes exists only in a handful of galaxies [...] Nevertheless, it is commonly accepted that the center of nearly every galaxy contains a supermassive black hole. [...] Although most galaxies with no supermassive black holes are very small, dwarf galaxies, one discovery remains mysterious: The supergiant elliptical cD galaxy A2261-BCG has not been found to contain an active supermassive black hole, despite the galaxy being one of the largest galaxies known $\endgroup$
    – PM 2Ring
    Commented Sep 27, 2019 at 7:02
  • $\begingroup$ Instead of adding links to your question you can (and should) write an answer. Self answering questions is encouraged! $\endgroup$
    – James K
    Commented Sep 28, 2019 at 6:20
  • $\begingroup$ @JamesK I would try to give an answer, thanks :) $\endgroup$
    – Bilbo Baggins
    Commented Sep 30, 2019 at 5:26

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Answering your first question:

If I understand it correctly then the black hole is created by collapse of a super massive star at the end of its life. This star will have a finite mass. Then what leads to the black hole to have such massive gravity so that nothing can escape it (because if it was a star before with a finite mass then it would also have some finite gravity, right)?

You are right that a black hole will have a finite mass that is smaller than that of the massive star that collapsed (some matter is carried off by the supernova). At large distances the gravitational field of this black hole will be no different than that of a star with the same mass.

The difference arises because in a black hole this mass is compressed into a much smaller volume. Consequently, you can get much closer to the concentration of mass, than you would be able to for a star (where the mass is scattered over a relatively large volume). It is here where the gravitational fields can become really "strong" and prevent light from escaping.

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  • $\begingroup$ what leads to such high compression of mass? And I will separate out the questions :) $\endgroup$
    – Bilbo Baggins
    Commented Sep 27, 2019 at 6:53
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    $\begingroup$ Gravity. The gravity of the mass itself compresses the mass. When the mechanisms that counter-act this compression (such as pressure caused by radiation inside a star) a remove, the mass is free to collapse under its own weight and form a black hole. $\endgroup$
    – TimRias
    Commented Sep 27, 2019 at 7:09
  • $\begingroup$ so kind of a chain reaction due to gravity, right? Once the mass falls due to its own gravity the mass is more dense and generates more gravity and hence forth, correct? $\endgroup$
    – Bilbo Baggins
    Commented Sep 30, 2019 at 12:30

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